Edition: U.S. / Global

Seas and Soils Emerge as Keys to Climate

Published: May 16, 2000

Over the last 150 years, the burning of coal, oil and natural gas has released some 270 billion tons of carbon into the air in the form of heat-trapping carbon dioxide. If all that had stayed in the atmosphere, much of the substantial global warming predicted for a century from now might have already taken place.

Luckily, more than half the carbon dioxide emitted by fossil-fuel burning is absorbed from the air by the oceans, by plants (which use it in converting sunlight to chemical energy) and by soils. Fewer than half the emissions, scientists believe, remain airborne to warm the earth.

All of this is part of one of nature's grand and endlessly complicated global recycling networks. In it, carbon is constantly exchanged among the air, the terrestrial biosphere, the oceans and the solid rock of the earth at varying rates on time scales ranging from hours to millions of years.

Now, as the economic and political difficulty of reducing carbon dioxide emissions at the source becomes ever more obvious, a broad-based move is under way to manipulate the carbon cycle so that it will remove more of the heat-trapping gases from the air. It is an ambitious idea, but on closer inspection one surrounded by questions, potential risks and obstacles.

In one approach, researchers are investigating ways to remove carbon chemically from industrial and automotive emissions before they are released to the atmosphere, then to sequester it at some other way station in the natural carbon cycle -- by injecting the removed carbon back into underground reservoirs or into the ocean, for instance.

The United States Department of Energy has expanded research into carbon-removal technologies at eight of its national laboratories. But this option, the department said in a recent study, is ''truly radical'' and most experts believe it is some distance from providing a practical alternative.

More promising in the near term, many scientists believe, are prospects for increasing the absorption of atmospheric carbon dioxide by what is called the terrestrial biosphere: that is, the whole complex of living things on land, mainly plants and various tiny inhabitants and detritus of the soil.

Every time a plant sprouts and grows to maturity, it absorbs carbon dioxide from the atmosphere, breaks it down chemically and uses carbon as construction material for roots, stem, branches, flowers and leaves. When the plant dies, some of the carbon goes back into the atmosphere, but some is also released into the soil as the dead plant decays. Decomposing animals, including people, further add to the carbon reservoir in the soil. Globally, soil contains about five times as much carbon as vegetation.

The expansion of these carbon reservoirs, or ''sinks,'' in vegetation and soil has been sanctioned as one way industrialized countries might meet their obligations under the 1997 Kyoto Protocol, which is aimed at reducing emissions of carbon dioxide and other heat-trapping greenhouse gases.

Lately, intense argument has developed over the degree to which countries like the United States, which may have great potential for expanding its carbon sinks, ought to be allowed to meet their emissions reduction targets under the protocol by means like planting trees, improving forest management and conserving soils rather than reducing carbon dioxide emissions from cars, factories and power generating stations.

Delegates from more than 100 countries are to grapple with the issue when they meet in November in The Hague for what is widely seen as a make-or-break effort to make the Kyoto Protocol operational.

The rules governing carbon sinks are among the many sets of yet-to-be adopted regulations necessary to put the protocol into effect, and only a few countries have ratified the agreement pending action on them. The United States, where the protocol has encountered strong opposition in Congress on several grounds, is one of those holding back.

In preparation for the critical November meeting, scientists of the Intergovernmental Panel on Climate Change, created by the United Nations more than a decade ago to advise the world's governments, issued a report on the carbon issue at a meeting last week in Montreal. The analysis suggested that expansion of terrestrial carbon sinks in the industrialized countries could potentially more than enable them to meet their entire combined emissions reduction quota under the protocol.

But experts associated with the report questioned how much of that potential could be achieved, given the difficulty of actually expanding the world's carbon sinks in practice.

''Judging the technical potential is easy,'' said Dr. Robert T. Watson, chairman of the intergovernmental panel, who is also the chief atmospheric scientist and environmental adviser of the World Bank. ''The real question is, what is the likelihood of realizing that technical potential?''

Under the Kyoto Protocol, the rich industrial countries as a group are obligated to reduce their greenhouse gas emissions 5 percent below 1990 levels by about 2010. The reductions are widely seen as a first step in eventually stabilizing atmospheric concentrations. If emissions are not reduced, prominent scientists say, the average global temperature will rise by about 3.5 degrees Fahrenheit by the year 2100. (By comparison, the world is now 5 to 9 degrees warmer than in the depths of the last ice age 18,000 to 20,000 years ago.)