Decomposing drivers of global temperature change after net zero

Annika Högner, Zebedee Nicholls, Gaurav Ganti

Net-zero CO2 (NZCO2) and greenhouse gas emission (GHG) targets are central to the development of mitigation scenarios. Global surface air temperature (GSAT) change after NZCO2 depends on several factors that broadly fall into two categories: scenario-specific factors that account for the diversity of possible mitigation pathways to net zero and beyond, and model-specific factors that describe the climate system response, captured in the zero emissions commitment (ZEC). Current established climate assessment approaches do not separate contributions from these two categories to post-net-zero GSAT change in GHG emission scenarios. To address this shortcoming, we propose a diagnostic protocol that decomposes the contributions from the model-specific ZEC and from key scenario-characteristics including non-CO2 GHG reductions, net-negative CO2 emissions, and other climate forcers. We assess the GSAT outcomes from a large collection of scenarios that achieve NZCO2 using the MAGICC model. We find a peak warming increase of 0.09°C (likely 0.06 to 0.14°C) per decade delay in reaching NZCO2 and a -0.1°C (likely -0.3 to 0.03°C) post-NZCO2 GSAT decline by 2100. Based on our decomposition approach we find that the model-specific negative ZEC contributes about -0.05°C across the full scenario set. Isolating this allows us to test the sensitivity of our outcomes. We apply the IPCC AR6 WGI assessed value of 0°C (likely range ±0.19°C) for ZEC and find that the median likelihood of no further warming after achieving and maintaining NZCO2 drops from 76% to just about 50%. Achieving and maintaining NZGHG, however, remains ‘very likely (>90%)’ to lead to no further warming outcomes for different ZEC configurations. This underlines the need for setting preventive mitigation targets to account for potential stronger than expected Earth system feedbacks and demonstrates the utility of a decomposition approach.