As more countries make net zero greenhouse gas emissions pledges, it is crucial to understand the effects on global climate after achieving net zero emissions. The climate has been found to continue to evolve even after the abrupt cessation of CO2 emissions, with some models simulating a small warming and others simulating a small cooling. In this study, we analyse if the temperature and precipitation changes post abrupt cessation of CO2 emissions are significant compared to natural climate variations. We find that the temperature changes are outside of natural variability for most models, whilst the precipitation changes are mostly non-significant. We also demonstrate that post-net zero temperature changes have implications for the remaining carbon budget. The possibility of further global warming post-net zero adds to the evidence supporting more rapid emissions reductions in the near-term.

Over the last decades, climate science has branched out into many smaller expert communities across the carbon cycle, radiative forcings, climate feedbacks or ocean heat uptake domains. Our best tools to capture state-of-the-art knowledge are the increasingly complex fully coupled Earth System Models (ESMs). However, computational limitations and the structural rigidity of ESMs mean that the full range of uncertainties are difficult to capture with multi-model ESM ensembles or single ESM perturbed parameter ensembles. The tools of choice are hence more computationally efficient reduced complexity models (RCMs), which are structurally flexible and can span the response dynamics across a range of domain-specific models and/or ESM experiments. Here, we provide the first comprehensive intercomparison of multiple RCMs that are probabilistically calibrated to key benchmark ranges from specialised research communities. This exercise constitutes Phase 2 of the Reduced Complexity Model Intercomparison Project (RCMIP Phase 2). We find that even if RCMs perform similarly against historical benchmarks, their future projections can still diverge. Under the low-emissions SSP1-1.9 scenario, across the RCMs, median 2081-2100 warming projections range from 1.1 to 1.4{degree sign}C while median peak warming projections range from 1.3 to 1.7{degree sign}C (relative to 1850-1900, using an observationally-based historical warming estimate of 0.8{degree sign}C between 1850-1900 and 1995-2014). Our findings suggest that users of RCMs should carefully evaluate the RCM they are using, specifically its skill against key benchmarks and consider the need to include future projections benchmarks either from ESM results or other assessments to reduce such divergence.

The IPCC’s scientific assessment of the timing of net-zero emissions and 2030 emission reduction targets consistent with limiting warming to 1.5C or 2C rests on large scenario databases. Updates to this assessment, such as between the IPCC’s Special Report on Global Warming of 1.5C (SR1.5) of warming and the Sixth Assessment Report (AR6), are the result of intertwined, sometimes opaque, factors. Here we isolate one factor: the Earth System Model emulators used to estimate the global warming implications of scenarios. We show that warming projections using AR6-calibrated emulators are consistent, to within around 0.1C, with projections made by the emulators used in SR1.5. The consistency is due to two almost compensating changes: the increase in assessed historical warming between the IPCC’s Fifth Assessment Report (AR5) and AR6, and a reduction in projected warming due to improved agreement between the emulators’ response to emissions and the underlying assessment.