PAGES 2k Global Mean Reconstructions Nature Geoscience 2019

Additional information

Here you will find additional information related to the 2k Network paper published in Nature Geoscience.

Title: Consistent multidecadal variability in global temperature reconstructions and simulations over the Common Era
Authors: PAGES 2k Consortium*
DOI: https://doi.org/10.1038/s41561-019-0400-0
SharedIt: Access a publicly available, full-text, view-only version
Publication date: 24 July 2019
Available on the PAGES Product Database here.

*The PAGES 2k Consortium authoring this study is a subgroup of the larger PAGES 2k Network. It comprises 19 scientists from eight countries. A full list of the authors is provided below.

This paper is a contribution to the PAGES 2k Network. This study uses the PAGES 2k temperature database to generate a new database of global climate field reconstructions over the past 2000 years using seven different reconstruction methods.

Access the database of PAGES 2k paleo-temperature data used to generate the reconstructions

Figshare: https://figshare.com/collections/A_global_multiproxy_database_for_temperature_reconstructions_of_the_Common_Era/3285353
NOAA: https://www.ncdc.noaa.gov/paleo/study/21171

Access the temperature reconstructions

Figshare: https://doi.org/10.6084/m9.figshare.c.4507043
NOAA: https://www.ncdc.noaa.gov/paleo/study/26872

Access the following official Press Releases (PDFs)

> The University of Bern, Switzerland (English)
> University of Maryland, USA (English)
> University of Melbourne, Australia (English)
> Northern Arizona University, USA (English) (link to website story)

> Read the Nature Editorial: The great climate conundrum

Audio interviews

> Listen to the Nature podcast
> Listen to a radio interview with co-author Michael Erb on "This Green Earth" KPCW 91.7 FM, Utah, USA

Short video

Watch a presentation, explaining the hockey stick graph, by University of Southern California climate scientist and contributing author Julien Emile-Geay. In the video, Dr. Emile-Geay also discusses the significance of the latest findings in this journal article and the companion Nature article.

 

Corresponding authors

Raphael Neukom (Corresponding author, German, English and Spanish contact)
Oeschger Centre for Climate Change Research and Institute of Geography
University of Bern, Switzerland
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
Telephone: +41 77 956 18 48

Feng Shi (Chinese contact)
Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics
Chinese Academy of Sciences, Beijing, China
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
Telephone: +86-10-82998898, Cell phone:+86 13911393443

Kira Rehfeld (German contact)
Institute of Environmental Physics, Ruprecht-Karls-Universität Heidelberg
Heidelberg, Germany
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
Telephone: +49 (0)6221 54 6353
 
Benjamin Henley (Australian contact)
School of Earth Sciences, University of Melbourne, Australia
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Telephone: +61 404 278 355

Michael N. Evans (US contact)
Department of Geology and ESSIC
University of Maryland, College Park, MD, USA
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Telephone: +1 301 405 8763

Andrew Schurer (UK contact)
School of Geosciences, University of Edinburgh, Edinburgh, UK
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Telephone: +44 (0) 131 650 8744

Fredrik Charpentier Ljungqvist (Scandinavian contact)
Department of History, Bolin Centre for Climate Research, Stockholm University, Sweden
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The PAGES International Project Office (This email address is being protected from spambots. You need JavaScript enabled to view it.) can provide contact details for consortium members for other languages.

 
Nature

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Lisa Boucher
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Raphael Neukom1*, Luis A. Barboza2, Michael P. Erb3, Feng Shi4,5, Julien Emile-Geay6, Michael N. Evans7, Jörg Franke1, Darrell S. Kaufman3, Lucie Lücke8, Kira Rehfeld9,10 Andrew Schurer8, Feng Zhu6, Stefan Brönnimann1, Gregory J. Hakim11, Benjamin J. Henley12, Fredrik Charpentier Ljungqvist13,14,15, Nicholas McKay3, Veronika Valler1 and Lucien von Gunten16

