PAGES Magazine articles

Erin McClymont1, P. Dekens2, H. Dowsett3, L. Dupont4, A. Haywood5, A. Rosell-Melé6 and U. Salzmann7

The Pliocene epoch (~2.6-5.3 million years ago) is arguably the best-resolved example of a climate state in long-term equilibrium with current or predicted near-future atmospheric CO2 concentrations. It was characterized by a globally warmer climate (Fig. 1), reduced continental ice volume, and reduced ocean/atmosphere circulation intensity. Data derived from natural archives can be constrained in time to the Pliocene by multiple stratigraphic frameworks. Orbital forcing of solar radiation is known precisely, and many of the species extant today were also present then. As a result, detailed understanding of climate forcings and feedbacks is possible through both data analysis and data-model integration.

Chronis Tzedakis1, E. Capron2, A. de Vernal3, B. Otto-Bliesner4 and E. Wolff5

Past interglacials can be thought of as a series of natural experiments in which boundary conditions, such as the seasonal and latitudinal distribution of insolation, the extent of continental ice sheets and atmospheric greenhouse gas concentrations, varied considerably with consequent effects on the character of climate change. Documenting interglacial climate variability, therefore, can provide a deeper understanding of the physical climate responses to underlying forcing and feedbacks, and of the capabilities of Earth System Models to capture the patterns and amplitudes of the responses. These considerations provided the impetus for a comprehensive comparison of interglacials of the last 800,000 years within the context of the PAGES working group on past interglacials (PIGS; 2008-2015). While PIGS synthesized the current state of understanding on interglacials of the last 800,000 years, it also identified a number of research issues that need to be solved if further breakthroughs are to be made (Past Interglacials Working Group of PAGES, in press):

• There is no simple astronomical cause for differences in the intensity of interglacials, which seems to arise at least partly from the patterns observed in atmospheric CO2 concentrations. This emphasizes the need to better understand and model the carbon cycle across glacial cycles.

• Chronological advances, both in assessing absolute ages relative to astronomical forcing, and in aligning different proxies and locations, are essential if we are to assess the dynamics of interglacials and their termination and inception.

• The paucity of long and continuous terrestrial records precludes the assessment of many important aspects of the climate.

• While existing records suggest that sea level was higher than present in some interglacials, better knowledge of the contribution of the Greenland and Antarctic ice sheets is critically needed.

• Identifying the controls on intra-interglacial variability remains a challenge.

Figure 1: “LR04” stack (graphic correlation of 57 records) benthic δ18O over the Quaternary (modified from Lisiecki and Raymo 2005). Interglacials occur above the dotted horizontal line. See Past Interglacials Working Group of PAGES (in press) for more information.

Within the so-called “zoo” of interglacials (Fig. 1), the Last Interglacial (LIG, MIS 5e) has been the most intensively studied, but modeling of earlier interglacials remains limited. In particular, much more needs to be done to better characterize and understand (1) warm extremes, (2) cool versus warm interglacials of the last 800 ka, and (3) interglacials in the 41 ka-world vs those in the 100ka-world.

The objectives of QUIGS are to:

• document and synthesize data on the temporal and spatial patterns of climate responses during Quaternary interglacials and assess the governing processes using numerical models;

• assess the relevance of interglacials to understanding future climate change.

The drive towards a systematic understanding of interglacials requires targeted model exercises as well specific data sets with improved chronologies (some of which are not available yet). QUIGS will promote closer collaboration between the modeling (Paleoclimate Modelling Intercomparison Project; PMIP) and data communities, who together will provide expertise on experimental design, data compilations and syntheses, model-data comparisons, and interpretation of results.

The first workshop on "Warm extremes" took place in Cambridge, UK, 9-11 November 2015. It specifically examined the LIG and MIS 11, identified by PIGS as the warmest interglacials of the last 800 ka. While both interglacials have been considered by previous projects, our aim is to stimulate the work needed for PMIP. Thus, during this first workshop, we assessed emerging data syntheses and recent model experiments. This will allow us to highlight data gaps, and promote and initiate specific efforts to fill these gaps. In particular, we will identify critical datasets needed and define the much needed model protocols (including transient simulations).

affiliations

1Department of Geography, University College London, UK
2British Antarctic Survey, Cambridge, UK
3Geotop, Université du Québec à Montréal, Canada
4National Center for Atmospheric Research, Boulder, USA
5Department of Earth Sciences, University of Cambridge, UK

contact

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

references

Lisiecki LE, Raymo ME (2005) Paleoceanography 20, doi:10.1029/2004PA001071

Past Interglacials Working Group of PAGES (in press) Rev Geophys, doi:10.1002/2015RG000482

Belen Martrat1,2, P.C. Tzedakis3, V. Margari3, L.C. Skinner2, D.A. Hodell2 and J.O. Grimalt1

The Iberian margin provides climatic and environmental sediment records with multi-decadal resolution over the last two deglaciations and interglacials. These records allow us to identify climatic structures and discuss inter-hemispherical connections.

