PAGES Magazine articles

Stella J. Alexandroff1, A. Bonk2, M.J. Mette3 and T. Trofimova4

Early-career researchers (ECRs) are an important driving force of past global change research and often responsible for the bulk of data production and scientific output. ECRs of today also play a major role in shaping the future of science as they advance in their careers toward leadership positions. It is therefore in the interest of the community to enable ECRs to develop their full potential. Yet, ECRs across the globe tend to work under precarious conditions, lacking visibility, opportunity, and recognition amid an uncertain job market. Ways to support them include setting up networking and collaboration opportunities, providing training, and sharing advice on how to navigate the science world within and outside of academia. In addition, an important effort we can make as a community is to recognize and highlight the value of the work ECRs are doing today.

Over the course of the past 30 years, PAGES has been increasingly proactive in providing opportunities for ECRs. For instance, a large portion of financial support for PAGES' meetings is now designated for ECR attendance, particularly for those from developing countries, to encourage their participation in working groups (WGs). Perhaps the best example to illustrate the efforts by PAGES to support ECRs is the Young Scientists Meeting (YSM). Starting in 2009 and occurring every four years, the YSM brings ECRs from different parts of the world together to provide training and networking opportunities. It was at the third YSM (Zaragoza, Spain, 2017) that discussions about the need for stronger ECR representation within PAGES led to the creation of the PAGES Early-Career Network (ECN). The ECN officially launched in 2018 with the main goals of connecting ECRs to promote the exchange of ideas and skills, and to provide a framework for community support and collaboration.

Figure 1: Early-career researchers are an important part of the PAGES community. This special section of the magazine puts a spotlight on their work.

The articles in this section illustrate advances in past global change research by ECRs active in the PAGES community. The individual contributions were selected in an effort to represent the diversity in scientific scope and geographic distribution of PAGES' members.

The first eight contributions in this section summarize recent developments and findings in original research. Grant and Naish kick things off with a visit to the Pliocene and new estimates for global sea-level variability and ice-volume sensitivity (p. 34). Next, King and Tetzner explain how novel ice-core proxies in the form of marine-sourced organic compounds and diatoms can improve our understanding of sub-Antarctic climate (p. 36). From here, we move to the Northern Hemisphere, where Chaudhary assesses peatland carbon dynamics across the pan-Arctic and its potential effects on climate (p. 38), while Liang et al. share recent advances in eolian processes and landscape dynamics research in Chinese deserts (p. 40). Three contributions present the multifaceted applications of lake sediments – from erosion patterns and flood chronicles in Europe (Rapuc et al. p. 42), to baselines for conservation efforts in Mount Kenya (Omuombo p. 44), to the question of the onset and magnitude of human influence in central Chile (Fuentealba et al. p. 46). Lawman et al. conclude the original data contributions with a look into coral proxy system modeling and the fidelity of tropical Pacific corals as archives of ENSO variability (p. 48).

The final two articles demonstrate excellent ways for ECRs to collaborate and advance their respective research fields. In an elegant metadata analysis, Kaushal et al. assessed the availability of terrestrial Indian paleoclimate records to identify data gaps and list recommendations on how these can be improved (p. 50). In the closing article of this section, Mette et al. describe their experience coordinating a horizon-scanning project in which they defined priority research questions in the field of sclerochronology with input from the research community (p. 52).

For ECRs, it is particularly important to build a track record of international collaborations beyond their own research departments. This can seem like a daunting task for those who have yet to establish a wide research network or access to ongoing projects. Fortunately, PAGES and the PAGES ECN provide organized structures that lend themselves to establishing international science projects. The potential for high-impact and cutting-edge research coordinated and driven by ECRs within PAGES is growing, and will no doubt continue to do so in future generations.

affiliations

¹College of Life and Environmental Sciences, University of Exeter, UK

²Division of Geomorphology and Quaternary Geology, Institute of Geography, University of Gdańsk, Poland

³US Geological Survey, St. Petersburg Coastal and Marine Science Center, FL, USA

⁴NORCE Norwegian Research Centre, Bjerknes Centre for Climate Research, Bergen, Norway

contact

Stella Alexandroff: s.alexandroffbangor.ac.uk

Dr. Govind Ballabh Pant passed away suddenly on 18 November 2020. Govind was an outstanding scientist and science leader with a wide range of contributions across the whole climate spectrum, from paleoclimate to future climate scenarios. Govind served as a member of the PAGES Scientific Steering Committee from 1997 to 1999 and played a key role in building and nurturing a strong PAGES community in South Asia.

Govind worked at the Indian Institute of Tropical Meteorology (IITM) in various capacities for more than three decades, including as Director from 1997 to 2005. After his retirement from IITM in 2007, he returned to teaching as a Visiting Professor at the School of Environment and Natural Resources at Doon University, Dehradun, India, and subsequently as a Distinguished Professor in the Department of Atmospheric and Space Sciences, Savitribai Phule Pune University, Pune, India.

Govind's research interests included atmospheric energetics, monsoon dynamics, the ENSO-monsoon relationship, seasonal prediction, climate and climate change, and especially paleoclimatology. He was a fellow of the Maharashtra Academy of Sciences and the Indian Meteorological Society (IMS) as well as the recipient of the K.R. Ramanathan gold medal of the Indian Geophysical Union and an IITM Silver Jubilee award. He was the principal author of two books: Climates of South Asia and Climate Change in the Himalayas. Along with having served as the President of the IMS and as a member on the editorial boards of many research journals including the International Journal of Climatology, he contributed to many national and international bodies in climate science and published numerous research papers in reputed scientific journals.

Govind's international leadership contributions to the International Geosphere Biosphere Programme (IGBP), World Climate Research Programme (WCRP), Intergovernmental Panel on Climate Change (IPCC) and PAGES are highly acclaimed. Govind was associated with the IPCC right from its inception and was the first Indian climate scientist invited to contribute to the First Assessment Report. He continued to support the subsequent assessments and served as the review editor for the Fourth Assessment Report of IPCC WGI in 2007; he received a certificate of appreciation from the IPCC for his contribution to the report when the Panel was awarded the Nobel Peace Prize in 2007. Govind made special efforts to promote these international programs within the Indian scientific community, including through organizing meetings of the governing bodies of PAGES, WCRP and IGBP at IITM. He hosted a PAGES SSC meeting in February 2000, which was attended by the entire PAGES leadership at that time, along with a PAGES workshop on South Asian paleoenvironments.

Trained in tree-ring labs in Tucson and Palisades, USA, under a UNDP fellowship, Govind established the first dendroclimatology laboratory in India at IITM in 1982 and built a multi-institutional team to reconstruct monsoon variations over the past few centuries with an interannual resolution. He passionately nurtured it over the years, and it grew into a leading international leading dendroclimatology laboratory. He also played a pivotal role in bringing the paleoclimatological community closer to the meteorological community, which facilitated a more consolidated view of the entire spectrum of climate variability across India.

Govind published pioneering work in 1981 on the quantitative evaluation of the relationship between the Southern Oscillation and Indian summer monsoon rainfall. His visionary contributions to climate change research at IITM led it to be recognized internationally as an authentic source for global and regional climate change projections. In fact, the seeds for the establishment of the Centre for Climate Change Research at IITM were sown during the implementation of the Indo-UK program of research in which he secured two high-profile projects for IITM.

Govind's international network is far and wide, and he will be remembered more as a dear friend than as a professional collaborator. His sudden passing is certainly a great loss to the climate community, particularly to the dendroclimatic community in India. Govind leaves behind his wife Gita, son Saurabh and daughter Aparna, who played host to many a climate scientist and provided a unique family touch to his collaborations.

affiliation

International CLIVAR Monsoon Project Office, Indian Institute of Tropical Meteorology, Pune, India

contact

Rupa Kumar Kolli: rkollitropmet.res.in

references

Pant GB, Rupa Kumar K (1997) Climates of South Asia. Wiley, 344 pp

Pant GB et al. (2018) Climate Change in the Himalayas. Springer 145 pp

Boris Vannière1,2, D. Colombaroli3 and M.J. Power4,5

Paleofire research, which was the focus of the PAGES Global Paleofire Working Group over the past 12 years, offers a unique approach to understanding the environmental and social implications of large-scale disturbances associated with changing fire regimes at regional and continental scales.

Recent episodes of destructive fires, seen in media worldwide, have been referred to as "mega-fires" (Williams 2013). In the past decade, nearly every continent has experienced fires of unusual magnitude, calling into question humanity's ability to accept fire as a natural process with which we should coexist (Moritz et al. 2014). Fire scientists are beginning to recognize how humans have been responsible, in many ways, for patterns and consequences of fire occurrence that pervade ecosystems today. Even more critical is acknowledging how our species has progressively promoted conditions for fires to occur over the past centuries and millennia by the sustained conversion of landscapes into fire-prone ecosystems. Humans have become one of the greatest sources of fire while simultaneously creating more fire-prone weather through changing the Earth's climate (Pyne 2015).

Global Paleofire Working Group

Over the past two decades, the number of scientific papers on past fire regimes has increased steadily (e.g. Aleman et al. 2018). Some of these publications were products of an ambitious research project driven by the PAGES Global Paleofire Working Group (GPWG; Power et al. 2008a; Vannière et al. 2016a). Indeed, the long-term perspective offered by paleofire research provides a unique approach to understanding environmental dynamics through time, including the ecological consequences of large-scale disturbances like mega-fires. Such long-term perspectives highlight the multiple factors driving fire regimes and capture the long-lasting effects on ecosystems.

Improving our knowledge of ecological legacies is one of the many opportunities that paleoarchives offer (Whitlock et al. 2010; Power and Vannière 2018). Ecologists have long understood that fire regimes evolve over long timescales, often beyond the ability of modern observations to disentangle forcings and responses, justifying the need for paleofire perspectives. This is most evident in recent trends of increased occurrence of catastrophic fires, emphasizing the critical need to understand and contextualize these transformative processes in the modern world. Interrogating and disseminating knowledge on the history of fire and its role in shaping ecosystems is a fundamental objective for maintaining a habitable Earth where all species may thrive, despite the destructive nature of these fine-scale processes with global consequences.

The GPWG was formally launched in 2008 after several years of collaborative work around the implementation of a global fire history database (Power et al. 2008b; Marlon et al. 2008). The main objective was to centralize a growing volume of fire history data, scattered throughout publications, laboratories and research programs around the world. This unique dataset made novel estimations of millennial-scale changes in biomass burning at global scales possible, as fire scientists began to understand the causes and responses of those changes (e.g. Marlon et al. 2013; Vannière et al. 2016b). This empirically based understanding of fire allowed GPWG to test new hypotheses while evaluating and improving climate models that integrate fire as a key element of the global carbon cycle (Harrison et al. 2018). Additionally, global paleofire data have become a critical resource for estimating the probability of fire occurrence under the constraints of past and future climate change scenarios (Daniau et al. 2012; Lestienne et al. 2020).

Figure 1: Location of Global Paleofire Working Group workshops and congress sessions over the past 12 years.

