A tract of Amazon rainforest, which has been cleared by loggers and farmers for agriculture, is seen near the city of Santarem, Para State April 20, 2013. Scientists worry that forest degradation due to drought could surpass deforestation as the biggest source of carbon emissions from the Amazon. REUTERS/Nacho Doce

When thinking about greenhouse gas emissions, there are a few obvious sources that come to mind, such as cars or coal-burning plants. But there are also some huge natural reservoirs of carbon on Earth, known as “carbon sinks,” which could pour huge amounts of carbon dioxide into the atmosphere if they were unleashed. The world’s forests are one major example.

Trees store large amounts of carbon while they’re alive — but when they die, they release all that carbon back into the atmosphere. So as concern over global climate change continues to grow, preserving the world’s forests is also becoming a greater priority. And perhaps no area has posed more concern for scientists than the Amazon rain forest.

While the Brazilian government, in particular, has taken major steps to cut down on agriculture-related deforestation in the Amazon, climate change itself has become a serious threat to the trees. Rising temperatures will likely cause widespread drought throughout the Amazon basin, which could cause forests to degrade and release their carbon stores.

That’s the message of a study published today in Proceedings of the National Academy of Sciences, which explores the extent to which drought will increase in the Amazon throughout the century and finds that the total area affected by severe drought will triple by the end of the century.

There have been three severe droughts in the Amazon in the past decade — one in 2005, one in 2010 and another one currently ongoing — that helped to inspire the study, said lead author Philip Duffy, currently president of the Woods Hole Research Center and a visiting scientist at the Carnegie Institution for Science at the time the study was initiated. Research estimated that the amount of carbon released as a result of the 2005 drought was more than the annual emissions of Europe and Japan combined, and the 2010 drought’s effects were even more severe, Duffy said. “So if there’s going to be more and more drought, it has the potential to really make the Amazon a big source of carbon to the atmosphere,” he said.

The release of carbon from forests has the potential to contribute to a kind of climate feedback loop: Warming temperatures cause drought, which kills trees and releases carbon into the atmosphere, which contributes to greater warming, which brings about more drought — and so on. So coming up with accurate projections for the future is important for gaining insight into not only the future of the Amazon, but the future of the world’s climate in general.

In this case, the researchers used models, assuming a business-as-usual trajectory for greenhouse gas emissions (meaning they assumed greenhouse gas emissions will stay on their current levels throughout the century), to project future precipitation patterns in the Amazon. They found that, overall, mean annual precipitation will decrease in the eastern Amazon, while it will increase in the western Amazon.

However, it’s important to note that the eastern Amazon composes about three quarters of the Amazon basin, so precipitation declines will be much more widespread than precipitation gains. “Different things are going on in different regions, but they don’t balance out by any means,” Duffy said.

The regional differences largely stem from differential warming in the Atlantic Ocean versus the Pacific, Duffy noted. Sea surface temperatures can help drive weather patterns, so if the ocean on one side of the Amazon basin warms differently than the ocean on the other side, it makes sense that the two regions would see different precipitation responses.

Altogether, the models predict that the area affected by mild drought will double and the area affected by severe drought will triple by 2100. Additionally, the Amazon could see an overall increase in not just drought, but also periods of extreme wetness — in other words, a general increase in precipitation extremes. In fact, the models show that some areas which will be more greatly affected by drought in the future will also be more greatly affected by extreme rainfall (although, naturally, these two extremes won’t occur at the same time).

The paper “generally fits well with what we know already of how the Amazon is currently changing,” said Oliver Phillips, chair in tropical ecology at the University of Leeds School of Geography (who was not involved in the study), in an e-mail to The Post.  “It is also consistent with what earlier modeling studies have indicated for this century, but goes beyond in also showing how periods of both drought and wetness may increase (and at different rates in different locations).”

But while it might seem like extreme rainfall during one part of the year could balance out extreme drought at a different time, precipitation extremes are just bad for forests in general, Duffy said. Extreme rainfall can cause floods, which can be damaging. And it also tends to be difficult for soil to capture because it runs off so quickly. That means all the extra water might not actually help offset a drought because it doesn’t get adequately absorbed into the soil.

When considering the ecological effects of these precipitation extremes, “we can expect both winners and losers,” Phillips said by e-mail. “Diversity is so high (with more than 15,000 tree species alone for example) that predictions for individual species are almost impossible, but those which prefer it wet year-round are going to be challenged (if not already).”

As far as the implications for the trees’ carbon storage abilities go, Phillips said the Amazon’s resilience is still poorly understood. “Droughts kill trees,” he said. “But in the long-term, how well the Amazon forests survive and thrive will also depend on whether or not the dry-adapted species can ‘step up to the plate’, and how much the rising levels of carbon dioxide also help to buffer the impacts of moisture stress for plants.”

However, since past droughts have caused such impressive releases of carbon, the issue is still a great concern. And severe drought also has the potential to indirectly kill off trees through other related consequences — for example, wildfires may become more common as the Amazon continues to dry out.

And there are other factors that can affect forest health that this study didn’t take into account. For instance, this study only included projections for precipitation patterns — but, as Duffy pointed out, continued warming could also decrease the amount of moisture already present in the soil, which would only exacerbate the drying effects caused by declines in rainfall.

The optimistic point in all of this is that the study assumes a business-as-usual trajectory, meaning that if the world is able to implement a more aggressive mitigation strategy in the coming decades, some of these effects might not end up being so severe. With so much work already going into the preservation of the Amazon, it’s an important motivation for keeping global climate change under control.

“What’s worrisome is that if drought becomes more frequent, then all of the work on preventing deforestation could be swamped by just the huge emissions from these droughts, which become really difficult to control,” Duffy said. “Because, really, the only way to control them is to reduce global greenhouse gas emissions.”