Rainforests are too degraded to act as carbon sinks any longer, a new paper reports. Averaged across the globe, rainforests now have a positive output of greenhouse gases, prompting the authors to call for urgent conservation efforts that will allow rainforests to re-don the mantle of carbon sinks.

The team, composed of scientists at the Woods Hole Research Center and Boston University, took a different approach in assessing the health of rainforests. Unlike previous research, which generally focused on deforestation (complete removal of the forests), they worked to account for more subtle changes in the form of disturbance and degradation, both natural and anthropic. These changes include small-scale tree mortality or removal, or forest gains through natural or human-assisted growth.

Sadly, they report that when taking such changes in forest density into account, tropical forests lose their ability to act as net carbon sinks, meaning they emit more carbon that they can capture.

Net producers

The study quantified changes in aboveground forest carbon across tropical America, Africa, and Asia. These areas were selected as the sheer scale of their rainforests provide the greatest ability to act as carbon stores. They’re also the most biodiverse places on the planet, providing a wealth of ecosystem services such as food, fuel, and materials to millions of people — meaning they see a lot of human activity.

The team used 12 years’ worth of satellite imagery (taken between 2003-2014), laser remote sensing technology, and measurements taken in the field to calculate losses in forest carbon from flat-out deforestation as well as the more subtle and fine-grain degradation and disturbance processes, which have previously remained unaccounted-for over large swaths of rainforest. Their findings point to a worrying, death-by-a-thousand-cuts scenario playing out in Earth’s richest ecosystems.

Overall, tropical regions have become a net source of atmospheric carbon, they report. Forests saw an increase in capture power of roughly 437 teragrams of carbon annually (expressed as ‘carbon gains’), but losses amounted to a whopping 862 teragrams — meaning rainforests contribute a roughly 425 teragrams of atmospheric CO2 yearly. Each teragram is equivalent to one trillion grams, one million metric tons, or 1.102.331 short tons. To put that number into context, China and the US emitted some 10,600, respectively 5,100 teragrams of CO2 in 2015 (29.5% and 14.3% of world emissions).

“Gains result from forest growth; losses result from deforestation and from reductions in carbon density within standing forests (degradation/disturbance), with the latter accounting for 68.9% of overall losses,” the team writes.

“In many cases throughout the tropics you have selective logging, or smallholder farmers removing individual trees for fuel wood. These losses can be relatively small in any one place, but added up across large areas they become considerable,” said WHRC scientist Wayne Walker, one of the paper’s coauthors.

Losses and gains aren’t evenly distributed, however. On a by-continent basis, the majority of losses occurred in Latin America (some 60% of loss), in the Amazon forest. Some 24% of loss was seen in Africa, and Asia experienced the least share of total losses, with a little over 16%. Degradation and disturbance were responsible for the lion’s share of continental losses in both the Americas (70% of losses) and Africa (81%), but under half (46%) in Asia. Gains were also predominantly centered in the Americas with nearly 43% of total gains, followed by Africa with 30%, and lastly Asia with 26%.

Such results are worrying, especially at a time when governments around the world are scrambling to meet their commitments to the Paris Agreement and curb climate change. The authors note that ending deforestation, degradation, and disturbance in the tropics and allowing the ecosystem to regrow would cut at least 862 teragrams of carbon per year, some 8% of global emissions. The UN already has a project set in place to help preserve natural carbon sinks — the REDD+ (Reducing Emissions from Deforestation and Forest Degradation), which offers incentives for countries to maintain forests intact. However, it depends on regular access to accurate measurements of incremental gains and losses in forest carbon density, and research such as this one will give us a better understanding of how forests function.

“These findings provide the world with a wakeup call on forests,” said WHRC scientist Alessandro Baccini, the paper’s lead author. “If we’re to keep global temperatures from rising to dangerous levels, we need to drastically reduce emissions and greatly increase forests’ ability to absorb and store carbon.”

“Forests are the only carbon capture and storage ‘technology’ we have in our grasp that is safe, proven, inexpensive, immediately available at scale, and capable of providing beneficial ripple effects — from regulating rainfall patterns to providing livelihoods to indigenous communities.”

The paper “Tropical forests are a net carbon source based on aboveground measurements of gain and loss” has been published in the journal Science.

Enjoyed this article? Join 40,000+ subscribers to the ZME Science newsletter. Subscribe now!