Study Casts Doubt on Uses of Carbon Sinks

By Cat Lazaroff

WASHINGTON, DC, November 8, 2001 (ENS) - The earth's land based ecosystems absorbed all of the carbon released by deforestation plus another 1.4 billion tons emitted by fossil fuel burning during the 1990s, a new study suggests. But the study also warns that so called carbon sinks cannot be counted on to mop up carbon dioxide emissions indefinitely.


Pollution from a U.S. power plant using coal to generate electricity - one of the world's major sources of carbon dioxide emissions (Photo courtesy National Renewable Energy Lab)
Carbon dioxide (CO2) is the primary greenhouse gas entering the atmosphere from human activities. Ongoing negotiations regarding international efforts to reduce the impacts of greenhouse gases have included discussions about carbon sinks - natural and manmade areas where plant growth absorbs carbon emitted from human sources.

A study published this week in the journal "Nature" indicates that carbon sinks were able to mop up most of the naturally and artificially emitted carbon dioxide over the past decade. However, with carbon emissions on the rise, and vegetated areas continuing to shrink, that situation is likely to change, the researchers said.

"We could easily see this robust transfer of carbon out of the atmosphere and into land based ecosystems that occurred in the 1990s slow down in the future," said the paper's lead author, David Schimel, of the National Center for Atmospheric Research (NCAR).

Fossil fuel burning, cement manufacture, and deforestation gave off about seven billion tons of carbon per year during the 1980s and eight billion tons each year during the 1990s, about half of it ending up in the earth's atmosphere, the study shows.


Tests at Brookhaven National Laboratory examine how extra CO2 affects plant growth and diversity (Photo courtesy Brookhaven National Laboratory)
In the 1980s the amount of carbon released to the atmosphere from deforestation about equaled that taken up by land ecosystems into various sinks. During the 1990s the balance tipped, and 1.4 billion tons more carbon ended up in land based ecosystems than in the atmosphere, despite continuing deforestation.

"Land and ocean processes have, in essence, provided a major, but far from permanent, subsidy to humans, protecting the atmosphere from many of the consequences of deforestation and burning fossil fuels," said co-author professor Chris Field from the Carnegie Institution of Washington.

Land use changes in the Northern Hemisphere have been partly responsible for carbon uptake during the 1990s, the researchers found. In the United States, trees and other growth expanded on abandoned agricultural land, while a reduction in fires allowed forests to spread. Increased plant growth spurred by increasing carbon dioxide and nitrogen deposits - a process more noticeable in Europe and Asia - also helped clear the air of CO2 buildup.

"Forests can only replace farms for so long," explained Schimel. "Eventually new trees and grasses reach maturity and soak up less carbon dioxide. Similarly, there's a limit to how much forests can fill in and spread, even with successful fire suppression."


Northern forests like the United Kingdom's New Forest are getting greener due to rising temperatures and carbon dioxide levels (Photos by Ian Britton, courtesy
Over time, the effects of climate change on ecosystems will probably reduce sinks globally, write the authors. Meanwhile, carbon dioxide emissions are expected to continue to rise because of human activities.

The results have potential implications for the Kyoto Protocol negotiations now underway in Morocco. Under the Kyoto Protocol, an addition to the United Nations framework climate change treaty, 38 industrialized nations have agreed to cut their emissions of six greenhouse gases linked to global warming.

Recent difficulties in talks over the Protocol have included a lack of scientific knowledge about the strength and distribution of carbon sinks and how they vary from year to year.

The new "Nature" study, produced by a team of 30 leading carbon scientists from around the world, draws on a large body of research to build up a new and comprehensive picture of carbon sinks on land.

"It is a major step forward in understanding where terrestrial carbon sinks actually are, why they are there, and how long they will operate into the future," said Dr Will Steffen, one of the authors and executive director of the International Geosphere-Biosphere Programme (IGBP), which coordinated the production of the paper.


Emissions rising from the Chemical Works, Seal Sands, Teesside, United Kingdom (Photo by Ian Britton courtesy
The paper represents a major advance in terms of reconciling two different approaches to measuring the strength of carbon sinks and sources. Historically, researchers have used two types of measurements: 1) direct measurement of CO2 in the atmosphere, and 2) on the ground measurements on the basis of forest growth and soil uptake etc.

Until now, these two techniques have provided inconsistent results. In this study the authors show that, on the broad scale, they are consistent.

The authors point out that there are many regional differences in the strength of terrestrial carbon sinks. Much of Siberia, for example, has been warming at a rate of about 0.5° C per decade since the 1960s and an increase in wild fires and insect damage appears to have converted this region from a sink into a temporary carbon source with considerable year to year variability.

"Although carbon sinks have a role to play in absorbing excess carbon dioxide, it is possible that the net global terrestrial carbon sink may disappear altogether in the future," said lead author Professor David Schimel from the Max Planck Institute for Biogeochemistry in Jena, Germany.


The United States is hoping to get credit under the Kyoto Protocol for replanting clearcuts like this one in Mt. Hood National Forest, Oregon (Photo courtesy American Lands)
There is also considerable annual variability in sink strength associated with climatic variations such as the El Niņo Southern Oscillation in tropical and nontropical regions. Globally, there appears to be a net release of carbon to the atmosphere during warm, dry years and a net uptake during cooler years.

"This observation gives a hint of how terrestrial sinks may respond to longer term climate changes such as increased temperatures," said Dr Steffen.