Russian heat wave had both manmade and natural causes

WASHINGTON, DC— The heat wave that struck western Russia in summer 2010, killing 55,000 people, broke July temperatures records and caused $15 billion in damage. Searching for a culprit for the soaring temperatures, research teams have identified either natural or manmade causes. But a new study concludes the devastating heat wave had both. Soaring temperatures were within the natural range for a Russian summer, the researchers found, but due to human-induced climate change, the chance of such an extreme heat wave has tripled over the past several decades.

Smoke from forest fires blankets much of Russia during the 2010 heat wave that struck the country, killing 55,000 people. (Credit: NASA)

“Natural variability could lead to such a heat wave,” said Friederike Otto of the Environmental Change Institute, Oxford University, lead author of the paper. “However due to the global warming trend, the frequency of occurrence of such a heat wave has increased.”

Otto and her colleagues used a climate model that was able to run thousands of simulations. The model was part of the weatherathome.net project, which uses volunteers’ idle computers in order to increase the computing power and answer complex questions – like the role of climate change in a particular extreme event.

The researchers ran two sets of simulations. One set looked at heat waves that would be expected using 1960s climate conditions including temperature, sea ice thickness and extent, and greenhouse gas forcing. The second set examined heat waves under those conditions for the 2000s. Under the earlier set of conditions, the simulated July temperatures for western Russia did reach the recent heat wave’s actual maximum, which topped 40 degrees Celsius (104 degrees Fahrenheit) and had a comparable pattern of daily above-average temperatures that peaked at about 12 degrees Celsius (22 degrees Fahrenheit) beyond the mean. So the magnitude of the heat wave could be due to natural factors, the researchers concluded.

However when the researchers ran their computer simulations, 2010 temperatures were only reached every 99 years or so under the pre-global warming conditions. Under the current, warmer, climate of the 2000s, those extreme temperatures popped up in the model every 33 years – a three-fold increase over just four decades.

‘These results show that the same weather event can be both “mostly natural” in terms of magnitude and “mostly human-induced” in terms of probability,’ explained Neil Massey of the Smith School of Enterprise and the Environment, Oxford University, who was also an author of the study. ‘Thinking in these terms makes it possible to calculate, for instance, how much human-induced climate change cost the Russian economy in the summer of 2010.’

The research will be published in an upcoming Geophysical Research Letters, a publication of the American Geophysical Union.

This new way of approaching the problem reconciles apparently contradictory results from two separate 2011 studies, Otto said. In the first of those studies, by Randall Dole of NOAA’s Earth System Research Laboratory (ESRL) in Boulder, Colo, and his colleagues, researchers focused on why the heat wave was as large as it was – coming up with a natural explanation. The second, by Stefan Rahmstorf and Dim Coumou with the Potsdam Institute for Climate Impact Research in Potsdam, Germany, asked the question of how frequently such heat waves will occur, finding a manmade acceleration.

The new study stresses the need to examine both the magnitude and frequency of extreme events, Otto said. “It’s necessary to analyse both, to really judge the role of global warming,” she said.

And the results will vary with respect to the event. In a separate study, when the researchers looked at flood risk in the United Kingdom, they found that the risk for fall floods increased under warmer climate conditions, but decreased for spring floods.

Given the cost of extreme weather events, determining how the risks are changing allows scientists to better quantify the events and possibly to help build resilience in society’s responses to them, said Myles Allen, a professor in the School of Geography and Environment, Oxford University, co-author of the GRL paper and the principal investigator of the weatherathome.net and climateprediction.net projects.

“People deserve to know how much climate change is affecting them,” Allen said, “and we have the methods to answer the question: How is human influence loading the weather dice?”