Energy Evolution: September 12, 2005
Katrina Reignites Debate Over Global Warming And Hurricanes
A Canadian weather expert says there’s no conclusive evidence to support claims by some scientists that global warming is a major cause of increasing incidences of devastating hurricanes, like that of Katrina, which slammed into Louisiana, Mississippi and Alabama in late August.
David Phillips, senior climatologist with Environment Canada, told Energy Evolution there seems little doubt storms like Katrina, which caused billions of dollars in damage and killed hundreds on the Gulf of Mexico coast, will occur more frequently in the next decade.
But he says that's more likely because there's a natural storm cycle that occurs and hurricane-prone areas in the Atlantic will experience more of them in the next few years.
"It's a grey area, as so much of climate change [science] is, because climate change tends to occur over a long period of time," he notes.
"You can be seduced into thinking that every time there is an event [like Katrina] it's our [mankind's] fault.
"But 120 years ago, when there were a number of tornadoes [in North America], it was blamed on [the air pollution from] locomotives."
However, some scientists are arguing that global warming is playing a role in pumping up the power of big Atlantic storms.
This year is on track to be the worst-ever for hurricanes, according to experts measuring ocean temperatures and trade winds -- the two big factors that breed these storms in the Caribbean and tropical North Atlantic.
Earlier this month, Tropical Storm Risk, a London-based consortium of experts, predicted that the region would see 22 tropical storms during the six-month June to November season, the most ever recorded and more than twice the average annual tally since records began in 1851.
Seven of these storms would strike the United States, of which three would be hurricanes, it said.
Already, 2004 and 2003 were exceptional years: they marked the highest two-year totals ever recorded for overall hurricane activity in the North Atlantic.
This increase has also coincided with a rise in the Earth's surface temperature in recent years, driven by greenhouse gases that cause the sun's heat to be stored in the sea, land and air rather than radiate back out to space.
But experts are cautious, also noting that hurricane numbers seem to undergo swings, over decades.
About 90 tropical storms -- a term that includes hurricanes and their Asian counterparts, typhoons -- occur each year.
The global total seems to be stable, although regional tallies vary a lot, and in particular seem to be influenced by the El Nino weather pattern in the Western Pacific.
"[Atlantic] cyclones have been increasing in numbers since 1995, but one can't say with certainty that there is a link to global warming," says Patrick Galois with the French weather service Meteo-France.
"There have been other high-frequency periods for storms, such as in the 1950s and 60s, and it could be that what we are seeing now is simply part of a cycle, with highs and lows."
On the other hand, more and more scientists believe that global warming, while not necessarily making hurricanes more frequent or likelier to make landfall, is making them more vicious.
A key factor in ferocity is the temperature differential between the sea surface and the air above the storm. The warmer the sea, the bigger the differential and the bigger the potential to "pump up" the storm.
Just a tiny increase in surface temperature can have an extraordinary effect, says researcher Kerry Emanuel of the Massachusetts Institute of Technology (MIT).
In a study published in the journal Nature in July, Emanuel found that the destructive power of North Atlantic storms had doubled over the past 30 years, during which the sea-surface temperature rose by only 0.5 C.
Emanuel's yardstick is storm duration and wind power: hurricanes lasted longer and packed higher wind speeds than before.
Another factor in destructiveness is flooding. Kevin Trenberth of the U.S. National Center for Atmospheric Research suggests that hurricanes are dumping more rainfall as warmer seas result in more moisture into the air, swelling the storm clouds.
The indirect evidence for this is that water vapour over oceans worldwide has increased by about two per cent since 1988. But data is sketchy for precipitation dropped by recent hurricanes.
"The intensity of, and rainfalls from, hurricanes are probably increasing, even if this increase cannot yet be proven with a formal statistical test," Trenberth wrote in the journal Science in June. He said computer models "suggest a shift" toward the extreme in hurricane intensities.
Environment Canada’s Phillips says he suspects that the increasing number and ferocity of hurricanes is related to the cyclical nature of storms and to global warming.
"I think it's both, but whether we're the big perpetrators, we just don't know," he says.
"Would Katrina have been less of a hurricane because we burn fossil fuels? We don't know."
Phillips says water temperature is a big factor in causing hurricanes.
“Hurricanes need 26 degree water, but you always have 26 degree water in that area at this time of the year. It’s not that simple to say global warming is causing these destructive hurricanes.”
Phillips says there are hundreds of thunderstorms every year over the Atlantic that could develop into hurricanes, but only six do on average.
He says there seems little doubt major storms are cyclical.
“Prior to 1995 we had 25 years of quiet times and nine of the last 11 years have been active.”
Yet, even in “quiet” years major hurricanes can strike. For instance, Andrew, which caused devastation on a par with Katrina, struck in 1992, considered a quiet hurricane year.
Although Canadians often complain about their weather, anything but tropical for most of the year, Phillips says Canada is actually very fortunate in the area of weather.
That’s because storms that do occur in Canada aren’t “relentless,” as they tend to be in the slow moving air of the tropics.
“Katrina was moving at 13 miles an hour, but the storms that hit Canada move at highway speeds.”
On both Canada’s Atlantic and Pacific coasts the water is too cold to feed hurricanes.
“When they [hurricanes] get over cooler water it’s like turning down the thermostat,” he says.
Tornadoes, which are more localized events caused by hot air meeting cold air, can and do occur in Canada, but they don’t cover the same wide swath and cause the same devastation as hurricanes.
While some hurricanes have struck Eastern Canada -- for instance, Juan hit Nova Scotia a couple of years ago -- they are usually downgraded to storm status by the time they make Canadian landfall.
Meanwhile, on Canada’s Pacific coast, where the water temperature gets no warmer than 22 degrees, there has never been a hurricane recorded.
“They may have Pineapple Express storms [fast-moving storms], but there’s no cold air meeting warm air and clashing to give them a hurricane.”
In fact, on the entire Western Pacific coast, even to the tropical coasts of Mexico, hurricanes are very unlikely to be as devastating as they are in the Atlantic, where the Caribbean and the southern U.S. gets hit often.
“In the Pacific [even in tropical Mexico and in southern California] the winds aren’t favourable [to spawn large hurricanes],” says Phillips. “They blow the storms out to the sea.”