Simulating the Earth system response to negative emissions

Natural carbon sinks currently absorb approximately half of the anthropogenic CO2 emitted by fossilfuel burning, cement production and land-use change. However, this airborne fraction may change inthe future depending on the emissions scenario. An important issue in developing carbon budgets toachieve climate stabilisation targets is the behaviour of natural carbon sinks, particularly under lowemissions mitigation scenarios as required to meet the goals of the Paris Agreement. A key requirementfor low carbon pathways is to quantify the effectiveness of negative emissions technologies which will bestrongly affected by carbon cyclefeedbacks. Here we find that Earth system models suggest significantweakening, even potential reversal, of the ocean and land sinks under future low emission scenarios.For the RCP2.6 concentration pathway, models project land and ocean sinks to weaken to 0.8 ± 0.9and 1.1 ± 0.3 GtC yr−1 respectively for the second half of the 21st century and to−0.4 ± 0.4 and0.1 ± 0.2 GtC yr−1 respectivelyfor the second half of the 23rd century.Weakening of natural carbonsinks will hinder the effectiveness of negative emissions technologies and therefore increase theirrequired deployment to achieve a given climate stabilisation target.We introduce a new metric, theperturbation airborne fraction, to measure and assess the effectiveness of negative emissions.