Tropical storms more intense, new research shows
Scripps Howard News Service, July 31, 2005
Tropical storms have become significantly more intense in both the Atlantic and Pacific oceans during the past 30 years, according to an analysis published Sunday.
Using a new "power dissipation index" that reflects both the duration of storms and their maximum wind speeds, Kerry Emanuel, an atmospheric researcher at the Massachusetts Institute of Technology, reports that tropical storms' overall intensity has increased by about 50 percent since the mid-1970s.
Although many of the fiercest storms of the past three decades haven't made landfall when they were at peak intensity, "the near-doubling of hurricanes' power during this period should be a matter of some concern, as it's a measure of the (future) destructive potential of these storms," Emanuel said.
His paper, published online by the journal Nature, illustrates that the increases in storm intensity have been mirrored by increases in the average temperatures at the surface of tropical oceans.
And while much of this warming has been attributed to decades-long swings that come and go in the Atlantic and Pacific, Emanuel said his research shows there have been increases in tropical sea surface temperatures worldwide, even outside zones that have been affected by the established patterns.
Moreover, he pointed out in an interview that "the intensity of hurricanes depends both on how much heat can be transferred from the ocean to the atmosphere, which depends on the temperature of the ocean, and on how high air rising in the eyewall can go. This depends on the temperature profile of the atmosphere."
So if climate change continues to warm both seawater and the air above it during the rest of this century, as most scientists expect, "future warming may lead to an upward trend in tropical cyclone destructive potential, and, taking into account an increasing coastal population, (also) lead to a substantial increase in hurricane-related losses in the 21st century,' Emanuel warned.
The research, sponsored by the National Science Foundation, "is an innovative application of a theoretical concept and has produced a new analysis of hurricanes' strength and destructive potential,' said Jay Fein, director of the foundation's climate dynamics program.
Emanuel also has a theory that increased tropical cyclone activity would increase the transport of heat through the oceans from the tropics to high latitudes. If this turns out to be true, then this "feedback" could contribute to making things even warmer around the northern Pacific and Atlantic basins.
Recent simulations done by scientists at the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory indicate that the global warming anticipated over the next 80 years would increase average hurricane wind speeds by 5 percent to 10 percent, or by about half a category on the five-category Saffir-Simpson scale of hurricane intensity.
While a number of other climate and hurricane researchers have been debating whether a warming climate could make hurricanes more frequent, Emanuel is one of relatively few who has considered the impact on intensity.
Reliable records of hurricanes, particularly those with measures of intensity, go back only a few decades, and many researchers argue that it's too early to tell if warming temperatures will make a significant difference. Some climate models suggest that changing tropical wind patterns in a warming world will cause hurricanes to break up more easily, making the storms less frequent, even if temperatures that fuel the systems are higher. And there's no evidence that climate change will affect the tracks of hurricanes to make them any more or less likely to hit land, virtually all experts in the field say.
Emanuel agrees that the question of climate control on hurricane frequency is still open and subject to a lot more study.
But hurricane watchers, particularly those with connections to insurers, note that despite the freak pileup of four hurricanes on Florida last year, some of the biggest losses from recent hurricanes - like Hugo in 1989 and Andrew in 1992 - came in seasons where there weren't a large number of storms.
Storm Turns Focus to Global Warming
The Los Angeles Times, Aug.30, 2005
Though some scientists connect the growing severity of hurricanes to climate change, most insist that there's not enough proof.
Is the rash of powerful Atlantic storms in recent years a symptom of global warming?
Although most mainstream hurricane scientists are skeptical of any connection between global warming and heightened storm activity, the growing intensity of hurricanes and the frequency of large storms are leading some to rethink long-held views.
Most hurricane scientists maintain that linking global warming to more-frequent severe storms, such as Hurricane Katrina, is premature, at best.
Though warmer sea-surface temperatures caused by climate change theoretically could boost the frequency and potency of hurricanes, scientists say the 150-year record of Atlantic storms shows ample precedent for recent events.
But a paper published last month in the journal Nature by meteorologist Kerry Emanuel of the Massachusetts Institute of Technology is part of an emerging body of research challenging the prevailing view.
It concluded that the destructive power of hurricanes had increased 50% over the last half a century, and that a rise in surface temperatures linked to global warming was at least partly responsible.
