Heat Waves May Compound Global Warming

Europe's devastating heat wave of 2003 killed an estimated 35,000 people. Research published in the journal Nature says the heat also put huge amounts of heat-trapping carbon dioxide into the atmosphere, causing concern that it accelerated global warming.

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The death toll from Hurricane Katrina currently stands at more than 1,000, but that is by no means the biggest climate disaster of this decade. Two summers ago, Europe was gripped by a summer heat wave that killed an estimated 35,000 people. Research published in the journal Nature says that heat wave also ended up putting a huge amount of carbon dioxide into the atmosphere. That could contribute further to global warming, as NPR's Richard Harris reports.

RICHARD HARRIS reporting:

Europe's killer heat wave also caused massive wildfires. It withered crops and forests, it brought a severe drought and it raised summer temperatures by an average of 12 degrees across the region.

Mr. PHILIPPE CIAIS (Laboratory of Climate Science and the Environment): I was in France during the heat wave and it was quite impressive to see the impact on health and also the trees becoming brown and some of them even dying.

HARRIS: Philippe Ciais has a special interest since he studies how plants take up carbon dioxide and give it off into the environment. He works at France's Laboratory of Climate Science and the Environment.

Mr. CIAIS: So I got the idea of trying to estimate with my colleagues the impact of this climate spell on plants.

HARRIS: The researchers looked at monitors from throughout Europe that measure the amount of carbon dioxide coming up from the ground, and they gathered information about crops from the region. Normally, plants in Europe soak up a lot more carbon dioxide than they produce. But Ciais now reports that was clearly not the case during the heat wave and drought of 2003.

Mr. CIAIS: We have estimated that during that particular year the consequence of these extreme summer conditions--the plants in Europe, they have outgassed about a half a billion ton of carbon into the atmosphere.

HARRIS: To put that in perspective, human activities in Europe usually contribute 1 billion tons a year. So this is half as much as the carbon dioxide that pours from tail pipes, chimneys and smokestacks. It normally takes the green plants in Europe four years to soak up that much carbon dioxide.

Mr. CIAIS: So suddenly you have a climate spell and you are undoing, essentially, everything which was taken up during four consecutive years before.

HARRIS: Ciais cares about carbon dioxide because this gas traps heat in the atmosphere. It's been building up rapidly as a result of human activities. The rate would be even faster if plants didn't capture a lot of the carbon dioxide that we emit. Now this finding suggests that droughts and heat waves can actually accelerate the process of climate change. Dennis Baldocchi at the University of California Berkeley says climate modelers expect to see a lot more summers like that in Europe.

Mr. DENNIS BALDOCCHI (University of California Berkeley): This extreme event here will be somewhat normal in 30 or 50 years from now. The big question we all want to know is: Can forests adapt to this rapid change in warming?

HARRIS: Baldocchi notes that forests in the United States have adapted to much warmer temperatures than those typical in Europe. And it's also true that carbon dioxide is a plant fertilizer. So in the absence of extreme events some scientists say plants might actually grow more vigorously as this gas continues to build up. Baldocchi says that may or may not prove to be the case, and he notes that the drought also seems to have harmed the soils in Europe and, as a result, the plants didn't bounce back entirely the following year even though the weather was much more favorable.

Mr. BALDOCCHI: So these are also interesting questions. You know, if you have an extreme event, does it have lingering effects down the road? And the initial data's starting to show that.

HARRIS: So it remains to be seen whether plants will help slow the buildup of carbon dioxide in the environment or whether eventually they will become another part of the problem. Richard Harris, NPR News.

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