Is the climate response to CO 2 emissions path dependent?

Recent studies with coupled climate‐carbon cycle models suggest that global mean temperature change is proportional to cumulative CO 2 emissions, independent of the timing of those emissions. This finding has prompted the suggestion that climate stabilization targets, such as the 2°C target adopted by the Copenhagen Accord, can be expressed in terms of cumulative CO 2 emissions. Here we examine the simulated response of a range of global and regional climate variables to the same cumulative CO 2 emissions (2500 PgC) released along different pathways using a complex Earth system model. We find that the response of most surface climate variables is largely independent of the emissions pathway once emissions cease, with the exception of variables with response timescales of centuries, such as ocean heat content and thermosteric sea level rise. Peak responses of many climate variables, such as global mean temperature, precipitation and sea ice, are also largely independent of the emissions pathway, except for scenarios with cumulative emissions overshoot which require net removal of CO 2 from the atmosphere. By contrast, peak responses of atmospheric CO 2 and surface ocean pH are found to be dependent on the emissions pathway. We conclude that a CO 2 mitigation framework based on cumulative emissions is well suited for limiting changes in many impact‐relevant climate variables, but is less effective in avoiding impacts directly associated with atmospheric CO 2 , whose peak response is dependent on the rate of emissions.