As temperatures rise worldwide from the effect of greenhouse gases in the atmosphere, hurricanes may become fewer in number, according to a paper published this week in the journal Nature Geoscience (" Tropical cyclones and climate change," by Thomas R. Knutson et al). However, the storms that do occur are likely to be more potent, the paper says. The Washington Post has a summary here (" Study: Warming to bring stronger hurricanes," by Seth Borenstein, Associated Press). The paper's ten authors make up an expert team established by the World Meteorological Organization to provide advice to national meteorological and hydrological services on tropical cyclones and climate change. Their paper is based on an extensive review of recent research into global warming and hurricane formation, including studies on past historical storm records as well as a selection of mathematical models designed to project future trends. Although the issue of global warming is politically contentious, the discussion among experts is less so. Climate scientists generally agree that the earth's temperature has been warming over the past century, and that this warming is driven in large part by "greenhouse gases," such as carbon dioxide, dumped into the atmosphere by burning of fossil fuels. But even among scientists who agree on the basics of global warming, the effect, if any, of this temperature rise on weather events such as hurricanes remains unclear. And the Nature Geoscience expert report adds little in the way of certainty. Hindsight, according to the authors, is less than 20-20: the available historical data don't clearly show any link between global warming and hurricanes in the last century. For one thing, hurricane frequency appears to rise and fall over many decades in a natural cycle (especially in the Atlantic basin), making any longer-term warming-related trend hard to spot even if it does exist, the scientists say. And the historical storm record is spotty in any case: only recently have satellite data or extensive ship and airplane records been available to analyze. So, the paper reports, "it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes." What about specific storm effects, such as storm surge flooding and heavy rainfall? Here again, past history offers no clear lessons. Theory predicts that warming should lead to heavier rains, but "a detectable change in tropical-cyclone-related rainfall has not been established by existing studies," the paper says. The same goes for storm surges: "There is no conclusive evidence that any observed changes in tropical cyclone genesis, tracks, duration and surge flooding exceed the variability expected from natural causes," says the paper, and "a detectable increase in storm-surge flooding from tropical cyclones has not been established." What about the future? Here again, the scientists are not sure. The team reviewed seven different climate models in their study. However, models that seek to predict global temperature trends do not themselves model storm patterns. So the temperature output from the global-warming models has to be fed into different weather-modeling programs in order to analyze the possible influence temperature may have on weather events. And some of these weather models are too coarse, or "low resolution," to evaluate the likelihood of localized storms such as hurricanes. The results can be confusing. When the scientists put temperature predictions from different global climate models into any given weather-prediction model, the output is inconsistent. Even though all the climate models predict a general rise in temperature, they differ in their small-scale regional and local temperature projections. And that leads to differences in the predicted storm events — so that the final result can be either an increase in storm frequency, or a decrease, depending on which climate model is used to provide the input values. Most of the various model combinations tend to support the paper's general conclusion: that storms are likely to decrease in number, while increasing in power. But the uncertainties in the climate models, coupled with the uncertainties in the storm-simulation models, lead the scientists to have relatively little confidence in this composite average prediction. Instead, they fall back on the familiar phrase: "More research is strongly recommended." But whether hurricane frequency and the associated wind speeds and rainfall increase, decrease, or stay the same, the scientists note, coastal communities will remain vulnerable for reasons that are independent of weather or climate. "Recent decades have seen very large increases in the economic damage and disruption caused by tropical cyclones," they observe. "Historical analyses indicate that this has been caused primarily by rising coastal populations and the increasing value of infrastructure in coastal areas." The authors refer to a 2008 economic analysis of storm damage (" Normalized Hurricane Damage in the United States: 1900–2005," by Roger A. Pielke, Jr., et al), which concludes, "Such growth in vulnerability is expected to continue for the foreseeable future, in the United States and around the world, and without effective disaster mitigation efforts, ever-escalating hurricane damage will be the inevitable result."
