Is the tropical carbon sink weakening?

The ability of  forests to remove carbon from the atmosphere is found to be decreasing in tropical regions of the world, according to new research led by the University of Leeds.

Ivindo National Park, central Gabon. Source: Kath Jeffery

The global research effort, led by researchers from the School of Geography, saw  a long-term analysis of the carbon stored in tropical forests. The study, published this month in Nature, tracked 300,000 trees over 30 years, from 565 undisturbed forest sites in Africa and the Amazon to examine the rate of carbon uptake in tropical forests over time.

Intact tropical forests are crucial to regulation of the climate, mitigating climate change through the sequestration and storage of carbon. These forests are estimated to have sequestered approximately 50% of global terrestrial carbon uptake during the 1990s and early 2000s. Tropical forests also provide a range of other ecosystem services and are home to high proportions of global biodiversity.

A census of the intact forest plots was carried out, taking measurements of trunk diameter and tree height for each individual tree across the 565 sites every few years. These measurements were used to monitor the growth of the trees and estimate change in carbon storage over time. Key drivers of change are found to be temperature, drought, forest dynamics and atmospheric concentrations of CO2. Increasing concentrations of CO2 are found to boost productivity of forests over time, however the CO2 fertilisation effect is only partially mitigating of the negative effects that temperature rise, and drought episodes have on both tree growth and mortality.

Measuring tree height for biomass estimates. Source: Roel Brienen

The research is one of a kind in terms of its spatial extent and provides large scale evidence for the trends in tropical forest carbon stocks. It is found that the shift from carbon sink to carbon source in tropical forests has already begun, with peak carbon storage observed in the 1990s. The ability of tropical forests to sequester carbon is estimated to have fallen by one third, mainly due to increased tree mortality, with removal of carbon dioxide estimated at 46 billion tonnes in 1990s, compared to just 25 billion in 2010s.

Study lead author Dr Wannes Hubau, a former post-doctoral researcher at the University of Leeds now based at the Royal Museum for Central Africa in Belgium, 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.”

The Amazonian sink was found to have begun weakening much earlier than the African sink, around 1990s compared to mid-2000s. Whilst both regions show evidence of increasing tree growth attributed to increasing atmospheric concentrations of CO2, the overall carbon uptake has peaked. This is of concern due to the global impacts that small scale changes in individual biomes can have. Whilst the atmospheric impact of tree mortality is not felt immediately, due to the time taken for decomposition, gradual changes to the carbon sink will have implications for interactions between the climate and carbon cycle, and may further exacerbate the rise of atmospheric CO2, causing a positive feedback effect.

Tropical forest, Colombia. Source: Peter Vitzum

The study shows that tropical forests are losing their ability to help slow climate change. Whilst the forests continue to be major stores of carbon, the results demonstrate the need for urgent action to support the functionality of these carbon stores into the future.

Professor Simon Lewis, the study’s other lead author added, 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.

Hubau and colleagues recognise the immediate and direct threats posed by human activities, namely logging, fires and deforestation; efforts to restrict these activities will be vital to the maintenance of intact tropical forests. In addition, the study confirms that  global greenhouse gas emissions must be reduced to net-zero as soon as possible, in order to limit the warming that is driving this decline of carbon uptake in  tropical forests.

Read the full study here.