First results from weatherathome experiment

February 2012

In the summer 2010, Western Russia experienced the warmest July since records began. Whether or not this event was caused by man-made climate change has been a source of controversy. Dole et al. (2011) reported that it was “mainly natural in origin”, whereas Rahmstorf and Coumou (2011) wrote that with a probability of 80% “the 2010 July heat record would not have occurred” without large-scale climate warming since 1980. Most of this large scale warming has been attributed to the man-made increase in greenhouse gas concentrations. The Rahmstorf study explicitly states that their results “contradict those of Dole et al. (2011)”.

Results from the weatherathome experiment show that there is less contradiction between these two papers than it may seem. The same event can be generated largely by natural variability, but the probability of a similar event occurring is increasing. The two papers’ conclusions differ as they asked different questions. We argue you need to ask whether the magnitude was inside the range of natural variability, as well as whether the frequency of such heatwaves has changed. Our data shows that the magnitude of the heatwave was mainly natural in origin, but the possibility of such a heatwave occurring has increased by a factor of 3-4 times due to anthropogenic climate change.

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