Climateprediction: Publication of first results from Weather At Home experiment
In the summer 2010 Western Russia was hit by an extraordinary heat wave, with the region experiencing by far the warmest July since records began. Whether or not this event was caused (in parts) by anthropogenic climate change has been a source controversy. Dole et al. (2011) reported that the 2010 Russian heat wave 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 anthropogenic increase in greenhouse gas concentrations. The latter study explicitly states that their results "contradict those of Dole et al. (2011)".
By using the results from the Weather At Home experiment we show that there is no substantive contradiction between these two papers. The same event can be both mostly internally-generated in terms of magnitude and mostly externally-driven in terms of the probability of such an event occurring. The difference in conclusion between these two papers illustrates the importance of specifying precisely what question is being asked. We argue you need to ask both questions, whether the magnitude was inside the natural variability and whether the frequency of such heatwaves to occur has changed. Our answer is that the magnitude of the heat wave was mainly natural in origin, but the possibility of such a heat wave occurring has increased by a factor of 3-4 times due to anthropogenic climate change.