The GPWG operated for 12 years in two distinct phases (Power et al. 2008a; Vannière et al. 2016a): GPWG (2008–2015) and GPWG2 (2016–2019). During this time, 18 workshops and congress sessions were organized in 11 countries and 16 cities, bringing together scholars from more than 60 countries (Fig. 1). Based on these scientific meetings, which ranged from day-long to week-long events, more than 50 scientific papers emerged from the new collaborations promoted by the GPWG. PAGES news and Past Global Changes Magazine published 12 meeting-outcome papers and dedicated a full issue to paleofire research (Whitlock et al. 2010), including topics such as regional paleofire reconstructions, calibration, and data-model comparisons. A full issue of Quaternary International reported on the PAGES-GPWG session at INQUA 2015 at Nagoya, Japan (Power and Vannière 2018).

The main objectives of the community workshops were to collectively define priority research areas in paleofire science, to collect data through the sharing of the regional expertise of the participants, to support the emergence of early-career researchers, and to reach out to as many researchers as possible from countries where paleofire research had received limited support.

Data, expertise, and outreach

As the GPWG transitioned from the early community-growth phase into phase two, new challenges and research agendas emerged. During the workshops of the GPWG2 phase, a reflection on intellectual gaps in knowledge and a need for additional collaborative work was carried out with a focus on targeting policy makers and environmental managers. The priority for GPWG2 was to apply and transmit scholarly research into action, by emphasizing theoretical reference frames and quantified estimates of biomass burning, and by connecting areas of expertise on long-term environmental processes associated with past and current fire regime changes (Marcisz et al. 2018). One of the long-standing challenges in this community has been the integration of more applied research in communicating with stakeholders. In 2019, the GPWG released a first policy brief (Colombaroli et al. 2019) to identify best practices for sustainable ecosystem management, including how transdisciplinary knowledge (such as paleoecology and Indigenous knowledge) can better inform fire management and policy.

As the paleofire community has evolved, knowledge has been acquired about the drivers and circumstances of fire regimes, the role of anthropogenic fire practices since prehistoric times, and fire ecology on a range of spatial and temporal scales. Translating this knowledge for practitioners has opened new dialogs on sustainable fire risk preparedness. Since fire is viewed by many as a dramatic and dangerous phenomenon, it naturally raises societal fears. Considerable national and local resources are focused on firefighting and suppression policies, which, unfortunately, in the current context of global change, have become insufficient for protecting human populations and the resources we depend on. Moreover, fire was the first of the natural elements – water, earth, fire, and air – to have been significantly altered by our species. Unlike most other elements, fire transcends spatial scales, from the smallest hearth to the largest mega-fires, and operates on all temporal scales, from rapidly changing ecosystems over a few minutes to shaping landscapes over millennia (Pyne 2015).

The large majority of the work and results facilitated by the GPWG are based on the Global Paleofire Database (https://database.paleofire.org). The original goal of the Global Charcoal Database was to integrate all dated, quantitative sedimentary fire-history series (i.e. records of sedimentary charcoal) previously published in the scientific literature. Numerous efforts were put forth to synthesize and compare these fire-history series at regional, continental, and global scales to reconstruct temporal changes in biomass burning (Power et al. 2008b; Vannière et al. 2014).

Fire history, drivers, and impacts

Several key discoveries have emerged over the past decade because of these efforts. The first lesson was that for a very large majority of the world's ecosystems, biomass burning has increased continuously since the Last Glacial Maximum (~21,000 years ago) in response to long-term changes in (1) climate; (2) vegetation, i.e. the amount of biomass available; and (3) human land use. In contrast to the last ice age when the fire signal was very weak in most of the world's ecosystems, the Holocene shows increased spatial heterogeneity in fire activity from one region of the globe to another (Power et al. 2008b).

A second lesson from these efforts was that increasing temperatures is the most important driver of past fire activity. Additionally, abrupt increases in fire activity are linked to intermediate moisture levels that, on the one hand, favor vegetation growth and, on the other hand, can lead to periods of fire-prone drought (Colombaroli et al. 2014; Daniau et al. 2012). For example, during the last glacial-interglacial transition, and at the beginning of the Holocene, a time of maximum solar insolation, many ecosystems on the planet burned regularly, depending on the biomass availability, and in a relatively sustained manner when compared to the modern period (for example Lestienne et al. 2020).

A third lesson has emerged about the role of anthropogenic fire: during the middle and late Holocene, vegetation communities were increasingly modified by human activities; at this time, anthropogenic activities began to override climate as the major player in maintaining and modifying fire regimes in many ecosystems. Evidence from the boreal region (Blarquez et al. 2015), the equatorial region (Colombaroli et al. 2014), the temperate region in Europe (Dietze et al. 2018), and the Mediterranean (Vannière et al. 2016b) supports these findings.

Perhaps the most significant lesson derived from the efforts of the GPWG was that across the planet's biomes and ecosystems, it remains challenging to disentangle natural from anthropogenic drivers of fire and related feedbacks. Similarly, it is still unclear whether vegetation ultimately drives a particular type of fire regime or whether the introduction of fire encourages the expansion of fire-adapted plant formations (Feurdean et al. 2020). However, the emerging collaborative work on these challenges suggests that following a shift in fire regime and/or vegetation composition, a new dynamic balance is established, at least until changes in climate and/or human activities disrupt the system once again.

Figure 2: A slash-and-burn plot in the tropical dry forest of south-Yucatan, Mexico (Image credit: Boris Vannière, 2004).

Lessons from the past

Today, paleofire research suggests that the spatial expression of burning has become more regionally heterogeneous throughout the past 10,000 years, particularly as humans increasingly altered natural fire regimes (Fig. 2). Although the precise timing and regional chronologies of human impacts on fire remain highly variable in space, these findings agree with regional histories of land colonization and cultural changes (Connor et al. 2019). Increasing evidence for regional and even continental-scale human-fire legacies on long timescales are beginning to question old paradigms (Blarquez et al. 2015; Colombaroli et al. 2014).

For example, in Europe, the human footprint on fire regimes extends at least to the beginning of the Neolithic period, i.e. between 9000 and 7000 years ago (Dietze et al. 2018). This may have taken the form of increased fire frequency in exploited ecosystems, which indirectly caused a decrease in the magnitude of large-scale events (extent and intensity). As landscapes became more fragmented, fuel loads were altered and fire regimes were permanently changed from pre-human intervention. (Fig. 3; Vannière et al. 2016b).

Novel fire and vegetation reconstructions are also challenging assumptions regarding biodiversity. It has recently been recognized, for example, that human activities may promote and maintain optimum fire conditions, which in turn maximize plant diversity in ecosystems; in this way, long-term anthropogenic behavior can have a positive impact on biodiversity. For example, in the Iberian Peninsula, Connor et al. (2019) demonstrated that changes in fire regime and vegetation diversity correspond with long-term human-environment interactions beginning as early as 7500 years ago. This new evidence suggests that Neolithic burning promoted vegetation openness and increased woodland diversity ~5000 years and again ~2000 years ago, when intensification and acceleration of the human landscape transformation led to permanent transitions in ecosystem state. In this case, human-driven fires favored open vegetation diversity, disrupted woodland diversity, and meaningfully decreased landscape richness on a regional scale.

During the Holocene, the frequency, size, and intensity of fires may have been much greater or more intense than even the "mega-fires" observed in recent years (Lestienne et al. 2020). The media's portrayal of mega-fires promotes frightening news summaries with discussions of the unprecedented nature of recent events. Considering the amplitude of past climate changes and the occurrence of hundreds of major cultural transitions around the world, the paleofire community has much to add to these discussions, yet what makes forest fires gain media attention worldwide today is the socio-ecological context in which they occur. The expansion of private and commercial properties and infrastructure into the wildland–urban interface all but ensures future clashes between large-scale wildfires and an expanding human population. In addition, modern land management and resource exploitation, far removed from traditional land-use systems, has abruptly changed rates of fuel accumulation and fuel structure, often leading to fire-prone conditions in anthropogenic landscapes.

Figure 3: Density distribution of (A) biomass burned and (B) fire frequency proxies for 16 high-resolution records of south-western Europe (adapted from Vannière et al. 2016b). The colors represent the density of the proxy site-records at each time step; cold (blue and green) colors are indicative of highly dispersed data and thus capture the heterogeneity among sites; hot (yellow to dark red) colors indicate sites with homogeneous responses and thus spatial uniformity. The white dashed lines separate time periods with significant differences in the proxies' mean and variance. Ages are expressed in calibrated kiloyears before present (1950 CE).

As an example, on the island of Corsica in the Mediterranean Sea, Lestienne et al. (2020) coupled data and models to show that currently, and likely for the first time in the Holocene, the fire regime is constrained by both climatic and anthropogenic factors. Climatic conditions may lead to events similar to the maxima in the paleofire record, but human activities may also increase their frequency. Moreover, these events will take place in very different ecosystems than in the past that are possibly not adapted to such events, therefore posing different levels of risk. At the beginning of the Holocene, summer climatic conditions promoted an extended fire season and large fires in pine forests. About 7000 years ago, climatic conditions became much less favorable for the natural spread of fires, and human land uses explain the recorded fire events. Today, based on the same criteria and markers, it appears that the conditions and length of the summer drought season are reaching levels equivalent to those at the beginning of the Holocene and may exceed them in the coming years. In addition to this, human pressure on ecosystems, as we know them today, is far greater compared to the beginning of the Holocene.

Summary and outlook

Over the past decade, the GPWG has contributed to the international community effort to understand present fire patterns in the context of the long-term changes, with:

• estimates of baseline trends and variability in fire regimes on orbital to decadal timescales and at regional to global spatial scales;
• the online sharing and public dissemination of all fire history data collated at https://database.paleofire.org;
• data-model integration studies that have been used for future projection assessment based on long-term archive observations;
• the different roles of climate, humans, and vegetation as the co-drivers of past fire regimes;
• the development of projects that addressed challenges in conservation, restoration, and biodiversity maintenance under changing climate and land-use conditions;
• the growth and advancement of early-career paleofire scientists; and
• an emphasis on improving the dialog with fire managers and sustainable fire management practices.

Several key challenges remain for the global paleofire community. Many regions of the world remain insufficiently documented in terms of fire history and changing fire regimes through time (for example equatorial Africa and tropical environments). These knowledge gaps require further research to better inform the response to future environmental challenges in terms of how these systems will evolve with management that either includes or excludes policies regarding fire. The paleofire community must intensify efforts to identify knowledge gaps and promote research in critical regions of future change. Stimulating additional paleofire data generation, data synthesis, and novel research are imperative across the following themes:

• Investing in resources to implement new fire-proxy calibration in underrepresented regions, for example by promoting research activities and network building in Asia or Africa;
• A concerted effort of cross-disciplinary integration to promote more diverse knowledge for environmental policy assessment, particularly focusing on local/Indigenous knowledge (Colombaroli et al. 2019);
• Improving our understanding of global fire variability and impacts by integrating the existing fire database (https://database.paleofire.org) with modern observations in a way that can be accessed by other non-specialists, including ecosystem managers and policy makers.

As the paleofire community moves forward, more investment in programs similar to PAGES' recently launched DiverseK working group (pastglobalchanges.org/diversek), which will pursue initiatives related to recent GPWG activities (Colombaroli et al. 2018), is critical. Finally, the PAGES-endorsed International Paleofire Network (https://paleofire.org; Adolf et al. 2020) will make significant contributions toward addressing these challenges in the coming years.

