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 up changes in the climate, researchers reported today in the British journal Nature.
The researchers, led by Dr. Chris Freeman of the University of Wales, 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 some 460 billion tons of carbon, or about 60 percent of the amount already 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.
Wetzel, who was not involved in the research, said the mechanism identified by Freeman "would accelerate the release from this enormous pool."
But he and other scientists cautioned that the possibility outlined in the study depends on predictions that are based on uncertain climate models.
Still, several scientists said that the findings would make it even more urgent to try to reduce emissions of greenhouse gases.
The new study, said Dr. Peter Frumhoff, an official of the global environmental program at the Union of Concerned Scientists, emphasized "the need to be taking a precautionary approach and moving forward quickly with reducing atmospheric concentrations of greenhouse gases."
Other scientists did not dispute that assessment but said the results demonstrated the great 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 Earth's climate will not be known until that effect can be studied longer, said the authors of the study.
"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 [carbon dioxide] concentration."
About half of carbon emissions are eventually absorbed by natural carbon sinks in the oceans and on land. The rest remains in the atmosphere.
On land, the carbon is stored when plants grow, die and decay into organic matter in the soil. When that organic matter finally decomposes, nutrients such as 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, Freeman and his co-authors studied the biochemistry of northern peatland soils, which make up about 6 percent of Earth's land, including wetlands, tundra and other moist soils.
The team found that within those soils, the action of a single enzyme, phenol oxydase, could trigger a sudden 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.