Land carbon sinks in response to zero and negative emissions across Earth system models

Land carbon sinks are responsible for removing about a quarter of anthropogenic CO$_2$ emissions, and make up approximately half of total global carbon sinks. Uncertainty in the response of land carbon sinks to climate and increasing CO$_2$ emissions are large, and dominate the uncertainty in total carbon sinks over the next century. Understanding the carbon cycle response to net-zero and net-negative emissions has important implications for projecting future climate. Experiments in the `flat10' model intercomparison (flat10MIP) were designed for directly estimating key climate metrics that underlie carbon budgeting frameworks. Here we characterize the response of land carbon pools and fluxes from ten emissions-driven Earth system models (ESMs) under positive, net-zero, and net-negative CO$_2$ emissions. Although there are many differences in simulated land carbon pools and fluxes across models, we find some consistent behavior across ESMs. 1) During the positive emissions phase, carbon is gained on land -primarily in vegetation pools- in both the tropics and mid-latitudes. 2) Following net-negative emissions to the point of cumulative zero emissions, vegetation carbon is lost from land. 3) In tropical latitudes, total carbon is lost coming primarily from vegetation pools, but in mid-latitudes nearly all models show net land carbon gain, primarily in soil pools. 4) Following an extended period of net-zero emissions, a majority of models again show carbon gain in mid-latitudes and vegetation carbon loss in the tropics. Under net-negative emissions the timing of vegetation carbon response relative to peak emissions is relatively consistent across ESMs, but timing of soil carbon response varies widely, implying larger intermodel disagreement associated with the longer timescale responses of land carbon.