Acknowledgements

The authors warmly thank all students and scientists who contributed to the GPWG's activities over the past 12 years.

affiliations

1MSHE, CNRS, Université Bourgogne Franche-Comté, Besançon, France

2Chrono-environnement, CNRS, Université Bourgogne Franche-Comté, Besançon, France

3Centre for Quaternary Research, Department of Geography, Royal Holloway University of London, UK

4Natural History Museum of Utah, University of Utah, Salt Lake City, USA

5Department of Geography, University of Utah, Salt Lake City, USA

contact

Boris Vannière: boris.vanniereuniv-fcomte.fr

references

Adolf C et al. (2020) PAGES Mag 28: 62

Aleman J et al. (2018) Fire 1: 7

Blarquez O et al. (2015) Sci Rep 5: 13356

Colombaroli D et al. (2014) Glob Change Biol 20: 2903-2914

Colombaroli D et al. (2019) Diverse knowledge informing fire policy and biodiversity conservation (policy brief). Royal Holloway University of London, 8 pp

Colombaroli D et al. (2018) PAGES Mag 26: 89

Connor S et al. (2019) Holocene 29: 886-901

Daniau AL et al. (2012) Global Biogeochem Cycles 26: GB4007

Dietze E et al. (2018). Quat Sci Rev 201: 44-56

Feurdean A et al. (2020) Biogeosciences 17: 1213-1230

Harrison SP et al. (2018) Earth Syst Dyn 9: 663-677

Lestienne M et al. (2020) Fire 3: 8

Marlon JR et al. (2008) Nat Geosci 1: 697-702

Marlon JR et al. (2013) Quat Sci Rev 65: 5-25

Marcisz K et al. (2018) Open Quat 4: 1-7

Moritz MA et al. (2014) Nature 515: 58-66

Power M et al. (2008a) PAGES news 16: 39-40

Power M et al. (2008b) Clim Dyn 30: 887-907

Power M, Vannière B (2018) Quat Int 488: 1-2

Pyne S (2015) The fire age. Aeon, https://aeon.co/essays/how-humans-made-fire-and-fire-made-us-human

Vannière B et al. (2014) PAGES Mag 22: 40

Vannière B et al. (2016a) PAGES Mag 24: 31

Vannière B et al. (2016b) Quat Sci Rev 132: 206-212

Whitlock C et al. (2010) PAGES news 18: 53-96

Williams J (2013) Forest Ecol Manage 294: 4-10

Chronis Tzedakis 1, L. Menviel2, E. Capron3, B.L. Otto-Bliesner4, J.F. McManus5, D. Raynaud3 and E. Wolff6

Part of the scientific rationale for pursuing studies of past interglacials is that they provide a baseline against which to assess the climatic evolution of the current interglacial and the impact of anthropogenic interference. Here, we trace the history of the PAGES working groups on interglacials (PIGS and QUIGS).

Prelude

When we look at the past, our attention is often captured by the allure of the recent (the last couple of millennia) or the shock of the extreme (a planet plunging into an ice age). However, although the past provides no exact analog for the next century and beyond, it is interglacials that provide examples that are most relevant for assessing the current anthropogenic warming, including its influence on the cryosphere and the feedbacks associated with biogeochemical cycles. For that reason, the study of the full range of past interglacials, their climate variability and impacts, their initiation and their ending, are a cornerstone of PAGES' research.

PIGS

On the evening of 30 January 2007, Jerry McManus, Dominique Raynaud and Chronis Tzedakis were talking quietly in a corner of the Captain Kidd bar at Woods Hole Village, on Cape Cod, MA, USA. It had been a good scientific meeting. After two days of talks on Marine Isotope Stage (MIS) 11, the workshop organized by McManus was drawing to a close. In fact, the entire working group on MIS 11, sponsored by the International Union for Quaternary Research (INQUA; inqua.org), was drawing to a close. "Where do we go from here?" the trio asked. "Perhaps we could learn more, if we looked at the whole ensemble of interglacials in a systematic way," said Raynaud. Not entirely clear-headed, they called it "PIGS" for Past Interglacials, and the name stuck (pastglobalchanges.org/science/wg/former/pigs/intro).

Figure 1: Interglacials of the last 800 kyr. (A) Precession parameter, plotted on an inverse vertical axis (Berger 1978). (B) Obliquity (Berger 1978). (C) Atmospheric CO2 concentration from Antarctic ice cores (Lüthi et al. 2008). (D) δD of ice in the EPICA EDC ice core, Antarctica (Jouzel et al. 2007). (E) δ18Obenthic record from the LR04 stack (Lisiecki and Raymo 2005). Marine isotope stages and substages of interglacial status are shown. Vertical dotted lines indicate the timing of precession minima (red) and obliquity maxima (blue). Note the three longer interglacials (MIS 11c, 13a, 17) where precession and obliquity are nearly opposite in phase (Tzedakis et al. 2012). Modified from Tzedakis et al. (2009).

They decided to approach PAGES for sponsorship. In its earlier days, PAGES had been focused primarily on the Holocene, but it was now under new management. Paleoceanographer Thorsten Kiefer had recently become the executive director and was possibly amenable to expanding the scope of PAGES to longer timescales. The plan for PIGS was to focus on the last 800 thousand years (kyr), as Antarctic ice-cores were furnishing information on atmospheric concentrations of greenhouse gases over that interval, thereby providing constraints on global boundary conditions. Kiefer was indeed receptive to such a prospect, and later in 2007 a proposal for a PAGES working group on Past Interglacials was accepted by the PAGES Scientific Steering Committee (SSC).

The first PIGS workshop (2-4 October 2008; pastglobalchanges.org/calendar/past/2008-past/127-pages/1024) assessed the then state of knowledge and defined specific priority topics that would form the agenda of subsequent workshops. A PIGS-community progress article (Tzedakis et al. 2009) pointed to the large diversity among interglacials in terms of their intensity, structure, and duration (Fig. 1), but also noted that a general theory accounting for this remained elusive. In essence, an underlying ambition of all PAGES interglacial working groups has been to elucidate some general principles governing this diversity.

The second PIGS workshop (24-27 August 2009; pastglobalchanges.org/calendar/past/2009-past/127-pages/1023) proved a resounding success and failure, in equal measure. Over the course of four days, it showed the full potential of small group meetings (~30 participants), as an interglacial community of like-minded scientists began to form, freely exchanging ideas, showing each other unpublished data, and intensely debating general issues and finer points. A modus operandi emerged where community papers would be planned, but otherwise participants were free to pursue their own avenues of research independently, energized by the discussions at the meetings. This caused some consternation at PAGES headquarters, as some papers never formally acknowledged PIGS, although others did (e.g. Mokeddem et al. 2014), but overall the science moved forward. One of the ideas pursued during the meeting was the role of millennial-scale variability as an intrinsic feature of the past five glacial terminations, and plans were made for a PIGS-community paper on this topic. All the initial excitement, however, evaporated a few weeks later, after a brilliant paper on Ice Age Terminations was published by a different group (Cheng et al. 2009), making essentially the same point. The PIGS paper was abandoned.

The third PIGS workshop (20-22 October 2010; pastglobalchanges.org/calendar/past/2010-past/127-pages/1022) focused on interglacial duration and glacial inception. Although estimates of interglacial durations are sensitive to the definition of interglacial conditions in different proxies and archives, it was thought that robust patterns could emerge from a systematic comparison of interglacials. One outcome was a paper (Tzedakis et al. 2012) arguing that the fundamental concept underlying the terminology of an interglacial is that of the sea-level highstand, a measure of integrated global climate effects that lead to the loss of continental ice; by extension, interglacial length was linked to the duration of the highstand. On this basis, it suggested that over the last 800 kyr, the phasing of precession and obliquity influenced the persistence of interglacial conditions over one or two insolation peaks, leading to shorter (~13 kyr) and longer (~28 kyr) interglacials (Fig. 1).

The fourth PIGS workshop (2-5 July 2012; pastglobalchanges.org/calendar/past/2012-past/127-pages/1012) focused on how well we can explain the diversity of interglacials from the forcing and feedbacks and attempted to place interglacials within the wider context of ice-age cycles and the extent to which these are deterministic. A community paper to develop the major themes considered over the course of PIGS was planned, and a follow-up writing workshop brought together the lead authors for each section of the paper at Louvain la Neuve, Belgium, in March 2013. Eric Wolff took up the gargantuan task of editing and putting the different sections together. The landmark paper "Interglacials of the last 800,000 years" appeared in 2016 (Past Interglacials Working Group of PAGES 2016), condensing in 58 pages the then state of knowledge. It proposed that an objective definition of an interglacial is the absence of substantial Northern Hemisphere ice outside Greenland. A corollary of this is the occurrence of more than one interglacial within MIS 7 and MIS 15 (Fig. 1). Thus, interglacials of the past 800 kyr do not occur every 100 kyr, and therefore attempts to predict the onset of interglacials need to account for this irregular return time. The review corroborated the crucial role that millennial-scale climate change (involving rapid changes in Atlantic Meridional Overturning Circulation strength) plays in each glacial termination. It highlighted MIS 5e as the interglacial that experienced the warmest conditions of the last 800 kyr across the globe. Taking a look into the future, the paper concluded that the next glacial inception is unlikely to occur in the next 50 kyr, given the combined effect of the current low eccentricity and high atmospheric greenhouse gas concentrations.

QUIGS

The PAGES-PMIP Working Group on Quaternary Interglacials (QUIGS; pastglobalchanges.org/quigs) arose from an initiative of Bette Otto-Bliesner, who envisaged a more formal connection between the successor to PIGS and the Paleoclimate Modelling Intercomparison Project (PMIP; https://pmip.lsce.ipsl.fr). More specifically, QUIGS would promote a closer collaboration between modelers and the data community to provide expertise on experimental design, data compilations and model-data comparisons, and to assess the relevance of interglacials to understanding future climate change.

With the guidance of PAGES SSC members Hubertus Fischer and Michal Kucera, an ambitious QUIGS working group structure, comprising two three-year QUIGS phases with a one-year gap, was envisaged: in Phase 1, QUIGS would formulate research questions, identify knowledge gaps, and plan how to fill these gaps. In close collaboration with PMIP, it would define model protocols and initiate the model runs and data collection needed. In Phase 2, the working group would return to the research questions identified in Phase 1, with better datasets and new model experiments, ultimately aiming to gain a quantitative understanding of interglacial controls. The four-year return period (between the beginning of Phase 1 and the beginning of Phase 2) would therefore provide the necessary time to complete the tasks.

QUIGS, led by Bette Otto-Bliesner, Emilie Capron, Anne de Vernal, Eric Wolff, and Chronis Tzedakis, was formally approved by the PAGES SSC in May 2015. A little later, Andrea Dutton, Anders Carlson, and Laurie Menviel joined the team.

The first QUIGS workshop on Warm Extremes (9-11 November 2015; pastglobalchanges.org/calendar/2015/127-pages/1520) assessed the current knowledge and research needs on the temporal and spatial patterns of climate forcing, responses, and feedbacks during MIS 5e and MIS 11. Paleorecords and climate model simulations highlighted the need for an improved understanding of the magnitude and drivers of the enhanced warmth during MIS 5e and 11. This led to the definition of model protocols for CMIP6 and PMIP4 Last Interglacial simulations (Otto-Bliesner et al. 2017), and also of surface-climate data benchmarks for high-latitude regions (Capron et al. 2017).