1Oeschger Centre for Climate Change Research and Institute of Geography, University of Bern, Bern, Switzerland.
2Escuela de Matematica-CIMPA, Universidad de Costa Rica, San Jose, Costa Rica.
3School of Earth and Sustainability, Northern Arizona University, Flagstaff, AZ, USA.
4Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China.
5CAS Center for Excellence in Life and Paleoenvironment, Beijing, China.
6Department of Earth Sciences and Center for Applied Mathematical Sciences, University of Southern California, Los Angeles, CA, USA.
7Department of Geology and ESSIC, University of Maryland, College Park, MD, USA.
8School of Geosciences, University of Edinburgh, Edinburgh, UK.
9British Antarctic Survey, Cambridge, UK.
10Institute of Environmental Physics, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany.
11Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA.
12School of Earth Sciences, University of Melbourne, Melbourne, Victoria, Australia.
13Department of History, Stockholm University, Stockholm, Sweden.
14Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden.
15Department of Geography, University of Cambridge, Cambridge, UK.
16PAGES International Project Office, Bern, Switzerland.
*Corresponding author

 

1. The most rapid warming of the past 2000 years occurred during the second half of the 20th century, highlighting the extraordinary character of current climate change, due mostly to human emissions of heat-trapping gases.

2. Prior to the industrial revolution of the 19th century, global-mean temperature fluctuations at multi-decadal timescales were mainly controlled by major volcanic eruptions - not variations in the Sun’s output.

3. Global climate models accurately represent both the magnitude of the cooling effect of volcanic eruptions and the natural variability intrinsic to the climate system on global mean temperature.

 

This study used an extensive new collection of paleo-temperature records and seven different statistical methods to generate a state-of-the-art reconstruction of global average temperature changes over the past 2000 years. In the past, various studies had attempted to do this, using different input data or methods (often both), leading to a "spaghetti diagram" with widely disparate estimates. By using a new, highly-curated set of input data and comparing seven statistical methods within a common protocol, the study is the first to cleanly isolate the impact of methodological choices. The seven methods lead to different results on timescales longer than about 50-100 years, but remarkably similar estimates on timescales of a few decades.

This gave us confidence in the validity of the results, allowing to test the same global climate models used to predict what will happen over the next few decades under continued man-made emissions of heat-trapping gases. This allowed us to quantify the relative importance of various causes of global temperature changes during the past, including volcanic eruptions, solar variability, and natural ups and downs inherent to the climate system.

 

The new reconstructions put the rate of global warming into a long-term context, showing that the pace of recent warming is clearly exceptional over the past 2000 years.

Multi-decadal temperature changes may be caused by both natural and human causes. Distinguishing these factors is essential for predicting the response of temperature to different types of climate forcing.

Multidecadal variations may also happen without being forced. The results of our study show that the magnitude of the unforced variation is not inconsistent between reconstructions and simulations.

The paleo-temperature reconstructions agree with climate model simulations, which boosts confidence in the ability of these models to predict climate change over the coming decades.

 

Twenty years ago, the famed "Hockey Stick" curve, published by Mann, Bradley & Hughes (https://www.nature.com/articles/33859), made the case that recent warming was exceptional in the context of the past millennium. The study was widely disseminated by the Intergovermental Panel on Climate Change (IPCC), and came under heavy fire from climate skeptics, triggering two inquiries in the US congress and National Academy of Sciences. The criticisms, many of which were legitimate, had to do with the paucity of records of past temperature over the past 2,000 years, as well as the statistical methods used to infer temperature from these sparse and indirect observations.  

For the past 20 years, the paleoclimate community has worked to improve both the data and methods underlying the reconstruction of global climate over the past 1 to 2 millennia. A particularly active group of scientists on this front was the 2k Network consortium of the Past Global Changes (PAGES) project, an international non-profit organization coordinating the work of the international research community on all aspects of the paleosciences.

