Droughts caused by global warming could set off a biochemical process in northern soils that would release large amounts of carbon dioxide into the air and possibly speed changes in the climate, researchers are reporting today in the journal Nature.
The researchers, led by Dr. Chris Freeman of the University of Wales in Bangor, said the increase in droughts predicted by some climate models could abruptly activate a dormant enzyme in moist, peaty northern soils, triggering decomposition of their organic matter.
This decay would release large amounts of carbon dioxide, a "greenhouse gas" thought to cause global warming. The soils are believed to hold 460 billion tons of carbon, or about 60 percent of the amount in the atmosphere as carbon dioxide.
"It is an enormous reservoir that potentially can be released into the atmosphere as another climate change factor," said Dr. Robert G. Wetzel, a professor of biological sciences at the University of Alabama in Tuscaloosa.
Dr. Wetzel, who was not involved in the research, said the mechanism identified by Dr. Freeman "would accelerate the release from this enormous pool."
But he and other scientists cautioned that the possibility outlined in the study depended on predictions based on uncertain climate models.
Dr. Sandra Brown, an ecologist, said, "The best predictions I've seen say it will be a warmer and drier climate." But Dr. Brown added, "I always worry when I see these papers making broad extrapolations across the entire world."
Dr. Brown, of Winrock International, a private environmental organization in Arlington, Va., is a member of the Intergovernmental Panel on Climate Change, an organization of hundreds of scientists that was created by the United Nations.
Still, several scientists said that the findings would make it even more urgent to try to reduce emissions of greenhouse gases. Late last year at The Hague, negotiations on a worldwide treaty to reduce global warming collapsed in part because of disagreements over the role of natural ecosystems like forests in sopping up carbon dioxide from the atmosphere and reducing warming.
The new study, said Dr. Peter Frumhoff, an official of the global environmental program at the Union of Concerned Scientists, who was present during the negotiations at The Hague, emphasized "the need to be taking a precautionary approach and moving forward quickly with reducing atmospheric concentrations of greenhouse gases." The organization backs efforts to limit emissions of the gases.
Other scientists did not dispute that assessment but said the results demonstrated the uncertainties in models of the global carbon cycle.
As if to underline that point, a second study published in the same issue of Nature suggested that under some circumstances higher amounts of carbon dioxide in the atmosphere could inhibit decomposition of material in grassland soils, presumably enhancing carbon storage there.
Any direct implications for the earth's climate will not be known until that effect can be studied longer, said the authors of the study, who include Dr. Shuijin Hu of the department of plant pathology at North Carolina State University and Dr. Christopher B. Field, a plant biologist at the Carnegie Institution of Washington, at Stanford University.
"It reminds us that there are still surprises," said Dr. Bruce A. Hungate, an ecologist at Northern Arizona University who was not involved in the work, "that we don't completely understand all the dynamics that control atmospheric CO2 concentration."
About half of carbon emissions are eventually absorbed by natural carbon sinks in the oceans and on land. The rest of the carbon remains in the atmosphere. On land, it is stored when plants grow, die and decay into organic matter in the soil. When that organic matter finally decomposes, nutrients like nitrogen are released in forms that another generation of growing plants can absorb, and the carbon escapes into the atmosphere as carbon dioxide.
In experiments at the University of Wales, Dr. Freeman and his co- authors — Dr. Hojeong Kang, now at the University of Wisconsin, and Dr. Nick Ostle, now at the Centre for Ecology and Hydrology at the Natural Environment and Research Council in England — studied the biochemistry of northern peatland soils, which make up about 6 percent of the earth's land, including wetlands, tundra and other moist soils.
The team found that within those soils, the action of an enzyme called phenol oxydase could trigger decomposition of the stored organic matter. But the enzyme is normally dormant because the water in those soils keeps them free of oxygen, which the enzyme needs to function.
But frequent droughts could dry the soil and let oxygen in. The enzyme would then begin decomposing organic matter; it would also break down compounds that inhibit the activity of other enzymes, leading to a sort of chain reaction.
Dr. Hu and Dr. Field's research found that increased carbon dioxide levels over several years led indirectly to a starvation of microbial activity in grassland soils, lowering decomposition rates. But both scientists said that over longer periods, the effect could also reduce plant growth, canceling any enhancement of the soil's ability to store carbon.
"The ecosystems are big players, but they're almost certainly not going to save the day and result in a situation where emissions cuts aren't necessary," Dr. Field said.