The buildup of human-induced greenhouse gases in the atmosphere appears to be the primary driving force behind warmer oceans that fuel more powerful hurricanes, according to a new study by 19 top climate scientists from the United States and Europe.
Previous studies had already suggested a connection between warming ocean temperatures and stronger hurricanes. This study provides a new and important link needed to show that global warming, not natural cycles, is responsible, according to the authors and other hurricane researchers.
"Clearly, this is a result of the increasing concentrations of greenhouse gas emissions in the atmosphere," said co-author Tom Wigley, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo. "The work that we've done kind of closes the loop here."
The study used 22 sophisticated computer climate models to examine the tropical regions of the Atlantic and Pacific oceans, where hurricanes, also called tropical cyclones, are born. In those areas, the temperatures have risen an average of between a half degree Fahrenheit and 1.2 degrees Fahrenheit, over the last century.
Humans are very likely to blame for at least 67 percent that warming, the report said
The report provides further evidence, said scientists, that people are changing the way Earth's climate responds to an ever-increasing amount of carbon dioxide and other greenhouse gases put there by the burning of fossil fuels.
The paper appears today in the online edition of the Proceedings of the National Academy of Sciences. The research was led by Benjamin Santer of the Lawrence Livermore National Laboratory and funded by the U.S. Department of Energy.
For the new study, scientists first tested the ability of the computer models to accurately simulate real-world climate conditions where hurricanes form.
After running computer simulations of a period of time that's already passed -- called "hindcasting" -- the researchers compared the simulation to what actually happened to see how well the computer performed.
"If the models can simulate the observations, that tells us about the credibility and skill of the models, and that's important," Wigley said. He noted the models performed "exceptionally well," accurately predicting long-term climate trends, natural weather patterns, and even the effect of volcanic eruptions.
Having established their confidence in the computer models, researchers then used them to examine what was making the oceans warmer in those hurricane-forming regions.
In more than 80 simulations, they systematically introduced the varied factors that might influence hurricane formation into the simulation -- including greenhouse gases, ozone and even solar activity.
The key was being able to run the simulation by selectively turning on and off those influences to see how they individually influenced the climate, Wigley said. By taking only the greenhouse gas buildup out of the simulation, for example, scientists were able to see how Earth's climate might have evolved without the Industrial Revolution.
The models revealed that while natural cycles including volcanoes do have some influence on sea surface temperatures, the chief culprit is global warming.
"Greenhouse gases really are the dominant cause of the forcing of the climate system by human influences," Wigley said.
Hurricane scientist Greg Holland agreed, saying the results contradict the idea that natural cycles are responsible for stronger storms.
"One cannot say that [increased hurricane activity] is due to natural variability. It's just simply impossible to say that," said Holland, who also works at the National Center for Atmospheric Research but was not a co-author. "The data show that there is an increasing trend, which is associated with human-induced climate change."
A number of scientific studies have long predicted that extreme weather events, from drought to heavy rains, would result from increasing global temperatures. In the past year, a flurry of about a dozen papers have established the scientific basis for linking warmer oceans and stronger hurricanes.
Kerry Emanuel, a climate scientist and hurricane specialist at MIT, published an influential paper in August 2005 that correlated longer and more powerful hurricanes with rising temperatures in tropical oceans.
"The observations leave no doubt at all that over the last 30 years or so there have been important increases in hurricane activity in most of the world's oceans," Emanuel said last week in a conference call with journalists.
"The next obvious question is, why is the ocean temperature changing? Initially, we thought the changes were perfectly natural," Emanuel said. "But we have found that much of the changes we have seen, particularly in the Atlantic, are associated with global or hemispheric climate change, much of which is manmade. And that is what has us all very concerned."
Part of the concern is that, according to a 2005 study by scientists at Georgia Tech University and the National Center for Atmospheric Research, the number of Category 4 and 5 hurricanes has doubled in the past 35 years. Those storms pack an enormous amount of destructive power, with winds anywhere from 131 miles to more than 155 miles per hour.
Not all aspects of hurricane science are settled, however, although most researchers do agree that warmer oceans are important in hurricane formation.
"It's not one of the key areas of controversy," said Chris Landsea, a hurricane researcher at the National Oceanic and Atmospheric Administration's National Hurricane Center.
However, Landsea said that one of the main questions is not whether but how much impact global warming is having on hurricane intensity and frequency, something the new study does not address.
Thomas Knutson, a climate model scientist at the NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, N.J., said there is disagreement among scientists about whether historical data show clear trends in hurricane activity.
"I think that has a higher degree of uncertainty about it," Knutson told ABC News. "The disagreement is about whether there is actually a significant change that's observable in the historical record or not."
Last year's Atlantic hurricane season set a record of 27 named storms, including Katrina. This season has been quieter so far. Researchers said that is not unusual given year-to-year variability.
Emanuel said that hurricanes will get stronger as oceans continue to warm, but predicting the future long-term trends in hurricane activity is complicated. It would be naïve, he said, to try and predict exactly how hurricanes would change if sea surface temperatures continue to go up.
"Although it's tempting to do that," he said, "I think we have to hold back until we understand things better."
Scientists said the exact effects of global warming on hurricanes may not be known yet, but change is coming.
"We're increasing greenhouse gases and forcing the climate system in ways that we expect there are going to be very substantial changes in the coming century," Knutson said. "We're trying to figure out what it means in terms of weather and things that impact people day to day, such as storms and heat waves and droughts. This is just one part of that overall puzzle that we're working on."