Corresponding authors

Raphael Neukom
Oeschger Centre for Climate Change Research and
Institute of Geography University of Bern, Switzerland
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Telephone: +41 77 956 18 48

Nathan Steiger
Lamont-Doherty Earth Observatory
Columbia University
New York, USA
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Juan José Gómez-Navarro
Department of Physics
University of Murcia, Spain
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Telephone: +34 868 88 84 35
Mobile: +34 600 46 31 10

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Additional authors

Jianghao Wang
The MathWorks, Inc.
Natick, USA
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Johannes Werner
Bjerknes Center for Climate Research
Bergen, Norway
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1. This study uses the PAGES 2k temperature database to generate a new database of global climate field reconstructions over the past 2000 years using six different reconstruction methods. It is the first time that so many different methods are used in a coordinated way to quantify past global temperatures and their uncertainties.

2. The results revise the common understanding of global synchronous climatic epochs over the last 2000 years: we find that they were not as spatially consistent as previously thought.

3. The warmest multi-decadal periods of the "Medieval Climate Anomaly" and "Roman Warm period" occur at different times in different places of the Earth.

4. Similarly, the coldest periods of the "Little Ice Age" and the "Dark Ages Cold Period" were not globally simultaneous.

5. The spatial coherence of extreme temperatures during these pre-industrial epochs is consistent with internal climate variability and noise. This suggests that external forcing factors such as volcanic eruptions and solar irradiation variability were not strong enough to cause globally synchronous multi-decadal extreme temperatures.

6. In contrast, the likely warmest multi-decadal periods of the past 2000 years occurred simultaneously in the 20th century over more than 98% of the globe. The spatial coherence of warming during this period cannot be explained by internal climate variability alone.

 

Q1: What is the most important new conclusion from the study?

A: We find that previously named climate epochs of the Common Era were not globally coherent phenomena. This is in contrast to the contemporary anthropogenic warming which we do find is globally coherent to an extent that hasn’t happened in the past 2000 years.

Q2: Earlier studies have found cold temperatures during solar downturns or periods of active volcanic activity and global cooling during the "Little Ice Age". Are the new results contradicting these?

A: Our results show that the interpretation of climate epochs, such as the Little Ice Age, as being globally coherent phenomena is incorrect. At long time scales and at large spatial scales the effect of averaging can give the impression that there was global cooling but our analysis of both climate reconstructions as well as the underlying proxy data do not support the idea that multi-decadal or centennial temperature minima occurred simultaneously all over the globe. The same applies for multi-decadal warm peaks, with the notable exception of the current warm phase, where we find globally synchronous maximum temperatures.

Q3: Are you recommending that scientists not use terms such as the "Little Ice Age"?

A: Not necessarily. In general, having simplified conceptual models of natural phenomena can be very useful and even essential in the pursuit of scientific understanding. It’s when the conceptual models get in the way of accurate science that problems arise. For example, when one labels any Common Era proxy time series with terms like the "Little Ice Age" or the "Medieval Climate Anomaly", they are usually implicitly assuming that such epochs were global phenomena over well-defined time intervals. Our results show that both of these assumptions are incorrect.

Q4: Does this study contradict the idea of a global cooling trend over the pre-industrial Common Era?

A: Here we have tested only the spatial coherency of previously identified climate epochs. We have not analyzed multi-centennial trends in the climate reconstructions and underlying proxy data. Parallel efforts based on the same input data found that for the global mean, such cooling trend can be robustly identified (PAGES 2k Consortium, Nature Geoscience).

Q5: Does this study show that there were no warm temperatures in medieval times?

A: No. Many regions experienced relatively warm temperatures at some time during medieval times. In some regions temperatures may have even been at similar levels as during the late 20th century. However, the timing of these warm extremes was not globally synchronous. This means that maximum warming occurred at different times in different places.

Q6: Does this study prove that humans are causing the current, globally consistent warm phase?

A: Not explicitly. We show that the non-synchronicity of previous warm or cold extremes is consistent with stochastic climate variability. This means that conditions during medieval times or during the "Little Ice Age" are expected to occur naturally. The large spatial consistency of the present warm phase cannot be explained by natural variability. This result corroborates numerous existing studies that have shown that humans are causing global temperatures to rise since the beginning of the industrial period.

Q7: 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 available through the scientific literature or online databanks.

Q8: 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 time. 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. The available proxy network is also not sampled uniformly across the globe so some areas are more accurately reconstructed than others.

Q9: What is the difference between this study and the reconstructions provided in the PAGES 2k Consortium paper in Nature Geoscience?

A: This study reconstructs gridded global temperatures, this means at every location in a 5°x5° spatial resolution. The PAGES 2k Consortium focuses exclusively on the global mean. 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. Therefore, this study examines spatial patterns of temperature change over the Common Era. 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 spatially explicit and global mean temperatures, respectively.

 

Figure 1, above: Locations of paleoclimate data used in this study.

Image 1, above: Coral coring off northwest Australia. (Image credit: Eric Matson, AIMS.)

Image 2, above: Ice core camp, Mount Brown South, Antarctica, 2018. (Image credit: With kind permission of Nerilie J. Abram.)

Image 3, above: Tree-ring data from remote forests all over the globe - such as in South American Patagonia - contributed to the 2,000-year-long climate reconstructions. (Image credit: Ricardo Villalba.)

Image 4, above: Illustration of "Little Ice Age" glacier advance in the Swiss Alps. (Lower Grindelwald Glacier. Painting by Caspar Wolf. Image credit: Samuel Nussbaumer.)

Figure 2, above: Most likely timing of the coldest 51-year period of the last Millennium. Colours indicate the century, in which this "Little Ice Age" minimum took place at each location of the globe.

Figure 3, above: Most likely timing of the warmest 51-year period during medieval times (700-1400 CE). Colours indicate the century, in which this “medieval Warm Period” maximum took place at each location of the globe.

Figure 4, above: Most likely timing of the warmest 51-year period over the past 2,000 years. Colours indicate the century, in which this warm-peak took place at each location of the globe.

 

Global T CFR is a project of the third phase of the PAGES 2k Network. Find out more about it here: http://pastglobalchanges.org/ini/wg/2k-network/projects/gtcfr-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

 

Please find a selection of articles below. This will be updated as stories come to hand.

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	Schweizer Forscher entkräften Argument von Klimawandel-Leugnern spiegel.de
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	Modern climate change is more intense than anything the Earth has seen in the past 2,000 years theweek.com
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	Global warming dwarfs climate variations of past 2,000 years: Study channelnewsasia.com
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