More than a decade has passed since it was verified that major temperature changes in Atlantic surface and deep waters at the Mediterranean latitudes were closely connected with Greenlandic and Antarctic climatic variability (Shackleton et al. 2000). Since then, deep sea sediments retrieved at the Iberian continental margin (e.g. Martrat et al. 2007; Hodell et al. 2013; Margari et al. 2010, 2014) have been adding further clues, showing that episodic abrupt change is a fundamental aspect of the Earth’s climate. Anomalies were observed to take place rapidly enough to be noticed in the time frame of a regular human life and persist long enough to cause substantial disruptions in natural, and potentially socioeconomic, systems. Hence, far from only being of academic interest, the long-term management of our livelihoods now require pushing the data to the limits and focusing on fine-scale details (Shackleton, 2006).

A recent study of site ODP-976 (Martrat et al. 2014) has provided such detailed records over the present interglacial (Holocene, initiated at 11.7 ka before present), the last interglacial (LIG, onset approximately at 129 ka), and the respective deglaciations. The marine records obtained for the penultimate deglaciation and the LIG onset are particularly relevant, given the difficulties in obtaining an undisturbed ice core record from Greenland for this interval (NEEM community members, 2013). In this regard, the fact that the bipolar effect is well illustrated at the Iberian margin (Martrat et al. 2007; Margari et al. 2010) provides us with a robust basis for a Holocene-to-LIG comparison. Alkenone measurements enabled reconstruction of a sea surface temperature (SST) profile with a temporal resolution of 60 to 90 years and an associated uncertainty lower than 0.5°C. Events and transitions described and published before on the basis multi-proxy evidence (isotopes, vegetation, ice-rafted debris, etc.) from other Iberian sites (ODP-977, MD95-2043, MD95-2042, and MD01-2444) were essential for establishing hypotheses regarding long-distance climatic connections. Chronological uncertainties are commonly less than four centuries for the Holocene, but significantly higher – from two to even six millennia – for the LIG, when astronomical calibration of time scales is used as the main reference. In the paleotemperature record, three types of structures relevant to inter-hemispherical connections stand out: ‘‘Ws’’, ‘‘saddles”, and a “cooling trend”. We discuss each of these in turn below.

The ‘‘Ws’’: Heinrich stadials less static than previously apparent

Figure 1: The penultimate (left) and last (right) deglaciations in Greenland, off Iberia, and Antarctica. From top to bottom: the precessional oscillation; climate variability traced by the Greenland NGRIP ice core (75ºN); alkenone-derived sea surface temperatures (SST) and total alkenone amount from marine sediment core ODP-976 (36ºN); and climate variability registered in the EPICA Dronning Maud Land ice core (75ºS). Two main structures relevant to inter-hemispherical connections stand out: the ‘‘Ws’’ and the ‘‘saddles”.

Heinrich (H) events are identified in marine sediments of the mid-latitude North Atlantic as layers with a concentration of ice-rafted debris and scarcity of foraminifera. As a first-order description, H events are flat cold anomalies between some of the Dansgaard-Oeschger warm interstadials, which modelers simulate by putting freshwater perturbations or icebergs into Arctic latitudes (e.g. Jongma et al. 2013). However, increasingly detailed SST reconstructions at Mediterranean latitudes, specifically from sites ODP-976 and MD01-2444, suggest that cold stadial periods associated with H11 and H1 were anything but static. A sharp warming occurred halfway their progression, causing a characteristic ‘‘W’’ shape in the SST records during these episodes (Fig. 1). Long-term vegetation patterns in the Mediterranean show that extreme dry and cold episodes took place during periods around perihelion passage in Northern Hemisphere (NH) spring equinox (Magri and Tzedakis, 2000). The cold spells observed within the stadials associated with H11 and H1 are placed around this orbital signature, i.e., ca 133 ka and 17 ka, respectively, including the abrupt warming events within them (up to 4ºC in less than eight centuries; Fig. 1).

Skinner and Elderfield (2007) suggest that the occurrence of sharp warming events at the centre of the stadials associated with H events indicates the potential energy storage of the deep North Atlantic. The warmings appear linked to the culmination of a large reduction in the Atlantic meridional overturning circulation, ice surge phases with moderate rise in sea level, and possible sub-surface warming feedbacks (Flückiger et al. 2006). These multi-decadal scale oscillations within H events may have played an active role in the progressive glacial-to-interglacial re-activation of convective deep-water formation in the North Atlantic, adding a new element to the bipolar-seesaw between the Northern and the Southern Hemispheres.

The “saddles’’ as a reference for deglacial processes

Deposition of organic rich layers, showning up as alkenone accumulation maxima, characterize the later part of the last two deglaciations when perihelion moves from alignment with the NH spring equinox to the summer solstice (from 132-126 ka and 15-9 ka, respectively; Fig. 1). These layers are not comparable with the sapropels known from the eastern Mediterranean, neither in magnitude, nor timing, or mode of formation. They are features unique to the western Mediterranean. Their deposition histories show different maxima, the youngest ones separated by a significant ‘‘saddle’’ (Rogerson et al. 2008). Alkenone accumulation compares well between the last and penultimate deglaciations, but the derived SSTs differ (Fig. 1). Essentially, a cooling is recorded during the last deglaciation, around 12 ka (during the Younger Dryas; YD), while there is no analogous cooling over the penultimate deglaciation around 130 ka. This difference proves dissimilarities in the developments of the last two deglaciations. Surface and bottom water temperature records from off Iberia reflect the temperature changes over Greenland and Antarctica, respectively. They can thus be used to study temporal relationships between the Iberian and the polar regions. Maxima in the Antarctic water isotopic record (Masson-Delmotte et al. 2011) suggest mild climate in Antarctica during the deposition of both deglacial organic rich layers in the western Mediterranean. This is interesting in that both deglaciations are otherwise remarkably dissimilar in Antarctica, pointing to different configurations of ice sheets and varying strengths in thermohaline circulation during the last two deglaciations, with a dissimilar impact on SSTs across both hemispheres.