The second QUIGS workshop (18-20 October 2016; pastglobalchanges.org/calendar/2016/127-pages/1592) examined patterns of climate forcing, feedbacks, and responses characterizing glacial terminations. It assessed common features and differences between Terminations I and II (TI and TII), and highlighted the need for improved chronologies and for constraining the size and spatial distribution of ice sheets during the penultimate glacial maximum. This led to an article presenting a protocol for transient simulations of TII (140-127 kyr BP) under the auspices of PMIP4, as well as a selection of records, providing appropriate benchmarks for subsequent model-data comparisons (Menviel et al. 2019).

Figure 2: Effective energy for deglaciation (= peak caloric summer insolation + [(time elapsed since interglacial onset) × (discount rate)]) at each insolation peak during the past 2.6 Myr. Each insolation peak is plotted as the onset of an interglacial (red circles), a continued interglacial (black diamonds) or an interstadial (light blue triangles); open symbols correspond to uncertain assignments. The dotted line separates complete deglaciations from incomplete and missed deglaciations; the ramp indicates a gradual rise in the threshold required for a complete deglaciation. Numbers refer to Marine Isotope Stages. Modified from Tzedakis et al. (2017).

Our understanding of glacial-interglacial cycles has been built on a large body of evidence from Middle and Late Pleistocene environments, dominated by ~100-kyr ice-volume variations. However, any theory of ice ages remains incomplete if it does not include an adequate description and understanding of the mode and tempo of climate variability during the Early Pleistocene (the so-called "41-kyr world") and the transition into the "100-kyr world" (Mid-Pleistocene Transition, MPT). With this in mind, the third QUIGS workshop (28-30 September 2017; pastglobalchanges.org/calendar/2017/127-pages/1655) explored the characteristics of interglacials of the 41-kyr world and considered causes of the MPT (Ford and Chalk 2020). Despite the emergence of boron-based CO2 data from marine cores, progress in modeling 41-kyr cycles in sea level and the causes of the MPT requires CO2 reconstructions with reduced uncertainties. The planned drilling of an "Oldest Ice" core back to 1.5 million years ago (Myr BP) will eventually provide increased confidence on the evolution of the climate-carbon cycle interactions for this period.

One of the outstanding questions identified by PIGS in their final review paper was: "Given the astronomical forcing and the feedbacks that are present, is the occurrence and character of interglacials predictable? In other words, … is it inevitable that we would find ourselves in today's interglacial climate following the same sequence of glacial and interglacials that has occurred?" (Past Interglacials Working Group of PAGES 2016, p. 206). An initial answer to this was provided by Tzedakis et al. (2017), who proposed that an interglacial onset occurs when a peak in insolation exceeds a threshold that decreases with time elapsed since the previous deglaciation, as ice sheets become more unstable. This correctly predicted the deglaciation history during the Quaternary and identified a gradual rise in the deglaciation threshold from ~1.5 Myr BP that led to an increase in the frequency of skipped insolation peaks after 1 Myr BP (Fig. 2). The emergence of longer glacials then allowed the accumulation of larger and increasingly unstable ice sheets. The analysis also showed that the succession of interglacials is not chaotic; the sequence that has occurred is one among a very small set of possibilities, suggesting a degree of probabilistic determinism.

Between Phases 1 and 2, QUIGS and another PAGES working group, PALeo constraints on SEA-level rise (PALSEA), held a joint workshop from 24-27 September 2018 (pastglobalchanges.org/calendar/2018/127-pages/1759). The goal of the workshop was to identify the state of our understanding on the interplay between climate, polar ice sheets, and sea level during past interglacial periods. A position paper identified eight research areas as critical for an improved understanding of climate and ice-sheet responses to astronomical and greenhouse gas forcing, and by extension, responses to conditions similar to or warmer than the pre-industrial climate (Capron et al. 2019).

2019 marked the start of the second phase of QUIGS in which improved datasets and new model experiments are being used to address research questions and knowledge gaps identified during Phase 1. A workshop on "Warm extremes - MIS 5e and its relevance to the future" (1-4 July 2019; pastglobalchanges.org/calendar/2019/127-pages/1910) played a key role in focusing community efforts (model and data) to publish relevant science for the forthcoming 6th Assessment Report of the IPCC. In particular, papers comparing the ensemble of new CMIP6-PMIP4 lig127k simulations and proxy reconstructions of surface temperature and sea ice were developed (Kageyama et al. 2021; Otto-Bliesner et al. 2021). The latter paper showed that the model ensemble was able to simulate the reconstructed 127-kyr BP JJA temperature anomalies over Canada, Scandinavia, parts of midlatitude Europe, and much of the North Atlantic (Fig. 3). The exceptions are in the northwestern North Atlantic and Nordic seas, where the marine reconstruction suggests significant cooling (Capron et al. 2017). Potential reasons for mismatches include dating uncertainties, a lingering memory of the H11 event in marine records (Marino et al. 2015), and/or the design of the CMIP6-PMIP4 lig127k protocol without meltwater from potential remnant ice sheets over Canada and Scandinavia.

Figure 3: Comparison of results of the CMIP6-PMIP4 lig127k simulations and proxy records. High-latitude surface temperature anomaly comparing 127 kyr BP to the preindustrial period from models (ensemble average in colors) and proxies (circles for the compilation by Hoffman et al. (2017); squares and diamonds for marine sites and ice cores, respectively, from the compilation by Capron et al. (2014, 2017); pluses for the compilation of Brewer et al. (2008); and triangles for the Arctic compilation, https://doi.org/10.5194/cp-17-63-2021-supplement): (A) 40°-90°N June-July-August, (B) 40°-90°S annual. The preindustrial reference is 1850 CE for model anomalies and for the data is 1870–1899. Modified from Otto-Bliesner et al. (2021), courtesy of A. Zhao.

In the midst of the COVID-19 pandemic, a virtual meeting on glacial termination processes and feedbacks was held on 10 and 12 November 2020 (pastglobalchanges.org/calendar/2016/127-pages/1592). Seventy-five percent of the talks were given by early career researchers (ECRs), who presented advances in understanding of deglacial changes in climate, ice sheets, the carbon cycle, and vegetation. Further meetings on terminations and the MPT are planned. A final workshop on one of the most challenging issues, the causes of interglacial intensity, will provide a fitting close to the PAGES interglacial effort.

Coda

Looking back from today's perspective of accelerating global warming, initiating a working group with a specific focus on interglacials appears the obvious thing to have done. But in January 2007, with much attention centered on glacial climate variability, this was not necessarily obvious. From a small group of friends, the PAGES interglacial community grew to involve 69 (PIGS) and 95 (QUIGS) scientists, while ECR participation increased from 10% to over 40%. We are grateful to PAGES, and especially Thorsten Kiefer and Marie-France Loutre, for their encouragement and continued support in this endeavor. It has been a fabulous ride.

affiliations

1Environmental Change Research Centre, Department of Geography, University College London, UK

2Climate Change Research, Centre, PANGEA, University of New South Wales, Sydney, Australia

3Institut des Géosciences de l'Environnement, CNRS, Grenoble, France

4Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO, USA

5Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA

6Department of Earth Sciences, University of Cambridge, UK

contact

Chronis Tzedakis: p.c.tzedakisucl.ac.uk

references

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Capron E et al. (2019) Quat Sci Rev 219: 308-311

Cheng H et al. (2009) Science 326: 248-252

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Kageyama M et al. (2021) Clim Past 17: 37-62

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Lüthi D et al. (2008) Nature 453: 379-382

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Alessio Rovere1 and Andrea Dutton2

We provide an account of the past 13 years of activity of PALSEA, the PALeo constraints on SEA level rise (PALSEA) working group supported by PAGES and INQUA.

Prelude

Sea-level rise due to polar ice-sheet retreat in a warming world is one of the most important aspects associated with future climate change, yet remains challenging to project due to uncertainties in the dynamics of rapid ice-sheet retreat. The geologic record features major, and sometimes rapid, changes in ice sheets and sea level that offer an excellent opportunity to assess the rates, magnitudes, and processes involved in ice-sheet and sea-level change and how they are connected to climate forcing. The PALeo constraints on SEA level rise working group (PALSEA; pastglobalchanges.org/palsea) has developed an interdisciplinary network of paleoscientists who seek to pair the development and synthesis of datasets with geophysical modeling of ice and sea-level proxies. The overarching goal of PALSEA is to improve our understanding of the physical processes involved in ice-sheet dynamics and solid Earth responses, and to provide improved constraints for predicting sea-level rise in the future.

PALSEA started its activities in 2008 after the IPCC 4th Assessment Report: the working group was largely initiated by Mark Siddall, who gathered a group of paleoclimate scientists united by the goal of having a more coordinated role in the 5th Assessment Report. Today, PALSEA is a PAGES working group and an International Focus Group of INQUA (International Union for Quaternary Sciences; https://www.inqua.org). The following is the story of how PALSEA evolved over the past 13 years and its achievements in pushing the boundaries on paleo sea-level and ice-sheet science.

Figure 1: An illustrative sketch of how the relative sea-level record can vary at different sites across the globe across an interglacial sea-level highstand (modified from Siddall et al. 2010). Understanding the links between relative sea level and global mean sea level has been a theme within PALSEA since its inception.

2008–2012: The early years

The first meeting of the newly formed PALSEA group was held 25-29 August 2008 in Bern, Switzerland (pastglobalchanges.org/calendar/past/2008-past/127-pages/1082), and was organized by Mark Siddall, Thomas Stocker, Bill Thompson, and Claire Waelbroeck. It brought together experts from across the community to address how studying past records of sea-level change can add to our understanding of the climate system, and in turn inform future projections of sea-level rise. To foster interdisciplinary discussions, scientists with diverse areas of expertise were invited to attend: Earth and climate modelers, field geologists, and geochronologists. The idea was to facilitate a meeting of these experts, together with a mix of early-career researchers (ECRs), in a friendly and informal environment, to develop new interdisciplinary collaborations. Participants realized the meeting only scratched the surface regarding the various issues on paleo sea-level and ice-sheet reconstructions, some of which were summarized in a review paper (Siddall et al. 2010; Fig. 1).

One year later (21-25 September 2009; pastglobalchanges.org/calendar/past/2009-past/127-pages/1085), at Woods Hole, MA, USA (meeting organized by Bill Thompson, Mark Siddall, and Claire Waelbroeck), the working group met again to discuss the challenges of dating past interglacials. What had initially emerged during discussions at the first meeting became very clear: there was a need to establish a comprehensive Quaternary sea-level database, including standardized descriptions of dated samples and sea-level proxies. This goal would characterize PALSEA activities for the years to come.

The relaxed atmosphere of the first two meetings was in part attributable to the never-written "ground rule" of PALSEA: everyone should leave their ego at the door and should be ready to challenge and be intellectually challenged by others. The emphasis of PALSEA meetings was (and still is) community-building, and PALSEA strives to bring together people who are thinking about the same problem(s) but from different angles and using very different methodologies. Also, PALSEA has always had a strong emphasis on including a large contingent of ECRs.

