Climate negotiators will discuss measures to slow global warming at June 6 meet in Germany
Rising forest density in many countries is helping to offset climate change caused by deforestation from the Amazon basin to Indonesia, a study showed on Sunday.
The report indicated that the size of trees in a forest - rather than just the area covered - needed to be taken into account more in UN-led efforts to put a price on forests as part of a nascent market to slow global warming.
"Higher density means world forests are capturing more carbon," experts in Finland and the United States said of the study in the online journal PLoS One, issued on June 5 which is World Environment Day in the UN calendar.
Trees soak up carbon dioxide, the main greenhouse gas, as they grow and release it when they burn or rot. Deforestation in places from the Congo basin to Papua New Guinea is blamed for perhaps 12 to 20 percent of all emissions by human activities.
The report, based on a survey of 68 nations, found that the amount of carbon stored in forests increased in Europe and North America from 2000-10 despite little change in forest area.
And in Africa and South America, the total amount of carbon stored in forests fell at a slower rate than the loss of area, indicating that they had grown denser.
And some countries still had big losses of carbon, including Indonesia and Argentina. The study did not try to estimate the overall trend, saying there was not yet enough data.
Greater density in some countries, including China, was probably linked to past forest plantings, lead author Aapo Rautiainen of the University of Helsinki said.
"Forests that were established in China a few decades ago are now starting to reach their fast-growing phase. That is a reason for rising density now," he said.
Global warming, blamed by the UN panel of climate experts mainly on human use of fossil fuels, might itself be improving growth conditions for trees in some regions. Warming is projected to cause heatwaves, droughts and rising sea levels.
The US has had among the most striking shifts - timberland area expanded by just one percent between 1953 and 2007 but the volume of growing stock surged by 51 percent.
A shift toward farming in the Midwestern US meant that forests in the east had been left to grow, and get denser.
The report also suggested that forest managers might rotate fellings less frequently since trees kept thickening.
But it could complicate efforts to design market mechanisms to encourage developing nations to safeguard tropical forests. Under the UN-led effort, people would get tradable credits for slowing the rate of deforestation.
Measuring the density of a forest requires more complex monitoring than just measuring the extent of a forest by photographing it from a plane or by satellite.
"There does need to be a greater sampling to be able to come to a legitimate and credible number for the carbon," said Iddo Wernick, a co-author at the Rockefeller University in New York.
Negotiators from about 180 nations will meet in Bonn, Germany, from June 6-17 to discuss measures to slow global warming, including the protection of tropical forests.
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