Nitrogen availability reduces CMIP5 projections of Twenty-First-Century land carbon uptake

Coupled carbon cycle--climate models in the Coupled Model Intercomparison Project, phase 5 (CMIP5), Earth system model ensemble simulate the effects of changes in anthropogenic fossil-fuel emissions and ensuing climatic changes on the global carbon (C) balance but largely ignore the consequences of widespread terrestrial nitrogen (N) limitation. Based on plausible ranges of terrestrial C:N stoichiometry, this study investigates whether the terrestrial C sequestration projections of nine CMIP5 models for four representative concentration pathways (RCPs) are consistent with estimates of N supply from increased biological fixation, atmospheric deposition, and reduced ecosystem N losses. Discrepancies between the timing and places of N demand and supply indicated increases in terrestrial N implicit to the projections of all nine CMIP5 models under all scenarios that are larger than the estimated N supply. Omitting N constraints leads to an overestimation of land C sequestration in these models between the years 1860 and 2100 by between 97 Pg C (69-252 Pg C; RCP 2.6) and 150 Pg C (57-323 Pg C; RCP 8.5), with a large spread across models. The CMIP5 models overestimated the average 2006–2100 fossil-fuel emissions required to keep atmospheric CO2 levels on the trajectories described in the RCP scenarios by between 0.6 Pg C yr⁻¹ (0.4-2.2 Pg C yr⁻¹; RCP 2.6) and 1.2 Pg C yr⁻¹ (0.5–3.3 Pg C yr⁻¹; RCP 8.5). If unabated, reduced land C sequestration would enhance CO₂ accumulation in the ocean and atmosphere, increasing atmospheric CO₂ burden by 26 ppm (16-88 ppm; RCP 2.6) to 61 ppm (29-147 ppm; RCP 8.5) by the year 2100.