The effectiveness of net negative carbon dioxide emissions in reversing anthropogenic climate change

Natural carbon sinks currently absorb approximately half of the anthropogenicCO2 emitted by fossil fuel burning, cement production and land-use change.However, this airborne fractionmay change in the future depending on the emissions scenario. An important issue in developing carbonbudgets to achieve climate stabilisation targets is the behaviour of natural carbon sinks, particularly under low emissionsmitigation scenarios as required tomeet the goals of the ParisAgreement. A key requirement for low carbonpathways is to quantify the effectiveness of negative emissions technologieswhichwill be strongly affected by carbon cycle feedbacks.Herewefind that Earth systemmodels suggest significant weakening, 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 toweaken to 0.8±0.9 and 1.1±0.3GtC yr respectively for the secondhalf of the 21st century and to−0.4±0.4 and 0.1±0.2GtC yr respectively for the secondhalf of the 23rd century.Weakening of natural carbon sinkswill hinder the effectiveness of negative emissions technologies and therefore increase their required deployment to achieve a given climate stabilisation target.We introduce a newmetric, the perturbation airborne fraction, tomeasure and assess the effectiveness of negative emissions.