Thursday, August 08, 2013

Hurricanes and Climate Change: Huge Dangers, Huge Unknowns

Posted by: Dr. Jeff Masters, 9:57 AM GMT on August 05, 2013

Hurricane Sandy's enormous $65 billion price tag put that great storm in third place for the most expensive weather-related disaster in U.S. (and world) history. Indeed, when we look at the list of most expensive U.S. weather-related disasters since 1980, hurricanes dominate, claiming six of the top ten spots. Drought is also a formidable presence, accounting for three of the other top-ten budget-busting disasters. Thus, a critical question for society is: how will our dangerous uncontrolled experiment of dumping massive amounts of heat-trapping carbon dioxide into the air affect droughts and hurricanes? For droughts, the answer is relatively straightforward--in general, dry parts of the world like the Southwest U.S. and Southern Europe are predicted to get drier. The worst droughts will get more extreme, wherever they occur, since it will be hotter. But for hurricanes, the uncertainties are greater. Since hurricanes are heat engines that extract heat energy from the oceans to power themselves, hurricane scientists are confident that the very strongest storms will get stronger by the end of the century, when Earth's land and ocean temperatures are expected to warm to levels unmatched since the Eemian Era, 115,000 years ago. Computer modelling work consistently indicates that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. But hurricanes are fussy creations, and are sensitive to wind shear and dry air. Although the strongest storms should get stronger when "perfect storm" conditions are present, these "perfect storm" conditions may become less frequent in the future, due to the presence of higher wind shear and/or more dry air. Indeed, the climate models used to formulate the 2007 IPCC report suggested that we might see the strongest hurricanes getting stronger, but a decrease in the total number of hurricanes in the Atlantic (and worldwide) later this century. However, the latest set of models used to formulate the 2013 IPCC report, due out in September, show that the total number of hurricanes both globally and in the Atlantic may increase in both number and intensity, according to a paper published in July 2013 by one of our top hurricane scientists, Dr. Kerry Emanuel of MIT. Hurricane intensity may also increase, leading to a global increase of 40% of Category 3 and stronger hurricanes by 2100. However, the uncertainties in this research are great. We really don't know how global warming will affect the number of hurricanes and their intensity.

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