* China's forests offset 28-37 percent of CO2 in 1980s-90s
* Levels drop as emissions rise, despite more tree-planting
OSLO, April 22 (Reuters) - China's forests and other vegetation absorbed around a third of its greenhouse gases in the late 20th century, but the rate may now be falling because of a surge in industrial emissions, scientists said.
A study by Peking University said that increased summer rains, efforts to plant forests, an expansion of shrubland, shifts in crop use and higher bamboo mass soaked up between 28 and 37 percent of industrial emissions in the 1980s and 1990s.
The study gave the first estimate of the impact of plants in offsetting carbon dioxide emissions in China, which has recently overtaken the United States as top emitter. Plants soak up carbon as they grow and release it when they burn or rot.
The report, in the journal Nature, also said that China's plants and soils soaked up more carbon per square metre than in Europe but less than in the United States.
But a U.S. scientist said the percentage of emissions absorbed by plants was falling because a surge in economic growth in recent years meant China's industrial emissions were expanding faster than vegetation.
"It's dropping like a rock," Kevin Robert Gurney, a carbon expert at Purdue University in Indiana, said of the percentage absorbed. He wrote an opinion piece accompanying the Chinese study in Nature.
For 2007, vegetation would have offset just 10-15 percent of China's emissions, he told Reuters. China is opening a coal-fired power plant at a rate of more than one a week, U.N. officials say.
And projections of Chinese energy use in 2030 by the International Energy Agency would cut the level to 6-8 percent, assuming stable rates of vegetation uptake.
Vegetation worldwide absorbs between 10-60 percent of greenhouse gas emissions from human sources.
The Chinese-led study also said that a shift by people to cities meant that less firewood and charcoal was being burnt in the countryside, but people were also burning more fossil fuels.
Forests cover about 14 percent of China.
Gurney welcomed the study as helping understanding of China. "We haven't had a really good handle on Chinese emissions until now," he said.
More than 190 nations aim to agree a new U.N. climate treaty by the end of 2009 that may include credits for policies to slow deforestation. Under the new treaty, Gurney said that China could only claim credit for a tiny amount of the absorbtion -- from deliberate forest plantings.
The U.N. Climate Panel says that greenhouse gases are heating the planet and will bring heatwaves, more powerful storms, extinctions of animals and plants and rising ocean levels. -- For Reuters latest environment blogs click on:
Editing by Louise Ireland
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