November 29, 2007
Thursday AM

The 2002 North American drought left an extra 360 million tons of heat-trapping carbon in the air, equivalent to the pollution caused that year by 200 million U.S. cars, according to a study released this week by the National Oceanic and Atmospheric Administration.

The prolonged drought cut by half the continent's ability to absorb carbon dioxide, said the study.

Most atmospheric scientists say increased carbon dioxide is the main reason the planet's average temperatures are creeping up.

NOAA used its powerful new modeling system CarbonTracker to analyze data.

CarbonTracker found that in North America, humans released 1.9 billion metric tons of carbon into the atmosphere each year -- through burning fossil fuels.

Typically, forests, grasslands, crops and soil would be expected to absorb about one-third of those emissions in North America.

But that natural ratio slumped in 2002 when the continent had one of its largest droughts in a century.

Conditions over almost half of the United States were deemed "extreme" or "exceptional."

Vegetation and soil took up only 330 million metric tons of carbon, down from a yearly average of 650 million metric tons.

Humans and other animals breathe in oxygen and emit carbon dioxide. The burning of fossil fuels also emits carbon dioxide.

Plants absorb carbon dioxide and emit oxygen, as a way of keeping the Earth in balance. (But as plants die and decompose, much of the carbon dioxide is re-released).

But industrialization and the proliferation of automobiles has skewed that balance, resulting in a steady increase in the amount of carbon dioxide in the Earth's atmosphere.

So-called greenhouse gases have been linked to global warming.

"Scientists often look at the role of greenhouse gases in producing climate extremes," said Wouter Peters, who led the study at NOAA's Earth Systems Research Lab in Boulder, Colo.

"Here, we show the reverse is also true. Climate extremes can have a major impact on the amount of carbon dioxide in Earth's atmosphere," he said.

Peters is presenting the study at the 50th anniversary of the Global Carbon Dioxide Record Symposium and Celebration this week in Kona, Hawaii.

Drought and other variations in climate disturb the natural absorption of carbon dioxide by changing temperatures, rainfall, soil moisture and the length of the growing season, scientists say.

A widespread drought in Europe in 2003 left more than 500 million tons of extra carbon in the air.

Disruptions to natural carbon intake can have enormous environmental, as well as economic, impacts, Peters said.

Land-use decisions are key drivers in a world in which the carbon dioxide-oxygen exchange already is out of balance.

The fact that big stretches of Midwest farm land are reverting back to forests likely will increase overall carbon uptake and give at least a little nudge in the right direction, NOAA said.

Fire suppression is double- edged. On the one hand, it keeps trees alive, and that means more biomass to absorb carbon dioxide. On the other hand, it leads to thinner trees and ultimately larger fires that can't be stopped by human intervention.

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