SINGAPORE - The world's forests are still soaking up billions of tonnes of planet-warming carbon dioxide (CO2) every year, a global study has found, despite millions of hectares being burned and cleared for agriculture.
The findings show that forests remain a key brake on the pace of climate change by locking away large amounts of CO2 from industry, power stations and cars even after decades of destruction.
But the analysis shows that some forests, especially those in South-East Asia and the Amazon, are in trouble, becoming major sources of greenhouse gas emissions. It is a situation that could worsen unless land clearing and fires are rapidly reined in. Failure to do so could significantly undermine global efforts to fight climate change, scientists say.
The study, published late on Thursday (Jan 21) in the journal Nature Climate Change, involved researchers from US space agency Nasa, the World Resources Institute(WRI), the Centre for International Forestry Research and others.
To get a better idea of the role of forests in regulating global CO2 emissions, they came up with a method of calculating how much of the gas forests soak up naturally every year and how much of it is produced through deforestation, fires, clearing and draining peatlands and other disturbances.
The researchers created a map of greenhouse gas emissions (sources) and CO2 removals (sinks) from global forests at a resolution of 30m by 30m between 2001 and 2019.
They found that the world's forests sequestered about twice as much CO2 as they emitted between 2001 and 2019. This "carbon sink" totalled a net 7.6 billion tonnes of CO2 per year, 1.5 times more than the United States emits annually.
In all, forests absorbed about 16 billion tonnes of CO2, or about 30 per cent of mankind's greenhouse gas emissions.
But deforestation, burning and other damage released more than 8 billion tonnes of emissions every year, the researchers found. And that has been taking a toll on some of the world's richest and largest carbon stores.
"Over the past 20 years, forests across South-east Asia have collectively become a net source of carbon emissions due to clearing for plantations, uncontrolled fires and drainage of peat soils," co-authors Nancy Harris and David Gibbs of WRI said in a blog post.
The region has the world's third largest stretch of tropical rainforest, yet these forests now emit a net 490 million tonnes of CO2 a year.
For the Amazon too, the picture is grim.
"The Amazon River basin, which stretches across nine countries in South America, is still a net carbon sink, but teeters on the edge of becoming a net source if forest loss continues at current rates," Dr Harris and Mr Gibbs said.
Over the past four years, clearing for cattle pasture and degradation from fires have caused a big jump in deforestation and degradation in the Amazon Basin.
According to the study, the Amazon now locks away a net 100 million tonnes of CO2, or roughly twice Singapore's annual CO2 emissions, but is also a huge source of emissions.
Of the world's three largest tropical rainforests, only the Congo Basin in Africa has enough standing forest left to remain a strong net carbon sink, sequestering 600 million tonnes more CO2 per year than it emits.
"Protecting the remaining forests in all three regions is critical to mitigating climate change," Dr Harris and Mr Gibbs said.
Overall, they said, the maps show that keeping existing forests standing remains "our best hope for maintaining the vast amount of carbon forests store and continuing the carbon sequestration that, if halted, will worsen the effects of climate change".