Is Climate Change Reversible? CDRMIP simulations of the Earth system response to a massive CO2 increase and decrease (emissions followed by negative emissions).

To stabilize long-term climate change at well-below 2°C (ideally below 1.5°C) above pre-industrial levels, large and sustained CO₂ emission reductions are needed.  Despite pledges from numerous governments, the world is not on track to achieve the required reductions within the timeframes outlined in the Paris Agreement, and it appears increasingly likely that an overshoot of the 1.5 or 2 °C temperature target will occur.  If this happens, it may be possible to use carbon dioxide removal methods to return atmospheric CO₂ concentrations to lower levels or even to reduce the magnitude of the overshoot, with the hope that lower CO₂ will rapidly lead to lower temperatures and reverse or limit other climate change impacts.  Here we present a multi-model analysis of how the Earth system and climate respond during the CMIP6 CDRMIP cdr-reversibility experiment, an idealized overshoot scenario, where CO₂ increases from a pre-industrial level by 1% yr^-1 until it is 4 times the initial value, then decrease again at 1% yr^-1 until the pre-industrial level is again reached, at which point CO₂ is held constant.  For many modelled quantities climate change appears to eventually be reversible, at least when viewed at the global mean level.  However, at a local level the results suggest some changes may be irreversible, although spatial patterns of change differ considerably between models.  For many variables the response time-scales to the CO₂ increase are very different than to the decrease in CO₂ with a many properties exhibiting long time lags before responding to decreasing CO₂, and much longer again to return to their unperturbed values (if this occurs).