This particular study builds on continent-by-continent temperature reconstructions that were generated by PAGES 2k scientists in 2013 (https://www.nature.com/articles/ngeo1797). Since then, the PAGES 2k consortium redoubled its efforts to generate a robust reconstruction of global mean temperature. The group assembled the largest available paleo-temperature proxy data collection, which was published in the journal Scientific Data in 2017 (https://www.nature.com/articles/sdata201788). They then used this dataset for testing the similarities and differences of different statistical methods of temperature reconstruction. After five years of steady progress, the PAGES 2k consortium has accomplished that major goal in time for consideration in the upcoming climate assessment report of the Intergovernmental Panel on Climate Change.

The study was authored by 19 paleoclimate scientists from eight countries, and was led by Raphael Neukom (Oeschger Centre for Climate Change Research and Institute of Geography, University of Bern, Switzerland).

 

Q1: What features of global temperature over the past 2000 years are analyzed in this study?

A: We focus on global mean surface temperatures, so regional variations are not assessed (but they are in a parallel study, see below Q6). The study primarily analyzes temperature changes on multi-decadal time scales. These time scales are particularly relevant for projections of 21st century climate change and to study interactions between external drivers of climate change (for instance greenhouse gas concentrations, volcanic eruptions or changes in solar irradiation) and chaotic fluctuations within the climate system (so called internal variability). On these multi-decadal time scales we analyze:
- Temperature fluctuations from different reconstruction methods
- The agreement of reconstructions with climate model simulations
- The most important drivers of (pre-industrial) temperature changes
- The rates of global warming and cooling (i.e. the velocity of warming and cooling)

Q2: What are the key findings from these analyses?

A: Analyzing temperature fluctuations from different reconstruction methods we find:
The differences between temperature fluctuations from different statistical methods are small and show a clear improvement compared to earlier large-scale reconstructions.
This indicates that, given the new dataset of paleoclimate observations that we use, our results are robust to methodological choices and more robust than earlier estimates.

Analyzing the agreement of reconstructions with climate model simulations we find:
Also between climate reconstructions and model simulations, there is good agreement in the timing and magnitude of global temperature fluctuations. In addition, observations and models agree about the magnitude of unforced natural variability of temperature and about the maximum warming rates that are possible under pre-industrial (natural) conditions.
This indicates that model simulations are capable of simulating forced and unforced temperature variability under natural conditions. This increases our confidence in these models to project future temperature variations, superimposed on the anthropogenic warming trends on the policy-relevant multi-decadal time scales.

Analyzing the most important drivers of (pre-industrial) temperature changes we find:
Pre-industrial temperature fluctuations are to a significant extent driven by changes in external forcing (i.e. driving factors that influence the climate system from outside, such as greenhouse gas concentrations, volcanic eruptions, solar irradiation). Analyzing the relative importance of these factors, we find that volcanic eruptions played the most important role in influencing temperature variations over the pre-industrial period. In contrast, we find no significant influence of solar variability on global mean temperatures. Greenhouse gas concentrations do play a role, particularly over the industrial period (the last 150 years), when they are the strongest driver of temperature change.

Analyzing the rates of global warming and cooling we find:
In pre-industrial times, the periods of fastest cooling are mostly related to large volcanic eruptions, which often lead to cold periods in many places of the world. The most rapid pre-industrial warming mostly occurred at the times when the recovery from such volcanic cold phases took place. The fastest global mean warming over the last 2000 years is observed within the 20th century. This is the case for warming rates on all timescales between 20 and 150 years. The recent warming rates are larger than what can be expected from natural variability alone. For example the current global mean warming rate (1967-2017) is 1.7 °C per century, and the maximum warming rates expected from pre-industrial conditions with minor human influence on the climate system is roughly 0.6 °C per century.

Q3: What type of evidence was used to infer past temperatures?

A: The PAGES 2k dataset includes nearly 700 individual time series from natural archives that reflect temperature changes through various biological and physical processes. Most of the information comes from tree-ring measurements, but additional evidence comes from glacier ice, speleothems, corals, sediments from lake and ocean bottoms, and historical documents. The individual records in this data compilation were publicly available from online data repositories.

