OSLO Lakes and rivers emit far more of a powerful greenhouse gas than previously thought, counteracting the overall role of nature in soaking up climate-warming gases, a study showed on Thursday.
A review of 474 freshwater systems indicated they emitted methane equivalent to 25 percent of all carbon dioxide -- the main greenhouse gas blamed for stoking climate change -- absorbed by the world's land areas every year.
Trees and other plants absorb carbon dioxide as they grow.
"Methane emissions from freshwater sources were greater than expected," David Bastviken, lead author of the study at Linkoping University in Sweden, told Reuters.
"Some of the carbon that is being captured and stored by the Earth will be counteracted by methane from these freshwater sources," according to the study by experts in Sweden, the United States and Brazil in the journal Science.
Emissions of methane, released by decaying vegetation and other organic matter in rivers, reservoirs, lakes and streams, have not previously been properly built into models used to understand global warming, Bastviken said.
The findings indicate that other parts of the landscape, led by forests, should be prized more as the most robust natural stores of greenhouse gases, he said.
"This means that forests and other local environments, being carbon sinks, are even more important" in helping offset global warming, he said. Land-based stores "may be more rare than expected."
Bastviken said the freshwater methane emissions were not a new environmental threat since the presence of the gas in the atmosphere was previously known, even if scientists were unsure where it came from.
"This has always happened. We just haven't paid attention," he said. Even so, he said a thaw of permafrost in places from Siberia to Alaska may also be releasing more methane from once frozen soils.
A U.N. climate conference in Cancun, Mexico, last month agreed to set up a system to slow deforestation, from the Amazon to the Congo basin, to help slow climate change.
The plan envisages incentives for developing nations to safeguard forests rather than clear them to make way for farmland, towns or roads. Deforestation accounts for perhaps 10 percent of greenhouse gases from human activities.
A build-up of greenhouse gases, mainly from burning fossil fuels in power plants, factories and cars, will cause more heatwaves, floods, droughts and rising sea levels, according to the U.N. panel of climate scientists.
Methane is about 25 times more powerful than carbon dioxide as a greenhouse gas.
Bastviken said the findings were not an argument for draining wetlands or lakes to limit methane emissions -- that might well backfire and release carbon stored in sediments.
(Editing by Janet Lawrence)