Can scientists now say with a fair amount of confidence that extreme weather events such as the Texas heat wave last year, the Russian heat wave of 2010 and the European heat wave of 2003 are directly linked to global warming caused by humans?
In a new paper entitled ‘Perception of Climate Change’ NASA scientist James Hansen and two colleagues describe how climate change has increased the occurrence of unusually warm weather events in the past 30 years. There is only a small chance that the shift towards both higher temperatures and increased weather extremes would have happened in the absence of climate change. Periods of extreme summertime temperatures have increased in geographic scope from less than 1% of the land’s surface in the base period of 1951 to 1980 to covering 13% in recent years.
Hansen and colleagues have come under much criticism for attempting to link extreme weather events to climate change, a connection thought impossible to make. But they think their data speaks for itself. They looked at surface-air temperature data from 1951 to 2011 in cells covering the world’s land masses. For each cell an average summer temperature was obtained for the base period. Deviations from this average were recorded and plotted for each decade. This resulted in a set of 6 bell curves whereby for each curve the peak represents the mean temperature and the width, the deviation from this mean. As decades have progressed the peaks of the bell curves have shifted to the right representing warmer average temperatures. Moreover the curves have also gotten wider or, in other words, warmer summer temperatures outside of the average (or in scientific terms three standard deviations away from the mean of the reference curve) are increasing. From Dr Hansen’s definition, extreme weather events, at any one time, could be expected to be occurring in 8% of the world.
Although the increasing frequency of extreme events is thought to be linked to human-caused climate change, and the evidence from Dr. Hansen’s paper is illuminating, a robust and direct connection has yet to be made. In the meantime, the trend for warmer and more extreme weather events, set out plainly in this paper, will hopefully be enough to spark action from the global community to tackle climate change, just in case.