The Amazon rainforest dieback scenario
By JUSTIN GILLIS
7 October 2011 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. Scientists are out in the field trying to gain a better understanding of how trees and forests respond to changes in climate. And they are using that knowledge to devise more sophisticated computer projections that narrow the range of possible outcomes. One of the scarier possibilities to emerge from this body of work is worth dwelling on a bit, simply because it would be so devastating if it came to pass: the so-called Amazon dieback scenario. Many scientists were deeply skeptical of the idea when it was first published, but events in the last few years have made them less dismissive. The scenario emerged most clearly in computer analyses in Britain led by Peter M. Cox of the University of Exeter and published in 2000 as a paper in the journal Nature. Running a large-scale computer simulation in which forests interacted with a changing global climate through the course of the 21st century, the Cox group found that forests would continue to take up carbon until about 2050. But then, their computer predicted, warmer temperatures and water stress would cause a huge dieback of the Amazon forest, which would stop absorbing carbon and start emitting it as a result. […]
