Current climate models are not capturing carbon dynamics in tropical forests
Recent research has shown that the carbon sink of tropical forests in Africa and Amazonia is saturated and will decline. However, current climate models are unable to reproduce these natural dynamics, according to a new study by the universities of Hong Kong, Leeds and the Royal Museum for Central Africa (RMCA). The models even predict a slightly increasing carbon uptake. It is necessary to adjust these models, as they are at the basis of global climate policy.
A forest giant in the Luki Man-and-the-Biosphere Reserve. This tree contains tonnes of carbon and absorbs significant quantities of CO2 from the atmosphere every year.
© Wannes Hubau
From carbon sink to carbon source
Tropical forests play a crucial role in absorbing carbon dioxide (CO2) from the atmosphere. Recent analyses show that intact tropical forests in Central Africa and Amazonia not only harbour an enormous carbon reservoir, but are also a large 'carbon sink'. These findings are based on direct measurements in an extensive network of research plots. However, after decades of close monitoring (1985-2014), scientists came to the alarming conclusion that the capacity of tropical forests to absorb carbon has been declining since the 1990s.
This research, published last year in Nature and led by the University of Leeds and the Royal Museum for Central Africa, suggests that the feared switch of tropical forests from a carbon sink to a carbon source has already begun.
Declining carbon dynamics not represented in climate models
Researchers now compared these field results with estimates from the two most recent generations of Global Climate Models (Earth System Models). The comparison shows that the model results do not match the observed carbon dynamics from 1985-2014.
"The decreasing capacity of the tropical carbon sink that we observe since the 1990s is not captured in the models. For the future, the models even predict a slightly increasing carbon sink, instead of a further decrease," says forest ecologist Wannes Hubau (RMCA/UGent). This difference is mainly due to the distorted way the models take tree mortality into account.
Crucial for future climate policy
Earth System Models are an important tool for predicting the evolution of the global carbon balance and climate on the basis of various future CO2 emission scenarios. "Climate models are directly at the basis of policy decisions with a global impact," Hubau explains. The most recent model (Climate Modelling Intercomparison Project, CMIP6), and its predecessor (CMIP5), examined in this study, directly influence the reporting of the authoritative United Nations Intergovernmental Panel on Climate Change (IPCC).
"It is therefore crucial to evaluate the models’ performance. There is an urgent need to adjust the models to reality in order to better inform future climate policy," concludes Wannes Hubau.
Koch, A., Hubau, W., & Lewis, S. L. (2021). Earth System Models are not capturing present-day tropical forest carbon dynamics. Earth's Future, 9, e2020EF001874. https://doi.org/10.1029/2020EF001874