A long-term “cooling trend” and bipolar-seesaw variability

Figure 2: The last (left) and present (right) interglacials in Greenland, off Iberia and Antarctica. The descriptions of the individual curves are the same as for Figure 1. The distinctive feature is the “cooling trend”, calculated between perihelion passage in the NH autumn equinox and winter solstice – and the bipolar-seesaw variability that ensues.

Some specific events during the interglacial progression capture our attention, though a trend towards colder climatic conditions dominates the observed SST variability quite prominently (Fig. 2). Interglacial multi-decadal scale events are superimposed upon this long-term trend towards colder SSTs. The ending of organic rich layer deposition in the western Mediterranean marks the onset of temperate Mediterranean conditions with relatively mild winters and winter rainfall, compared with the extreme seasonality of precipitation that characterized the interglacial onset. In Iberia, temperate intervals commence after the 8.2 ka-event and are over at 5.3 ka for the Holocene; for the LIG, they commence after 125 ka and are over at 121 ka (Fig. 2). During the LIG, the cooling trend is steeper (-0.4ºC/ka from 122 ka to 116 ka) than during the Holocene (-0.1ºC/ka from 6 ka to 0.7 ka). Trends simulated by an ensemble of climate models are qualitatively consistent with these Iberian cooling trends (Bakker et al., 2014). A cold spell of around eight centuries at 2.8 ka during the Holocene is possibly mimicked during the LIG at 118 ka by a fall of around 1ºC within a millennium. These events lead interglacial SST to stabilize at around 18ºC, i.e. at a value comparable to the present average annual in the western Mediterranean. The glacial inception at 115 ka commenced after perihelion passage in the NH winter solstice and culminated with a drop of at least 2ºC in a few millennia, placed in the Iberian cores at 111 ka, around perihelion passage in the NH spring equinox. The end of the LIG occurred late in the ice-sheet growth cycle and involved major re-activation of the bipolar-seesaw. The Little Ice Age (0.7 ka), which had strong impacts on European societies, also occurred after the latest perihelion passage in the NH winter solstice and may be an example of a glacial pre-inception event following an interglacial.

data

Data from ODP site 976 are available at http://doi.pangaea.de

affiliations

1Department of Environmental Chemistry, Institute of Environmental Assessment and Water Research (IDÆA-CSIC), Barcelona, Spain

2Department of Earth Sciences, University of Cambridge, UK

3Department of Geography, University College London, UK

contact

Belen Martrat: belen.martratidaea.csic.es

references

Bakker P et al. (2014) Quat Sci Rev 99: 224-243

Flückiger J et al. (2006) Paleoceanography 21, doi:10.1029/2005PA001204

Jongma JI et al. (2013) Clim Dyn 40: 1373-1385

Hodell DA et al (2013) Paleoceanography 128: 185-199

Margari V et al. (2010) Nat Geosci 3: 127-131

Margari V et al. (2014) Geology 42: 183-186

Magri D, Tzedakis PC (2000) Quat Int 73-74: 69-78

Martrat B et al. (2007) Science 317: 502-507

Martrat B et al. (2014) Quat Sci Rev 99: 122-134

Masson-Delmotte V et al. (2011) Clim Past 7: 397-423

NEEM community members (2013) Nature 493: 489-494

Rogerson M. et al. (2008) Geochem Geophys Geosyst 9, doi:10.1029/2007GC001936

Shackleton NJ et al. (2000) Paleoceanography 15: 565-569

Shackleton NJ (2006) Quat Sci Rev 25: 3458–3462

Skinner LC, Elderfield H (2007) Paleoceanography 22, doi:10.1029/2006PA001338

J. Ignacio Martínez1, C. González2, M. Grosjean3 and R. Villalba4

LOTRED-SA 3rd Symposium and Training Course, Medellín, Colombia, 7-12 July 2014

Within the framework of the PAGES 2k Consortium, which aims to reconstruct large-scale global temperature patterns for the past two millennia, the LOng-Term multi-proxy climate REconstructions and Dynamics in South America (LOTRED-SA) initiative has produced new high-quality datasets for millennial-long quantitative climate reconstructions for South America. Former LOTRED-SA symposia, held in Malargüe (2006) and Valdivia (2010), focused mostly on southern South America. Two special issues edited by Villalba et al. (2009) and Masiokas et al. (2012) featured key datasets from these meetings.

