Plants both absorb and exhale carbon dioxide, but healthy forests — especially those in the tropics — take up far more of the greenhouse gas than they give off. When they are damaged, get sick or die, that stored carbon is released.

“We normally think of forests as putting the brakes on global warming,” said Risto Seppala, a professor at the Finnish Forest Research Institute

and head of the expert panel that produced the report. “But in fact over the next few decades, damage induced by climate change could cause forests to release huge quantities of carbon and create a situation in which they do more to accelerate warming than slow it down.”

Authored by 35 of the world’s top forestry scientists, the study provides the first global assessment of the ability of forests to adapt to climate change.

Manmade warming to date — about 0.7°C since the mid-19th century — has already slowed regeneration of tropical forests, and made them more vulnerable to fire, disease and insect infestations. Increasingly violent and frequent storms have added to the destruction. If temperatures climb even further, the consequences could be devastating, according to the report by the Vienna-based International Union of Forest Research Organisations (IUFRO).

“The current carbon-regulating functions of forests are at risk of being lost entirely unless carbon emissions are reduced drastically,” said Alexander Buck, IUFRO’s deputy director and coordinator of the report.

“With a global warming of 2.5°C compared to pre-industrial times, the forest ecosystems would begin to turn into a net source of carbon, adding significantly to emissions from fossil fuels and deforestation,” he said. The UN’s Intergovernmental Panel on Climate Change predicted in 2007 that average global temperatures would go up before 2100 by 1.1°C to 6.4°C, depending on efforts to curb the greenhouse gases.