Al Gore’s new movie on global warming, “An Inconvenient Truth,” opens with scenes from Hurricane Katrina slamming into New Orleans. The former vice president says unequivocally that because of global warming, it is all but certain that future hurricanes will be more violent and destructive than those in the past.
Inconvenient or not, the nation’s top hurricane scientists are divided on whether it’s the truth.
With the official start of hurricane season days away, meteorologists are unanimous that the 2006 tropical storm season, which runs from June 1 through November, is likely to be a doozy. The first tropical storm of this season showered light rain yesterday on Acapulco, a Mexican Pacific resort, but forecasters said the weather could worsen. Tropical storm Aletta was stalled 135 miles from Acapulco, with maximum winds of 45 mph, according to the U.S. National Hurricane Center in Miami, which said the storm could move toward land today.
The 2004 and 2005 Atlantic hurricane seasons broke many records, and as forecasters predict 15 named storms, nine or 10 making it to hurricane strength and four or five of those major, 2006 is shaping up as another bad one.
The top names and brightest minds in hurricane science are divided, writing papers and publishing rebuttals regarding the nature and causes of the current “active period” that began in 1995 and is expected to run at least another 10 to 15 years. They study the same facts, but draw opposite conclusions.
In one corner, subscribing to the theory that the Atlantic Basin is in a busy cycle that occurs naturally every 25 to 40 years, are Chris Landsea, science and operations officer at the National Hurricane Center in Miami, and William Gray and Phil Klotzbach of Colorado State University, who pioneered much of modern hurricane-prediction theory.
“There has been no change in the number and intensity of Category 4 or Category 5 hurricanes around the world in the last 15 years,” Mr. Landsea said, in a telephone interview from Miami.
On the other side are Kerry Emanuel of the Massachusetts Institute of Technology, one of the most respected hurricane scientists in the world, a team of meteorologists from Georgia Tech led by Peter Webster, an MIT-educated monsoon specialist, and Greg Holland, who earned his doctorate at Colorado State under Mr. Gray.
“You cannot blame any single storm or even a single season on global warming. … Gore’s statement in the movie is that we can expect more storms like Katrina in a greenhouse-warmed world. I would agree with this,” said Judith Curry. She is chairwoman of Georgia Tech’s School of Earth and Atmospheric Sciences, and is co-author, with Mr. Webster, Mr. Holland and H.R. Chang, of a paper titled “Changes in Tropical Cyclones,” in the Sept. 16 issue of Science, a weekly publication of the American Association for the Advancement of Science.
The paper concluded that there has been an 80 percent increase in Category 4 and Category 5 hurricanes worldwide.
Balancing the atmosphere
Tropical cyclones, rotating wind systems that include hurricanes, are heat engines — nature’s way of balancing extremes, mechanisms for taking heat from one place and taking it to another, as part of balancing the Earth’s atmosphere.
All agree that in the past 30 years, the waters off the West Coast of Africa, where most Atlantic hurricanes are born, have warmed by about 1 degree, to about 81 degrees Fahrenheit, and as much as two-thirds of that increase is attributable to greenhouse gases, or global warming. Where the scientists disagree is what that means for the number and intensity of hurricanes.
Mr. Emanuel of MIT said that, globally, the number and intensity of hurricanes are unchanged over the past 30 years, and that according to Japanese models, global warming could even lead to a modest decline in the number of hurricanes worldwide. However, he said that in the Atlantic Basin, where just 11 percent of all tropical storms occur, there is a “quite nice correlation” between the rise in sea surface temperature (SST) and an increase in the number and intensity of hurricanes.
Mr. Emanuel, author of the book “Divine Wind: The History and Science of Hurricanes,” said that while the 1-degree rise in ocean temperature has been recorded globally, the correlation between SST and hurricane frequency does not appear in other parts of the world.
“Since 1981, the number and intensity of [Atlantic] hurricanes has almost doubled,” said Mr. Emanuel, based on his research published in a letter in the Aug. 4 issue of Nature, which surveyed 55 years of hurricane and ocean-temperature data in the Atlantic and Pacific. “In my mind, the jury is not out. The upswing since the 1980s is largely a global-warming signal. But if you polled my colleagues, I think you’d find they are divided on the issue.”
The Georgia Tech meteorologists used data collected around the world and arrived at similar conclusions.
