Is Anthropogenic Climate Change Reversible? Results from CDRMIP on transient Earth system response under atmospheric CO 2 increase and decrease

To limit global warming at well-below 2°C above pre-industrial levels, large and sustained CO2 emission reductions are needed and it may be possible to use alongside carbon dioxide removal (CDR) methods to return atmospheric CO2 concentrations to lower levels, with the assumption that lower CO2 will rapidly lead to lower temperatures and reverse or limit other climate change impacts. Despite such assumption, a comprehensive understanding of the reversibility of the Earth system components is a persistent knowledge gap, which hinders our ability to assess future outcomes of decreasing anthropogenic forcing. Here we evaluate the reversibility of climate change through a multi-model analysis of how the Earth system and climate respond in the CMIP6 Carbon Dioxide Removal Model Intercomparison Project (CDRMIP) cdr-reversibility experiment. cdr-reversibility is an idealized CO2 concentration peak and decline experiment where atmospheric CO2 increases at 1% yr-1 from a pre-industrial level until 4 times its initial value, then decreases at 1% yr-1 until pre-industrial levels are again reached, at which point CO2 is held constant. A few modelled quantities reverse or overshoot initial levels, on the same timescale as the changes in CO2 forcing, such as the land and ocean net primary productivity or the Atlantic Meridional Overturning Circulation – at least when viewed at the global annual mean level. However, when responding to decreasing CO2 concentration many properties exhibit time lags and hysteresis, e.g. changes in ocean and land carbon stocks or sea level. While some variables respond to mainly one specific driver, most show a hysteresis behaviour relative to all the considered driving forcers, namely surface air temperature, atmospheric CO2 or cumulative emissions. Due to such, the majority of the considered environmental variables would take long, many times longer than a human lifetime, to return to their unperturbed values (if this ever occurs). Therefore, exceeding and then reversing a climate target would have a clear legacy in the climate system even if CO2 concentrations return to pre-industrial levels.