Since 1750, human activities have dramatically increased atmospheric greenhouse gas (GHG) concentrations, causing global warming. The rate of warming has accelerated, with global mean surface air temperature rising by 1.1 °C above pre-industrial levels. This warming increases thermal and latent energy, boosting extreme weather events like heat waves, torrential rains, and flooding. To mitigate these negative impacts, over 110 countries have committed to carbon neutrality by the mid-21st century, aiming to meet the Paris Agreement's target of limiting global warming to well below 2 °C, preferably to 1.5 °C by 2100. Carbon neutrality involves substantial emission reductions, offering co-benefits for air quality. For instance, China's carbon neutrality targets are expected to significantly reduce emissions of various pollutants. Global GHG mitigation also reduces air pollution by slowing climate change, which alters meteorology conducive to pollution. However, changes in air pollutants can affect the climate. Tropospheric O₃ has positive radiative forcing, while PM components have varying effects. Therefore, reducing air pollution toward carbon neutrality may either dampen or amplify the effects of GHG mitigations. Previous studies primarily focused on mean climate changes, overlooking the more impactful changes in extreme weather events. This study, using the SSP1-1.9 scenario (limiting warming to 1.5 °C), investigates the individual impacts of GHGs, aerosols, and tropospheric O₃ changes on climate and extreme weather events under a carbon neutrality scenario, contrasting it with findings from the medium emission pathway (SSP2-4.5).

Existing literature extensively documents the increase in GHGs and resulting global warming, along with the rise in frequency and intensity of extreme weather events. Studies highlight the co-benefits of carbon neutrality for air quality, projecting significant reductions in various pollutants under ambitious emission reduction pathways. Research also acknowledges the complex interplay between GHGs, aerosols, and tropospheric ozone, recognizing their varying radiative effects and influence on climate change. However, past research primarily focuses on mean climate changes, with less emphasis on the more direct and impactful changes in extreme weather events. While the DAMIP project within CMIP6 has explored the individual contributions of various forcings to observed and projected climate changes under the SSP2-4.5 scenario, a dedicated analysis focusing on the carbon neutrality pathway (SSP1-1.9) with a comprehensive assessment of extreme weather events is lacking. This gap in the literature underscores the need for this study.

This study employs the fully coupled Community Earth System Model version 1 (CESM1) to investigate the climate impacts of anthropogenic emission changes under a carbon neutrality scenario. CESM1, with its Community Atmosphere Model version 5 (CAM5), incorporates detailed aerosol representation and aerosol-cloud interactions. The model's performance in simulating present-day climate is validated against observational data. Five sets of climate simulations are conducted: Baseline (2020 levels), GHG2050 (2050 GHG levels, 2020 other forcings), AerGHG2050 (2050 GHG and aerosol levels, 2020 O3), ALL2050 (2050 levels for all forcings), and ALL2100 (2100 levels for all forcings). The SSP1-1.9 scenario is used to represent the carbon neutrality pathway. The study focuses on changes in annual mean surface air temperature, precipitation, and extreme weather events (heat waves, humid heat waves, and extreme precipitation indices) between these simulations and the baseline. Global land regions are divided into 21 subregions for regional-scale analysis. The study also includes multi-model analysis from DAMIP experiments under SSP2-4.5 for comparison. Extreme indices analyzed include heat wave frequency, duration, and amplitude; humid heat wave counterparts; annual total precipitation on wet days; maximum consecutive wet days; and heavy precipitation days (R10mm). Present-day climatology is evaluated against ERA5 reanalysis data.

The study's key findings demonstrate that aerosol reductions significantly contribute to future warming and increased frequency and intensity of extreme weather events under the carbon neutrality scenario. Specifically: 1. **Aerosol reduction-driven warming dominates:** Aerosol reductions cause much more surface air temperature increases (0.5–1.4 °C) than GHG increases (below 0.2 °C) by 2050. This warming is further amplified by 2100. This contrasts with findings under the SSP2-4.5 scenario where GHG changes dominate warming. 2. **Amplified impacts on extreme weather:** Aerosol reductions lead to a dramatic increase in the frequency, duration, and amplitude of both dry and humid heat waves. These increases are far more pronounced than those attributable to GHG changes or tropospheric ozone reductions. 3. **Significant changes in precipitation:** Aerosol reductions lead to increased precipitation globally, particularly in the Northern Hemisphere, shifting the intertropical convergence zone northward. This effect is larger than that caused by GHGs or O3 changes. 4. **Effective radiative forcing (ERF):** The changes in ERF at the top of the atmosphere and at the surface further support the dominance of aerosol reductions in driving future warming. Aerosol-caused ERF changes are considerably larger than those from GHGs or O3. 5. **Consistency with multi-model results:** While CESM1 shows stronger responses than the CMIP6 multi-model mean under SSP1-1.9, both show consistent spatial patterns of increased temperature and precipitation in the Northern Hemisphere, highlighting the significant role of emission changes. 6. **Model Sensitivity:** CESM1's higher sensitivity to anthropogenic forcings, particularly aerosol-cloud interactions, results in relatively stronger responses than the CMIP6 multi-model mean. However, even accounting for this, the dominance of aerosol reduction effects remains evident.

This study's findings highlight the critical importance of considering aerosol reductions when planning for carbon neutrality. The previously held assumption that GHGs would be the primary driver of future climate change under carbon neutrality is challenged. The results show that aerosol reductions counteract the benefits of GHG mitigation, leading to unexpected and potentially severe climate impacts. The amplified warming and increased extreme weather events driven by aerosol reductions pose significant challenges to achieving the 1.5°C warming target. This underscores the urgent need for comprehensive mitigation strategies addressing both GHGs and aerosols, particularly their precursors. The study's findings have significant implications for climate change adaptation and mitigation policies, emphasizing the need for integrated strategies that account for the complex interplay of different forcing agents.

This study demonstrates that aerosol reductions significantly amplify climate warming and increase extreme weather events under carbon neutrality, surpassing the effects of GHGs and O3 changes. This necessitates integrated mitigation strategies that address both GHGs and aerosols to achieve climate goals. Future research should focus on refining aerosol-cloud interaction modeling, exploring the complex interactions among different pollutants, and considering transient climate responses to better understand the long-term implications of carbon neutrality pathways. The unexpected dominance of aerosol effects underscores the need for more comprehensive modeling and mitigation strategies.

The study acknowledges several limitations. The equilibrium climate response to GHG changes may be underestimated due to the model spin-up time. CESM1 shows higher sensitivity to anthropogenic forcings than some other models, potentially overestimating the responses to aerosol reductions. The interactions of absorbing and scattering aerosols, their complex effects on climate, and the feedback mechanisms between aerosols and tropospheric gas-phase species are not fully accounted for. Finally, the interactions between climate change and air pollutants, including feedback mechanisms, are not comprehensively explored.