Global warming may have caused Katrina, but proof is lacking, researchers say
July 13, 2005
Special to World Science
A study to be published tomorrow provides strong new evidence linking giant hurricanes such as Katrina—which devastated the Gulf of Mexico last month—to rising ocean temperatures, scientists say.
That, scientists added, provides additional reason to study whether global warming is making hurricanes stronger, as some suspect.
The evidence to date, while intriguing, doesn’t prove the case, scientists said. This is partly because studies to date include only a few decades’ worth of data, which isn’t enough.
It’s also because scientists lack a detailed explanation of how global warming would cause the hurricane trends seen so far. For instance, hurricanes are getting stronger, but not more frequent, and scientists don’t know why.
In general, it makes sense that higher temperatures would boost hurricane strength, many scientists say. Heat is energy, and energy drives hurricanes.
The study, to appear tomorrow in the research journal Science, is the second to link stronger hurricanes with rising temperatures. The link is statistical: as temperatures have risen, hurricanes have become more violent, the researchers said. Whether the first causes the second remains unproven.
"What we found was rather astonishing," said Peter Webster of the Georgia Institute of Technology in Atlanta, Ga., lead author of the study.
"In the 1970's, there was an average of about 10 Category 4 and 5 hurricanes per year globally. Since 1990, the number of Category 4 and 5 hurricanes has almost doubled, averaging 18 per year globally."
Category 4 hurricanes have sustained winds from 131 to 155 miles per hour; Category 5 systems, such as Hurricane Katrina at its peak over the Gulf of Mexico, feature winds of 156 mph or more.
"Category 4 and 5 hurricanes made up about 20 percent of all hurricanes in the 1970's, but over the last decade they account for about 35 percent of these storms," said Georgia Tech’s Judith Curry, a co-author of the study.
All this is happening as sea-surface temperatures are rising across the globe-anywhere from around one-half to one degree Fahrenheit, depending on the region, for hurricane seasons since the 1970's, the researchers said.
"Our work is consistent with the concept that there is a relationship between increasing sea surface temperature and hurricane intensity," said Webster. "However, it's not a simple relationship. In fact, it's difficult to explain why the total number of hurricanes and their longevity has decreased during the last decade, when sea surface temperatures have risen the most."
The only region that is experiencing more hurricanes overall is the North Atlantic, where they have become more numerous and longer-lasting, especially since 1995, Webster said.
The North Atlantic has averaged eight to nine hurricanes per year in the last decade, compared to the six to seven per year before the increase, the authors reported. Category 4 and 5 hurricanes in the North Atlantic have increased at an even faster clip: from 16 in the period of 1975-89 to 25 in the period of 1990-2004, a rise of 56 percent.
A study published in July in the journal Nature came to a similar conclusion. Focusing on North Atlantic and North Pacific hurricanes, Kerry Emanuel of the Massachusetts Institute of Technology in Cambridge, Mass. found an increase in their duration and power, although it used a different measurement to determine a storm's power.
To prove whether human-induced warming is cause the trend will require "a longer data record of hurricane statistics,” Webster said. Also, “we need to understand more about the role hurricanes play in regulating the heat balance and circulation in the atmosphere and oceans."
Computer simulations do show global warming would produce stronger hurricanes, researchers said.
"Basic physical reasoning and climate model simulations and projections motivated this study," said
The new findings “will stimulate further research” into both natural and human-driven processes influencing hurricane trends, said Jay Fein, director of climate and large scale dynamics program at the National Science Foundation in Arlington, Va., which funded the research.
Webster is studying the role of hurricanes in climate.
"The thing they do more than anything is cool the oceans by evaporating the water and then redistributing the oceans' tropical heat to higher latitudes," he said. "But we don't know a lot about how evaporation from the oceans' surface works when the winds get up to around 100 miles per hour, as they do in hurricanes." Understanding this will help answer whether global warming is indeed fueling hurricanes.
"If we can understand why the world sees about 85 named storms a year and not, for example, 200 or 25, then we might be able to say that what we're seeing is consistent with what we'd expect in a global warming scenario. Without this understanding, a forecast of the number and intensity of tropical storms in a future warmer world would be merely statistical extrapolation."
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