Will Global Warming Lead to Stronger Hurricanes?

Profile: Thomas Knutson, National Oceanic and Atmospheric Administration -- by Seth Shulman

Thomas Knutson

Thomas Knutson’s research projects that global warming is likely to mean fewer Atlantic hurricanes but stronger ones, with the possibility of twice as many powerful category-4 and category-5 hurricanes by the end of this century.

As the East Coast of the United States warily awaits end of another hurricane season, teams of meteorologists are working around the clock to track the next potentially threatening tropical storm as it forms in the Atlantic Ocean. Thomas Knutson, a meteorologist at the National Oceanic and Atmospheric Administration (NOAA), tracks hurricanes too, but with a difference. Knutson’s research seeks to answer one of climate change’s thorniest and most consequential questions: what effect will global warming have on the frequency, structure, and intensity of hurricanes in the years to come?

Five years after Hurricane Katrina devastated the Gulf Coast, causing more than 1,800 deaths and upward of $100 billion in damage, there is little doubt about the urgency of Knutson’s research. He says he vividly remembers his feeling of awe when he visited Gulfport, Mississippi, a few years ago and saw the nearly 30-foot high-water marks of the storm surge Katrina caused there. “Storms like Katrina are thankfully rare,” he says. “But when you see how hard they hit, it reminds you how bad things can be and how vital it is to prepare and plan ahead.”

Knutson, whose team works at NOAA’s Geophysical Fluid Dynamics Laboratory, on a suburban campus of Princeton University in New Jersey, says he is spurred on in his research by the dangerous threat posed by hurricanes. But understanding the relationship between global warming and hurricanes has been a long-term fascination for him.

Growing up in a tiny town in rural Virginia, Knutson first encountered the subject of climate change in an article in the Washington Post that he read as a teenager. “It captured my imagination to learn about past ice ages and the prospect that we were going to be perturbing the climate system with heat-trapping gases,” he recalls.   Knutson was influenced too, he says, by his father’s work as a geologist at a local zinc mine. As he puts it, “In my upbringing, I think I was steeped in a certain ‘geological’ way of looking at the world.”

Knutson’s scientific bent and his fascination with climate change have served him well, first as an undergraduate computer science major, then in his graduate studies in meteorology at the University of Wisconsin at Madison, and, for the past two decades, as a research scientist at NOAA. But as Knutson is quick to acknowledge, teasing apart the relationship between global warming and hurricanes is an exceedingly complex business.

First, Knutson explains, there is a data problem. “We would, of course, very much like to have a perfect data set of all past tropical storms. But reliable satellite data for hurricane intensities exists only as far back as about 1980.” Before that, Knutson and his colleagues have to rely upon what he calls “a hodgepodge of data.”

“We know a lot about hurricanes that made landfall for the past century or more,” he says, “but if a hurricane didn’t hit land, it’s a different story, and, of course, many don’t hit land.” Prior to about the mid-1940s, we would know about a tropical storm or hurricane that never struck land if a ship happened to be in the vicinity and noted the storm-strength winds in its log. I

n recent decades there have been just eleven named tropical storms each year in the Atlantic, on average, and only a few (2.3 on average) have developed into hurricanes in category 3 or higher. The relative rarity of these events—what Knutson calls “the small size of the signal”—makes statistical analysis trickier, especially for landfalling hurricanes. 

Today, the evidence is overwhelming that carbon dioxide and other heat-trapping gases accumulating in the atmosphere from human activities are warming summer sea-surface temperatures to levels that allow hurricanes to develop. But as Knutson explains, while current computer climate models can project some degree of increase in global mean temperature and ocean-surface temperatures with a high level of confidence for each assumed global warming emissions path, they can’t predict more local conditions.

“To predict how hurricane activity may change,” Knutson says, “we need to know a lot of fine-grained detail about things like the local pattern of sea-surface temperature, not just the global average.”  Today’s climate change models have grids 100 to 200 kilometers in size, more or less the driving distance between New York City and Philadelphia, and that scale, he notes, is “roughly two orders of magnitude too crude for the purpose of simulating hurricane intensities.”

To overcome such formidable obstacles, Knutson and his team have developed an ingenious strategy: they take data about future climatic conditions from 18 separate world climate models and plug that information into the more detailed regional weather forecasting models that NOAA uses to track current storms. “Rather than wait a few decades until climate models offer enough resolution to predict storm formation,” Knutson says, his team “telescopes in,” using what he calls a “regional nested modeling approach.”

Using supercomputers, Knutson’s team tested the approach by entering atmospheric and sea-surface temperatures measured in recent years, including the 2005 hurricane season that produced Hurricane Katrina. Only after determining that their detailed regional model could offer a realistic picture of intense hurricane activity in those years did they run the model using anticipated future conditions.

Knutson’s results so far show both good news and bad news. On the positive side, global warming is likely to mean less frequent hurricanes in the Atlantic. Although ocean temperatures will continue to rise, most of the climate models project that atmospheric patterns in the tropical Pacific and Atlantic will shift toward something like a weak El Niño phase. Historically, El Niños are associated with wind patterns that make it more difficult for storms to grow into hurricanes in the Atlantic. As Knutson puts it, wind shear—that is, a distinct difference in wind speeds between the upper atmosphere and closer to the surface—will “inhibit the vertical coherency” of many forming storms, in effect disrupting the atmospheric machinery that allows the storms to develop into hurricanes.

The rest of Knutson’s findings, however, are considerably more worrisome. As he explains, in the warmer Atlantic of the future, more persistent wind shear will likely “weed out some storms, but not all.” And the models indicate that the storms that do materialize and grow will probably be more intense. Knutson’s latest modeling study projects that the number of category-4 and category-5 hurricanes in the Atlantic will double by the end of this century, making a catastrophic storm like Katrina considerably more likely.

Knutson notes several other worrisome trends for the hurricanes of the future as well. He warns that today’s climate models uniformly project that the hotter atmosphere brought about by global warming will retain more moisture. For developing hurricanes, this means more intense precipitation—some 20 percent higher rainfall rates, according to Knutson’s calculations. In addition, he says, current estimates of rising sea levels over the coming century mean that the storm surges caused by intense hurricanes are likely to be more devastating than ever before.

Knutson and his team are already refining their hurricane research. He says the group’s next generation of studies will streamline the process, and ultimately the global climate models should become fine-grained enough to predict the formation of storms. Meanwhile, coastal communities are paying close attention to Knutson’s findings so they can brace for these rare but intense hurricanes and make better plans to cope with them. “We live in a unique time,” Knutson says. “Unfortunately, we have been running a global experiment that is perturbing the climate system in profound ways, and climate affects many things in our natural world. I’m glad to work on a project that can at least help us predict and anticipate some of these effects so we can make more informed decisions.”

Bender, M.A. T. R. Knutson, R. E. Tuleya, J.J. Sirutis, G. A. Vecchi, S. T. Garner, I. M. Held. 2010. Modeled Impact of Anthropogenic Warming on the Frequency of Intense Atlantic Hurricanes, Science 327:454-458. DOI: 10.1126/science.1180568
Knutson, T. R., J.L. McBride, J. Chan, K. Emanuel, G. Holland, C. Landsea, I. Held, J.P. Kossin, A.K. Srivastava and M. Sugi. 2010. Tropical cyclones and climate change, Nature Geoscience, 3: 157-163. DOI: 10.1038/NGE0779.

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