Neukom et al. (2011) demonstrated that significant data gaps exist in (sub)tropical South America preventing a continent-wide paleoclimate reconstruction. Later assessments (PAGES 2k Consortium 2013; Neukom et al. 2014) have shown that pronounced climatic differences existed between South America and the Northern Hemisphere, e.g. the unique warm event in South America during the late 18th to early 19th centuries. Beyond discussing such interhemispheric differences, a big challenge ahead for LOTRED-SA is filling the data gap in the tropics. Available datasets from tree-rings and lake sediments, speleothems, historical documents, vegetation, pollen, and ice cores are mostly from southern South America. Therefore, more data from the tropics, including other proxy archives from marine and lowland areas need to be collected.

South America, extending from the northern tropics to the sub-Antarctic region and incorporating coastal and high Andean settings, offers a wealth of opportunities for studying the paleoclimate of the late Holocene. The LOTRED-SA 3rd Symposium achieved another of its key goals, which was to provide an up-to-date synoptic picture of South American climate dynamics. Over 115 researchers from 13 countries currently working on tropical and southern South America presented over a hundred contributions, including new findings from the Neotropics and the adjacent oceanic regions. Although the emphasis of the symposium was on the last 2 ka, contributions ranged from the late Holocene to modern climate and included lake and marine sediments, speleothems, tree-rings, and ice core paleoclimate records, in addition to documentary data and model results.

Figure 1: Panoramic view of the San Nicolás terrace in the Santa Fé–Sopetran Basin, northern Colombia.

Beyond the climatic aspects of the 2k initiative, contributions at the symposium also explored how ecosystems responded to and created feedbacks to climate change, and how humans have dealt with the variability. The wide diversity of processes operating in the region include the annual/decadal migration of the Intertropical Convergence Zone (ITCZ), the dynamics of El Niño-Southern (ENSO), and the multidecadal Pacific (PDO) and Atlantic (AMO) Oscillations. These regional dynamic features seem to explain the Medieval Climate Anomaly, the Little Ice Age, and the current warm period scenarios, all apparently connected through the South American Monsoon System.

An intensive two-day training course for young scientists took place prior to the symposium. It provided training on the building of radiocarbon age models, on the integration of archives, proxies, and sites from the Neotoma Paleoecology Database, and on using R software and Quantum GIS for statistical and spatial analyses. The course, attended by 30 young scientists from 12 countries, was taught by Maarten Blaauw (Queen's University, UK), Alexander Correa-Metrio (UNAM, Mexico), Suzette Flantua (University of Amsterdam, The Netherlands), and Ricardo Villalba (IANIGLA, Argentina).

After the meeting, a field trip took attendants to examine the geomorphology and paleolimnology of the Santa Fé–Sopetrán Basin, where the late Holocene San Nicolás terrace was visited (Fig. 1). This terrace contains a high-resolution succession of laminated sediments whose hydrological multi-decadal frequencies were controlled by the dynamics of the ITCZ.

affiliations

1Dept. Geología, Universidad EAFIT, Medellín, Colombia

2Dept. Ciencias Biológicas, Universidad de los Andes, Bogotá, Colombia

3Oeschger Centre for Climate Change Research, University, Bern, Switzerland

4Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales IANIGLA, Mendoza, Argentina

contact

Martin Grosjean: martin.grosjeanoeschger.unibe.ch

references

Masiokas et al. (2012) Clim Past 8, Spec Iss 42

Neukom R, Gergis J (2011) The Holocene 22: 501-524

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

Villalba R et al. (2009) Palaeogeog Palaeoclimatol Palaeoecol 281: 175-376

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

Thomas Hoffmann1, D. Penny2, G. Stinchcomb3, V. Vanacker4 and X.X. Lu5

Figure 1: Smallholder rain-fed agriculture in the Ethiopian Highlands, Amhara Region, Ethiopia. Photo by Veerle Vanacker.

Anthropogenic soil erosion reduces soil productivity, compromises freshwater ecosystem services, and drives geomorphic and ecological change in rivers and floodplains. It is now well accepted that the rate of anthropogenic soil erosion exceeds the rate of soil production by several orders of magnitude in many parts of Earth (Montgomery 2007), threatening the sustainability of food production that is so essential to human well-being. Deposition of the eroded soil downstream has profoundly altered the structure and function of fluvial and deltaic ecosystems, often with negative impacts on the societies and economies that depend on them (Hoffmann et al. 2010). The legacy of these impacts exerts strong influence over modern and future ecosystem functions. In many agricultural ecosystems, natural processes no longer primarily control soil erosion and deposition, and greatly altered sediment fluxes are a key marker of the Anthropocene (Syvitski and Kettner 2011).

The vulnerability of soils to human-induced erosion is highly variable in space and time; dependent on climate, geology, the nature and duration of land use, and topography. Our knowledge of the mechanistic relationships between soil erodibility, land use, and climate is well developed. However, the global heterogeneity of land use history and the co-occurrence of other erosion-relevant factors such as climate variability have prevented us from sufficiently understanding the global patterns of long-term soil erosion and fluvial sediment flux and storage, and quantifying their budgets.

