Quantifying CO2 and Non-CO2 Contributions to Climate Change Under 1.5°C and 2°C Adaptive Emission Scenarios

The individual contributions of various human-induced forcings under scenarios compatible with the Paris Agreement targets are highly uncertain. To quantify this uncertainty, we analyze three types of models with physical parameter perturbed large ensembles under global warming levels of 1.5 and 2.0°C. The scenarios use adaptive CO₂ emissions, while non-CO₂ emissions are prescribed. The residual emission budgets in the scenarios are measured in terms of CO₂ forcing equivalent (CO₂-fe). Our simulations quantify approximately 0.8 (0.2–1.3 for a 90% confidence interval) and 1.9 (0.9–3.0) TtCO₂-fe for the 1.5 and 2.0°C targets by the end of the 21st century. About 37.5% (73.7%) of the budget for 1.5°C (2.0°C) originates from the CO₂ emission pathways, highlighting the importance of non-CO₂ forcings. Aerosols dominate the uncertainty in non-CO₂ contributions to global responses in both temperature and precipitation. Our modeling results underline the need to constrain the response to each climate forcing, particularly aerosol, to build an accurate mitigation and adaptation plan under the pledges of the Paris Agreement. Moreover, we demonstrate robust differences in global and regional temperature and precipitation responses between the higher and lower CO₂ emission scenarios, highlighting the significance of carbon neutrality.