“The best available data shows that the number of Category 4 and 5 hurricanes globally has almost doubled since 1970,” said Ms. Curry at Georgia Tech, in an e-mail response to questions. “In the North Atlantic, there has been a comparable increase in intensity, and also a 50 percent increase in the total number of North Atlantic hurricanes. This increase in hurricane activity has been linked to a 1-degree Fahrenheit increase in global tropical sea-surface temperature since 1970. This global temperature increase since 1970 is attributed to global warming.”
Asked about Georgia Tech’s findings regarding Category 4 and Category 5 hurricanes, Mr. Emanuel said that he had reviewed their data.
“I came up with the same result. I think they are right,” Mr. Emanuel said.
But Mr. Landsea of the National Hurricane Center vigorously disagreed with Mr. Emanuel in a rebuttal to his paper, published in Nature in December, saying, in effect, that Mr. Emanuel tortured the data until it confessed what he wanted to hear.
Regarding Georgia Tech, Mr. Landsea’s argument is that primitive measuring techniques, here and especially in Asia, where most of the major tropical storms occur, made for imperfect data, and inaccurate “data sets” generate incorrect conclusions.
“At the beginning of the [Georgia Tech] study, in 1970, there wasn’t even a tool for determining wind speed and how strong a hurricane was. The Dvorak Technique [for measuring wind speed] did not come into being until 1972,” he said, adding that it wasn’t perfected until 1984. “The incomplete data sets artificially causes the number of Category 4 and Category 5 hurricanes to go up.”
Asked about the increase in Atlantic hurricanes, he said: “I think that is real, but the largest component of that is the natural cycle,” he said.
According to NOAA hurricane records going back into the mid-1800s, hurricanes come in cycles. There have been quiet periods, with less hurricane activity, followed every 25 to 40 years by active periods, that last about 25 years. The current active period began in 1995 and is expected to last another 10 to 15 years.
Mr. Landsea agreed that the 1-degree rise in ocean temperature is largely a product of greenhouse gases — that is to say global warming — but he said it was not a primary factor in determining the size and intensity of recent hurricanes.
“Models show a 2- to 4-degree temperature increase by the end of the 21st century, and hurricanes will get about 4 percent stronger for every 2-degree increase,” he said, citing Princeton’s Geophysical Fluid Dynamic Laboratory and Tom Knutson for the research in this area.
In other words, the 1-degree water temperature increase off the coast of Africa could fuel a Category 3 hurricane at landfall, like Katrina, with 130-mph winds, to increase by about 2 percent. Two or three miles per hour of Katrina’s winds could have been the result of global warming, Mr. Landsea said.
“One or 2 percent stronger? That is a very tiny change today, and even in 100 years from now, it is very small,” he said.
Offset in Pacific
At Colorado State University, Phil Klotzbach wrote a rebuttal, published in the Geophysical Research Letter last week, to the Georgia Tech and MIT papers, and concluded that where sea-surface temperature has increased, there is in fact a slight decrease in hurricane activity.
“With regards to the number of Category 4-5 hurricanes, there has been a large increase in North Atlantic storms and a large decrease in Northeast Pacific storms,” wrote Mr. Klotzbach in “talking points” for the paper on his Web site. “When these two regions are summed together, there has been virtually no increase in Category 4-5 hurricanes.”
Like Mr. Landsea, Mr. Klotzbach attributes the Georgia Tech findings to bad data. Ms. Curry of Georgia Tech says that while there have been inconsistencies in processing the data over time and in different regions, no one has demonstrated that there is actually a major problem with the data itself.
“The key issue is whether you can distinguish a Category 4 from a Category 1 hurricane from satellite [data], the answer is almost always yes,” she said.
Clearly, it is a busy period for the number and intensity of academic research papers being published on global warming and hurricanes, but Mr. Landsea said all remains collegial in the meteorological community.
“These are my friends and colleagues. The disagreements are civil. This debate is essence of science that is alive. Everyone here is doing science. This debate is confusing for us. I’m sure it is confusing for everyone else,” said Mr. Landsea.
Mr. Emanuel said that for all practical purposes, the real problem of hurricanes is not number and intensity, but demographics and the desire of people to live by the sea.
If a hurricane blows itself out over the ocean or hits an unpopulated area, there are few consequences to life or property. However, if a major hurricane hits a populated area, like New Orleans, Miami or the Outer Banks of North Carolina, it can be catastrophic.
“By regulating insurance, by holding insurance rates down, we are subsidizing risky behavior. We are underwriting the drive to [build and populate] the coastline. That is the big hurricane problem in the United States,” he said.