The Impact of Drought on Terrestrial Carbon in the West African Sahel: Implications for Natural Climate Solutions

Abstract

Terrestrial ecosystems store more than twice the carbon of the atmosphere, and are critical to climate change mitigation efforts. This has led to a proliferation of land-based carbon sequestration efforts, such as re/afforestation associated with the Great Green Wall in the West African Sahel (WAS GGW). However, we currently lack comprehensive assessments of the long-term viability of these ecosystems' carbon storage in the context of increasingly severe climate extremes. The WAS is particularly prone to recurrent and disruptive extremes, exemplified by the persistent and severe late-20th century drought. We assessed the response and recovery of WAS GGW carbon stocks and fluxes to this late-20th century drought, and the subsequent rainfall recovery, by leveraging a suite of terrestrial ecosystem models. While multi-model mean carbon fluxes (e.g., gross primary production, respiration) partly recovered to pre-drought levels, modeled total (above and below ground) ecosystem carbon stock falls to as much as two standard deviations below pre-drought levels and does not recover even ∼20 years after the maximum drought anomaly. Furthermore, to the extent that the modeled regional carbon stock recovers, it is nearly entirely driven by atmospheric CO₂ trends rather than the precipitation recovery. Uncertainties in regional ecosystem carbon simulation are high, as the models' carbon responses to drought displayed a nearly 10-standard deviation spread. Nevertheless, the multi-model average response highlights the strong and persistent impact of drought on terrestrial carbon storage, and the potential risks of relying on terrestrial ecosystems as a “natural climate solution” for climate change mitigation.