Quantifying the different forcing contributions to the climate under the adaptive emission scenarios.

The Intergovernmental Panel on Climate Change (IPCC) issued a special report on global warming of 1.5°, emphasizing the necessity of urgent and strict mitigation policies to achieve the warming levels of the Paris Agreement. To explore the possible worlds at the warming levels of the Paris Agreement, a brand-new experiment, the adaptive emission reduction approach (AERA), is launched, calibrating the future CO₂ emission pathways by considering the transient climate response of the model to the cumulative emission amount of CO₂ forcing equivalent (CO₂-fe).

While waiting for the AERA protocol results with expensive tools, Earth System Models (ESMs), we explore the likely range of adaptive emission scenario runs by utilizing the fastest and most effective tools. Here we suggest the simple emulator, FaIR, composed of three thermal energy boxes and four carbon-cycle pools. We tested thousands of FaIR ensembles to perform the AERA algorithm and confirm the capability of FaIR to estimate the likely ranges of ESMs. Notably, this FaIR can act as the bridge for inter-mediate complexity models (e.g., HadCM3 and CLIMBER-X) to interact with the AERA algorithm by emulating the carbon-cycle characteristics part of GCMs.

Despite different climate sensitivity in ensemble members, 2450 FaIR, nine HadCM3-FaIR, and six CLIMBER-FaIR ensembles well stabilized at the target temperatures of 1.5°C and 2.0°C by 2100. The residual cumulative emission amount of CO₂-fe since 2021 is about 0.71 and 1.77 TtCO₂-fe for 1.5°C and 2.0°C levels. We further quantified the contributions from each anthropogenic forcings to the climate of mitigated worlds with FaIR emulations. While CO₂ is the most dominant driver for 2.0°C warming (about 83% of total warming), anthropogenic aerosol plays a vital role in stabilizing a warming level at 1.5°C. The reduction of aerosols by 2100 additionally provides positive radiative forcings, and this size is equivalent to the increase in CO₂-fe emission amount of 1 TtCO₂-fe, which leads to the temperature rise by 0.57°C. The non-CO₂ GHG (like methane or fluorinated gases) offsets the emission amount of CO₂-fe about 0.57 TtCO₂-fe and reduces the warming levels by 0.35°C. Despite the considerable uncertainty in climate responses to anthropogenic aerosols, our results illustrate their notable contributions, at least for the 1.5°C world. In addition, we plan to explore the forcing-driven contributions to the changes in precipitation based on the equations reflecting the favorable atmospheric energy conditions to precipitate.