NOAA Researcher: 2002 Drought Left Millions of Tons of Extra Carbon Dioxide in Earth's Atmosphere

November 26, 2007

A new NOAA study, appearing in the current issue of the Proceedings of the National Academy of Sciences, shows how a prolonged drought in North America in 2002 cut the continent's natural uptake of carbon dioxide (CO2) in half, leaving more than 360 million tons (330 million metric tons) more of the heat-trapping greenhouse gas in Earth's atmosphere. The amount not absorbed that year is equivalent to annual emissions from more than 200 million U.S. automobiles.

Scientists from NOAA's Earth System Research Laboratory will present this and other findings on the North American carbon cycle at a world CO2 symposium in Hawaii this week. The results are the first from a powerful data and modeling system called CarbonTracker, released earlier this year by NOAA.

Map of carbon dioxide absorption for North America.
Credit: NOAA

Red areas denote reduced carbon dioxide absorption in the summer of 2002 with blue areas showing enhanced absorption.

The study presents the first objective estimate of net atmospheric CO2 exchange across North America every week from 2000 to 2005. The estimate is based on 28,000 global atmospheric observations.

In North America alone, humans release two billion tons (1.85 billion metric tons) of carbon as CO2 into the atmosphere each year through burning fossil fuels and manufacturing cement. Typically, forests, grasslands, crops, and soil absorb about a third of those emissions.

However, the natural ratio was upset in 2002, when North America experienced one of the largest droughts in more than a century. Conditions over nearly 45 percent of the United States were classified as "extreme" or "exceptional." The amount of carbon taken up by vegetation and soil plunged from an annual average of 650 million metric tons to 330 million metric tons.

"Scientists often look at the role of greenhouse gases in producing climate extremes," says scientist Wouter Peters, who led the study at ESRL and is also affiliated with Wageninen University and Research Center in The Netherlands. "Here we show the reverse is also true. Climate extremes can have a major affect on the amount of carbon dioxide in Earth's atmosphere."

Drought and other climate variations can disturb the natural uptake of CO2 by changing regional temperatures, rainfall, soil moisture, and even the length of the growing season. The problem is not unique to North America. The widespread drought and heat wave that struck Europe in 2003 left more than 500 million tons of extra carbon in the air that year.

"Disruptions to natural carbon uptake can have enormous environmental and economic effects, possibly even erasing efforts to reduce fossil fuel emissions in a given year," says Peters. "CarbonTracker not only tells us when and where such disruptions occur, but also suggests why."

In the long term, land use has been the major driver of CO2 uptake in the United States. Increased fire suppression and changes in farming methods have been widespread, as well as forests recovering from logging in the past century or those regrowing on abandoned croplands.

Combined, such land use changes may be the cause of a strong carbon uptake the NOAA scientists found over the U.S. East Coast, in the Canadian evergreen forests, and across the grass and croplands of the Midwest.

Note to Editors: For a copy of "An atmospheric perspective on North American carbon dioxide exchange: CarbonTracker," Wouter Peters, et al., please contact Jonathan Lifland, National Academy of Sciences, jlifland@nas.edu, 202-334-1293 or Anatta, NOAA Communications, anatta@noaa.gov, 303-497-6288.

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