Almost 4 years ago the Intergovernmental Panel on Climate change told us that climate change is "very likely" to increase the frequency of heat waves and heavy precipitation.1 The same may be true for hurricanes.2 It's now common after extreme weather events to hear some people suggest that those events were caused by global warming. Careful scientists, however, point out that weather is extremely variable. It always has been. It always will be. Climate change refers to a change in the average conditions, so attributing any one event to climate change will always be fraught with difficulty.Indeed, here's what the authors of a paper published in today's Nature have to say about attributing increased flooding to climate change:
Recent widespread UK floods--such as in spring 1998, autumn 2000, winter 2003 and summer 2007--have prompted debate as to whether these particular events are attributable to anthropogenic climate change6, 7, 13, 14, 15. This is an ill-posed question, given uncertainty in the antecedent conditions; many untraceable factors, anthropogenic or natural, may have contributed to any individual event13, 16. Indeed, observed UK fluvial-flood and high-flow trends for recent decades suggest no clear evidence for any change above that of natural variability17, 18, mirroring the mixed picture in observed precipitation changes19, 20.Translating that to English: Asking whether recent floods in the UK are the result of climate change is asking a question that can't be answered. Too many variables influence particular weather events in complicated ways to make any answer plausible.
Does that mean the IPCC was wrong? Not at all. In fact, both the paper just quoted and another one in the same issue suggest that "global warming resulting from anthropogenic emissions of greenhouse gases may have compounded the effects of such fluctuations", as Richard Allan writes in an accompanying commentary on the articles. And how can we be so sure that extreme weather is more likely when we can't attribute any single event to global warming? Read on.
The figure above is from the paper by Pardeep Paul and colleagues. The blue line shows the frequency with with different amounts of runoff in the UK were expected given atmospheric conditions present in the late 20th century. The brown, purple, pink, and yellow dots in the four different figures show the frequency with which those events would have been expected under four different scenarios with levels of carbon dioxide in the atmosphere held at their levels in 1900.
As you can see, large events (those higher in each graph) are more frequent under every scenario, and it appears that the very largest events are even more frequent, as shown by the sharp rise iine at the extreme right of each graph.
Our modification of earth's atmosphere has already had a large effect on the climate, and that effect will only grow as concentrations of carbon dioxide and other greenhouse gases increase in the future.
1"Very likely" is the phrase used in the IPCC synthesis report.
2See "Extreme Weather Advisory" by Kerry Emanuel for an accessible overview.
Pall, P., Aina, T., Stone, D., Stott, P., Nozawa, T., Hilberts, A., Lohmann, D., & Allen, M. (2011). Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000 Nature, 470 (7334), 382-385 DOI: 10.1038/nature09762
Min, S., Zhang, X., Zwiers, F., & Hegerl, G. (2011). Human contribution to more-intense precipitation extremes Nature, 470 (7334), 378-381 DOI: 10.1038/nature09763
Allan, R. (2011). Climate change: Human influence on rainfall Nature, 470 (7334), 344-345 DOI: 10.1038/470344a
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