Studies Measure Capacity of "Carbon Sinks"

By John Roach
for National Geographic News
June 21, 2001
After years of wide disagreement, scientists are getting a better grip
on how much carbon Earth's forests and other biological components suck
out of the atmosphere, thus acting as "carbon sinks." New research in
this area may be highly useful in efforts to devise international
strategies to address global warming.

The emission of carbon
dioxide from the combustion of fossil fuels is the leading cause of the
buildup of greenhouse gases in the atmosphere, which many people believe
is the main culprit behind an increase in Earth's temperatures.

For a long time, scientists have known that forests, crops, soils, and other organic matter soak up some of that carbon, thereby slowing down the rate of global warming. Yet their calculations of how much carbon is absorbed have differed, in some cases significantly.

A team of scientists led by Stephen Pacala, a professor of ecology and evolutionary biology at Princeton University in New Jersey, set out to resolve this discrepancy in calculations. Their research is reported in the June 22 issue of Science.

Different Measuring Techniques

While some carbon is absorbed by organic matter such as trees and shrubs, carbon is also regularly emitted into the atmosphere by activities on land such as the burning of fossil fuels.

Researchers' lack of agreement on how much carbon is "stored" has been rooted in the use of two different methods of measurement—one atmosphere based, the other land based.

The first method involves measuring concentrations of carbon dioxide in the air as the air moves across landmasses from Point A to Point B. The second method entails making an inventory of all the carbon in a given area of ground and calculating the difference between the levels of carbon recorded from year to year.

Although there is wide variation among different atmospheric models of carbon measurement, their results have consistently indicated that higher levels of carbon are absorbed than the land-based models show.

Pacala said his team's land-based analysis was more thorough than earlier studies. "We did the first exhaustive analysis of the land sink," he said.

Previous land-based models inventoried mainly the amount of carbon absorbed by trees, he explained. He and his colleagues included measures of carbon absorbed by landfills, soils, houses, and even silt at the bottom of reservoirs.

"We found out that the land sink was bigger than had been reported by other analyses, about twice as big, and the atmosphere [models] gave numbers that were consistent," he said.

The researchers used their results to help answer a major question that has been a subject of much contention: How big is the entire "carbon sink" of the continental United States?

According to their findings, the scientists estimate that U.S. forests and other terrestrial components absorb from one-third to two-thirds of a billion tons of carbon each year.

At the same time, reliable figures indicate that the United States emits more than two to four times that amount of carbon each year, about 1.4 billion tons.

Taking into account the carbon sink effect, 800 million to 1.1 billion tons of carbon accumulates annually in the atmosphere, the researchers say. This refutes the idea that the U.S carbon sink is big enough to equal the amount of carbon that U.S. factories emit through the burning of fossil fuels, as some studies have concluded.

The results of the Princeton-led study are particularly interesting because the 23 scientists who participated in the research and agreed on the conclusions initially held strongly differing views about the size of the U.S. carbon sink.

Diminishing Effect

Pacala and his colleagues say the main reason the United States is drawing in a large volume of carbon is because many forests and areas of land that were logged or converted to agriculture in the last 100 years are now recovering with the growth of new vegetation.

These trees and shrubs absorb carbon dioxide from the air and channel it into the growth of massive tree trunks, branches, and foliage. This, in turn, gradually expands the overall size of the U.S. carbon sink.

Pacala emphasizes, however, that the U.S. absorption of carbon does not fully offset the emissions of carbon from fossil fuels and should not be seen as a license to release more carbon. A large part of the current sink effect, he said, is the land re-absorbing large quantities of carbon that were released during heavy farming and logging of the past.

"When we chopped down the forests, we released carbon trapped in the trees into the atmosphere. When we plowed up the prairies, we released carbon from the grasslands and soils into the atmosphere," said Pacala. "Now the ecosystem is taking some of that back." But, he added, the sink effect will steadily decrease and eventually disappear—as U.S. ecosystems complete their recovery from past land use.

"The carbon sinks are going to decrease at the same time as our fossil fuel emissions increase," he said. "Thus, the greenhouse problem is going to get worse faster than we expected."

Carbon Sink in China

In a separate study in Science, researchers reported on a similar carbon sink effect in China, which they attribute to the regrowth of logged forests and intensive planting of new forests.

Jingyun Fang, an ecology professor at Peking University in Beijing, and his colleagues noted that Chinese forests were heavily exploited from 1949 to the end of the 1970s. Since then, however, the government has undertaken wide-scale forest planting and reforestation, mainly to combat erosion, flooding, desertification, and loss of biodiversity.

An unintended consequence of this increase in vegetation was the growth of a carbon sink that is estimated to be on par with that of North American forests.

(c) 2001 National Geographic Society

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