Asynchronous Saturation of the Carbon Sink in African and Amazonian tropical forests

Data and R-code from Hubau W et al. 2020. 'Asynchronous Saturation of the Carbon Sink in African and Amazonian tropical forests'. Nature 579, 80-87. 2020. DOI: 10.1038/s41586-020-2035-0. ABSTRACT: Structurally intact tropical forests sequestered ~50% of global terrestrial carbon uptake over the 1990s and early 2000s, offsetting ~15% of anthropogenic CO2 emissions1-3. Climate-driven vegetation models typically predict that this tropical forest ‘carbon sink’ will continue for decades4,5. However, recent inventories of intact Amazonian forests show declining carbon sequestration6. Here, we assess the trends in African forests and compare with Amazonia. Records from 244 multi-census plots spanning 11 countries reveal that the carbon sink in aboveground live biomass in intact African tropical forests has been stable for the three decades to 2015, at 0.66 Mg C ha-1 yr-1 (95% CI: 0.53-0.79). Thus, the carbon sink responses of Earth’s two largest expanses of tropical forest have diverged. As both continents show increasing tree growth (consistent with the expected net effect of rising atmospheric CO2 and air temperature on photosynthesis and respiration7), the divergence arises from differences in carbon losses from tree mortality (no detectable multi-decadal trend in Africa; monotonic increase in Amazonia). Despite the past stability of the African carbon sink, our data suggest a very recent (c. 2010) increase in carbon losses, delayed compared to Amazonia, indicating asynchronous carbon sink saturation on the two continents. A statistical model including CO2, temperature, drought, and forest dynamics can account for the observed trends. Extrapolating these predictor variables indicates a long-term decline in the African sink, being 18% smaller in 2030, while the Amazonian sink continues to rapidly weaken, reaching zero in the 2030s. Overall, the uptake of carbon into Earth’s intact tropical forests peaked in the 1990s. Furthermore, this tropical sink is set to end decades sooner than even the most pessimistic vegetation models predict4,5. Observations indicating greater recent carbon uptake into the Northern hemisphere landmass8 reinforce our conclusion that the intact tropical forest carbon sink has already saturated.

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