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Title: Tracking the shift of tropical forests from carbon sink to source
Date: 31-Jul-2018
Category: Climate Change
Source/Author: Mongabay.com
Description: Currently, tropical forests take up roughly the same amount of carbon as is released when they’re cleared or degraded. But climatic changes, which lead to more droughts and fires resulting in the loss of tropical trees, could shift the balance, making tropical forests a net source of atmospheric carbon.

  • Improved maps of carbon stocks, along with a better understanding of how tropical forests respond to climate change, are necessary to meet the challenge of keeping the global temperature below a 2-degree-Celsius (3.6-degree-Fahrenheit) rise, according to scientist Edward Mitchard of the University of Edinburgh.
  • Currently, tropical forests take up roughly the same amount of carbon as is released when they’re cleared or degraded.
  • But climatic changes, which lead to more droughts and fires resulting in the loss of tropical trees, could shift the balance, making tropical forests a net source of atmospheric carbon.

Scientists seeking the tools necessary to tackle climate change need to better understand both the current interplay between carbon and tropical forests and how those forests will respond as temperatures warm.

That’s one of the conclusions of a recent review by University of Edinburgh-based scientist Edward Mitchard, published July 25 in the journal Nature.

“Predicting how tropical forests will affect climate is a complex challenge — we do not know how climate will affect forests, nor if countries will meet their commitments to safeguard them,” Mitchard said in a statement. “Worryingly, research indicates that forests could soon stop counteracting warming, and instead become a major source of greenhouse gas.”

The 2015 Paris climate accord centers on the hope that humans could limit the global temperature rise to less than 2 degrees Celsius (3.6 degrees Fahrenheit) with a multi-pronged strategy that includes stopping deforestation and degradation in the tropics. The belt of equatorial forests in Africa, South America and Southeast Asia typically gets credit for siphoning off anywhere between 25 and 33 percent of global carbon emissions each year.

But at this point, scientists also figure that the continued loss and destruction of forest likely results in about as much carbon released into the atmosphere as they take up. And right now, they don’t know exactly how long that balance will last, leading concerns that we could be reaching a tipping point.

“Considering all sources of evidence, it appears probable that as the intact forest sink declines in size, tropical forests are in the process of switching from being approximately neutral to being a net source of carbon,” Mitchard writes.

“With both forest loss and climate change likely to accelerate over the 21st century, tropical forests are likely to release ever more carbon, which will make limiting global warming to less than [2 C] above pre-industrial levels very difficult,” he said.

Part of the challenge is predicting how a future with climate change will impact tropical forests. On the one hand, higher carbon dioxide levels in the atmosphere could encourage trees to grow bigger and faster, and the right set of policies in place that limit deforestation and promote forest regrowth could tip the balance back toward tropical forests as a net carbon sink.

On the other hand, warmer temperatures also encourage spikes in carbon-releasing respiration by plants and the soil and in tree-destroying fires and drought. If the need for more land for agriculture and livestock ranching to feed a growing human population, for building materials and for mines continues to drive deforestation, then a future in which forests emit more carbon than they pull out of the air is more likely.

Mitchard called for long-term experiments in which scientists turn up the temperatures on large plots of forests and measure the responses.

He said more detailed maps of carbon stocks would also help zero in on the nuanced movements of carbon. While satellite maps have helped researchers keep an eye on deforestation, peatlands aren’t as well mapped out. In part, that’s due to the difficult legwork involved in measuring the huge amounts of carbon they stash away. Scientists believe that, despite covering only about 5 percent of the area of tropical forests, peatlands hold as much as 65 percent of the total carbon that tropical trees do.

Already, Mitchard said, a lot of tropical countries have been working on bolstering their monitoring efforts as part of a strategy for “reducing emissions from deforestation and forest degradation, and the role of conservation, sustainable management of forests and enhancement of forest carbon stocks in developing countries.” REDD+, as it’s known, involves payments to these countries (typically from wealthier, more developed countries) to maintain the carbon stocks held in their forest. More precise maps will only further support the goals of this approach.

Models for predicting the future of the climate must also account for the continued demand that drives deforestation. Some countries, such as Brazil, have made strides toward addressing those drivers, but the country also offers a cautionary tale about potential shifts in policy and the prices of agricultural goods.

By 2012, the country had slashed its deforestation rate by 76 percent compared to what it was in 1990. Brazil’s leaders were lauded for shepherding the country’s share of the world’s largest rainforest back from the brink of destruction with targeted policies, aided by falling commodity prices, to keep forests standing. But the recent introduction of pro-development policies has put Brazil’s deforestation rates — and carbon emissions — back on an upward trajectory.

That trend has scientists like climatologist Antonio Donato Nobre of Brazil’s National Institute of Amazonian Research concerned about the coming changes to the Amazon rainforest — changes that ultimately could play out on a global stage.

“We are beginning to see extensive release of carbon from climate-related forest dieback,” Nobre told Mongabay in 2017. “So, from a precious net sink of carbon, the green ocean Amazon might very soon become a nightmarish carbon source.”

Banner image of a buttress tree in Trinidad by Edward Mitchard/University of Edinburgh.


Mitchard, E. T. A. (2018). The tropical forest carbon cycle and climate change. Nature, 559(7715), 527–534.

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