The conventional wisdom has been that you can’t do that. However, a new study in PNAS by Dr. James Hansen and colleagues argues that several dramatic heat waves in different parts of the world over the last decade are statistically so far away from the historical norm that they are virtually inconceivable in a world not experiencing anthropogenic climate change. According to their analysis of large surface temperature datasets, not only have mean temperatures increased, but their standard deviation has increased as well, with so many examples of high-temperature events exceeding three standard deviations relative to a historical baseline that a new normal distribution must be drawn (shifted towards higher temperatures AND spread out more widely). In the author’s own words:
This is not a climate model or a prediction but actual observations of weather events and temperatures that have happened…. The deadly European heat wave of 2003, the fiery Russian heat wave of 2010 and catastrophic droughts in Texas and Oklahoma last year can each be attributed to climate change…. These weather events are not simply an example of what climate change could bring. They are caused by climate change. The odds that natural variability created these extremes are minuscule, vanishingly small.
This represents a clear challenge to the existing mantra that individual weather events cannot be directly attributed to climate change, and it is likely that this viewpoint will face a lot of skepticism (as any new scientific theory should) both from within and outside of the scientific establishment. It is however consistent with the approach of some other recent high-profile studies that use pure observational statistics (completely independent of physical process models and their associated predictive uncertainties) in order to describe the occurrence of anthropogenic climate change and its negative effects on agriculture. It will interesting to see whether or not this study will contribute to forming a new consensus on the issue.