Strong Nonlinearity of Land Climate‐Carbon Cycle Feedback Under a High CO2 Growth Scenario

Abstract

AbstractProjections of future climate change for given CO2 and other greenhouse gas emission scenarios depend on the response of global climate‐carbon cycle feedback, which consists of carbon‐concentration feedback (e.g., CO2 physiology effect on land carbon sink) and carbon‐climate feedback (e.g., CO2 radiative effect on land carbon sink). Previous studies have assumed no significant interaction between these two feedbacks within the Earth system. This study quantifies the interaction of these two feedbacks, or the nonlinear feedback on land using the fully, biogeochemically, and radiatively coupled simulations under a 1% yr−1 CO2 increase path from nine Earth system models of the Coupled Model Intercomparison Project Phase 6 (CMIP6). The results show that the nonlinear feedback is 1.64 ± 2.92 × 10−2 GtC ppm−1 K−1 at the end of 140‐year simulation with a quadrupling CO2 (4 × CO2), where its strength is 11% ± 18% of the carbon‐concentration feedback or −27% ± 49% of the carbon‐climate feedback on land. Compared to previous assumptions that did not consider this interaction, the nonlinear feedback contributes about 8% ± 12% of the land carbon increase accumulated at the 4 × CO2. The nonlinear feedback largely results from the combined effect of increased CO2‐induced additional fertilization effect on warming‐induced additional leaf area index and vegetation productivity over the Northern Hemisphere. The magnitude of the nonlinear feedback on land decreases with an increase in atmospheric CO2 or warming under the high emission scenario. This study highlights the significance of land nonlinear climate‐carbon cycle feedback in increasing land carbon sink and slowing down future climate change.