"I was one of those skeptics myself - a year ago," Emanuel said Monday.
But after examining data on hurricanes in the Atlantic and typhoons in the Pacific, he said, "I was startled to see this upward trend" in duration and top wind speeds.
"People are beginning to seriously wonder whether there is a [global warming] signal there. I think you are going to see a lot more of a focus on this in coming years."
Hurricane activity in the Atlantic has been higher than normal in nine of the last 11 years, said the National Oceanic and Atmospheric Administration.
This month, the agency raised its already-high hurricane forecast for this year to 18 to 21 tropical storms, including as many as 11 that would become hurricanes and five to seven that would reach major-hurricane status. That could make 2005 one of the most violent hurricane seasons ever recorded. A typical storm year in the Atlantic results in six hurricanes.
But the agency believes that the increase in hurricanes is most likely the result of a confluence of cyclical ocean and atmospheric conditions that tend to produce heightened tropical storms every 20 to 30 years. If global warming is playing any role in the hurricanes, it is a minor one, the federal agency maintains.
Computer models have shown for years that rising sea-surface temperatures resulting from global warming could create more ideal conditions for hurricanes.
Yet before Emanuel's research there were few indications that hurricanes had become stronger or more frequent, despite well-documented increases in surface temperatures.
Moreover, skeptical hurricane scientists were quick to point out that worldwide weather records were too inadequate for a thorough examination of such trends. They said that would require an analysis of storm activity going back hundreds if not thousands of years.
"There is absolutely no empirical evidence. The people who have a bias in favor of the argument that humans are making the globe warmer will push any data that suggests that humans are making hurricanes worse, but it just isn't so," said William Gray, a Colorado State University meteorologist who is considered one of the fathers of modern tropical cyclone science and who sharply questions Emanuel's conclusions.
"A lot of my colleagues who have been around a long time are very skeptical of this idea that global warming is leading to more frequent or intense storms," Gray said. "In the Atlantic, there has been a change recently, sure. But if you go back to the 1930s, you see a lot of storms again. These are natural cycles, not related to changes in global temperature. I can't say there is no human signal there, but it's minute."
Nonetheless, some scientists have maintained that the rise in mean global temperatures over the last half a century - a well-documented trend widely linked to human activities such as the burning of fossil fuels - will inevitably have an effect on storms, if it hasn't already.
"It's the ocean temperatures and sea-surface temperatures that provide the fuel for hurricanes," said Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research who recently published a paper in the journal Science contending that climate change could cause hurricanes to produce more rain and thereby become more dangerous.
"It's the big guys, the more intense storms, that have been increasing," Trenberth said. Hurricane scientists have been "unduly influenced by what has been happening in their corner of the world in the Atlantic. But if you look more broadly, at what has been happening in the Indian Ocean and the Pacific Ocean, there is a clear trend."
Such views remain controversial among veteran hurricane scientists.
Chris Landsea, a hurricane expert with the National Oceanic and Atmospheric Administration, withdrew this year from the Intergovernmental Panel on Climate Change, an international scientific group that periodically sums up the consensus on global warming. Landsea said in a letter to scientific colleagues that he resigned because he strongly disagreed with public statements made by Trenberth, who was also part of the panel, suggesting that last year's Atlantic hurricanes were linked to global warming.
Despite the dispute among scientists, the prospect of stronger hurricanes has alarmed some insurance companies, which are concerned that disaster losses could increase in years to come.
Munich Re, the world's largest insurer of insurance companies, said that global warming was at least partly responsible for a rise in worldwide insurance losses over the last 50 years, including $114.5 billion in losses last year, the second-highest total ever.
Critics, including Roger Pielke Jr., a University of Colorado science professor, have attributed the losses to a simpler cause: more people living in harm's way in areas such as Florida and Louisiana.
Still, some experts believe that hurricane scientists will have to consider climate change more seriously if the streak of Atlantic storms persists.
"You are seeing more intense storms, which is consistent with what you would see" under global warming scenarios, said Richard Murnane, a hurricane expert with the Bermuda Biological Station for Research, which studies storms for insurance companies.
"The majority view is that if this keeps up for a few more years, we will be outside of natural variability. But people are still leery of saying that this is a result" of human-caused climate change, he said.