Yangambi Biosphere Reserve in the Democratic Republic of Congo. Ahtziri Gonzalez

While tropical forests remain one of the world’s most important weapons in the fight against climate change, their ability to remove carbon from the atmosphere is decreasing, shows a new study published today in the journal Nature.

Researchers from almost 100 academic institutions studied the Amazon and the Congo Basin forests for three decades, observing a sharp decline in their carbon sink that started around 20 years ago. While intact tropical forests removed 17 percent of human-made carbon dioxide emissions in the 1990s, this figure was reduced to 9 percent in the 2000s, and just 6 percent in the 2010s.

The main reason behind this phenomenon is dying trees, according to the scientists. While extra carbon dioxide from man-made emissions boosts tree growth, this effect is being increasingly countered by negative climate change effects such as higher temperatures and droughts.

The authors also warn that unless action is taken, tropical forests’ carbon sink will continue to rapidly weaken, and some of them, particularly in the Amazon, might even become a carbon source in the mid-2030s.

“This study provides the first large-scale evidence that carbon uptake by the world’s tropical forests has already started a worrying downward trend,” explained Wannes Hubau, scientist at the Royal Museum for Central Africa (RMCA) and lead author.

Tropical forests are still huge reservoirs of carbon, storing 250 billion tones in their trees alone. “But the study reveals that unless policies are put in place to stabilize Earth’s climate, it is only a matter of time until they are no longer able to sequester carbon,” said Simon Lewis, a professor at Britain’s University of Leeds and senior author.

These findings are of utmost importance for policymakers, as a smaller tropical forest carbon sink means that carbon budgets, which are the cumulative amounts of carbon dioxide emissions permitted over a period of time to remain within temperature thresholds, and emissions targets need to be reassessed to ensure an adequate response to climate change.

A tale of two forests

To reach their findings, the scientists tracked 300,000 trees in 565 patches of forest over a period of 30 years – an ambitious and time-consuming endeavor. Scientists measured the diameter and estimated the carbon in the biomass of every individual tree, returning every few years to follow up. By calculating the carbon stored in the trees that survived and those that died, they could quantify the changes in carbon storage over time.

Results show that the Amazon sink began weakening first, starting in the mid-1990s, followed by a reduction in the African sink about 15 years later.

The authors argue that this difference arises because Amazon forests are both more dynamic than those in Africa, and that they also face stronger climate impacts. Compared to African forests, typical Amazonian forests are exposed to higher temperatures, faster temperature increases and more regular and severe droughts, which slow tree growth and increase mortality. However, the study concludes that more research is needed to better understand how underlying environmental drivers affect carbon gains and losses, and to fully explain the divergent patterns.

Uncovering the secrets of forests

Despite the contribution of forests to climate regulation, there are still huge research gaps to fully understand their functioning, according to the authors.

“Continued on-the-ground monitoring of intact tropical forests is required to track the effects of accelerating environmental change,” said Hubau, urging international backing for more collaborative studies. “We need this more than ever, as our planet’s last great tropical forests are threatened as never before.”

However, this does not only require funding for new studies, but also support to local scientists, who can be closer to forests and contribute to monitor their evolution. “For too long the skills and potential of African and Amazonian scientists have been undervalued,” said Oliver Phillips, a professor at the University of Leeds. “It will fall to the next generation of African and Amazonian scientists to monitor these remarkable forests to help manage and protect them,” he added.

A new forestry hub

A protected reserve in the Democratic Republic of Congo (DRC) is fast emerging as a global forestry hub, already responding to the challenges detailed in the study. Located in the northeast of the country, the Yangambi Biosphere Reserve is at the forefront of ground-breaking research supported by the Center for International Forestry Research (CIFOR) to measure the Congo Basin’s carbon sinks.

Backed by the European Union, in 2019 the RMCA launched a new wood biology laboratory in Yangambi. The first of its kind in sub-Saharan Africa, the laboratory facilitates relevant research on wood anatomy and dendrochronology (tree-ring dating) to better understand forests’ contribution to climate change mitigation and adaptation.

“This laboratory provides much-needed equipment for African and visiting researchers,” said Hans Beeckman, head of the RMCA’s Wood Biology Service, who contributed to the research. Previously, scientists had to take wood samples to Europe to be analyzed, but now they can do it right next to the forest, he explained. “Having a laboratory in the Congo Basin will make studies cheaper, easier, and more inclusive.”

The RMCA is also measuring carbon uptake by intact tropical forests through periodic censuses in permanent sample plots in Yangambi and the neighboring Yoko Forest Reserve. Currently, 25 hectares of permanent sample plots are being followed, but more will be established soon.

“These measurements provide precise information on the vital rates of a forest ecosystem, such as tree growth, regeneration and mortality,” explained Hubau. “This is valuable data for the models that analyze and predict climate change and inform international climate negotiations.”

Yangambi will also soon host the Congo Basin’s first Eddy Covariance Flux Tower. Reaching a height above the forest canopy, this structure will deliver continuous and accurate data on greenhouse gas exchanges between the atmosphere and the forest, filling a huge data gap.

“Long-term flux tower data will allow to measure the evolution of net carbon dioxide uptake by the Congo Basin’s lowland humid forests, and proof or disproof the extrapolations of this study,” said Pascal Boeckx, professor at Belgium’s Ghent University and contributing author.

“The findings of this study reveal a worrying trend for tropical forest carbon sinks,” said Paolo Cerutti, a CIFOR scientist who coordinates its activities in DRC. “We hope that our interventions in Yangambi will shed light on how to reverse it.”