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Abstract

Coupled carbon-cycle climate models in the 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. Using plausible ranges of terrestrial C:N stoichiometry, we investigated 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. We calculate that omitting N constraints leads to an overestimation of land C sequestration in these models between the years 1860 and 2100 by between 97 (69-252) Pg C (RCP 2.6) and 150 (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 (0.4-2.2) Pg C yr-1 (RCP 2.6) and 1.2 (0.5-3.3) Pg C yr-1 (RCP 8.5). If unabated, reduced land C sequestration would enhance CO2 accumulation in ocean and atmosphere, increasing atmospheric CO2 burden by 26 (16-88; RCP 2.6) to 61 (29-147; RCP 8.5) ppm by the year 2100.