As the World Gets Hotter, Soils May Emit More CO2 Than Previously Thought

Climate change may release a new stream of planet-warming carbon dioxide and other gases now locked in the ground, possibly boosting global emissions by 30 percent, research out today suggests.

Scientists at California's Lawrence Berkeley National Laboratory tried to simulate what would happen when meter-thick (3.25 feet) layers of soil were exposed to temperatures 4 degrees Celsius (7.2 degrees Fahrenheit) warmer than normal - about where global average temperatures as expected to be by 2100 without efforts to hold back man-made emissions. They found the soil started giving up its trapped carbon even at depths well underground.

The question of how much trapped CO2 and other carbon compounds like methane - which punches far above its weight as a greenhouse gas - could be released from warming soil has been the subject of intense scrutiny in recent years. Releasing more underground carbon could create a carbon feedback loop, producing more warming and releasing more emissions in the process. But how likely that is - or determining how much carbon could be released - remains uncertain.

Since about three times as much organic carbon is locked into the ground as is now floating around in the air, the new study suggests we may have a bigger problem underfoot than previously believed.

"There's an assumption that carbon in the subsoil is more stable and not as responsive to warming as in the topsoil, but we've learned that's not the case," Margaret Torn, an ecologist and biochemist at the lab's environmental science center, said in a statement announcing the results. "Deeper soil layers contain a lot of carbon, and our work indicates it's a key missing component in our understanding of the potential feedback of soils to the planet's climate."

The findings were published Thursday in the research journal Science. The Department of Energy-funded project monitored six plots of woodland at a research station in the Sierra Nevada foothills for two years. Three of the plots were warmed up using cables sunk into the ground; the rest were left unheated.

The researchers planted instruments to measure CO2 concentrations at various depths and at the surface. They found that the warmer soil yielded between 34 and 37 percent more carbon than the unheated control patches, and 40 percent of it came from more than 15 cm (six inches) underground. The carbon came not only from the ground, but from increased activity by bacteria and other microbes when they warmed up.

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The research station is in a temperate forest, similar to those that cover about 13 percent of Earth's land area. If the LBNL findings are applied to similar terrain, those soils could be coughing up carbon at a "significantly higher" rate by 2100 - "Perhaps even as high as 30 percent of today's human-caused annual carbon emissions, depending on the assumptions on which the estimate is based," said Caitlin Hicks Pries, the study's lead author.

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Image: Melting permafrost in places like Alaska could become a major contributor to climate change as temperatures increase.