Agricultural expansion a major cause to doubling of annual tropical carbon loss over past two decades

Using multiple high-resolution satellite datasets, researchers from the Department of Civil Engineering at the University of Hong Kong (HKU) and Southern University of Science and Technology (SUSTech) found that tropical carbon loss has doubled over the past two decades due to excessive forest removal in the tropics.

The tropics are an important ecosystem as they store massive amounts of carbon in their woody vegetation and soil -- but they have suffered from extensive forest clearance since 2001. The researchers analysed the gross forest carbon loss associated with forest removal in the tropics (between 23.5° N and 23.5 S but excluding northern Australia) during the 21st century. They revealed a two-fold increase in gross tropical forest carbon loss worldwide from 0.97 gigatons of carbon per year in 2001-2005 to 1.99 gigatons of carbon per year in 2015-2019 due to rapid forest loss.

The study has been published in the academic journal Nature Sustainability in an article entitled "Doubling of annual forest carbon loss over the tropics during the early twenty-first century."

Given the key role of the tropics in the carbon cycle, the study poses serious implications. "The findings are critical because they suggest that existing strategies to reduce forest loss are questionable; this failure underscores the importance of monitoring deforestation trends following one of the new pledges made -- to halt and reverse deforestation -- by UN climate summit-the twenty-sixth Conference of the Parties (COP26) in Glasgow in November 2021," said Professor Ji CHEN from HKU's Department of Civil Engineering.

Tropical forests are the largest terrestrial component of the global carbon cycle, storing about 250 gigatons of biomass carbon in its woody vegetation and absorbing about 70 gigatons of atmospheric carbon per year through photosynthesis. The rapid and steady loss of forests could be devastating because it leads to the loss of stored carbon in biomass and soil. Deforestation also obstructs carbon sequestration or the process of capturing and retaining carbon dioxide.

"The doubling and acceleration in the loss of forest carbon, including biomass and soil organic carbon, is primarily driven by agricultural expansion which differs from current estimates of land-use change emissions in the assessments of the global carbon budget that shows a flat or decreasing trend. In addition to carbon, conversion of forests to agricultural lands also induces other environmental consequences, like biodiversity extinction and land degradation," said Yu FENG, a PhD candidate of the HKU and SUSTech joint programme.

Most of the tropical forest carbon loss (82%) was set off by agricultural expansion, for example shifting cultivation, particularly in Africa.

"While some agricultural lands may reappear as forested due to abandonment or policies, we still observed about 70% of former forest lands converted to agriculture in 2001-2019 remained so in 2020, confirming a dominant role of agriculture in long-term pan-tropical carbon reductions on formerly forested landscapes," said research team member Dr Zhenzhong Zeng, Associate Professor at SUSTech.

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