Objectives

GloSS will analyze the global pattern of past and present anthropogenic soil erosion, and the transfer and deposition of sediment. It aims to determine the sensitivity of soil resources and sediment routing systems to varying land use types during the period of agriculture, under a range of climate regimes and socio-ecological settings.

To achieve this objective, GloSS will integrate the scientific domains of geomorphology, paleoecology, archaeology, and history. GloSS focuses on the local and regional impact of anthropogenic activities on soil erosion and sediment transfer through fluvial systems in different socio-ecological contexts since the onset of agriculture, which began in Eurasia as early as approximately 8,000 years ago.

GloSS therefore aims to:

• Update the global network of scientists developing long-term soil erosion and sediment flux histories within socio-ecological systems, building on the work of the former Land Use and Climate Impacts on Fluvial Systems (LUCIFS) working group;

• Develop proxies and indices for human impact on rates of soil erosion and fluvial sediment transfer that are applicable on a global scale and throughout the Holocene;

• Create a global database of long-term (102-104 years) human-accelerated soil erosion and sediment flux records;

• Identify hot spots of soil erosion and sediment deposition during the Anthropocene;

• Locate data-poor regions where particular socio-ecological systems are not well understood, as strategic foci for future work.

The objectives and goals of the GloSS working group sit at the nexus of climate, environment, and humanity and thus contribute to the interdisciplinary activities at the heart of the revised PAGES science structure and the Future Earth initiative.

Visit the GloSS webpage at: http://pastglobalchanges.org/science/wg/former/gloss/intro and sign up to our mailing list to keep up to date with the group’s activities.

affiliations

1Department of Geography, University of Bonn, Germany
2School of Geosciences, University of Sydney, Australia
3Department of Geography, Louvain, Belgium
4Watershed Studies Institute, Murray State University, USA
5Department of Geography, National University of Singapore

contact

Thomas Hoffmann: thomas.hoffmannuni-bonn.de

references

Hoffmann T et al. (2010) Global Planet Change 72: 87-98

Montgomery DR (2007) PNAS 104(33): 13268–13272

Syvistki JP, Kettner AJ (2011) Phil Trans R Soc London, Ser A 369: 957-975

Marie-José Gaillard1 and LandCover6k Interim Steering Group members2

There is today a general understanding of the need for powerful climate models to inform societies on the climate’s possible development in the future. Climate models help us to understand the climate system as a whole and envisage our future. They have existed for many decades and have developed progressively into very complex Earth system models (ESMs) in which the atmosphere, the ocean and land-surface processes are coupled. Although already powerful, many of these ESMs are still under development. By using a model-data comparison approach, i.e. comparing model outputs with actual climate data over decades, centuries, and millennia back in time (paleoclimate data), both model outputs and paleodata can be better understood and evaluated, which also contributes to model improvements.

Land cover (here referring essentially to vegetation cover, but also bare soils and rocks) is an inherent part of the climate system. Natural, primarily climate-driven vegetation and ecosystem processes interact with human land use to determine vegetation cover on earth and its development through time. The resulting land-surface properties feed back to climate by modulating exchanges of energy, water, and greenhouse gases with the atmosphere through biogeochemical feedbacks (affecting sources and sinks of greenhouse gases, aerosols, pollutants, and other gases) and biogeophysical feedbacks (affecting heat and water fluxes, and wind direction and magnitude). The sum of these feedbacks may be either positive, i.e. amplifying changes in climate (e.g. amplifying a warming or a cooling trend), or negative, i.e. slowing trends in climate (e.g. slowing a warming or a cooling trend). Biogeochemical feedbacks, especially involving the carbon cycle, have received particular attention. However, biogeophysical feedbacks can have an effect of comparable magnitude; but because biogeophysical feedbacks generally operate at the regional scale they may be missed or underestimated at the relatively coarse resolution of Global ESMs. These feedbacks still represent a major source of uncertainty in climate projections under rising greenhouse gas concentrations. Therefore, the incorporation of dynamic vegetation into ESMs currently is one of the high priorities among climate modelers.

Figure 1: Grid-based REVEALS estimates for the plant functional type (PFT) grassland (GL) for three Holocene time-windows. The scale is percentage cover, with the different colors indicating different percentage intervals: >0–10% in 2% intervals, 10–20% in a 10% interval, and 20–100% in 20% intervals. The category 0 (grey) corresponds to the grid cells with pollen records but no pollen data for the actual PFT and, therefore, no REVEALS estimates. The category >0–2 corresponds to REVEALS estimates different from zero (can be less than 1%) up to 2%. The uncertainties of PFT REVEALS estimates are shown by circles of various sizes in each grid cell with an estimate. The circles represent the coefficient of variation (CV; the standard error divided by the REVEALS estimate). When SE ≥ REVEALS estimate, the circle fills the entire grid cell and the REVEALS estimate is considered unreliable. This occurs mainly where REVEALS estimates are low. GL (all most common herbs): Artemisia species, Cyperaceae, Filipendula species, Plantago lanceolata, Plantago media, Plantago montana, Rumex acetosa-type (several species). Modified from Trondman et al. (in press).

