Using records dating back to 1855, hurricane researchers say they have uncovered an ongoing rise in the number of Atlantic hurricanes that tracks the increase in sea surface temperature related to climate change. Critics of such a link argue that this trend is merely because of better observations since the dawn of the satellite era in the 1970s. But the authors of the new study say the conclusion is hard to dodge.
"Even if we take the extreme of these error estimates, we are left with a significant trend since 1890 and a significant trend in major hurricanes starting anytime before 1920," say atmospheric scientists Greg Holland of the National Center for Atmospheric Research in Boulder, Colo., and Peter Webster of the Georgia Institute of Technology in Atlanta.
Looking at data from 1855 through 2005, Webster and Holland found that the total number of tropical cyclones per year doubled in that time, from an average of six at the beginning of last century to 14 over the past decade. And the present regime has yet to stabilize: "With increasingly higher sea surface temperatures it is hard to imagine anything lower than 15 storms per year" going forward, the two conclude.
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Globally, areas of warm ocean have nearly tripled in size since the beginning of the 20th century, from roughly 17 million square miles to more than 46 million square miles, Webster and Holland note. "There has been an average of one additional tropical cyclone for each 0.1-degree Celsius increase in sea surface temperature and one hurricane for each 0.2-degree Celsius rise," they write in Philosophical Transactions of the Royal Society A.
Critics note that although there has been an increase in the number of storms since 1995, the effects of Atlantic currents, rather than climate change, could be responsible. And improvements in detecting tropical cyclones may be more to blame for this trend than anything else.
"In 2005 we had 27 major storms, the most ever measured. But six or seven of them were exclusively in the central Atlantic," notes William Gray, a meteorologist at Colorado State University in Fort Collins. "The record it broke was 1933 that had 21 major storms, but there was no satellite or aircraft data then. Had they had those measurements we probably would have had a comparable number of storms that year."
But Holland and Webster assumed that measurements taken before aircraft and satellites made storms easier to spot could have missed as many as five hurricanes per year. "Including these errors in our analysis changes the magnitude of the trend but leaves all the substantial conclusions unaffected," they say.
Gray's team will come out with its latest forecast for this year's hurricane season at the end of this week, having previously predicted 17 storms this season. "We will continue to predict an above average year," he says. "There's a chance we might lower the numbers slightly but if we change it, we're not going to change it by much." And Gray predicts that a global cooling will set in within the next decade, finally putting the climate change link on ice.
The experts agree that natural variability is largely to blame for the relative intensity of various hurricanes, but Holland and Webster note that the locations of such storms have changed. "As more storms form near the equator, they are experiencing much better conditions for intensification and they are experiencing these conditions for a much longer period," the pair note. And that means more numerous and stronger hurricanes in the foreseeable future, whether the forecast is from a computer model or a meteorologist's instincts.