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OSLO (Reuters) - Brazilian farmers are setting more fires in parts of the Amazon where deforestation has slowed, according to a study on Thursday that shows weaknesses in a U.N. plan for slowing climate change.
Big fires, set by farmers to clear land for agriculture, are the main cause of deforestation but they continue to set smaller fires to maintain their plots -- and the damage is often hidden from satellite imaging because they burn under the tree canopy.
"This was a big surprise," Luiz Aragao, lead author of the study at the University of Exeter in England, told Reuters. "We thought that fires would decrease with less deforestation."
The extra blazes, used to clear regrowing trees and undergrowth and add nutrients to the soil, release large amounts of greenhouse gases stored in vegetation and so partly negate the climate benefits of a drive to brake deforestation.
"Fire occurrence has increased in 59 percent of the area that has experienced reduced deforestation rates," according to the scientists writing in the journal Science, based mainly on satellite images.
Aragao said that a pattern of more fires might also be true in other tropical forests, such as the Congo basin or parts of Asia which are seeking to slow forest loss under U.N. schemes.
In Brazil, extra fires often burned undergrowth in forests alongside farmland but the damage went unnoticed by satellites if the big trees were mostly intact.
"Farmers plant pasture under the large trees and you can't see it from a satellite. It still counts as a forest because of the canopy cover," Aragao said.
U.N. estimates are that deforestation accounts for up to a fifth of all greenhouse gases released by human activities, led by the burning of fossil fuels. Plants soak up heat-trapping carbon as they grow and release it when they rot or are burned.
Climate negotiators from 185 nations are meeting in Bonn from May 31-June 11 to work on a U.N. deal to slow global warming. The talks include a project known as REDD -- Reducing Emissions from Deforestation and forest Degradation.
Aragao said any U.N. deal should take account of the risks of more fires -- especially since most models of climate change predict a drying of the Amazon region in coming decades. Most Amazon fires are set by people, rather than by lightning.
He said any U.N. REDD programmes should encourage farmers to drop traditional reliance on fires. "That would increase the costs of REDD," he said, since farmers would need training and access to machinery such as tractors.
Deforestation in the Brazilian Amazon has meant a loss of 19,000 sq km (7,300 sq mile) of forest a year from 1998-2007, releasing 280 million tonnes of carbon a year, the study said.
In recent years of drought, extra fires probably emitted as much carbon as deliberate clearing of trees, the study said. That total was in turn comparable to an estimated 450 million tonnes of carbon soaked up by the forest every year.