Recently we have seen a spate of extreme climate and weather events that have drawn intense media interest, including this winter’s intense storms affecting the US and Canadian eastern seaboard, similarly extreme winter storms last year, the Russian heat wave and Pakistani flooding of summer 2010, the extraordinary Australian flooding event of this past January. These events have certainly tested our ability to cope with weather and climate variations, have had significant negative impacts, and pose the question as to whether human influence on the climate system has played a role. While the research required to answer this question specifically in the context of recent events is yet to be completed, two new papers in Nature (Min et al., 2011; Pall et al., 2011) have presented evidence that changes in the intensity of extreme precipitation since the middle of the 20th century may be linked to human induced global warming, and that in at least in one instance, that human influence on climate had likely substantially increased the risk of flooding.
Changes in extreme temperature and the intensification of extreme precipitation events are natural consequences of a warming climate. A warmer climate would inevitably have more intense warm temperature extremes than the present climate, including longer and more intense heat waves, and less intense cold temperature extremes.
The clear answer to this question, and one that is underscored by the Meehl et al. (2009) study of the occurrence of record breaking temperatures, is that individual extreme events cannot be ascribed to human influence on the climate system in the sense that the event could not have occurred if it were not for human influence. It is, however, possible to assess how human influence on climate may be “loading the weather dice”, making some events more likely, and others less likely.
The weather events that do most damage are very often those that are most difficult to predict: we can, however, assess the impact of an external factor like human influence on climate on the odds of a weather event occurring, even if we cannot predict when it will occur (if you load a dice to double the odds of a six, you still cannot predict precisely the result of any particular roll). Hence the fact that seasonal forecasting of extreme weather is clearly very difficult does not prevent us from assessing the role of long-term drivers in extreme weather risk or attempting to predict seasonal variations in risk.
Any human influence on extreme weather risk combines with these episodic variations and the chance fluctuations that are inevitable when dealing with rare events: hence we should not assume that, if human influence is making a particular event more likely on average, it will necessarily do so every year.
However, the “smoking gun” evidence from these studies suggests that human influence is now affecting the frequency and intensity of high impact events that put people and their livelihoods at risk. While assessments of the abilities of climate models to simulate temperature and precipitation extremes (e.g., Kharin et al., 2007) are sobering, there is a firm physical basis for the expectation that increasing greenhouse gases will intensify warm temperature extremes, moderate cold temperature extremes, and intensify extreme damaging precipitation events.
