The study was published Monday in the Proceedings of the National Academy of Sciences, written by top climate researcher Kerry Emanuel of the Massachusetts Institute of Technology.
On average, about 90 tropical cyclones form each year around the world, Emanuel says. “Tropical cyclone” is an umbrella term that includes hurricanes, typhoons and cyclones, which are all the same type of storm that have different labels depending on where they form.
One of the biggest debates in the climate change research community in recent years has been the projected impact of global warming on hurricanes. Will it make them stronger? More frequent? Longer lasting?
Emanuel’s study used six newly upgraded global climate computer models to simulate future hurricane activity around the world. His study found that these killer storms will not only increase in intensity during the 21st century, as many previous studies had predicted, but will also increase in frequency in most locations.
What’s different about this new PNAS study? Improved computer models, Emanuel says: “Studies using the same technique applied to the previous generation of global climate models showed very little change in global frequency, but an increase in intensity,” he says.
“Our study suggests that the largest increases might occur in the western North Pacific region, but with noticeable increases in the South Indian Ocean and in the North Atlantic region,” he says. Most hurricanes that affect the USA form in the Atlantic.
“It is important to emphasize that most studies suggest that the frequency of the highest category tropical cyclones (those of Category 3 and higher on the Saffir-Simpson scale) should increase as the globe warms,” Emanuel says. “There is less agreement about the frequency of the weakest category of storms.”
Category 3 storms have sustained wind speeds of at least 111 mph. Any Category 3, 4 or 5 hurricane is classified as a “major” hurricane.
What will cause the additional tropical cyclones to form? “Our study has not established a cause,” Emanuel says, “but we suspect that the projected decrease in man-made aerosol particles may be at least partly responsible.”
Aerosol particles from pollution can act to cool the globe, counteracting the influence of climate change.
Georgia Tech climatologist Judith Curry, who was not part of the study, warns, “The conclusions from this study rely on a large number of assumptions, many of which only have limited support from theory and observations and hence are associated with substantial uncertainties. Personally, I take studies that project future tropical cyclone activity from climate models with a grain of salt.”
Another researcher, Roger Pielke, Jr., a professor of environmental studies at the University of Colorado, who was also not part of the PNAS study, says, “Kerry Emanuel is a smart scientist; I’ll trust that he has done good work here.” However, Pielke says his own research — along with Emanuel’s other work — suggests that the ability to detect any signal of human-caused climate change on the impacts of hurricanes on society will take many decades, and maybe centuries.
He says the increases predicted in Emanuel’s study are projected over a century into the future. However, “over the past century, no such increase in frequency or intensity has been observed (in the U.S. or globally) and in fact, the U.S. is currently in the midst of the longest stretch with no Category 3+ hurricane landfall since at least 1900.”
“We should not be lulled into a false sense of security by our recent good luck (yes, good luck) with respect to hurricanes,” Pielke says. “We do not need studies of the distant future to tell us that we are going be be hit hard and often in the coming years, just as a matter of historical precedent.
“It is important to recognize that this study adds one stone to a bigger pile. It won’t be the last word. No study is.”