Reply on RC2

Abstract

Land and ocean carbon sinks play a major role in regulating atmospheric CO2 concentration and climate. However, their future efficiency depends on feedbacks in response to changes in atmospheric CO2 concentration and climate, namely the concentration-carbon and climate-carbon feedbacks. Since carbon dioxide removal is a key mitigation measure in emission scenarios consistent with global temperature goals in the Paris agreement, understanding carbon cycle feedbacks under negative CO2 emissions is essential. This study investigates land carbon cycle feedbacks under positive and negative CO2 emissions using an Earth system model driven with idealized scenarios of atmospheric CO2 increase and decrease, run in three modes. Our results show that the magnitude of carbon cycle feedbacks differs between the atmospheric CO2 ramp-up (positive emissions) and ramp-down (negative emissions) phases. These differences are likely largely due to climate system inertia: the response in the ramp-down phase represents the response to both the prior positive emissions and negative emissions. To isolate carbon cycle feedbacks under negative emissions and quantify these feedbacks more accurately, we propose a novel approach that uses zero emissions simulations to reduce this inertia. We find that the magnitudes of the concentration-carbon and climate-carbon feedbacks under negative emissions are larger in our novel approach than in the standard approach. This has two implications: using feedback parameters from the standard approach will (1) underestimate carbon release under negative emissions due to the concentration-carbon feedback, and (2) underestimate carbon gain due to the climate-carbon feedback. Given that the concentration-carbon feedback is the dominant feedback, quantifying carbon cycle feedbacks with the standard approach will result in the underestimation of carbon loss under negative emissions, thereby overestimating the effectiveness of negative emissions in drawing down CO2.