Whether or not our planet’s rainforests are a net sink of carbon — meaning they sequester more than their destruction by human activities causes them to emit — is a much-debated issue.
Research released today suggests an answer, however: due to deforestation and forest degradation and disturbance, tropical forests in Africa, the Americas, and Asia now emit more carbon into the atmosphere than they sequester on an annual basis, according to scientists with the Woods Hole Research Center (WHRC) and Boston University.
Over the study period, the rainforests of Africa, the Americas, and Asia were found to have gained approximately 437 teragrams of carbon every year, but to have lost about 862 teragrams of carbon. That means they were a net source of some 425 teragrams of carbon annually.
Earth’s tropical forests store an incredible amount of carbon, and as they grow they are continually absorbing even more of the carbon in the atmosphere, which is why forest protection is considered such a crucial element of strategies to mitigate global climate change.
At the same time, however, deforestation is responsible for as much as 10 percent of total global carbon emissions.
Whether or not our planet’s rainforests are a net sink of carbon — meaning they sequester more than their destruction by human activities causes them to emit — is a much-debated issue. Research released today suggests an answer, however: due to complete deforestation as well as forest degradation and disturbance, tropical forests in Africa, the Americas, and Asia now emit more carbon into the atmosphere than they sequester on an annual basis, according to scientists with the Woods Hole Research Center (WHRC) and Boston University.
Using satellite imagery, laser remote sensing technology, and field measurements collected between 2003 and 2014, the researchers examined aboveground carbon density change not only in forests that were completely cleared, which previous analyses mostly focused on, but also in tropical forests impacted to a lesser degree by activities like selective logging, small-scale agriculture, and collection of wood for fuel in addition to natural causes of loss like tree mortality.
“It can be a challenge to map the forests that have been completely lost,” WHRC scientist and study co-author Wayne Walker said in a statement. “However, it’s even more difficult to measure small and more subtle losses of forest. 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.”
The team also looked at forest carbon gains in order to determine whether the study regions were a net source or sink. Over the study period, the rainforests of Africa, the Americas, and Asia were found to have gained approximately 437 teragrams of carbon every year, but to have lost about 862 teragrams of carbon. That means they were a net source of some 425 teragrams of carbon emissions annually, which the authors of the study note is more than the amount of carbon pumped into the atmosphere by all of the cars and trucks in the United States.
Alessandro Baccini, a scientist with WHRC and lead author of the study, said that the team’s findings serve as a “wakeup call” on the implications of tropical forest destruction and degradation in an era of manmade global climate change.
“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,” Baccini said in a statement. “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.”
Daniel Zarin, program director at the Climate and Land Use Alliance, said that Baccini et al.’s findings show that more attention needs to be paid to activities that degrade forests, even if they don’t lead to wholesale deforestation. “Illegal logging should top this list for many tropical forest countries where it is prevalent, pernicious, and linked to a host of other illegal activities ranging from land, labor, and human rights violations to drug trafficking, financial crimes, and corruption,” he told Mongabay.
“This is a significant finding, arrived at with a novel combination of remote sensing technologies,” Zarin, who was not involved in the study, added. “The details of the spatial analyses associated with these global estimates should be reviewed by governments and businesses interested in better understanding and reducing greenhouse gas emissions in their jurisdictions and sourcing regions.”
Nancy Harris, a research manager for the World Resources institute who was also not a member of the research team behind the study, told Mongabay that Baccini and colleagues are the first to quantify net carbon losses and gains in a geographically explicit manner.
“Tropical forest carbon dynamics are complicated, and answering the question about whether tropical forests are a carbon source or a sink has always been hampered by a lack of data. Studies have traditionally had to rely on very few ground measurements, which were then scaled up through massive extrapolation to conclude things about all tropical forests worldwide,” Harris said. “In contrast to that, these authors have developed innovative methods that takes advantage of different types of satellite imagery to essentially extend what we can measure on the ground in a few plots around the world to be able to say something about tropical forests that’s rooted in a more empirical basis than just a few ground plots.”
Harris noted that the focus of conservation initiatives under the UN’s Reducing Emissions from Deforestation and forest Degradation program (REDD+) have generally focused on deforestation, mainly due to the fact that the complete clearance of forests is easier to measure using the tools and methods we have available. “Emissions from forest degradation and enhancement of forest carbon stocks in standing and regrowing forests have been more difficult to measure, and this study sheds light on what we have generally failed to capture to date,” she added.
Uneven distribution of losses and gains across tropics
Baccini and team’s findings are generally in agreement with previous research that also found we are greatly underestimating emissions from forest degradation, leading to calls for a more concerted effort to design forest conservation interventions that specifically address forest degradation and disturbance. But there are a number of other important policy implications of the present research.
For instance, the pattern of forest carbon losses and gains across the tropics uncovered by the researchers could help guide any future efforts to rehabilitate degraded forests as countries seek to meet the climate goals they committed to by signing the Paris Agreement in December 2015. The best place to start implementing such policies might be in Latin America, home to the world’s largest remaining intact rainforest, the Amazon, given that the majority of losses — nearly 60 percent — occurred there. Another 24 percent occurred in Africa, while Asia’s tropical forests were responsible for just 16 percent.
About 70 percent of the losses in the Americas were the result of degradation and disturbance, showing just how important those drivers of forest carbon emissions really are. That’s even more true in Africa, where degradation and disturbance caused 81 percent of losses. (Asia is once again the statistical outlier in this measure, with less than half, or 46 percent, of its losses linked to degradation and disturbance.)
Latin America led the way with the most forest gained over the study period, at about 43 percent, whereas 30 percent of tropical forest gains were made in Africa and 26 percent in Asia.
“With this study, countries are now able not only to identify where degradation is taking place, but also, given the potential to now measure gains from growth, they can demonstrate their contribution to returning tropical forests to their more beneficial role as a carbon sink,” Baccini said. “We envision this tool improving the way countries across the tropics tackle the challenges of deforestation and degradation.”
- Baccini, A., Walker, W., Farina, M., Houghton, R., Carvalho, L., & Sulla-Menashe, D. (2017). Tropical forests are a net carbon source based on aboveground measurements of gain and loss. Science. doi:10.1126/science.aam5962
- Pearson, R.H.T., Brown, S., Murray, L., Sidman, G. (2017). Greenhouse gas emissions from tropical forest degradation: an underestimated source. Carbon Balance and Management, 12(3). doi:10.1186/s13021-017-0072-2
Follow Mike Gaworecki on Twitter: @mikeg2001