As evacuees galvanized by Katrina fled the Gulf Coast in advance of Hurricane Rita this week, only to sit idling for hours on clogged freeways, they had plenty of time to wonder who or what was to blame for the storm-tossed mess. But while many have attributed the size and violence of Katrina and Rita to global warming, MIT climatologist Kerry Emanuel says it would be wrong to blame the devastation caused by any individual hurricane on long-term climate change.
There are troubling signs in the meteorological record of a link between global warming and hurricane intensity, says Emanuel, a professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences. But the best available science suggests that the now-scattered populations of the Texas, Louisiana, Mississippi, and Alabama coasts are the victims of mere happenstance.
There are simply too few examples of catastrophic hurricanes hitting U.S. shores to make out any statistical trend, says Emanuel. “It would be absurd to attribute the Katrina disaster to global warming,” Emanuel wrote on his website this month.
What Emanuel does believe is that the average power of many tropical cyclones – the blanket terms scientists use for hurricanes, typhoons, and cyclones – has risen sharply over the past several decades, at least in the North Atlantic and North Pacific. Moreover, the increase is closely tied to changes in the surface temperatures of the oceans where tropical cyclones are born. In other words, when the sea surface temperature rises, the energy of the cyclones above that surface also rises – and at an even faster rate.
And lately, ocean temperatures have been rising more than they’ve been falling. Emanuel’s examination of North Atlantic and North Pacific storm records over the last 50 years show a marked increase in the average intensity of tropical cyclones.
“It’s a big effect,” says Emanuel. “It’s gone up 50 to 80 percent over the last three decades or so.”
Emanuel published his findings in the August 4 issue of the journal Nature and has been at the center of his own storm of debate ever since. Politicans and pundits have latched onto his work as proof that global warming is starting to have an impact. And Emanuel’s fellow scientists are also asking tough question. Despite the appearance of corroborating results from a team of researchers at Georgia Tech and the National Center for Atmospheric Research in the September 16 issue of Science, some researchers have said they are surprised by the magnitude of the intensity increases, which don’t show up in existing climate-change theory or computer models.
But ironically, Emanuel and his group at MIT weren’t looking to generate debate when they began analyzing the northern-hemisphere storm records (which came from a commonly-used database compiled by the Hadley Center for Climate Prediction and Research in the UK). The group was mainly interested in how the upper layers of the ocean conduct heat from Earth’s lower latitudes to higher latitudes.
“The ocean transports between a quarter and a third of the total heat transported between lower and higher latitudes,” Emanuel says. “The atmosphere transports the rest. But the ocean’s mechanism is completely different from the mechanism the atmosphere uses. It’s been known for years that it’s done by the mixing of the upper oceans. But nobody knows what mixes the upper oceans.
“So I was testing the notion that tropical cyclones are primarily responsible for that. If that’s true, then the variability in cyclones should correlate with the variability in ocean surface temperatures.”
One of Emanuel’s early decisions was to measure the power of tropical cyclones, not their frequency.
“If you look globally there are about 90 storms per year,” Emanuel says. “That number, although variable, doesn’t show any long term trend. The particular metric I developed..iis sensitive to a storm’s intensity, meaning its wind speed and duration.”
Aside from the fact that the intensity measure might reveal previously undetected climate trends, Emanuel was attracted to it because it is the best predictor of the amount of damage a hurricane will inflict. (One Category 5 hurricane can wreak more havoc on land than dozens of Category 2 or 3 storms.)
Emanuel’s measure, called the power dissipation index, is calculated from cyclone wind speeds as observed by satellites, planes, ships, or land stations, and it turned out to provide a new lens on the Hadley data. At first, Emanuel was looking only for signs of “classical” climate signals in the data, such as the well-known El Nino ocean temperature oscillation. He found them – but then he saw the drastic increase in hurricane intensity, closely paralleling the sea surface temperature data.
“This database has existed for quite a long time, it just hasn’t been looked at in this particular way,” says Emanuel. “That happens a lot in science. The [Antarctic] ozone hole sat there in the data for years, and no one looked at it.”
The results excited Emanuel, since they supported his idea that tropical cyclones help to mix ocean water and redistribute heat around the globe.
But they were also open to another interpretation. In the blogosphere, everyone from the Democratic Women’s Caucus to prophets of impending Apocalypse has latched on to Emanuel’s findings as proof that that U.S. leaders are ignoring global warming.
Politicians such as former vice president Al Gore and even scientists such as Sir John Lawton, chairman of the UK’s Royal Commission on Environmental Pollution, have publicly warned that oceans roiled by global warming will produce more massive storms such as Katrina and Rita.
“Increasingly it looks like a smoking gun,” as Sir Lawton told the British press, referring to both Emanuel’s finding and the Science paper. “It’s a fair conclusion to draw that global warming, caused to a substantial extent by people, is driving increased sea surface temperatures and increasing the violence of hurricanes.”
Since his Nature paper appeared, Emanuel has been in great demand among journalists, policymakers, and other researchers. He says few have pressured him for his opinion on the real hot-button issue in climate change: how much of global warming can be attributed to human activity, and what, if anything, can be done to slow it.
“My experience is that most of the people who call me up, really want to know about the science, what do I really mean,” says Emanuel. “Most people, if they have a political agenda, they are good at keeping it hidden.”
But then there’s the ever-growing commentariat, which answers to no one.
“The people who are politicizing it are doing it behind my back – pundits writing on blogs or editorials whom I don’t actually talk to,” says Emanuel. “They don’t want to know the truth. They want to use something somebody wrote to advance their agenda.”