The effects of anthropogenic burning and deforestation on past global climate are not fully understood yet, and the question of whether humans had more impact than previously assumed on climate in prehistory (the Ruddiman hypothesis; Ruddiman 2003), is still a matter of debate. As long as the effects of land-use changes are not properly understood, mitigation strategies such as afforestation to sequester CO2 and cool the climate might be erroneous. Moreover, the scenarios of past ALCCs often used in climate modeling, such as HYDE (Klein Goldewijk et al. 2011), the KK scenarios (Kaplan et al. 2009), and others (e.g. Pongratz et al. 2008), show large differences between each other (Gaillard et al. 2010). Therefore, climate modeling in paleo-mode taking into account anthropogenic land-cover change (ALCC) is seriously hampered. Thus, there is an imminent need for independent descriptions of past vegetation cover based on empirical data and an improved ALCC history at regional scales and globally. Such independent descriptions can be provided by pollen-based quantitative reconstructions of past vegetation cover such as those recently achieved for a large part of Europe (Trondman et al., in press; Fig. 1).

The methodological starting point for LandCover6k

Objective, quantitative long-term records of past vegetation cover changes are, however, still limited globally. Although biomization of pollen data (Prentice et al. 1996) has become a robust tool to reconstruct the distribution of biomes and their boundaries over the globe, the methodology does not provide quantitative reconstructions of plant cover, e.g. fractions of deforested land or fractions of conifer trees versus deciduous trees. Until a few years ago, it was not possible to translate fossil pollen found in lake sediments or peat into a quantitative description of the past vegetation. However, Sugita (2007) developed an algorithm for inverse modeling of the relationship between pollen and vegetation (Regional Estimates of VEgetation Abundance from Large Sites; REVEALS) that makes it possible to translate fossil pollen data into vegetation cover at regional spatial scales. The LandCover6k working group aims to capitalize on the established REVEALS methodology in a large globally coordinated effort.

Scientifically, LandCover6k also builds on the European research project LANDCLIM (LAND cover – CLIMate interactions in NW Europe during the Holocene; Gaillard et al. 2010). This project applied a model-data comparison scheme that integrated a dynamic vegetation model (LPJGUESS), a regional climate model (RCA3), and the REVEALS model. The results indicate that past human-induced deforestation from Neolithic time (6 ka BP) did indeed have positive and negative biogeophysical feedbacks of +/- 1°C on the regional climate; the sign of the feedback varies between regions and seasons (Strandberg et al. 2014).

Figure 2: Reconstructions of proportion (% cover) of the three land-cover types coniferous forest, broadleaved forest and unforested for the 0.05 ka time window (modified from Pirzamanbein et al. 2014). From top to bottom, the pollen-based REVEALS estimates, the reconstruction from the intrinsic Gaussian Markov Random Field model (IGMRF), and the present day land-cover data extracted from the forest map of Europe compiled by the European Forest Institute (EFI-FM). For details, see text and Pirzamanbein et al. (2014).

Other LANDCLIM results on which LandCover6k will build include the existing reconstructions of land cover over large parts of Europe during five time windows of the Holocene (Trondman et al., in press; Fig. 1) and new spatial statistical models to turn REVEALS reconstructions into spatially continuous maps of past land cover (Pirzamanbein et al. 2014; Fig. 2).

LandCover6k's ambitions and strategy

The ultimate goal of LandCover6k is to produce useful outputs for ecologists, Earth system scientists, conservation bodies, land-use managers, and policy-makers. Broken down into specific goals, the working group aims to:

• produce pollen-based land-cover reconstructions for regions of the world where human impact has been particularly intense over the Holocene prior to AD 1500, i.e. North America, South America, Europe, Africa, Asia (China and India in particular), and Oceania (Australia, New Zealand, and other Pacific islands).

• evaluate the existing ALCC scenarios with the combined information from the pollen-based reconstructions, archeological and historical data, and other evidence of human-induced land-cover change such as paleofire reconstructions.

• improve the ALCC models and produce spatially continuous land-cover descriptions íntegrating the REVEALS-based reconstructions, biomization, dynamic vegetation modeling, ALCC modeling and spatial statistical modeling. We strive to achieve this final product within six years from now.

The ambitious and challenging plan of LandCover6k requires a large, well-organized group of devoted scientists. The group is coordinated by experts in the various disciplines and by one or two co-leaders for each of the six regional subgroups.

The tasks of the regional subgroups will be to:

• compile the fundamental information needed to produce pollen-based REVEALS reconstructions of past land cover, i.e. obtain new pollen records of past anthropogenic vegetation change, develop pollen databases, and estimate pollen productivities and fall speeds of the regionally prevailing plant taxa.

• develop datasets of archeological and historical information on past land cover.

• achieve as many REVEALS reconstructions as possible for each region.

• evaluate the REVEALS reconstructions by comparison with archeological and historical datasets (AHDs).

• evaluate the ALCCs for each region on the basis of the REVEALS reconstructions and AHDs.