During the 2010 meeting in Bristol, UK (organized by Glenn Milne, Mark Siddall, and David Richards; pastglobalchanges.org/calendar/past/2010-past/127-pages/1086), three other themes were brought to the table: (1) how to best use paleodata to constrain glacial-isostatic adjustment; (2) how to achieve better integration of archaeological archives of sea-level change in broader sea-level research; and (3) how to use data from past warm periods to better understand the response of sea level to warmer climates. The first theme was dissected into several overlapping topics one year later (24-26 August 2011; pastglobalchanges.org/calendar/past/2011-past/127-pages/1137) at Harvard University in Cambridge, MA, USA (meeting organized by Mark Siddall, Peter Huybers, and Jerry Mitrovica). For three days, the PALSEA community focused on maximum sea levels reached during past epochs, namely the Last Interglacial and the mid-Pliocene, and discussed the geologic evidence for or against rapid sea-level rises or falls in the Last Interglacial and since the Last Glacial Maximum. PALSEA also tackled the issues around the processes that (de)stabilize ice sheets and on the interactions between the cryosphere and the climate system. Last but not least, the PALSEA community started to direct its attention to the implications of paleo sea-level studies on our ability to understand modern ice sheets and sea-level changes.

It was with this focus in mind that the community met 4-8 June 2012 in Madison, WI, USA (meeting organized by Anders Carlson and Mark Siddall; pastglobalchanges.org/calendar/past/2012-past/127-pages/970). This meeting was centered on dissecting the current knowledge on ice-sheet and climate interactions at multiple timescales (Holocene to Pliocene) and in different regions (Greenland and Antarctica). Five years after the first PALSEA meeting, the Madison meeting was also the opportunity for the PALSEA founders to pass the baton to a new leadership team, who would lead PALSEA for the years to come.

The first five years of PALSEA ended with the Madison meeting. During the timeframe 2008–2012, lively discussions at the PALSEA workshops inspired several papers. Among them, an appraisal of ice-sheet responses to past climate forcings (e.g. Carlson and Winsor 2012; Gregoire et al. 2012) and several papers exploring past changes in sea level (e.g. Andersen et al. 2010; Raymo and Mitrovica 2012; Dutton and Lambeck 2012). Within its first five years, PALSEA also benefited from additional funding from the Worldwide Universities Network (WUN) and INQUA, which further enabled the group to support wide and diverse participation, particularly by ECRs. INQUA has remained a devoted supporter of PALSEA since these early years.

Figure 2: Comparing the magnitude of peak sea level during recent past warm periods. Modified from Dutton et al. (2015).

2012–2017: Exploring new grounds

The new PALSEA leaders were Anders Carlson, Andrea Dutton, Glenn Milne, and Antony Long. At the group meeting in Rome (21-24 October 2013, organized by Andrea Dutton and Marco Anzidei; pastglobalchanges.org/calendar/past/2013-past/127-pages/853), the community tackled the issue of estimating rates and sources of sea-level change during past warm periods and the Holocene. The workshop included a field excursion where participants had the opportunity to jump into the Mediterranean Sea and snorkel around fish tanks dating back to the Roman age that were often used as common-era sea-level indicators. Additional support was provided to ECRs with the help of CliC, the Cryosphere and Climate project within the World Climate Research Programme (WCRP). One outcome of this meeting was a review paper that summarized the current state of knowledge from an interdisciplinary perspective concerning sea level during past warm periods (Dutton et al. 2015; Fig. 2).

One year later (16-23 September 2014), PALSEA met in a slightly colder, yet equally interesting location. Antony Long and Natasha Barlow organized a workshop in north-west Scotland, in the remote town of Lochinver (pastglobalchanges.org/calendar/past/2014/127-pages/846). Here, the participants had the opportunity to work in a relaxed and informal atmosphere (the meeting was hosted in a lodge, and some participants decided to camp on the lake!), working out the best ways to tackle one of the long-lasting PALSEA goals: documenting paleo sea-level and ice-sheet extent and building sea-level/ice-sheet databases (Fig. 3). The discussions were intense and fruitful, leading to the draft of a paper on strategies and perspectives on sea-level databases that would become, in the following years, a handy guide for those wishing to build new sea-level databases (Düsterhus et al. 2016).

Figure 3: Map showing published regional sea-level databases as of 2014. Modified from Düsterhus et al. (2016).

Sea-level and ice-sheet databases have been (and still are) a central topic within PALSEA, mostly due to their importance for the validation of glacial isostatic adjustment (GIA) and ice-sheet models. For this reason, the Scotland meeting was followed by a workshop from 22-24 July 2015 focused on data-model integration and comparison (pastglobalchanges.org/calendar/2015/127-pages/1390; organized by Glenn Milne, Ayako Abe-Ouchi, and Yusuke Yokoyama). The trio took advantage of the 2015 INQUA conference in Nagoya, Japan, and hosted the workshop at the University of Tokyo. For the first time, a PALSEA conference was held outside of Europe or the US. In three intense days, the problems and opportunities related to using sea-level and ice-sheet data in conjunction with ice and GIA models were discussed for different timescales: the Pliocene, Pleistocene interglacials, and the Holocene.

One of the outcomes of this meeting was the understanding that PALSEA was missing one specific part of the community: scientists working with instrumental records of change. Therefore, for the 18-21 September 2016 meeting in Mt. Hood, OR, USA (organized by Anders Carlson; pastglobalchanges.org/calendar/2016/127-pages/1540), the participation of scientists working with modern sea-level and ice-sheet changes was encouraged. This led to a series of presentations which aimed to stimulate new ideas on the best ways to bridge paleo and modern records, delving mostly into data from the Late Holocene, the Common Era, and the last two centuries. The day before the official workshop start, Nicole Khan (leader of the HOLSEA project, under the umbrella of PALSEA within INQUA; https://www.holsea.org) united several colleagues interested in contributing to a global database of Holocene sea-level indicators. On that day, the group started to define what three years later would lead to the first standardized global sea-level database (Khan et al. 2019). Also the PALSEA team working on Pleistocene sea levels started to work on sea-level databases, with a series of papers dedicated to addressing issues on the data structure (Dutton et al. 2017; Rovere et al. 2016) and releasing a database of dated corals with associated sea-level metadata (Hibbert et al. 2016).

Pleistocene corals took center stage from 6-9 November 2017 in Playa del Carmen, Mexico, for the meeting closing PALSEA's first decade (organized by Andrea Dutton and Paul Blanchon; pastglobalchanges.org/calendar/2017/127-pages/1715). To delve into the issues related to the phasing of ice-sheet and sea-level responses to past climate change, participants explored the fossil reefs exposed at Xcaret, between talks and presentations. The lively discussions were centered on whether these reefs preserved imprints of sea-level oscillations, and how large these changes were. Once more, the possibility to have discussions in the field, among scientists at different career stages and from different backgrounds, proved a winning formula for PALSEA, and a source of inspiration for several new avenues of research. Therefore, it is not by chance that the second five years of PALSEA generated a large number of debates and ideas, which led to more than 80 scientific articles. The problems and advances fostered by PALSEA in its first decade are well summarized in a seminal paper by Dutton et al. (2015), that represents the outcome of several discussions and interactions within the PALSEA community.

2018–2020: Going forward

In Mexico, the transition was made to a new leadership, composed of then-ECRs who took part in several PALSEA activities in the previous years: Jacqueline Austermann, Natasha Barlow, Alessio Rovere, and Jeremy Shakun. The first meeting of this new cycle was organized in collaboration with QUIGS (PAGES-PMIP Working Group on Quaternary Interglacials; pastglobalchanges.org/quigs), another long-running PAGES working group. The meeting was held 24-27 September 2018 in Galloway, NJ, USA (organized by Emilie Capron, Robert E. Kopp, and Alessio Rovere; pastglobalchanges.org/calendar/2018/127-pages/1759). The efforts of these two communities were used to define "lessons learned" in these years, resulting in a paper with the self-explanatory title: "Challenges and research priorities to understand interactions between climate, ice sheets and global mean sea level during past interglacials" (Capron et al. 2019). A particularly refreshing aspect of this meeting was the input of new ideas from scientists from the QUIGS community, who had never before participated in a PALSEA meeting.

Figure 4: Tweet by PALSEA (@PALSEAgroup) summarizing the numbers of the 2020 virtual meeting.

Given the success of the New Jersey meeting, the 21-23 July 2019 PALSEA meeting in Dublin, Ireland (organized by Natasha Barlow and Robin Edwards; pastglobalchanges.org/calendar/2019/127-pages/1821) also aimed to expand the involvement of those with complementary expertise: several ecologists and geochemists were invited to discuss how to improve proxy-based paleo sea-level reconstructions. Embedded within this meeting was the presentation of the final version of the HOLSEA database (Khan et al. 2019) and the inception of the World Atlas of Last Interglacial Shorelines, an effort to standardize MIS 5e sea-level proxies, that is now underway (Rovere et al. 2020). Some members of the PALSEA community also participated in the INQUA-PAGES ECR workshop on impacts of sea-level rise from past to present (iSLR) in 2018.

The COVID-19 pandemic shocked the world and thwarted plans for the PALSEA meeting in Palisades, NY, USA, in September 2020. The ambitious aim for this meeting was to bring together the Earth and ice modeling communities to define and create a standardized way to share and analyze model results. The meeting was being co-organized with another very active community, SERCE (Solid Earth Response and influence on Cryospheric Evolution), and is currently postponed until September 2021. In order not to lose the possibility to meet and exchange ideas, PALSEA organized a virtual "express" meeting from 15-16 September 2020 (led by Jacky Austermann and Alexander Simms; pastglobalchanges.org/calendar/2020/127-pages/2043). The meeting was held at different times to allow people from different timezones to join. The result was the most well attended PALSEA meeting ever (Fig.4)!

affiliations

1MARUM, University of Bremen, Germany

2Department of Geoscience, University of Wisconsin-Madison, USA

contact

Alessio Rovere: aroveremarum.de

references

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Nerilie Abram1, D. Kaufman2, H. McGregor3, B. Martrat4, O. Bothe5 and H. Linderholm6

For the past 14 years, the PAGES 2k Network has brought together a large, interdisciplinary community to better understand pre-industrial climate and give context to recent human-caused climate change.

The past 2000 years of Earth's history provides a critical context for understanding climate variability and change. This is a period where climate changes occurred as a result of well characterized natural climate forcing, as well as unforced internal variability, and has now transitioned into a climate that is strongly forced by human factors. It is also a period where a range of paleoclimate proxy records, often with annual or better resolution, can be used to build up a comprehensive spatial understanding of our climate system. Recent step changes in computing capabilities now allow for ensembles of millennial-length climate-model simulations with which researchers can test and improve our knowledge of the climate system. All of these factors provide a rich scientific backdrop for the work of the PAGES 2k Network.

The 2k Network is one of the longest running working groups of PAGES. Now in its 14th year, the 2k Network has generated more than 54 journal articles, two major paleoclimate databases for temperature and hydrology reconstructions, and around 40 Past Global Changes Magazine articles, while fostering an open and collaborative work environment with an emphasis on FAIR data stewardship principles (Wilkinson et al. 2016).

Evolution of the 2k Network

When the 4th Assessment Report of the Intergovernmental Panel on Climate Change was released in 2007, it stated that "Palaeoclimate information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1,300 years" (IPCC 2007, p. 9). This was a finding that specifically related to the Northern Hemisphere, due to the low density of available proxy records from the Southern Hemisphere and tropics, and where questions remained over statistical reconstruction methods and the suitability of different proxy records.

