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 What is the link between hurricanes and global warming?Gore claims global warming is causing more frequent and severe hurricanes. However, hurricane expert Chris Landsea published a study on May 1 documenting that hurricane activity is no higher now than in decades past. Hurricane expert William Gray reported just a few days earlier, on April 27, that the number of major hurricanes making landfall on the U.S. Atlantic coast has declined in the past 40 years. Hurricane scientists reported in the April 18 Geophysical Research Letters that global warming enhances wind shear, which will prevent a significant increase in future hurricane activity. What the science says...It is unclear whether global warming is increasing hurricane frequency but there is increasing evidence that warming increases hurricane intensity. There are two aspects to hurricane activity that are often confused so it helps to consider them separately. Specifically, does global warming cause more frequent hurricanes and does it cause more intense hurricanes? Hurricane frequencyIn July 2007, Greg Holland of the National Center for Atmospheric Research and Peter Webster of Georgia Tech published a paper Heightened Tropical Cyclone Activity in the North Atlantic: Natural Variability or Climate Trend? that notes an increase in the number of observed hurricanes in the North Atlantic over the past century, concluding "increasing cyclone numbers has lead to a distinct trend in the number of major hurricanes and one that is clearly associated with greenhouse warming". However, this was refuted by Chris Landsea's Counting Atlantic Tropical Cyclones Back to 1900 stating "improved monitoring in recent years is responsible for most, if not all, of the observed trend in increasing frequency of tropical cyclones". In other words, the reason we're recording more hurricanes is due to our improved ability to observe them thanks to aircraft, radar and satellites.
In addition, a 2007 paper by Gabe Vecchi, Increased tropical Atlantic wind shear in model projections of global warming looked at the results of 18 climate models and concluded increased wind shear from rising sea surface temperatures make it more difficult for hurricanes to form and grow. Now it seems to me there's a dare-I-say hockey stick trend from the mid-80's, nearly two decades after all observing techniques have come into play. Nevertheless, I would tentatively conclude the topic of hurricane frequency is one area requiring further research and better data. Hurricane Intensity in the AtlanticKerry Emanuel wrote the definitive paper on hurricane intensity Increasing destructiveness of tropical cyclones over the past 30 years. Emanuel found a close correlation between hurricane intensity in the Tropical Atlantic and sea surface temperature. He also suggests wind shear has much less effect on hurricane intensity than increased sea temperatures. This is confirmed by the observed data:
Hurricane intensity in global satellite records Elsner 2008 used global satellite data from 1981 to find any long term trend in maximum wind speed against different strength hurricanes:
Figure 1: Trends in tropical cyclone maximum wind speeds by quantile, from 0.1 to 0.9 in increments of 0.1. The point-wise 90% confidence band is shown in grey. The solid red line is the overall trend and the dashed red lines delineate the 90% point-wise confidence band about this trend. Overall, there is a statistically significant upward trend (the horizontal red line). But more significantly, Elsner found weaker hurricanes showed little to no trend while stronger hurricanes showed a greater upward trend. In other words, stronger hurricanes are getting stronger. While Elsner found no trend in the overall number of hurricanes, the increasing intensity means there are an increasing number of storms with a maximum wind speed exceeding 210 kilometres per hour (category 4 and 5 storms). The inevitable conclusion is that as sea temperatures continue to rise, the number of Category 4 and 5 hurricanes hitting land will inexorably increase. That coupled with rising sea levels is a bleak prospect. More on Elsner's paper...
AcknowledgementsThe image North Atlantic Tropical Storms and Observing Techniques is courtesy of Global Warming Art. Emanuel's graph of PDI versus temperature was courtesy of Climate 411. Further reading
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The skeptic argument...