Tropical forests no longer a carbon sink

The world’s tropical forests are often described as the planet’s lungs; but now, due to human activity those lungs are emphysemic, with new research indicating they are no longer the globe’s great carbon sink. Instead, the rate of forest destruction, degradation and disturbance means they are emitting more carbon than they capture. 

This finding, by researchers from the Woods Hole Research Center and Boston University, both in Massachusetts, is based on developing more finely tuned measuring tools using satellite monitoring. As a result they have been able to make a far more accurate diagnosis of the global health of tropical forests, which they have published in a paper in Science.

The diagnostic tool involved using NASA light detection and ranging (LiDAR) data to calibrate a machine-learning algorithm to make carbon-stock estimates of individual patches of forest approximately 21 hectares in size. Then, using 12 years of NASA Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery, net gains or losses in carbon storage were calculated over time.

This new method means satellites can be used not just to identify where forests have been completely cleared but also the biomass lost in forests still standing.

Importantly, the analysis shows the clear dangers of deforestation – long recognised by the public and policymakers – are now overshadowed by the more subtle consequences of forests being degraded and disturbed. According to their calculations, degradation and disturbance account for 69% of total carbon losses from the world’s tropical forests.

“It can be a challenge to map the forests that have been completely lost,” says one of the paper’s authors, Wayne Walker, a scientist with WHRC. “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.”

By providing a far more accurate picture of the state of tropical forests and their stunted role in the global carbon cycle, the findings are significant, striking and credible, says Pep Canadell, a climate scientist with Australia’s CSIRO and  executive director of the Global Carbon Project.  

Bill Laurance, the director of the Centre for Tropical Environmental and Sustainability Studies based at James Cook University in Cairns, agrees: “It’s an important and interesting analysis.” 

The conclusion that tropical forests are now emitting carbon rather than removing it from the atmosphere is due to the carbon in the forest biomass lost being greater than the carbon that can be absorbed by the remaining forest. 

The paper notes that removed biomass may not necessarily be immediately released back into the atmosphere, as would occur if a tree was burnt. “Above-ground biomass may first transition to other carbon pools or be removed from the forest without release to the atmosphere.” But temporary carbon storage via the use of a tree trunk for wood products such as furniture and construction materials, they calculate, constitutes just 4-14% of losses. 

Bottom line: a tree lost in a forest stops being a carbon absorber and, sooner or later, ends up emitting its stored carbon, either quickly through incineration or slowly through decomposition. 

The paper’s lead author, Alessandro Baccini, says the study should allow countries not only to identify where degradation is taking place, but also, with the potential to better measure gains, “demonstrate their contribution to returning tropical forests to their more beneficial role as a carbon sink”.