These unprecedented levels of greenhouse gases are creating a new, hotter planet with weather that is much more extreme than in the recent past.
What does this mean for the 20 percent of the Earth's original forests that are still standing? Some scientists believe forests will grow faster in a warmer world. Others say they are more likely to burn, or suffer from disease or die from drought.
Laurance and his colleagues have shown that the higher levels of carbon dioxide in the atmosphere are changing the very nature of the existing forest in the Amazon.
"Trees in the rainforest are growing faster and dying faster, and changing in species composition," he said, adding that the long-term implications of these changes are not known.
Researchers predict that the Amazon region will become hotter and drier, much like last year's record drought where Amazonian rivers dried up and wildfires burned large areas of the surprisingly dry forest.
Rainforests are very vulnerable to consecutive years of drought, the U.S.-based Woods Hole Research Center warned recently. The Amazon cannot withstand more than two consecutive years of drought without breaking down, reported Woods Hole researchers in Santarem, a city on the Amazon River in Brazil.
One of the reasons is that rainforests are rain-making machines. About half of all the rainfall in the Amazon is almost immediately returned to the atmosphere as water vapor via plant respiration. That helps to maintain cloud cover and produce frequent rainfall, especially in the dry season when forests are most vulnerable to droughts and fires, says Laurance.
If the forests dry out too much, they cannot put water vapor into the air, creating a cycle of less and less precipitation.
"In the Amazon, some models suggest that the system could destabilize once more than 30 percent of the forest is lost," said Laurance, acknowledging that this idea is conjecture.
Brazilian scientist and climate expert Carlos Nobre, of the National Institute of Space Research (INPE), has said that 40 percent is the tipping point where the world's greatest forest will irreversibly turn into savanna. About 17 percent of the Amazon is already gone.
Although much smaller in area, the cloud forests of the Andes Mountains contain nearly twice as many plant species and four times as many endemic plants as other parts of the world. Conditions there are harsh, with cool to cold temperatures and poor soils, so plants grow slowly. Scientists believe that most plants there will not be able to survive the rising temperatures already under way -- which may climb five degrees Celsius by 2100.
"Trees and plants can't move up mountains very far because of the increasingly poorer soils," says Andreas Hamann, a forestry expert at the University of Alberta, Canada.
Species migration for trees can be as slow as a few meters percentury, but temperature hikes in North America would require a northward shift of 150 kilometers to 550 kilometers for many existing forest ecosystems by 2100.
Rapidly changing temperatures are pushing forest ecosystems out of equilibrium, says Hamann, who recently completed studies on the impacts of climate change on Canadian forests. "It could take 2,000 years to re-balance once temperatures stop climbing."
However, "it's not the temperature rise that is the big problem for forests in the short term, it is the changes that come with higher temperatures," Hamann explains.
Higher temperatures are changing weather patterns and producing more extremes, including longer droughts and huge forest fires.
In 2003 alone, Siberia lost 40,000 square kilometers of boreal forest to fires. Alaska and Canada experienced their worst fire year ever in 2004.
Canadian Forest Service researchers say 2.6 million hectares are being lost to fire every year, a huge increase from the more than 1 million hectares lost in the early 1970s. They have predicted that climate change will create still drier conditions in Canada's and Russia's northern boreal forests, making future increases in fires a virtual certainty.
Nadezhda Tchebakova, of the Russian Academy of Sciences Forest Research Institute, goes further, predicting that the boreal forests will become so dry by 2090 they will turn into steppe, or grassland.
But climate change will not be the end of forests.
Reforestation and natural regeneration have dramatically increased the amount of forest in at least 22 countries, according to the report published Nov. 13 by the Proceedings of the National Academy of Sciences. China and India, for example, have more forest cover than they did 15 years ago. Much of the U.S. Northeast is nearly all forest now, when 50 to 100 years ago it was farmland.
"Demand for paper and wood products are down and there is an increased interest in reforestation," says study co-author Jesse Ausubel, of Rockefeller University in New York. These new forests do not have the biodiversity of original, old-growth forests, but they "offer the chance for biodiversity to return."
This story is part of a series of features on sustainable development by IPS (Inter Press Service) and IFEJ (International Federation of Environmental Journalists).
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