A Climate Bomb Defused?

 A Climate Bomb Defused?

20 September 2007

published by sciencenow.sciencemag.org

Millions of square kilometers of frozen plant material stretching across Canada and Siberia were long believed to be a ticking time bomb of climate change. As temperatures rise, the thinking went, these permafrost-entrenched peatlands would thaw and exacerbate global warming by triggering soil bacteria to release enormous quantities of methane–a greenhouse gas 23 times more potent than carbon dioxide. However, new research shows that the thawing also fosters the growth of plants that suck up carbon, potentially offsetting rising methane emissions.

Nearly one-tenth of Earth’s vegetated land is in Canada’s and Siberia’s high-latitude forests, roughly a third of which contain peatlands. To find out just how these depositories of billions of tons of carbon-rich plant material contribute to climate change, ecologist Merritt Turetsky of Michigan State University in East Lansing and colleagues studied peatlands at three sites in Canada. The sites contained areas with intact permafrost inhabited by spruces and other peatland plants, as well as expanses of recently thawed permafrost. Over a 5-year period, the team used on-site measurements, aerial surveys, and other methods to monitor changes in carbon storage (as measured in new plant growth) and methane emissions.

As expected, thawing led to wetter conditions, which fostered the growth ofSphagnum mosses and other water-loving plants. “Peatlands previously underlain with permafrost turn ‘from terra firma to terra softa,’ ” explains Turetsky. These conditions were ideal for bacteria, which began decomposing thawing plant material in the peat, tripling methane emissions. But plants came to the rescue: Seeds and spores blown in by the wind and in thawing soil led to an explosion in new plant growth, allowing the peatlands to accumulate more than three times as much organic matter–about half of which is carbon–as before thawing. This carbon sequestration more than balances out the greenhouse impact of methane release, the researchers report in this month’s issue of Global Change Biology.

Still, plants will provide only a temporary fix. The team’s peat cores–records of thousands of years of climate history–indicate that carbon storage will decline as the wetlands fill in and soil accumulates, allowing the return of drier-adapted, slower-growing plants.

The study does a “stellar job at pulling together the links among carbon, water, and plant ecology,” says arctic biologist Jennifer Harden of the U.S. Geological Survey in Menlo Park, California. But the benefit of thawing peatlands could be even more ephemeral in the future. Increasing fire activity in boreal and subarctic regions–almost certainly a direct result of enhanced summer drought over the past 50 years–“doesn’t bode well for peatlands,” she says, noting that if peatlands dry out rapidly and burn, they could release loads of carbon dioxide.

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