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Past Global Changes project (PAGES) Press Release A Future Earth international research project Embargoed until Wednesday April 6 at 18:00 GMT, 13:00 US Eastern Time, 19:00 Central European Time. Climate models overestimate twentieth century wet and dry climate extremes For the first time, scientists have pieced together a 1200-­‐year long record of water availability, rainfall and drought across Europe, North Asia and North America. The research, published in the journal Nature is the first time scientists have been able to accurately see how rainfall patterns have changed during the twentieth century compared with the last twelve centuries. The findings show the Northern Hemisphere experienced considerably larger variations in rainfall and drought patterns during the past twelve centuries than in the twentieth century. The researchers, from Sweden, Germany and Switzerland, find that climate models overestimate the increase in wet and dry extremes as temperatures increased during the twentieth century. The new results, a contribution to the Past Global Changes (PAGES) 2k Network, can help improve how climate models represent future rainfall changes in a warmer world. A big uncertainty in climate research is what will happen to rainfall and drought when temperatures rise across the globe. This matters for water resources, crop yields and ecological change. The leading authority on the consensus of climate knowledge, the Intergovernmental Panel on Climate Change, concludes from climate model simulations that wet areas are likely to get wetter and dry areas drier in a warmer world. But the first reliable reconstruction of rainfall and drought across the Northern Hemisphere, looking back 1200 years, challenges the conclusions of these climate models. The results, published in the journal Nature (7 April 2016) indicate that the undisputed temperature rise of the twentieth century may not have affected the hydroclimate – rainfall and drought-­‐
related climate anomalies – to the same extent as earlier suspected. Lead author Fredrik Charpentier Ljungqvist, a historian and climate researcher at Stockholm University, said, “Despite strong twentieth century warming, we find that rainfall and drought extremes in the twentieth century have varied within the natural variability we can now see in earlier centuries. Several other centuries in the past 1200 years show stronger and more widespread extremes and deviations from the average.” “Climate models strongly overestimate the intensification of wet and dry extremes in the twentieth century,” added Ljungqvist. The team, working within a major international research initiative examining 2000 years of climate variability for the Past Global Changes project of Future Earth, used tree-­‐rings, lake sediment, historical data and other types of archives to piece together the new picture of past climate. The researchers found larger land areas with relatively wetter conditions in the ninth to eleventh and the twentieth centuries, whereas drier conditions than during the twentieth century were more widespread between the twelfth and nineteenth centuries. “The lack of agreement between the reconstruction and the climate models in the twentieth century indicates that the models can have limitations in realistically predicting which regions may get wetter and which may get drier in a warmer world. But one reason climate model predictions do not agree well with actual data could also be that twentieth century warming may not yet have been strong enough to trigger large-­‐scale hydroclimate changes.” To investigate the links between temperature and hydroclimate variations, the scientists compared their reconstructed hydroclimate variations with a new temperature reconstruction they also developed. The researchers conclude that only in a few regions is it possible to see clear correlations between changes in temperature and hydroclimate. For instance, drought was most widespread during both the relatively warm twelfth century and the relatively cold fifteenth century. “The study shows the importance of placing recent hydroclimate changes in a millennium-­‐
long perspective,” says Ljungqvist. “Actual measurements of precipitation and drought are too short to tell if the observed changes today fall outside the range of natural variability. Instrumental measurements are also too short to test the ability of state-­‐of-­‐the-­‐art climate models to predict which regions will get drier or wetter with global warming.” About the study: The article “Northern Hemisphere hydroclimate variability over the past twelve centuries” has been published in Nature, vol. 532, pp. 94–98, doi:10.1038/nature17418 Authors: Ljungqvist, Fredrik Charpentier, Krusic, Paul J, Sundqvist, Hanna S, Zorita, Eduardo, Brattström, Gudrun & Frank, David. Additional information: Fredrik Charpentier Ljungqvist, Department of History and Bolin Centre for Climate Research, Stockholm University, mobile phone: +4670-­‐662 07 28 E-­‐mail [email protected] For more information about PAGES, go to: www.pastglobalchanges.org For more information about Future Earth, go to: www.futureearth.org Changes in millimetre annual precipitation between 1951 and 2010
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Figure 1. At the top the trend in annual measured precipitation between 1951 and 2010 is shown from the last scientific report of the Intergovernmental Panel on Climate Change (IPCC). Below the relative hydroclimate changes for four different centuries are shown from the new hydroclimate reconstruction covering the past twelve centuries. The stronger the green colour is the wetter it was and the stronger the brown colour is the drier it was. The anomalies are shown with regard to the long-­‐term mean between AD 1000 and 1899. % reconstructed land area
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Figure 2. The percentage of the Northern Hemisphere land area with wet and dry anomalies, respectively, during the past the twelve centuries. The stronger the green colour is the wetter it was and the stronger the brown colour is the drier it was. The anomalies are shown with regard to the long-­‐term mean between AD 1000 and 1899. 800s
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Figure 3. Drought sensitive 1000-­‐year old trees from the mountains of Greece. Indirect recorders of past precipitation and drought variability such as tree-­‐ring width data were used by the researchers to reconstruct twelve centuries of Northern Hemisphere hydroclimate variability. Photo: Paul J. Krusic. 1900s