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

Artificial removal of CO 2 from the atmosphere (also referred to as negative emissions) has been proposed as a means to restore the climate system to a desirable state, should the impacts of climate change become 'dangerous'. Here we explore whether negative emissions are indeed effective in reversing climate change on human timescales, given the potentially counteracting effect of natural carbon sinks and the inertia of the climate system. We designed a range of CO 2 emission scenarios, which follow a gradual transition to a zero-carbon energy system and entail implementation of various amounts of net-negative emissions at technologically plausible rates. These scenarios are used to force an Earth System Model of intermediate complexity. Results suggest that while it is possible to revert to a desired level of warming (e.g. 2 °C above pre-industrial) after different levels of overshoot, thermosteric sea level rise is not reversible for at least several centuries, even under assumption of large amounts of negative CO 2 emissions. During the net-negative emission phase, artificial CO 2 removal is opposed by CO 2 outgassing from natural carbon sinks, with the efficiency of CO 2 removal—here defined as the drop in atmospheric CO 2 per unit negative emission—decreasing with the total amount of negative emissions.