In the aftermath of hurricane Katrina in 2005, a vigorous debate raged as to whether it was a "normal" natural disaster or a consequence of global warming. Al Gore depicted the devastation of New Orleans in his movie An Inconvenient Truth and linked it to climate change. I became involved during a case before the High Court in London challenging a U.K. government decision to distribute the movie to schools. I was asked to provide expert written evidence on the extent to which the film correctly represented scientific understanding at the time.
I liked the film and thought that Gore's presentation of the causes and likely effects of climate change was broadly accurate. As the Intergovernmental Panel on Climate Change concluded in its most recent assessment report: "warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level." And as data continue to pile up, the evidence gets ever stronger that human-induced emissions of greenhouse gases are the main cause of the observed warming over the past century.
But hurricanes are difficult. Climate models predict that they will become more intense. At the same time, considerable uncertainty remains. We have only about 40 years of reliable observational records, which precludes a clear determination of their variability. Given that different aspects of climate change could act to increase or decrease hurricane activity, whether or not Katrina can be ascribed to global warming is a challenge beset by difficulty.
It is not surprising, then, that in the aftermath of Katrina many scientists were reluctant to make definitive statements about its links with climate change. The same has happened after many other extreme weather events such as floods and droughts. When pressed, scientists often say that instances of extreme weather are consistent with the expected effects of climate change. But such statements are problematic. They can be misinterpreted to imply that every extreme flood or drought is due to climate change when this is manifestly not the case. And when events occur that climate change might make less likely, such as the record-breaking cold snap in the U.K. last December, it doesn't follow that climate predictions are inconsistent or wrong.
A clearer way of thinking about weather and climate is to consider the odds. After the European heat wave of 2003, I worked with Myles Allen and Dáithí Stone of the University of Oxford to show that human influence had very likely more than doubled the probability of such extreme temperatures. Since then, the concept that human influence could have "loaded the dice" in favor, or against, the occurrence of a particular heat wave, flood, or drought has become widely accepted by scientists and seems a relatively straightforward message to communicate to the public. But this doesn't mean that we are yet able to reliably quantify the changed odds of all extreme weather events.
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