A Past Global Changes Magazine article in 2005 highlighted the opportunities that dense networks of high-quality natural and documentary archives offer for moving beyond global average, or Northern Hemisphere average, temperature reconstructions (Wanner 2005) to resolve spatial patterns of climate variation. This effort coincided with an interest in testing the increasingly smaller-scale climate information being simulated by new generations of climate models. Wanner posited that "the complex dynamical processes leading to past and future climate and environmental change can only be understood if we also acquire insight into the regional dynamics." This, through the LOTRED (Long-Term Climate Reconstruction and Dynamics) approach, set in motion the beginnings of the PAGES 2k Network.

The first phase began in 2008, with the goal of assembling paleoclimate records over specific regions and using these to produce continent-scale temperature reconstructions. The work initially involved eight regional working groups covering each continent and its surroundings (including an Arctic2k group). Later, an Ocean2k working group was also established to collate paleoclimate data from the world's oceans. Each group used their own expertise to assess the suitability of paleoclimate records and the best methods for combining these proxies into continent- or ocean-basin-scale temperature reconstructions.

The second phase involved trans-regional projects that brought together the datasets compiled across the regional working groups to answer specific scientific questions about the timing and inter-hemispheric variability of natural and anthropogenic climate changes over the past 2000 years. Phase 2 also included efforts to unify and test the statistical methods of reconstructing climate, and assembled a global paleoclimate temperature database with best practices of data management and accessibility. Work of the regional working groups also continued with efforts to develop hydroclimate reconstructions and resolve spatial patterns in climate changes. A major product of Phase 2 was a special issue of Climate of the Past titled "Climate of the past 2000 years: regional and trans-regional syntheses". This special issue also focused on putting into practice open-access data stewardship principles (Kaufman et al. 2018).

Phase 3 of the PAGES 2k project has seen the conclusion of some trans-regional projects as they achieved their goals, and the development of new project groups under the 2k Network banner. These project groups are community-led projects that are working towards the Phase 3 goals to: (1) build further understanding of climate variability, modes and mechanisms, (2) improve reconstruction methods and reduce uncertainties, and (3) assess proxy-model agreement.

Figure 1: (A) Reconstruction of global mean temperature over the last 2000 years using multiple methods demonstrates the unprecedented rate of current warming. (B) Spatially resolved temperature reconstructions demonstrate that warm periods prior to the current warming were not globally synchronous. Modified from PAGES 2k Consortium (2019) and Neukom et al. (2019).

Highlights of 2k Network research

The first major synthesis of the PAGES 2k Network showed that across seven reconstructed continental regions, all experienced a long-term cooling trend during pre-industrial times over the past 2000 years (PAGES 2k Consortium 2013). Long-term cooling of the global ocean also characterized the last 2000 years (McGregor et al. 2015; Tierney et al. 2015). Reconstructed long-term pre-industrial cooling is robust across different reconstruction methods, is consistent with last millennium climate model simulations, especially for the Northern Hemisphere (PAGES 2k-PMIP3 group 2015), and is thought to be largely a response to accumulated volcanic forcing of pre-industrial climate (McGregor et al. 2015). Multidecadal fluctuations over the past 2000 years have a coherent magnitude and timing between global temperature reconstructions and appear to also be attributable to volcanic forcing during pre-industrial times (PAGES 2k Consortium 2019). Spatial reconstructions of this temperature variability have now found that there were no warm or cold periods during pre-industrial times of the past two millennia that occurred at a global scale (Neukom et al. 2019; Fig. 1).

The onset of industrial-era warming began in the mid to late 19th century in all ocean and land areas except Antarctica (Abram et al. 2016), i.e. earlier than can be assessed based on historical observations alone. Sustained warming began first over Northern Hemisphere land masses and in the tropical oceans, and was delayed in the Southern Hemisphere possibly due to Southern Ocean circulation processes, but this delayed southern warming onset is not currently reproduced by climate,model simulations. The 20th century warming interval is the only time in the past millennium when both hemispheres have experienced contemporaneous warm extremes (Neukom et al. 2014), and the second half of the 20th century has the largest global warming trends (at timescales of 20 years or longer) of any time in the past 2000 years – highlighting the unprecedented character of recent human-caused warming compared with natural climate variability in the past (PAGES 2k Consortium 2019).

The work behind these research highlights has led to, or been enabled by, the most well documented and extensive database of temperature sensitive proxies of the past 2000 years published to date (PAGES 2k Consortium 2017). The community-sourced database gathered 692 records from 648 locations, including all continental regions and major ocean basins, and is shared in the Linked Paleo Data (LiPD) format (McKay and Emile-Geay 2016) with options provided for accessing the database in multiple coding languages. A similar effort has now also resulted in a comprehensive global database of water isotope (δ18O and δD) proxies for investigating variability and trends in global hydroclimate (Konecky et al. 2020; Fig. 2).

Figure 2: (A) Schematic illustration of the global water cycle and key metadata fields in the Iso2k database, and (B) spatial distribution of isotope records in the Iso2k database. Adapted from Konecky et al. (2020).

Ongoing work within the 2k Network seeks to resolve natural versus anthropogenic trends in the global hydrological cycle, and linkages between the marine hydrological cycle and the terrestrial hydrological cycle. Several reconstructions have emerged over the past few years that shed new light on regional hydroclimate, including precipitation in Antarctica (Thomas et al. 2017) and Australia (Freund et al. 2017), the evolution of the Southern Annular Mode and its teleconnections (Dätwyler et al. 2018), and drought in Scandinavia (Seftigen et al. 2017). Regional studies also highlighted the problem of spatial gaps in hydroclimate data, which are obvious in the Southern Hemisphere (Nash et al. 2016; Gergis and Henley 2017), as well as parts of the Northern Hemisphere, including the Arctic (Linderholm et al. 2018). Others highlighted the importance of historical documents for our understanding of past hydroclimatic changes and their societal impacts (Guevara-Murua et al. 2018; Gil-Guirado et al. 2019).

A framework has been established for comparing modeled and reconstructed estimates of past hydroclimates in order to quantitatively constrain future hydroclimate risk (PAGES Hydro2k Consortium 2017). Proxy data-model comparisons have shown that northern hemispheric paleodata do not support the intensification of 20th century wet and dry anomalies produced by models (Ljungqvist et al. 2016), and paleodata from Europe further suggest that model simulations may overestimate the risk of temperature-driven droughts in Europe (Ljungqvist et al. 2019). Hydroclimatic perspectives on the Common Era have also been put in a longer context, demonstrating the potential for current weakening of the latitudinal temperature gradient in the Northern Hemisphere to reduce mid-latitude rainfall (Routson et al. 2019).

Future of the 2k Network

Discussions are underway within the community (PAGES 2k Network coordinators 2020) over the future of 2k activities after Phase 3 wraps up at the end of 2021. Throughout the history of the 2k Network, the scientific endeavors have been driven by an organic, grassroots approach. Individuals, often early-career researchers, have brought their ideas, enthusiasm, and leadership to the different activities. As specific projects have been completed, others have sprung up. As researchers have moved on to other priorities, others have joined and renewed the activities and direction of the 2k effort. The 2k Network has produced ground-breaking science, while also building scientific careers and fostering collaborations across an international scientific community.

It is clear that there is momentum within some of the current 2k projects that will continue to yield valuable scientific outcomes beyond 2021. Other 2k projects with important ambitions are still in the early phases. Over the coming months the 2k coordinators will continue the efforts already begun and work with the research community to develop a plan for future 2k research priorities. These include the curation of data products as well as the rescue of existing data sources, building stable bridges between the paleodata and paleomodeling communities, a more holistic 2k view of the climate system that goes beyond temperature, and, finally, using 2k research to provide information that aids society and guides policy decisions.

affiliations

1Research School of Earth Sciences, Australian National University, Canberra, Australia

2School of Earth and Sustainability, Northern Arizona University, Flagstaff, USA

3School of Earth and Environmental Sciences, University of Wollongong, Australia

4Institute of Environmental Assessment and Water Research (IDÆA-CSIC), Barcelona, Spain

5Helmholtz-Zentrum Hereon, Geesthacht, Germany

6Department of Earth Sciences, University of Gothenburg, Sweden

contact

Nerilie Abram: nerilie.abramanu.edu.au

references

Abram N et al. (2016) Nature 536: 411-418

Dätwyler et al. (2018) Clim Dyn 51: 2321-2339

Freund M et al. (2017) Clim Past 13: 1751-1770

Gergis J, Henley B (2017) Clim Dyn 48: 2087-2105

Gil-Guirado S et al. (2019) Clim Past 15: 1303–1325

Guevara-Murua A. (2018) Clim Past 14: 175–191

IPCC (2007) In: Solomon S et al. (Eds) Climate Change 2007: The Physical Science Basis. Cambridge University Press, 1-18

Kaufman D et al. (2018) Clim Past 14: 593-600

Konecky B et al. (2020) Earth Syst Sci Data 12: 2261-2288

Linderholm H et al. (2018) Clim Past 14: 473-514

Ljungqvist F et al. (2016) Nature 532: 94–98

Ljungqvist F et al. (2019) Environ Res Lett 14: 084015

McGregor H et al. (2015) Nat Geosci 8: 671-677

McKay N, Emile-Geay J (2016) Clim Past 12: 1093-1100

Nash D et al. (2016) Quat Sci Rev 154: 1-22

Neukom R et al. (2014) Nat Clim Change 4: 362-367

Neukom R et al. (2019) Nature 571: 550–554

PAGES 2k Consortium (2013) Nat Geosci 6: 339-346

PAGES 2k Consortium (2017) Sci Data 4: 170088

PAGES 2k Consortium (2019) Nat Geosci 12: 643-649

PAGES 2k Network coordinators (2020) PAGES Mag 28: 66

PAGES 2k-PMIP3 group (2015) Clim Past 11: 1673-1699

PAGES Hydro2k Consortium (2017) Clim Past 13: 1851-1900

Routson C et al. (2019) Nature 568: 83-87

Seftigen K et al. (2017) Clim Past 13: 1831-1850

Thomas ER et al. (2017) Clim Past 13: 1491-1513

Tierney JE et al. (2015) Paleoceanography 30: 226-252

Wanner H. (2005) PAGES news 13: 19-21

Wilkinson M et al. (2016) Sci Data 3: 160018

Zhengyu Liu1, B.L. Otto-Bliesner2, P.U. Clark3, J. Lynch-Stieglitz4 and J.M. Russell5

SynTRACE-21 initiated a comprehensive data-model comparison of the transient evolution of global climate during the last 21,000 years; this comparison improved our understanding of global and regional climate changes and also raised new challenges to both models and proxy data.

Background

The large magnitude of climate change over the last 21 thousand years (kyr), documented by an extensive array of well-dated paleoclimate records, has made this period one of the best paleoclimate targets for testing climate-model estimates of climate sensitivity and the ability of models to simulate abrupt climate change. Model-data comparisons have remained a challenge, however, because model simulations of global climate are usually limited to hundreds of years while proxy records that span the entire interval are limited in their spatial coverage.

To address these issues, model-data comparisons have traditionally used the "snapshot" strategy in which data representing a specific time slice (e.g. 21 kyr before present (BP), 6 kyr BP) are portrayed on a map for comparison to climate-model results for that time slice. This strategy greatly improved our understanding of global climate changes that are driven by external forcing to the coupled ocean-atmosphere system, notably orbital forcing, greenhouse gasses and ice sheets (COHMAP Members 1988), but it has several limitations.

