The ability of the world’s tropical forests to remove carbon from the atmosphere is decreasing, according to a study tracking 300,000 trees over 30 years, published in Nature. The global scientific collaboration, led by the Royal Museum for Central Africa and the University of Leeds, reveals that a feared switch of the world’s undisturbed tropical forests from a carbon sink to a carbon source has begun.
Intact tropical forests are well-known as a crucial global carbon sink, slowing climate change by removing carbon from the atmosphere and storing it in trees, a process known as carbon sequestration. Climate models typically predict that this tropical forest carbon sink will continue for decades. However, the new analysis of three decades of tree growth and death from 565 undisturbed tropical forests across Africa and the Amazon has found that the overall uptake of carbon into Earth’s intact tropical forests peaked in the 1990s.
By the 2010s, on average, the ability of a tropical forest to absorb carbon had dropped by one-third. The switch is largely driven by carbon losses from trees dying.The study by almost 100 institutions provides the first large-scale evidence that carbon uptake by the world’s tropical forests has already started a worrying downward trend.Study lead author Dr Wannes Hubau of the Royal Museum for Central Africa, said: “We show that peak carbon uptake into intact tropical forests occurred in the 1990s. By combining data from Africa and the Amazon we began to understand why these forests are changing, with carbon dioxide levels, temperature, drought, and forest dynamics being key. Extra carbon dioxide boosts tree growth, but every year this effect is being increasingly countered by the negative impacts of higher temperatures and droughts which slow growth and can kill trees. Our modelling of these factors shows a long-term future decline in the African sink and that the Amazonian sink will continue to rapidly weaken, which we predict to become a carbon source in the mid-2030s.”
In the 1990s intact tropical forests removed roughly 46 billion tonnes of carbon dioxide from the atmosphere, declining to an estimated 25 billion tonnes in the 2010s. The lost sink capacity in the 2010s compared to the 1990s is 21 billion tonnes carbon dioxide, equivalent to a decade of fossil fuel emissions from the UK, Germany, France and Canada combined. Overall, intact tropical forests removed 17% of human-made carbon dioxide emissions in the 1990s, reduced to just 6% in the 2010s. This decline is because these forests were less able to absorb carbon by 33% and the area of intact forest declined by 19%, while global carbon dioxide emissions soared by 46%.
Senior author Professor Simon Lewis, from the University of Leeds, said: “Intact tropical forests remain a vital carbon sink but this research reveals that unless policies are put in place to stabilise Earth’s climate it is only a matter of time until they are no longer able to sequester carbon. “One big concern for the future of humanity is when carbon-cycle feedbacks really kick in, with nature switching from slowing climate change to accelerating it. After years of work deep in the Congo and Amazon rainforests we’ve found that one of the most worrying impacts of climate change has already begun. This is decades ahead of even the most pessimistic climate models. There is no time to lose in terms of tackling climate change.”
- Wannes Hubau (Royal Museum for Central Africa, Tervuren ) et al., Nature, doi: 10.1038/s41586-020-2035-0