The heat wave

The year Europe felt the heat of global warming

At a zoo near Versailles, outside Paris, keepers kept twenty-seven polar bears cool by feeding them mackerel-flavored ice. In Alsace, the electricity company trained water cannons on the roof of a nuclear power reactor as temperatures outside soared to 118°F. In Rome, tourists queued up to pay the fine for bathing in Trevi Fountain. It seemed like a good deal, they said. Crops died; forests burned; power blacked out as office air conditioners were turned to full power; rivers from the Danube to the Po and the Rhine to the Rhone were at or near record lows.

This was by no standards an ordinary summer heat wave. For one thing, it killed at least 35,000 people: 20,000 in Italy and 15,000 in France. Old people, many of them abandoned in apartments without air conditioning as their families took their August holidays, suffered most. Dehydrated and short of breath, they died by the thousands in temperatures that often exceeded 1o4°F during the day and stuck close to 86°F at night. It was Europe's hottest summer in at least half a millennium. At the heat wave's peak, on August 13, 2003, the twenty-four-hour death toll in Paris was eight times the norm. In parts of the city, there was a three-week wait for funerals. More than 400 bodies were never claimed by relatives.

It wasn't just the mortuaries that were rewriting their record books. This was the first single weather event that climate scientists felt prepared to say was directly attributable to man-made climate change. In the past, the assumption had always been that any individual weather event could be the product of chance. But the 2003 heat wave was different, says the Oxford University climate scientist and statistician Myles Allen. "The immediate cause, I agree, was a series of anticyclones over Europe. They always raise temperatures in summer, and we can't say those were made any more likely by climate change. But we can say that climate change made the background temperatures within which those anticyclones operated that much higher."

There is no doubt that average temperatures have been rising strongly for years. In much of Europe, the summer average at the start of the new century was 0.9 to 1.8°F warmer than it was in the first half of the twentieth century. In the summer of 2003, temperatures averaged 4.1 degrees warmer. Judging from past averages, the heat wave was probably a once-in-a-thousand-years event. But, says Allen, "small changes in average temperatures make extreme events much more likely."

One of the nicest confirmations of how exceptional the summer of 2003 was came from a study published at the end of 2004. The French mathematician Pascal Yiou, of the Laboratoire des Sciences du Climat et de l'En- vironnement, had collected more than 600 years' worth of parish records showing when the Pinot Noir grape harvest began in the Burgundy vineyards of eastern France. There is a clear relationship between summer temperatures and the start of the harvest, so he extrapolated backward to produce a temperature graph from the present to 1370. The results showed that temperatures as high as those typical in the 1990s were unusual, but had happened several times before. "However," Yiou said, "the summer of 2003 appears to have been extraordinary, unique." Temperatures in Burgundy that year were almost 11°F above the long-term average. And if Yiou's formula was accurate, the highest previous temperature had been just 70 above the average. That happened in 1523, in a warm interlude during the little ice age.

"The 2003 heat wave was far outside the range of normal climate," says Allen. It was not impossible that it could have happened without global warming, but it was very improbable. "Our best estimates suggest the risk of such a heat wave has increased between four- and sixfold as a result of climate change." Many scientists continue to argue about how we might recognize "dangerous" climate change, he told me. "Well, for the thousands of victims in Europe in the summer of 2003, it is clear we have already passed that threshold."

And the big heat is only just beginning. Allen says that by mid- century, if current warming trends persist, the extreme temperatures experienced in 2003 in Europe could occur on average once every two years. Richard Betts, of Britain's Hadley Centre, says that for people living in cities, the risks are even greater. They are already feeling the worst of climate change, because they also suffer the "urban heat island effect." During heat waves, the concrete, bricks, and asphalt of buildings and roads hold on to heat much better than does the natural landscape in the countryside. In the typically windless, anticyclonic conditions of a European heat wave, the effect is even more marked. The air just stays in the streets and cooks. The effect is especially marked at night, which doctors say is a critical time for the human body to recover from daytime heat.

Betts says global warming will push the urban heat island effect into overdrive. Doubling carbon dioxide levels in the air will triple the effect, he calculates.

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