LandCover6k welcomes new members, particularly archeologists and historians, who are interested in this kind of work and feel they can provide useful information and make a contribution to the group’s goals. A launch meeting is planned in Paris, France from 18-20 February 2015, which aims to determine the organization, structure, and milestones of the group for 2015-2017. For more information visit the LandCover6k website at: http://pastglobalchanges.org/science/wg/landcover6k/intro

affiliations

1Pollen-vegetation modeling, Linnaeus University, Kalmar, Sweden
2See list of Interim Steering Group members at: http://pastglobalchanges.org/science/wg/landcover6k/people

contact

Marie-José Gaillard: marie-jose.gaillardlnu.se

references

Gaillard M-J et al. (2010) Clim Past 6: 483-499

Klein Goldewijk K et al. (2011) Glob Ecol Biogeogr 20: 73-86

Kaplan JO et al. (2009) Quat Sci Rev 28: 3016-3034

Pirzamanbein B et al. (2014) Ecol Complexity 20: 127-141

Pongratz J et al. (2008) Glob Biogeochem Cycles 22, doi:10.1029/2007GB003153

Prentice IC et al. (1996) Clim Dyn 12: 185-194

Ruddiman WF (2003) Clim Change 61: 261-293

Strandberg G et al. (2014) Clim Past 10: 661-680

Sugita S (2007) Holocene 17: 229-241

Trondman A-K et al. (in press) Glob Chang Biol, doi: 10.1111/gcb.12737

Ute Merkel1, D.-D. Rousseau2,3, J.-B. W. Stuut1,4 and G. Winckler3,5

In its recently published report, the Intergovernmental Panel on Climate Change identified the role of mineral dust in the Earth system and the uncertainties it introduces to the total aerosol radiative forcing and climate projections as key topics for future research (WG 1, chapters 5, 7 and 9). Achieving a thorough understanding of feedbacks associated with eolian dust is a challenge for a number of Earth science disciplines as mineral dust processes operate on a wide range of spatial and temporal scales. On the other hand, studies of mineral dust contribute significantly to research on past climatic and environmental conditions enabled by dust preservation in different kinds of depositional paleoclimate archives.

Such work has been the focus of PAGES’ recently concluded ADOM (Atmospheric Dust during the last glacial cycle: Observations and Modeling) working group, which was established in 2008 with the goal of combining reconstructions of climate and atmospheric circulation from terrestrial, marine and ice-core records with modern dust evidence and model simulations of past and present atmospheric circulation. To this end, ADOM considered processes ranging from the regional to (inter-)hemispheric scales and focused on fostering more detailed knowledge on dust-related dynamics. The idea of editing a dedicated PAGES Magazine on mineral dust was born during the 2nd ADOM workshop held at MARUM in Bremen, Germany, in November 2011. This issue with contributions from workshop participants and colleagues from the ADOM community provides an overview of the science ADOM has focused on during recent years and highlights challenges to state-of-the-art dust research.

Modern dust

Figure 1: Dust storm in Iwik, Mauritania (photo by Jutta Leyrer).

From a meteorological perspective the conditions governing the mobilization and entrainment of dust into the atmosphere (Fig. 1) and its long-range transport operate on daily to seasonal timescales. Schepanski et al. (p. 62) present an introduction to the modern mineral dust cycle and discuss on the basis of recent observational results how conditions for dust mobilization depend on daily meteorological conditions and on prevailing seasonal patterns. Lelli et al. (p. 64) show how seasonality impacts atmospheric aerosol content. Remote sensing of atmospheric aerosol content from satellites has now been carried out for more than three decades, which is sufficiently long to analyze interannual aerosol variations. However, developing algorithms to retrieve dust deposition information from the remotely sensed mineral dust content in the atmosphere is still a challenge (Lelli et al. p. 64). This is particularly relevant over the oceans where in-situ measurements of dust deposition are scarce (Fig. 2) but nevertheless required to put the dust deposition recorded in marine sediment into a quantitative perspective.

Figure 2: Dust collection at sea (photo by Jan-Berend Stuut).

Seasonal dust fluctuations are also superimposed by longer-term variations on interannual-to-decadal timescales. Shao (p. 66) sheds light on the links between dust fluctuations and climate variability modes and trends, and presents recent regional dust modeling results for Asia and Australia. Altogether, these results emphasize the need for long-term monitoring of dust deposition. To that effect, the most striking long-term effort discussed in this issue is the dust recordings from J. Prospero's Barbados observatory. The observations now cover half a century and provide a deep insight into West African dust source variations and trans-Atlantic dust transport (Prospero p. 68). This record revealed for example a less pronounced correlation between precipitation in the Sahel region and dust deposited on Barbados than suggested previously.

In addition to analyses of the total amount of dust deposited such as those discussed above, understanding the evolution of particle-size distributions along the transport path is another important target of dust research. It requires that sectoral observational studies are coordinated towards a holistic source-to-sink approach, connecting research on near-source dynamics of surface dust emission, on size-selective transport processes in the atmosphere, and on depositional processes including the sinking behavior of dust particles through the atmosphere and (ocean or lake) water column, and finally sediment formation. To achieve this, the different communities studying the dust cycle need to collaborate. Ideally, correlations between source area information and paleoclimatic records would emerge, taking into account the dust particle interactions and transformations on their way to the deposition site (Stuut et al. p. 70).

