Tropical forests now emit more carbon than they soak up

Boston

The world's tropical forests are so degraded that they now emit more carbon into the atmosphere than they capture, countering their role as a carbon sink, according to a study that highlights the urgent need for reducing greenhouse gas emissions.

Researchers at the Woods Hole Research Center (WHRC) and Boston University in the US found that tropical regions are a net source of about 425 teragrams of carbon annually, which is more than the emissions from all cars and trucks in the US.

A new, cutting-edge approach to measuring changes in aboveground forest carbon density helped scientists determine that tropical forests have undergone widespread deforestation, degradation and disturbance.

Previous measurements of forest carbon loss focused largely on areas subject to complete forest removal (deforestation).

This is the first time, however, that scientists have been able to account for changes from subtle natural and human-caused losses (degradation and disturbance) such as small-scale tree removal and mortality while also measuring gains from forest growth.

The findings add new urgency to the critical need for aggressive global and national-scale efforts to reduce greenhouse gas emissions in order to meet the climate goals of the Paris Agreement.

The study suggests there is a critical window of opportunity to reverse the trend in emissions by halting deforestation and degradation, and actively restoring forests to degraded lands.

The study quantifies changes in aboveground forest carbon across tropical America, Africa and Asia - the most threatened forests in the world — and those with the greatest ability to act as significant carbon stores as well as globally recognised hotspots of biodiversity and essential ecosystem services including food, fibre, and fuel.

"These findings provide the world with a wakeup call on forests," said WHRC scientist Alessandro Baccini, lead author of the study published in the journal Science.

"If we are 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.

Using 12 years (2003-2014) of satellite imagery, laser remote sensing technology and field measurements, Baccini and colleagues were able to capture losses in forest carbon from wholesale deforestation as well as from more difficult-to- measure fine-scale degradation and disturbance.

Gross annual losses were about 862 teragrams of carbon and while gains were approximately 437 teragrams of carbon.

Losses and gains of carbon are not evenly distributed across the tropical belt, the report finds. On a continental scale, the majority of the loss (nearly 60 per cent) occurred in Latin America, home to the Amazon - the world's largest remaining intact rainforest.

Nearly 24 per cent of the loss is attributable to Africa while the forests of Asia experienced the least losses — a little more than 16 per cent of the tropical total. PTI