From the data perspective, uncertainties in age models influence the map reconstruction, transferring uncertainties from the time domain to the space domain. From the perspective of mechanisms, while the snapshot strategy can be used to study the near-equilibrium surface responses, it cannot be used to study the response associated with the slow components of the climate system, such as the deep ocean, nor internal climate variability, such as the millennial-scale climate events and abrupt changes of the last deglaciation. The coarse temporal resolution between successive snapshots also makes it difficult for the snapshot approach to identify the complex temporal phasing relations between different climate events and thus assess mechanisms of climate change at regional and global scales.

Given these issues, the paleoclimate community recognized the importance of performing transient climate-model simulations that allow us to compare the results to the evolution of climate change recorded by data timeseries. In particular, such simulations should be conducted with synchronously coupled atmosphere and ocean components, as any asynchrony in the model, such as an acceleration in the forcing or a model component, will distort the response of the temporal evolution of the slow components, notably the deep ocean, and can thus only be used approximately for the quasi-equilibrium response of surface ocean and the associated climate variability to external forcing.

The rapid advance in high performance computing over the last decade has now made it possible to simulate the transient climate evolution on multimillennial timescales in state-of-the-art, synchronously coupled ocean-atmosphere models. Here we summarize the SynTRACE-21 project, in which the Community Climate System Model 3 was used to simulate the transient climate evolution of the last 21,000 years (TRACE-21) and promote model-data comparison. The model has a 3.75-degree horizontal resolution for the atmosphere, a variable resolution from ~3.6 degrees at high latitude to ~0.9 degrees in the ocean (Yeager et al. 2006).

Supported jointly by PAGES, the US National Science Foundation, the US Department of Energy, the US National Center for Atmospheric Research, and Brown University, SynTRACE-21 was led by a steering committee of US-based modelers (Z. Liu, Univ. Wisconsin-Madison; B. Otto-Bliesner, National Center for Atmospheric Research) and data developers (P.U. Clark, Oregon State Univ.; J. Lynch-Stieglitz, Georgia Tech.; J. Russell, Brown University) and ultimately involved dozens of scientists around the world. After two US NSF-funded community workshops in Madison, WI (2008), and Boulder, CO (2009), two PAGES workshops were held: the first at Timberline Lodge, Mt. Hood, OR (9-13 October 2010), and the second in Providence, RI (3-7 November 2012), with several other meetings coordinated with other projects and conferences.

Figure 1: Hemispheric temperatures. (A) Atmospheric CO2 concentration. (B) Northern Hemisphere (blue) and Southern Hemisphere (red) proxy temperature stacks. (C) Modeled Northern Hemisphere (blue) and Southern Hemisphere (red) temperature stacks from the TRACE simulation. (D) Northern Hemisphere minus Southern Hemisphere proxy temperature stacks (dark purple). North Atlantic minus South Atlantic region proxy temperature stacks (light purple). (E) Modeled Northern Hemisphere minus Southern Hemisphere temperature stacks in the TRACE (blue), CO2 (red) and ORB (green) simulations. (F) Modeled AMOC strength in the ALL (blue), CO2 (red), and ORB (green) simulations. (G) North Atlantic sediment core OCE326-GGC5 231Pa/230Th. Temperatures are given as deviations from the early Holocene (11.5–6.5 kyr BP) mean. Figure reproduced with permission from Shakun et al. (2012).

SynTRACE-21 outcomes

Using changing insolation, proxy data of greenhouse gas forcing, reconstructions of ice-sheet size and coastline, and an assumed history of freshwater water forcing in the North Atlantic, Liu et al. (2009) first simulated the transient climate evolution of the coupled atmosphere-ocean-terrestrial vegetation system for the last 21 kyr in a baseline experiment (TRACE-21). This experiment, along with additional sensitivity experiments, was then used for comparison with data and for understanding the mechanism of the response.

Global temperature changes

The deglacial evolution of global climate from the Last Glacial Maximum (LGM, ~21 kyr BP) to the early Holocene (~11 kyr BP) presents an outstanding opportunity to combine TRACE-21 simulations with data to better understand the transient response of Earth's climate system to major climate forcing factors. The forcing factors include the changes of the external forcing associated with the Earth's orbit, the ~80 ppm rise of atmospheric greenhouse gases (GHG), as well as internal forcing of continental ice sheet and meltwater inputs to the ocean that result in changes in the Atlantic Meridional Overturning Circulation (AMOC). A major effort has been made by the paleoclimate research community to characterize these changes through the development and synthesis of well-dated, high-resolution records from the deep and intermediate ocean as well as from the continents, as summarized in Clark et al. (2012). The synthesis indicates that the superposition of two leading modes of climate change explains much of the variability in regional and global climate during the last deglaciation, with a strong association between the first mode and variations in greenhouse gases, and between the second mode and variations in the AMOC.

Shakun et al. (2012) further reconstructed the global surface temperature (largely sea-surface temperature) from proxy records and compared the evolution of the reconstructed global and hemisphere mean temperatures (Fig. 1). They found that global surface temperature is correlated with and, furthermore, generally lags CO2 during the last deglaciation. The TRACE-21 simulation indicates that the large deglacial warming is caused by the large response of annual mean temperature to increasing GHGs, and the agreement with the data suggests comparable climate sensitivity in the model.

Differences between the respective temperature changes of the Northern and Southern Hemispheres parallel variations in the strength of the AMOC reconstructed from marine sediments. Consistent with the TRACE-21 simulations, these observations support the conclusion that an anti-phased hemispheric temperature response to the AMOC superimposed on globally in-phase warming driven by increasing CO2 concentrations can explain much of the temperature change during the last deglaciation (Fig. 1).

Figure 2: Evolution of the global surface temperature of the last 22,000 years: the reconstruction of Marcott et al. (2013) (blue) after 11.3 kyr BP and Shakun et al. (2012) (cyan) before 6.5 kyr BP, the model annual global temperature averaged over the global grid points (black) and the model seasonally biased temperature averaged over the proxy sites (red). The models are CCSM3, FAMOUS, and LOVECLIM, with the ensemble mean in heavy, solid lines and individual member in light, thin lines (LOVECLIM and FAMOUS marked by circles and squares, respectively). Each temperature curve is aligned at 1 kyr BP. The ensemble mean model annual temperature averaged over proxy sites is also shown (yellow); its similarity to the model grid average demonstrates the insensitivity of the temperature trend to the average scheme. The insert shows the expanded part after 2 kyr BP, with the addition of the last millennium experiment in CCSM4 (grey), which is forced additionally by volcanic aerosol and solar variability. Figure from Liu et al. (2014a).

Marcott et al. (2013) extended the annual global surface temperature reconstruction through the Holocene (~11–0 kyr BP; Fig. 2). The reconstruction showed that deglacial warming continued into the Holocene with temperatures plateauing in the early to mid-Holocene for global and hemispheric average temperatures, followed by a cooling of ~1°C through the middle to late Holocene. This Holocene cooling trend in annual mean global temperature, however, is physically puzzling.

Under the dual forcing of a declining residual ice sheet and rising atmospheric CO2, transient climate-model simulations, including TRACE-21, exhibit a warming trend in the Holocene, in contrast to the reconstructed late-Holocene cooling in proxy data (Fig. 2; Liu et al. 2014a). The Holocene cooling trend in the data is more consistent with a response to summer insolation in the Northern Hemisphere and tropics, and thus may be attributed to a summer seasonal bias of the temperature, as simulated in models.

This potential summer bias, however, can't explain the data cooling trend in the Southern Hemisphere, potentially indicating model shortcomings in the representation of certain feedback processes. Overall, TRACE-21 has improved our understanding of the mechanism of major global climate changes and, furthermore, has stimulated studies on the potential biases both in the model and data interpretation (e.g. Marsicek et al. 2018).

Regional hydroclimate changes

Comparisons of TRACE-21 with terrestrial proxy data also provided insights into mechanisms of regional hydroclimate changes over the last 21,000 years. For example, Otto-Bliesner et al. (2014) studied climate change during the last deglaciation in Africa (Fig. 3). Proxy data show that wet conditions developed abruptly ~14,700 years ago in southeastern equatorial and northern Africa and continued into the Holocene. The abrupt onset and coherence of this early African Humid Period, however, has been challenging to understand, because changes in seasonal insolation forcing in the southern tropics should weaken the austral monsoons (Otto-Bliesner et al. 2014).

Figure 3: African hydroclimate for the deglacial period 20 to 11 kyr BP. EOF1 and PC1 of (A) proxy data for moisture availability and (B) TRACE annual precipitation (millimeters per year). EOF1 explains 39.2% and 48.0% of the total variance of model precipitation and proxy data, respectively. (C) and (D) same as (A) and (B) except for EOF2 and PC2. EOF2 explains 17.2% and 16.2% of the total variance of model precipitation and proxy data, respectively. Model results and proxy records are interpolated to the same 100-year resolution. Figure reproduced with permission from Otto-Bliesner et al. (2014).

Comparing the data with TRACE-21 simulations shows that a meltwater-induced reduction of the AMOC during the early deglaciation suppressed precipitation in both regions (Fig. 3). Once the AMOC was reestablished, wetter conditions developed north of the equator in response to high summer insolation and increasing GHG concentrations, whereas wetter conditions south of the equator were a response primarily to the GHG increase.

The TRACE-21 simulations have provided similar constraints for a number of other studies of regional precipitation. For example, Liu et al. (2014b) investigated the relationships between deglacial evolution of the East Asian Summer Monsoon (EASM) and oxygen isotope records from speleothems. The δ18O records document a series of isotopic changes that vary coherently across the Asian monsoon region. This change is difficult to interpret as a response to local precipitation, which tends to change at regional scales.

Comparing the data with TRACE-21 simulations shows reasonable agreement between the speleothem δ18O records and southerly monsoon winds, demonstrating that the data can record large-scale changes in the EASM. The subtropical monsoon circulation exhibits a continental-scale response due to global climate forcing associated with insolation and AMOC, as well as atmospheric teleconnections. The δ18O values, however, are altered by changes in the upstream source region, as well as local precipitation changes. Thus, despite the inherent computational limitations in model resolution and complexity, the TRACE-21 simulations provide insights into the paleoclimate proxies and large-scale monsoon dynamics.

Perspective

TRACE-21 has now been widely used by the paleoclimate community, ushering in a new era of seamless model-data comparison of transient climate evolution and abrupt climate changes from seasonal to orbital timescales, from regional to global spatial scales, and from the atmosphere to the deep ocean (e.g. Marsicek et al. 2018; Kaufman et al. 2020).

SynTRACE-21 has also built upon earlier data-model comparisons in demonstrating the effectiveness of this approach for improving our understanding of the mechanisms responsible for the climate evolution recorded by the data, as well as in identifying potential shortcomings in models and data. The model-data comparison of transient climate evolution has also stimulated further studies on the stability of the climate system, such as the AMOC, in the past, as well as for the future (Liu et al. 2017).

With the continued development of high-performance computing and improvements and increase in the number of proxy records, paleoclimate research will further benefit from new model-data studies beyond SynTRACE-21. First, for a direct comparison with the observed proxy variables and model variables, models need to be improved to include paleo proxy tracers, such as stable isotope ratios in foraminifera and other geochemical tracers (Brady et al. 2019). Second, model resolution should be improved so that detailed regional conditions at the location of the proxy data can be better simulated, e.g. IsoROMS (Stevenson et al. 2015) and the isotope-enabled model WRF (Moore et al. 2016).

