Carbon cycle response to temperature overshoot beyond 2 °C – an analysis of CMIP6 models

There is a substantial gap between the current emissions of greenhouse gases and levels required for achieving the 2 and 1.5 °C temperature targets of the Paris Agreement. Understanding the implications of a temperature overshoot is thus an increasingly relevant research topic. We carry out a study as part of the “Achieving the Paris Agreement Temperature Targets after Overshoot (PRATO)” project of the MOPGA programme on the 2 °C overshoot of the Paris Agreement temperature target. We explore the carbon cycle feedbacks over land and ocean in the SSP5-3.4-OS overshoot scenario by using an ensemble of Coupled Model Intercomparison Project 6 Earth system models. Models show that after the CO₂ concentration and air temperature peaks, land and ocean are decreasing carbon sinks from the 2040s and become sources for a limited time in the 22^nd century. The decrease in the carbon uptake precedes the CO₂ concentration peak. The early peak of the ocean uptake stems from its dependency on the atmospheric CO₂ growth rate. The early peak of the land uptake occurs due to a larger increase in ecosystem respiration than the increase in gross primary production, as well as due to a concomitant increase in land-use change emissions primarily attributed to the wide implementation of biofuel croplands. The carbon cycle feedback parameters amplify after the CO₂ concentration and temperature peaks, so that land and ocean absorb more carbon per unit change in the atmospheric CO₂ change (stronger negative feedback) and lose more carbon per unit temperature change (stronger positive feedback) compared to if the feedbacks stayed unchanged. The increased negative CO₂ feedback outperforms the increased positive climate feedback. This feature should be investigated under other scenarios and reflected in simple climate models.