Make climate impact models fit for extreme events, says study

01/03/2019 - Computer models used for assessing future global climate change risks are doing a good job when it comes to gradual changes, but they may be underestimating the severity of extreme events, finds an international team of scientists led by the Potsdam Institute for Climate Impact Research. They compared, for the first time, a large number of different simulations across sectors with the measured impacts of the 2003 European heat wave. In reality, the heat wave and drought in 2003 hit agriculture and ecosystems hard, and caused thousands of excess fatalities. These extreme impacts were not captured by most of the simulations. This means that there is an urgent need to develop models that better represent impacts from climate extremes. It also means that unmitigated global warming might be more costly and stabilizing our climate hence economically more rewarding than so far estimated.
Make climate impact models fit for extreme events, says study
Observed vegetation productivity anomalies in 2003 heat wave. Cutout from figure 4a, Schewe et al 2019

“Global climate impact models are an indispensable tool for estimating the risks that societies face due to climate change,” says lead-author Jacob Schewe from the Potsdam-Institute. “The models we use today have been developed by researchers around the world over many years. Often, their original purpose was simply to understand natural processes such as plant growth or the water cycle. Now we apply these models to assess climate change risks. And when it comes to long-term changes in average conditions, they tend to do a remarkably good job. However, damages from the increase of weather extremes are much harder to capture.” For instance, just a few days of heat in a city – a rather small change of average temperature – can under certain circumstances result in more people dying, namely among the very young and the very old ones. This has to be taken into account when calculating the costs and benefits of greenhouse gas emissions reduction.

The decrease of maize yields in France and Italy, for example, due to the 2003 heatwave has been underestimated by the computer simulations, the researchers show. The same is true for the severity of the extreme event for wheat in Western Europe. Plant-growth in eco-systems also suffered more in reality than estimated by the simulations. As for heat-related mortality, the simulations provide numbers that are similar to what has been observed in Barcelona, London, and Rome. Yet in Paris, which saw by far the most fatalities, the real number of additional deaths was double the number produced by the simulations. Results are different when it comes to water cycles. Observed low water levels in the Rhine and Danube rivers are matched very well by the simulations, and for Weser, Elbe and Oder rivers projected flow reductions did not realize. Yet all in all, a majority of the observed impacts in the sectors studied are greater than what would be expected from the simulations.

“Our study is unique in assessing the ability of several state-of-the-art impact models, for multiple sectors like water, human health and agriculture, to replicate the impacts from a recent important climatic extreme event,” says Simon Gosling from the University of Nottingham in the UK, co-author of the study. “Based on our findings, if I was being precautious, I might consider the more pessimistic seeming projections to be as plausible as those that sit in the middle ground, in terms of what they suggest about the future impacts of climate change on society.” The study is part of the big innovative Intersectoral Impacts Model Intercomparison project (ISIMIP) which aims at systematically helping climate simulation quality. It is led by PIK and funded by the German Federal Ministry of Education and Research, but has more than 100 research groups worldwide participating.

“We now investigated the 2003 European heat wave, but our findings are relevant beyond this case,” explains Schewe. “In 2015 and 2018, Europe was again hit by extremely hot and dry summer conditions, and we worry that such extremes will likely occur more and more often. We’re therefore glad to help making the tools to look into the future, computer simulations, better and better. And we should not be afraid of insights showing the severity of impacts, because we still have it in our hands to turn down greenhouse gas emissions and stabilize our common climate.”

Article: Jacob Schewe, Simon N. Gosling, Christopher Reyer, Fang Zhao, Philippe Ciais, Joshua Elliott, Louis Francois, Veronika Huber, Heike K. Lotze, Sonia I. Seneviratne, Michelle T. H. van Vliet, Robert Vautard, Yoshihide Wada, Lutz Breuer, Matthias Büchner, David A. Carozza, Jinfeng Chang, Marta Coll, Delphine Deryng, Allard de Wit, Tyler D. Eddy, Christian Folberth, Katja Frieler, Andrew D. Friend, Dieter Gerten, Lukas Gudmundsson, Naota Hanasaki, Akihiko Ito, Nikolay Khabarov, Hyungjun Kim, Peter Lawrence, Catherine Morfopoulos, Christoph Müller, Hannes Müller Schmied, René Orth, Sebastian Ostberg, Yadu Pokhrel, Thomas A. M. Pugh, Gen Sakurai, Yusuke Satoh, Erwin Schmid, Tobias Stacke, Jeroen Steenbeek, Jörg Steinkamp, Qiuhong Tang, Hanqin Tian, Derek P. Tittensor, Jan Volkholz, Xuhui Wang, Lila Warszawski (2019): State-of-the-art global models underestimate impacts from climate extremes. Nature Communications [DOI:10.1038/s41467-019-08745-6]

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