Forests moderate global warming by pulling carbon dioxide out of the atmosphere. But scientists can't predict whether warming will cause major forests to die back, causing a vicious cycle that will make global warming worse.

"In an article last weekend about rising stress in the world’s forests, I briefly mentioned that computer projections regarding the future of forests are still in a primitive state.

Scientists cannot really say whether trees will continue to take up a big proportion of our carbon emissions through the rest of this century, or whether they will instead succumb to climate change on a large scale.

You can find reports in the scientific literature to support both outcomes and every prospect in between. Which prediction is right has big implications for how fast carbon dioxide will build up in the atmosphere, and therefore for how fast the climate will warm. The stakes are also high for many beloved landscapes in the United States.

“Is it true that the forests of the Eastern U.S. will continue to take up carbon?” said Paul R. Moorcroft, a Harvard professor who framed many of the issues for me. “What will happen in the West if, as predicted, the climate continues to warm and becomes increasingly arid, as it has done over the past couple of decades?”

The difficulty of predicting the future of forests under a rapidly changing climate means it is hard to know what to make of the current signs of distress."

Justin Gillis reports for the New York Times' Green blog October 7, 2011. [2]