Paleo archives of dust

Looking into the past enables us to detect amplitudes, ranges, and timescales unseen in modern observational records. In spite of the many challenges associated with understanding atmospheric processes and dust particle dynamics on their source-to-sink pathway, paleo-dust reconstructions have provided elucidating insights into the global dust cycle and its variations in the past. Paleoclimatic archives such as ice cores, terrestrial loess deposits, peat bogs, and marine sediments reveal direct information about variations of dust deposition processes over time. In addition they provide hints about dust-related facets of the Earth system, such as changes in vegetation cover, atmospheric circulation patterns and wind strength.

A central issue of paleo-dust studies is the question of provenance, i.e. what was the origin of the dust that eventually got deposited and preserved as sediment. The question becomes especially intriguing when paleo-dust archives are remote from established source areas such as it is the case for polar ice cores. These provide highly detailed information about gradual and abrupt climate change due to their high temporal resolution, but their interpretation requires assumptions about the long-range dust transport pathways. Based on the mineralogical and isotopic signatures of the dust, the Taklamakan desert of western China was recently identified as the main dust provenance region for Greenland ice cores (Bory p. 72). Mineral dust in East Antarctic ice cores has largely been attributed to Patagonian sources, in particular during glacial times. However, for the late Holocene, Australia and also Antarctica itself have now been identified as secondary dust sources to Antarctica (Vallelonga p. 74). Furthermore, dust signatures at West and East Antarctic deposition sites have been shown to be impacted by topographic elevation effects (Koffman and Kreutz p. 76). Similarities detected recently in the geochemical fingerprint of Australian and Southern South American dust (Gili and Gaiero p. 78) further complicate the interpretation of dust in Antarctica.

A large amount of mineral dust is deposited over the open ocean and leaves an imprint in marine sediments. Recent methodological developments have added non-destructive fine-scale elemental scanning and geochemical fingerprinting methods to the portfolio of sediment core analysis. The gain in information detail reveals new insight into dust provenance and transport mechanisms, as demonstrated with a marine sediment core off southern Australia (De Deckker p. 80). Reconstructing dust deposition over the ocean is of particular interest because of the potential role of dust input in fertilizing high-nutrient, low-chlorophyll regions of the surface ocean with the micronutrient iron, thereby stimulating phytoplankton growth and organic carbon export to the deep ocean. Iron fertilization as traced in marine sediments from the Southern Ocean (Martínez-García and Winckler p. 82) may help explain variations in atmospheric CO2 over past glacial climate cycles, and help to asses the potential of artificial iron fertilization as geo-engineering strategy.

Understanding the variability of the dust cycle in the past is closely tied to knowledge about past atmospheric circulation. The continental loess deposits at mid-latitudes around the world provide information about paleowind direction and intensity. Recent examples demonstrate that loess properties such as coarse-to-fine particle ratios and alterations in paleosol-loess sequences can provide insight into glacial-interglacial variations in eolian particle transport (Muhs et al. p. 84). Furthermore, an overview of global loess deposits and a comparison between climate model results and loess records in Europe highlights the potential for studying the role of changes in the seasonal cycle for the millennial-scale abrupt changes of Marine Isotope Stages 2 and 3 (Rousseau and Sima p. 86). During the past decade, peat bogs and the mineral dust they contain have turned out to be a new valuable natural archive for reconstructing the role of dust in climate change and during abrupt events, in particular during the Holocene (De Vleeschouwer et al. p. 88).

Encouraging collaboration

As emphasized by the contributions in this dust issue of PAGES Magazine, the evidence from proxies and model simulations about dust variations in the past is increasing rapidly. These are good conditions to foster cross-disciplinary approaches that combine more advanced characterizations of dust sources with the dust signatures at deposition sites. Concerted efforts are needed to produce quantitative proxy records and a synopsis of available records. Such an improved data basis would not only benefit the paleo-dust reconstruction community but climate modelers, who require quality-controlled globally gridded datasets as model input or for meaningful model-data comparisons. As an example of such concerted efforts, this issue provides news from the DIRTMAP initiative (Maher and Leedal, p. 90) and the new PAGES working group, DICE (Dust Impacts on Climate and Environment; Winckler and Mahowald, p. 61). DICE will build on the successful legacy of ADOM and provide a collaborative platform to build a tight and well-coordinated link from (paleo) observations to (paleo) climate modeling.

affiliations

1MARUM - Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, Germany

2Laboratoire de Météorologie Dynamique & CERES-ERTI, CNRS - Ecole Normale Supérieure, Paris, France

3Lamont-Doherty Earth Observatory, Columbia University, Palisades, USA

4NIOZ Royal Netherlands Institute for Sea Research, Texel, The Netherlands

5Department of Earth and Environmental Sciences, Columbia University, New York, USA

contact

Ute Merkel: umerkelmarum.de

Gisela Winckler1,2 and Natalie Mahowald3

Natural and human contributions of aerosols and dust are critically important components of climate and Earth system dynamics. Mineral dust aerosols, emitted through wind erosion, affect the radiative budget of the planet, precipitation patterns, biogeochemical cycles, the chemistry of the atmosphere, air pollution and human health. Emission patterns, transport and impact of aerosols on societies are almost certain to change under ongoing climate and environmental change, and it is thus increasingly important to improve our understanding of the impact of dust on climate and environment.