One ultimate objective of combining data with models is the data assimilation of paleo proxies in advanced climate models, which requires further improvement of the estimation of the uncertainty of the proxy records as well as models (Tierney et al. 2020). These assimilation products will not only provide dynamically consistent reanalyses of the state of past climate, but may also help to constrain parameters and processes in future generations of Earth system models, thus further enhancing our ability to predict the future response of Earth's climate to GHG emissions.

affiliations

1Department of Geography, Ohio State University, Columbus, USA

2National Center for Atmospheric Research, Boulder, CO, USA

3College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, USA

4School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, USA

5Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA

contact

Zhengyu Liu: liu.7022osu.edu

references

Brady E et al. (2019) J Adv Model Earth Syst 11: 2547-2566

Clark PU et al. (2012) Proc Natl Acad Sci USA 109: E1134-E1142

COHMAP Members (1988) Science 241: 1042-1052

Kaufman D et al. (2020) Sci Data 7: 201

Liu W et al. (2017) Sci Adv 3: e1601666

Liu Z et al. (2009) Science 325: 310-314

Liu Z et al. (2014a) Proc Natl Acad Sci USA 111: E3501-E3505

Liu Z et al. (2014b) Quat Sci Rev 83: 115-128

Marcott SA et al. (2013) Science 339: 1198-1201

Marsicek J et al. (2018) Nature 554: 92-96

Moore M et al. (2016) J Geophys Res Atmos 121: 7235-7253

Otto-Bliesner BL et al. (2014) Science 346: 1223-1227

Shakun JD et al. (2012) Nature 484: 49-54

Stevenson S et al. (2015) Paleoceanogr Paleoclimatol 30: 1573-1593

Tierney JE et al. (2020) Nature 584: 569-573

Yeager SG et al. (2006) J Climate 19: 2545-2566

Michael N. Evans1,
W. Tinner2,
Z. Jian3,
B. Vannière4,
S. Eggleston5 and
M.-F. Loutre5

The future of PAGES is in the construction of a more global and diverse paleoscience community, expansion of links with other, complementary initiatives, and in the support of community-driven science. PAGES should challenge itself with bold new initiatives, lean administration, a smaller carbon footprint, and open and inclusive activities, with the central theme of time threaded through the effort.

What will the PAGES community do in its next phase? May we borrow your crystal ball? Our view of PAGES is, at best, educated guesswork but is informed by the trajectory that is evident in the timeline of PAGES' activities, achievements, support over the past 30 years (Fig. 1), as well as recent initiatives. The future may also be reflected in the new science and organizational diagram that we present here and which replaces the venerable PAGES triangle (adopted in 2015), and, before that, the science plan and implementation strategy of 2009 (Fig. 2).

Figure 1: Timeline of PAGES’ research themes, working groups, contributions to other global research activities, and funding. Vertical lines represent structural transitions.

Science

With its unique perspective and focus on the element of time within Earth system dynamics, the PAGES community will continue the development of process understanding by the integrated analysis of experiments, observations, and reconstructions (What happened? What is possible? How likely is it?) with simulations (Why did it happen? Can it be generated from known processes? How likely is it?). What mechanisms are most consistent with observations, reconstructions, and available simulations that arise from a variety of different experimental designs? Leveraging the increasing precision and accuracy of multivariate data streams and increasingly realistic Earth system simulations, we see PAGES' working groups moving from analysis of global means to regionally resolved patterns. We see renewed focus on moisture dynamics and integrated tracers, climate services, the understanding of ecosystem dynamics, and the ways in which the oceans, atmosphere, land surface, ice, biosphere, and human activities are transiently coupled on timescales of decades to centuries (Fig. 3). These initiatives are already happening. All are useful contributions that enable forecasting of Earth system changes over the extended time horizon of the next ca. 500 years.

Support

PAGES is extremely fortunate to have had dedicated, continuous support over the past 30 years, including from the US and Swiss National Science Foundations, the Swiss and Chinese Academies of Sciences, and the University of Bern. The effect of steady funding has been cumulative, helping to increase PAGES' momentum, as can be observed in the rise in the number and diversity of supported products. In the next 30 years, PAGES may seek more diverse international support to become deeply rooted on all continents through various national funding instruments. It is up to all scientists involved in PAGES' activities to explore new funding possibilities, such as those currently provided by the Swiss and Chinese Academies of Sciences, in their countries of residence, to secure and promote PAGES' future prosperity.

Figure 2: PAGES' scientific structure (A) 1994–1998; (B) 2009–2014 (2015–2021: see Fischer et al. this issue, Fig. 3).

Mission

PAGES' objectives1 continue to center on the natural sciences but evolve toward inclusion of social-science perspectives, with continued support for climate, ecosystem and land use reconstructions to discover past processes and mechanisms of environmental and societal dynamics. PAGES' working groups will continue to provide quantitative forcing and long-term data for model validation. To further strengthen the societal perspective, PAGES fosters the development of working groups that seek to more fully integrate social-science disciplines and societal archives, including those from Indigenous records, and of climatic events and their impacts. For example, we might begin to understand the reasons underlying the human imprint on the environment, and this might support the development of the element of time in integrated assessment simulations (Beckage et al. 2018). With input from practitioners, the potential for paleo-informed policy should improve. This trend is evident in the scope of integrative activities on warm state climates and societal risks associated with thresholds and extreme events.

Figure 3: PAGES' proposed new science diagram (2021–). Concentric circles represent the building blocks of PAGES, which at its core is composed of working groups and integrative activities within PAGES' scope. Their research supports partner programs, informs strategies for sustainability, and provides outreach opportunities. The arrow represents the integration of information and learning that propels the paleosciences forwards and provides context for future projections.

Activities

PAGES will continue to be community-driven, with the support of its lean, efficient, and productive International Project Office. This includes even more global collection, dissemination, and synthesis across spatial and temporal scales, for phenomenologically meaningful regions and dimensions (e.g. patterns within and across elements in Fig. 3) by means of Open Science Meetings2 and clustered meetings, in which multiple working groups convene to consider shared interests and opportunities, such as the Topical Science Meetings.3 Although we acknowledge that personal contacts are at the heart of international science, we anticipate that meetings will become more and more internet-enabled, and virtual, to reduce their carbon footprint. This will enhance the goals of building a community, but also improve accessibility, which is especially important for an increasingly global PAGES.

It remains a challenge to make PAGES a truly global and diverse community, and to improve the representation of that community in its leadership. Emerging initiatives include visiting fellowships designed to mentor and support African and Latin American scientists; bias training and measures to enable us all to feel safe and welcomed at PAGES Open Science Meetings; web-enabled, recorded, and close-captioned meetings and webinars; and in-person workshops held in a more diverse set of locales. We must lead more in these regards, because it has been the community which has not only created awareness of problems, but also contributed ideas and initiatives to improving PAGES for all.

Further activities will continue to follow FAIR (findable, accessible, interoperable and reusable) practices in the curation and stewardship of paleodata, metadata, and code compilations. This will be achieved through the use of public repositories, advanced databasing technologies, webinars, and partnerships with associated and complementary global research networks such as the NCEI4, Neotoma5, Linked Earth6, PANGAEA7, Future Earth8, and WCRP.9 An achievable goal in the coming decades is the development of self-updating repositories that contain not only raw observations but also dynamic chronologies, reconstructions, and version-tracking code bases, which are indexed to associated direct observations and simulations of phenomena of interest.

Perhaps PAGES' most consequential future activities will be in the development of early-career scientists. Many of us can recall a pivotal PAGES "moment"; for one of us (MNE), it was observing a 2011 2k Network meeting, and leaving an hour or so later with the mandate to start Ocean2k. The PAGES Early-Career Network (ECN10) has recently formalized the interests and needs of emerging paleoscientists, and its dynamic, virtual community is a model for future working groups. PAGES' Scientific Steering Committee (SSC11) now encourages working groups to actively engage ECRs in their leadership, and includes an ECR representative on the SSC; this has improved its vision and creativity. PAGES' ECRs and workshop organizers have long integrated outreach activities, such as public events, but we envision a clear need to connect more directly with the public who supports our research and is interested in the science and consequences of global change. To this end, Past Global Changes Horizons (Fig. 4), a magazine for anyone interested in paleoscience, is designed to communicate and educate.

Figure 4: The first issue of PAGES' new magazine for anyone interested in paleoscience, Past Global Changes Horizons, was published in April 2021.19

Prospects

The ultimate goal of PAGES will remain its interest in three challenging global problems: climate change, biodiversity loss, and the sustainability of ecosystems and societies. To succeed, PAGES will need an even better integration of observations and reconstructions with process-based dynamic models to further understand long-term Earth system processes and how they impact sustainability. Additional effort will be needed to train new generations of paleoscientists and transfer knowledge from the PAGES community to the public and to decision-makers. And PAGES has an opportunity and important role to fulfill as its parent body, Future Earth, evolves. Future Earth's organization is becoming increasingly flat, simple, and representative, reflecting the wide range of ideas present in the community. Within that community, we imagine that PAGES will find synergies with other Future Earth Global Research Projects, such as AIMES12, BioDISCOVERY13, GMBA14, IHOPE15, SOLAS16, MRI17, the Emergent Risk and Extreme Events Knowledge Action Network18, and partners such as WCRP.9 PAGES can provide observations and modeling of what is possible on the seasonal to multi-million-year timescales that bracket those over which anthropogenic Earth system forcing are likely to be expressed. PAGES can also place concepts of risk, adaptation, resilience, and sustainability of societies within the context of what human civilizations have already managed, and the mechanisms by which they have either succeeded or failed. How might we learn from those past global changes, challenges, successes, and failures? Future global changes may not repeat past global changes, but perhaps they rhyme with them (Wittreich 1987; Gould 1988).

affiliations

1Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, US
2Institute of Plant Sciences, University of Bern, Switzerland
3State Key Laboratory of Marine Geology, Tongji University, Shanghai, China
4MSHE, CNRS, Université Bourgogne Franche-Comté, Besançon, France
5PAGES International Project Office, Bern, Switzerland

contact

Michael N. Evans: mnevansumd.edu

references

Beckage B et al. (2018) Nat Clim Change 8: 79-84

Gould SJ (1988) Time's Arrow, Time's Cycle: Myth and Metaphor in the Discovery of Geological Time. Harvard University Press, 222 pp

Wittreich J (1987) Feminist Milton. Cornell University Press, 192 pp

LINKS

1http://pastglobalchanges.org/about/mission

2https://pages-osm.org

3http://pastglobalchanges.org/tsm

4https://www.ncdc.noaa.gov/data-access/paleoclimatology-data

5https://www.neotomadb.org

6http://linked.earth

7https://pangaea.de

8https://futureearth.org

9https://www.wcrp-climate.org

10http://pastglobalchanges.org/ecn

11http://pastglobalchanges.org/about/structure/scientific-steering-committee

12https://aimesproject.org

13https://futureearth.org/networks/global-research-projects/biodiscovery

14https://futureearth.org/networks/global-research-projects/gmba-global-mountain-biodiversity-assessment

15https://futureearth.org/networks/global-research-projects/ihope-integrated-history-and-future-of-people-on-earth

16https://futureearth.org/networks/global-research-projects/solas-surface-ocean-lower-atmosphere-study

17https://www.mountainresearchinitiative.org

18https://futureearth.org/networks/knowledge-action-networks/risk

19http://pastglobalchanges.org/products/pages-horizons