Soil organic carbon in savannas decreases with anthropogenic climate change

- Rate of SOC loss in Kalahari savannas due to climate change is ~ 2.0 × 1012 g C yr− 1.
- Globally, savannas lose between ~ 28.4 Tg C y− 1 and 64.1 Tg C yr− 1 under RCP8.5.
- There is positive feedback between the SOC and atmospheric C cycles.
- These feedbacks are not adequately accounted for in existing Earth System Models (ESM) that are part of CMIP5.
- Revisions to these ESM would appear necessary to adequately account for this positive feedback.

Climate models indicate that climate change is likely to affect carbon (C) cycling in drylands, particularly savannas, but the magnitude and direction of change are not fully understood. In this study, we used the Century model to analyze how net primary productivity (NPP), soil respiration and soil C sequestration would respond to an increase in atmospheric CO2 and soil temperature. We also assessed the coupled effects of precipitation and temperature change on C dynamics under future climatic conditions, as well as the decoupled effects of each of the climate variables under three IPCC climate scenarios; historical, Representative Concentration Pathway 2.6 (RCP2.6) and RCP8.5. An increase in soil temperature results in loss of soil organic C (SOC), whereas doubling atmospheric CO2 concentration causes an increase in SOC. The increase in air temperature causes soil respiration to increase, while it causes NPP to decrease. We calculated the total SOC in the Kalahari savannas to be 0.9 Pg C (1 Pg = 1015 g) in the top meter, and the rate of SOC loss due to anthropogenic climate change to be ~ 1.1 Tg C yr− 1 (1 Tg = 1012 g) and ~ 2.0 Tg C yr− 1 under RCP2.6 and RCP8.5, respectively until the end of this century. If extrapolated to the global extent of savannas, our results imply net SOC loss of at least ~ 28.4 Tg C y− 1 and 64.1 Tg C yr− 1 under RCP2.6 and RCP8.5, respectively. The rapid loss of C from dryland soils predicted by Century could accelerate global warming and strengthen positive feedback mechanisms between climate change and processes controlling SOC. Our results vividly support the positive feedback between the SOC and atmospheric C cycles and further indicate that these feedbacks are not adequately accounted for in existing Earth System Models (ESM) that are part of CMIP5. Revisions to these ESM would appear necessary to adequately account for this positive feedback.