September 10, 2017

Word: Scientists' view of the relation between hurricanes and climate change

Scientific American - Whether or not we see more tropical storms, we know that the strongest storms are getting stronger, with roughly eight meters per second increase in wind speed per degree Celsius of warming. And so it is not likely to be a coincidence that almost all of the strongest hurricanes on record (as measured by sustained wind speeds) for the globe, the Northern Hemisphere, the Southern Hemisphere, the Pacific, and now, with Irma, in the open Atlantic, have occurred over the past two year....

Furthermore, a warmer ocean surface means more moisture in the atmosphere. A fundamental rule of atmospheric thermodynamics known as the Clausius-Clapeyron equation indicates an increase of roughly 7 percent more moisture in the air for each degree Celsius of increase in sea surface temperature (SST). Global SSTs have risen now the better part of a degree C and conditions in which SSTs are several degrees C above normal are now more common as a result.  Unusually warm SSTs contributed to the flooding power of both Hurricane Harvey and Hurricane Irene in 2011.

Other connections are more subtle. Part of what yielded the record flooding associated with Harvey and Irene was the slow-moving nature of the storms, which allowed for persistent rainfall over eastern Texas and New England respectively. The slow movement of these storms was favored by an expanded sub-tropical region of high pressure over the southern U.S. and a far northward-shifted jet stream, something that climate model simulations predict as a result of human-caused climate change. There is also some tentative evidence that the warming of the Arctic may favor the stalling of mid-latitude weather systems, though this is at the cutting edge of the science and still being studied.

The second approach to understanding the linkage between human activity and extreme weather involves a sort of climatological “CSI”—running simulations of a climate model both with and without the impact of human-generated greenhouse gas increases, seeking to detect a trend and attribute the event in question in part to those increases. As recently as a decade ago, climate scientists had a motto that “you can’t attribute any single extreme event to global warming.”

By the time politicians and journalists started repeating that line, however, the science had moved on, so that we now can attribute individual events in a probabilistic sense. For example, if a baseball player on steroids is hitting 20 percent more home runs, we can’t attribute a particular home run to steroids. But we can say steroids made it 20 percent more likely to have occurred. For some of the physical processes discussed here, one can view increasing carbon dioxide in the atmosphere as steroids for the storms.


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