Stephen Leahy

BROOKLIN, Canada, Sep 16 2005 (IPS) — The number of super-powerful storms like Hurricane Katrina has nearly doubled and there will be even more in the future as the world’s oceans continue to warm, scientists say.

Climate change is warming the surface of the oceans, and the additional heat provides the extra energy to generate more powerful hurricanes and cyclones.

The number of Category 4 and 5 hurricanes worldwide has nearly doubled over the past 35 years, according to a new study published Friday in the journal Science.

“Warmer sea surface temperatures have increased the amount of water vapour, which is the fuel for hurricanes,” said study co-author Peter Webster of Georgia Institute of Technology’s School of Earth and Atmospheric Sciences.

The largest increases in the number of intense hurricanes occurred in the North Pacific, Southwest Pacific and the North and South Indian Oceans, with slightly smaller increases in the North Atlantic Ocean.

The link between the global rise in sea surface temperatures and increased hurricane intensity is quite strong, Webster told IPS.

“I think it’s clear that global warming is causing oceans to warm,” he said.

Over the last 40 years, the top 300 metres of the world’s oceans have warmed about 0.5C on average. Earlier this year, Tim Barnett, a marine physicist at the Scripps Institution of Oceanography, published a study that provided clear evidence that emissions of greenhouse gases from burning fossil fuels was responsible for ocean warming.

Hurricane Katrina offers a good illustration of the role of warm water, Webster said.

Before it struck the U.S. Gulf Coast, Hurricane Katrina made landfall in Florida as a Category 1. However when it crossed over into the Gulf of Mexico, there was a huge, deep pool of very warm water that served as the storm’s high-octane fuel, he said.

Practically overnight, Katrina turned into a Category 5 super storm.

The Saffir-Simpson scale rates hurricanes from 1 to 5 according to wind speeds and destructive potential. A Category 1 storm has winds blowing continuously above 110 kilometres an hour: A Category 5 has continuous winds above 250 kilometres per hour.

At landfall, Katrina weakened to a Category 4. But with its exceptionally large size, the damage it caused will cost the U.S. at least 200 billion dollars.

“I wasn’t surprised by (Webster’s) results,” said Kevin Trenberth, head of the climate analysis section of the National Centre for Atmospheric Research in Boulder, Colorado.

Trenberth recently published his own paper in Science about the link between human-induced climate change and increased hurricane intensity and rainfall.

“Our estimate is that rainfall from Katrina was about seven percent enhanced by global warming,” Trenberth said in an interview.

He also points out that Kerry Emanuel, a leading hurricane expert at the Massachusetts Institute of Technology, released another study in Science showing that major storms have increased in intensity and duration by a whopping 70 percent in the North Atlantic and Northwest Pacific Oceans since the 1970s.

It’s important to note that Emanuel, Webster and Trenberth took different approaches to the issue, but all arrived at similar conclusions.

“We may differ on the details but there is no doubt there has been an increase in intensity of storms,” Trenberth said.

The North Atlantic ocean is exceptionally hot this year – about 1.5 degrees C warmer than average – and that’s why double the normal number of hurricanes and tropical storms have been forecast. That extra heat translates into an average intensity or power of these storms that is also likely to be 15 to 20 percent higher, he said.

Currently, the U.S. East Coast is being pummeled by a weakening Hurricane Ophelia, the fifteenth named storm of the hurricane season – which still has 10 more weeks to run.

Hurricanes and thunderstorms are climate regulating mechanisms for removing heat from the surface of oceans and land upwards and into space, Trenberth explains. With the extra heat that is trapped in the atmosphere and oceans by global warming, there has to be a corresponding increase either in the numbers or intensity of storms.

What will the future be like when the oceans warm another 0.5 degrees C, as they inevitably will even if all human emissions of greenhouse gases were cut off today?

More Category 4 and 5 storms and possibly beyond that towards what Emanuel and others have called “hypercanes”, said Webster.

Hypercanes is a speculative attempt to explain mass species extinctions 245 million years ago. Computer models showed that continent-sized super-storms with winds averaging 600 kilometres per hour could be produced if oceans warmed to an incredible 45 to 50 degrees C. Such temperatures are impossible today barring a massive meteor strike or gigantic underwater volcano eruption.

However coastal communities ought to be preparing for stronger storms than they’ve experienced in the past, Trenberth said.

And it should not be forgotten that as the oceans warm, sea levels rise due to thermal expansion, making storm surges even more damaging.

“The next 30 years will not be like the last 30 years. We’re in a new regime of stronger hurricanes and cyclones, we have to plan for that,” said Webster.

Otherwise, the U.S. can look forward to spending 100 billion dollars or more every few years to rebuild some section of its coast wiped out by storms, he said.

And that’s what just might happen, as Pres. George W. Bush made a strong commitment to rebuild the city of New Orleans and offered tax breaks to business to rebuild along the Gulf Coast.

“Mr. Bush said nothing about how to prepare for future storms,” Webster noted.

The Gulf Coast was completely smashed by a 28-foot storm surge and Hurricane Katrina was only the fourth-strongest storm to make landfall in the U.S.

“Would I rebuild New Orleans in its present location?” the climate scientist said. “No.”