Global Warming Likely Causing More Heat Waves, Scientists Say

John Roach
for National Geographic News
August 1, 2006
Global warming has loaded the dice in favor of heat waves and may be to blame for the scorching weather across much of the United States and Europe this summer, according to several of the world's leading climate scientists.

The U.S has already seen two severe heat waves this summer, and a third is currently frying the Midwest.

The U.K. just experienced what may be its hottest July on record. And August, often the warmest month of the year, is just beginning.

Scientists at the University of Oxford in England and the Hadley Centre for Climate Research and Prediction in Exeter, England, recently concluded that human-induced global warming has increased the odds by a factor of around six that Europe will see summer heat waves as extreme as that of 2003.

That heat wave killed an estimated 20,000 people.

And the odds are getting worse. By the middle of this century, every second European summer will be warmer than that of 2003, the scientists say. The study was published in the journal Nature in 2004.

"Some relatively rare events are being made a lot less rare by human influence on climate," said team leader Myles Allen, director of the Climate Dynamics Group at Oxford.

(Related news: "Rare Whales Appear off Scotland, Heat Wave Blamed" [July 28, 2006].)

A Safe Bet

"You can never say, but for global warming, this heat wave would never have happened," Allen said.

"What you can say is, global warming can increase the chance of a heat wave occurring, and a good analogy is loading the dice."

Loaded dice increase the odds of rolling a specific number. For example, by replacing the three on a die with a six, the odds of rolling a six are doubled. The odds of rolling two sixes in a row are quadrupled.

Michael Mann, a climatologist at Pennsylvania State University in University Park, also uses the analogy of loaded dice. If a die loaded in favor of sixes is rolled once and a six comes up, nobody can say whether the six showed because of the switch.

But when the die is rolled many times and six shows on a third of the rolls, the loading effect is clear.

"It's the same thing with heat waves," he said.

"It may be we are getting to see the effect of loading the die."

Complex Relationship

The link between the current heat waves and global warming is "a little complex," said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research in Boulder, Colorado.

According to Trenberth, the immediate cause of heat waves is a weather pattern known as an anticyclone, or a high-pressure ridge. Anticyclones lead to dry conditions.

"That means all the heat is going into raising temperature rather than evaporating moisture," he said. "If you have wet conditions—if the ground is wet—that tends to act somewhat as an air conditioner."

Trenberth said only 7 of the last 25 years have had above normal precipitation over land. And a recent statistical analysis of global temperature records indicates that the number of really hot days and really hot nights is increasing almost everywhere around the world.

"The extremes of temperature are generally increasing in line with what we expect with global warming," he said. "These heat waves are a particular manifestation of that."

Derrick Ryall, head of government meteorological research at England's Hadley Centre, says climate scientists expect extreme weather like heat waves and droughts to increase as Earth warms.

But the climate also has "lots of natural variability," he added.

The trick, he says, is to tease the climate change signals from the natural variability.

"Seeing hot conditions is perfectly consistent with our predictions [of global warming]," he said. "But we cannot blame any one event on climate change."

"It's a probability game."

Signs of Warming

According to climate scientists, other signs of global warming abound:

• Spring is arriving by February in New England, threatening the job of Pennsylvania's Punxsutawney Phil, the groundhog of Groundhog Day fame.

• Glacier ice is retreating all over the world, from the European Alps to Kenya's Mount Kilimanjaro to Glacier National Park in Montana.

• Warm climate zones have shifted higher in the mountains of Central America. Several species of frog have nowhere left to move and are rapidly going extinct.

• The National Oceanic and Atmospheric Administration recently said the first half of 2006 was the warmest ever for the U.S.

(See a photo gallery of global warming's effects.)

Mann, the Pennsylvania State climatologist, says proving an effect of global warming on day-to-day weather is difficult.

But amid all these other signs of global warming, he said, "to the extent it's ever possible to say individual events are due to climate change, this [summer's rash of heat waves] comes pretty close.

"It certainly looks like we are seeing those loaded dice."

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