Q4: What are the main limitations of the data and the results of this study?

A: Natural archives can be used to infer past temperatures, but are not thermometers per se. Translating proxy evidence to past temperatures involves making important assumptions. For instance, we assume that the relation between temperature and the proxy record that existed during the instrumental period is the same as that which existed during pre-historic times. Uncertainties about past temperature variability remain, especially during the first millennium when available information is sparser. Some of the temperature information relates to conditions during a particular season and some to annual averages. The two can differ, although they are generally correlated in meteorological records.

Q5: Why does the study make statements about global warming rates and not about absolute temperatures in modern and medieval times?

A: We find that the highest quality of our results is on multi-decadal time scales. Therefore we focus on analyses on these time-scales, which are best expressed by warming rates. On longer time scales, the results are more uncertain. However, comparisons of absolute temperatures in different periods are possible with our new reconstructions, and our study confirms earlier findings that current temperatures are likely higher than in any other period of the last 2000 years.

Q6: What is the difference between this study and the reconstructions provided in Neukom et al. in Nature?

A: This study focuses exclusively on the global mean. The Neukom et al. study reconstructs gridded global temperatures, this means at every location in a 5°x5° spatial resolution. The temperature history and response to external forcings (e.g. greenhouse gases, solar irradiation or volcanic eruptions) can be different from the global mean in certain regions of the globe. The two studies use the same underlying input data, but different methods to quantify past temperatures, each selected to best fit the purpose of reconstructing global mean and spatially explicit temperatures, respectively.

 

2k proxy map satel

Figure 1, above: Locations of paleoclimate data used in this study. 257 temperature records from natural climate archives were used for the 2,000 year long reconstructions of global mean temperature.

2k gmst fig2 19

Figure 2 (click here to enlarge), above: Rates of global mean warming or cooling over the Common Era. Temperatures (over time windows of 51-years) were increasing in years with red shading and decreasing during periods marked with blue shading. 20th century warming rates - also illustrated by the black line showing thermometer measurements - are larger than those of any previous century. The green solid and orange dashed lines represent represent the maximum warming rates expected from natural (pre-industrial) conditions. Climate models (orange dashed line) are doing a good job in simulating these natural warming rates obtained from the reconstructions (green line). The human-induces 20th century warming rates exceed these natural warming rates.

 

2k gmst ric vil trees

Image 1: Trees growing under extreme conditions, such as this Chilean cedar (Austrocedrus chilensis) in Patagonia, South America, are used to quantify changes in past climate (Image: Ricardo Villalba).

coral drilling christmas island credit Jason Turl

Image 2: Researchers drilling coral at Christmas Island (Image: Jason Turl).

ice core drill abram

Image 3: Antarctic ice core in drill (Image: Nerilie Abram).

 

GMST Reconstructions is a project of the third phase of the PAGES 2k Network. Find out more about it here: http://pastglobalchanges.org/science/wg/2k-network/projects/gmst-recon-2k/intro

Phase 3 of the PAGES 2k Network launched in May 2017, with all activities consolidated within a single working group. Find out more about the other projects here: http://pastglobalchanges.org/2k

The 2k temperature database was one outcome of Phase 2 of the PAGES 2k Network, which ended in May 2017 and comprised nine regional working groups, each one responsible for assembling the data from within their regions. Access information about phase 2 here: http://pastglobalchanges.org/ini/wg/2k-network/former-2k-activities

Past Global Changes (PAGES) was established in 1991 to facilitate international research into understanding past changes in the Earth system to improve projections of future climate and environment, and inform strategies for sustainability. Funding for this project was provided primarily through grants to PAGES IPO from the US NSF, Swiss NSF and Swiss Academy of Sciences. PAGES is a core project of Future Earth and a scientific partner of WCRP and WDS-Paleo.

More at: http://pastglobalchanges.org

 

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