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The Amazon Dieback Scenario

The Amazon Dieback Scenario

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.

That was a startling possibility for many reasons, not the least being that the Amazon is the richest single ecosystem left on the planet, and functioning as a major carbon sponge is only one of the critical roles it plays. Might the Amazon really die as a direct consequence of human-induced climate change?

The Nature paper set off a decade of research to pin down a better answer. Many scientists believe that the original paper made some overly pessimistic assumptions about temperature changes in the region and used a relatively crude representation of the Amazon forest.

Some groups came to the opposite conclusion from the Cox group, finding in their forecasts that the Amazon forest would remain robust through the coming century. Yet other papers supported the view that Amazon dieback was a real possibility, even if it might happen somewhat later in the century than predicted by the Cox group. The question remains unresolved.

It took on a new urgency in 2005, however, when a severe drought hit the Amazon region, killing many large trees. In 2010, there was an even larger drought with potentially worse damage - two "once a century" droughts just five years apart. The 2010 drought is still under study; some evidence suggests that the 2005 drought was linked to high Atlantic Ocean temperatures that may in turn be linked to human emissions of carbon dioxide.

The droughts raise a disturbing question: Could the great dieback predicted for midcentury already be starting?

Scientists do not know. They say the effects of the two droughts are likely to be transient, but only if similar events do not recur anytime soon. Oliver L. Phillips, a researcher at the University of Leeds, led a team that documented a huge loss of carbon in the Amazon because of the 2005 drought. "The most likely outcome is that the forest will gain all that carbon back, and then some," he said in an interview.

But he and other scientists say that if the Amazon starts experiencing such droughts every few years, all bets are off.

"It’s a worrisome moment for the Amazon," said Daniel C. Nepstad, an American scientist working at the Amazon Environmental Research Institute in Brazil to understand the pressures on the forest. "This is either just a big coincidence that we had these two severe droughts in close sequence, or it is a sign of things to come."

The dieback possibility has heightened the urgency that some policy makers and environmentalists feel about stopping deforestation in the region, an enormous problem that began decades ago. The rate of forest destruction has declined from its high point in the 1990s and early 2000s as a result of several factors, including stronger efforts by the Brazilian government to control the Amazon frontier.

But the level is still viewed as worrisome. Deforestation is a clear and present danger to the Amazon in itself, and the rising prices of soy and other commodities may add to the pressure by spurring new bouts of forest clearing.

In the majority of computer projections, continued human destruction of the forest tends to push the climatic balance in the region in a direction that makes the dieback situation more likely. Dr. Nepstad has led research suggesting that even without any feedback effects from climate change, half the Amazon forest could be destroyed or badly degraded by 2030 through interactions between human destruction, fire and drought unless stronger efforts are made to protect it.

The most ambitious proposal to slow deforestation, not just in the Amazon but throughout the tropics, goes by the acronym REDD, which stands for a mouthful of a program name: the Program on Reducing Emissions From Deforestation and Forest Degradation in Developing Countries.

It is basically a commitment by rich countries to pay countries in the tropics serious cash if they can slow their deforestation rates, with the results monitored intensively by satellites and airplanes before the big checks are cut.

But REDD has been slow to get off the ground. Rich countries have pledged nearly $5 billion so far, but that is considered just a down payment.

The original theory was that the rich countries would set up carbon trading schemes that would send some money abroad for forest preservation. "There was this expectation that there was going to be a whole lot of money flowing," said Ralph Ashton, head of the Terrestrial Carbon Group, which has worked on the issue.

Yet most European countries ultimately balked at sending money abroad, and climate legislation in the United States stalled amid fears about the economic and political repercussions. So it is not at all clear that REDD will ever get launched in anything like the ambitious form that was originally envisioned.

As I mentioned in my article, the state of California might allow some of its industries to comply with a forthcoming global warming policy by steering money into tropical forest protection, but even that is no certainty.

"If this linkage doesn’t take place, then the hopes for REDD to survive could be greatly diminished," Dr. Nepstad said. If California does commit to tropical forest protection, advocates of REDD hope other states and countries, and perhaps large companies that buy food commodities, will follow suit, creating a substantial flow of funds.

Even paying for the science needed to understand what is happening in the Amazon has been a challenge. One of the most significant sources of money has been the Gordon and Betty Moore Foundation, of Palo Alto, Calif., which has committed several hundred million dollars to scientific and conservation work in the region. See this project as an example. This write-up by an old classmate of mine, the Nature reporter Jeff Tollefson, offers a deeper look at a project supported by the Moore Foundation and led by the scientist Gregory P. Asner.

REDD, in addition to fomenting suspicions on the right about another big United Nations program, has picked up critics on the left who fear it will be hijacked for pay for tree-farming projects that actually harm real forests and may also undermine the rights of indigenous forest people.

Whether its advocates can steer REDD through this minefield and get it to work remains to be seen. And even if they do, will the Amazon forest - and forests in the rest of the world - remain resilient as human emissions accelerate in coming decades?