Is Anthropogenic Climate Change Reversible? Results from CDRMIP on transient Earth system response under atmospheric CO₂ increase and decrease

Abstract. 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.