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Carbon Brief Staff

22.02.2012 | 3:58pm
ScienceNew Russian heatwave study solves climate conundrum
SCIENCE | February 22. 2012. 15:58
New Russian heatwave study solves climate conundrum

Russia’s record-breaking heatwave of summer 2010 was down to both man-made climate change and natural variability according to a new study. In a nuanced conclusion, researchers found that the size of the heatwave was within natural limits, but the heatwave had been made three times more likely because of man-made climate change.

This finding reconciles two previous studies that reached conflicting conclusions as to the cause of the 60-day heatwave, which left tens of thousands of people dead from respiratory illnesses and heat stress, as well as causing numerous wildfires, failed wheat harvests, and an estimated $15 bn in financial losses.

Meteorologists seem to agree on the immediate trigger of the heatwave: a high pressure air mass settled over Russia for July and much of August, diverting the jet stream and the summer storms further north. At the same time warm continental air was able to spread up across the region, bolstering the high pressure air mass, and adding to the heat. The unusually high temperatures are depicted below:

NASA Russian heatwave

Source: NASA Earth Observatory

But whether this blocking high pressure system was the result of natural variability or man-made climate change seems to have been harder to resolve. In 2011 two separate groups of climate scientists published seemingly contradictory results. Randy Dole of the National Oceanographic and Atmospheric Administration, and colleagues concluded:

“the intense 2010 Russian heat wave was mainly due to natural internal atmospheric variability”

whereas Stefan Rahmstorf and Dim Coumou at the Potstam Institute for Climate Impact Research (PIK) wrote:

“we estimate … an approximate 80% probability that the 2010 July heat record would not have occurred without climate warming.”

Now another research team led by climate scientist Myles Allen of Oxford University has resolved the conflicting results in their upcoming paper, to appear in the journal Geophysical Research Letters.

They ran thousands of climate model simulations, half running the climate conditions of the 1960s, and the other half the climate conditions of the 2000s – including the effects of man-made climate change.

The simulations, run on volunteers’ home-PCs as part of the project, showed that heatwaves as big as that of Russia in 2010 occurred even in model simulations using only natural variation. But similar heatwaves occurred three times as frequently in the simulations which included man-made climate change.

The apparent contradiction between the two previous studies arises because Dole and colleagues focused their research on the size of the heatwave, whereas Rahmstorf and Coumou had looked at the likelihood of a similar heatwave occurring. As Neil Massey, University of Oxford and co-author of the study, puts it:

“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.”

And, Massey explains, this approach has useful practical implications:

“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.”

As is often the case with these issues, there’s no simple answer. Allen says:

“We have a tendency, whenever a weather event happens to say ‘it was caused by x’ but that’s never the case, you have multiple causes for an event. People just have to learn that there’s no such thing as a weather event that has only a single cause. This is a complicated, interacting system.”

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