The IPCC's 1.5°C Special Report highlights the urgent need for climate action to avoid significant socio-economic losses. China, the world's largest carbon emitter, pledged to peak carbon dioxide emissions before 2030 and achieve carbon neutrality before 2060. This commitment strengthens its Nationally Determined Contribution (NDC) under the Paris Agreement. Mitigation policies aimed at CO2 reduction can bring health co-benefits through reduced air pollution. While global studies suggest that the costs of GHG emission reduction could be offset by health co-benefits by 2050, cost-benefit ratios vary across countries. Developing nations like China are projected to gain significant health benefits from air pollution abatement. China's carbon emissions peak is anticipated earlier than the 2030 NDC target, driven by economic changes and emission reduction efforts. Existing literature, based on the 2030 target, may overestimate cumulative CO2 emissions and underestimate co-benefits. This study addresses the gap in understanding the social and economic impacts of an early carbon peak in China and the diverse climate change pathways.

Previous research has shown that China's CO2 emissions may peak before 2025, exceeding the Paris Agreement target. Other studies also indicate realistic paths to surpass the NDC target. However, the existing literature often bases its projections on the original 2030 target, potentially overestimating cumulative CO2 emissions and underestimating the associated co-benefits in air quality and public health. There is a scarcity of research assessing the societal and economic effects of an early carbon peak in China, along with insufficient consideration of diverse climate change pathways in existing studies on the synergistic effect of climate policy on air quality. This study addresses these gaps by applying a comprehensive integrated assessment framework.

This study utilizes an integrated assessment framework (illustrated in Figure 1) to estimate the air quality and health co-benefits of climate policies in China under different carbon mitigation pathways. The framework involves several key steps: 1. **Scenario Selection:** Nine scenarios are considered, combining Shared Socioeconomic Pathways (SSPs) representing different socio-economic developments and Representative Concentration Pathways (RCPs) representing varying climate policy ambitions (Table 1). These include SSP1 (sustainable low development), SSP2 (middle of the road), and SSP5 (fossil-fueled development), each paired with RCP2.6 (2°C target) and RCP1.9 (1.5°C target), along with respective reference scenarios (REF). 2. **Emission Projections:** The Global Change Analysis Model (GCAM) is used to estimate CO2 and air pollutant emissions under each scenario. This model considers factors such as energy structure adjustments, the role of non-fossil fuels (biomass, solar, wind, nuclear), and carbon capture and storage (CCS). 3. **Air Quality Modeling:** The Weather Research and Forecasting Model Coupled with Chemistry (WRF-Chem) is employed to quantify ambient PM2.5 concentrations under each scenario. This model incorporates meteorological conditions and detailed emission inventories derived from GCAM results. 4. **Health Impact Assessment:** The Global Exposure Mortality Model (GEMM) is used to quantify mortality from long-term PM2.5 exposure under each scenario, considering age-specific vulnerability and exposure-response relationships. Uncertainties in the exposure-response function are considered. 5. **Economic Valuation:** Health benefits are monetized using the Value of Statistical Life (VSL), and the results are compared to the mitigation costs from GCAM (Table 2). The analysis includes both local and internationally scaled VSL estimates.

The study's key findings are: 1. **Early Carbon Peak:** Under all mitigation scenarios (RCP2.6 and RCP1.9), China's CO2 emissions peak in or before 2030, meeting the NDC target. The 1.5°C scenarios (RCP1.9) show an even earlier peak, around 2020. 2. **Air Quality Co-benefits:** Climate policies lead to significant reductions in SO2 and NOx emissions, but the impact on VOCs, BC, and OC is less pronounced. Ammonia (NH3) emissions, primarily from agriculture, increase due to higher biomass fuel use and fertilizer demand in some scenarios, potentially offsetting some PM2.5 reduction in some regions. 3. **PM2.5 Reduction:** Climate policies reduce PM2.5 concentrations, with more stringent policies yielding greater improvements (Figure 4). However, co-benefits alone are insufficient to meet China's air quality goals by 2035, which aims to reduce PM2.5 to at most 35 µg/m³ in all cities. 4. **Health Benefits:** Implementing climate policies reduces PM2.5-related mortality (Figure 5 and 6). An early peak before 2030 (aligned with the 1.5°C target) could avoid substantial deaths. The more stringent the climate policy, the greater the health benefits. 5. **Economic Benefits:** The health co-benefits of CO2 emission reduction could fully offset mitigation costs in the long term (2050). SSP1_RCP2.6 and SSP5_RCP2.6 result in net economic benefits equivalent to 0.45% and 2.77% of China's 2050 GDP, respectively (Table 2). This study does not include potential cost savings in pollution control, representing an additional co-benefit. 6. **Population Aging:** Population aging is identified as the key factor driving the increase in PM2.5-related deaths from 2015 to 2050, highlighting that climate policy alone will not suffice to completely alleviate the health burden.

This study's findings demonstrate that China's commitment to peak carbon emissions before 2030 aligns with substantial public health benefits. The results highlight the synergistic potential of climate change mitigation and air pollution control policies. The significant net economic benefits under more stringent climate policies emphasize that sustainable, low-carbon development pathways can be both environmentally and economically advantageous. However, relying solely on climate policy co-benefits for air quality improvements is insufficient, underscoring the necessity of complementary air pollution control measures. The increase in NH3 emissions in some scenarios, driven by agricultural practices and biomass fuel use, highlights the need for integrated policy approaches that consider trade-offs between climate change and air pollution control. Future research should focus on strategies to manage NH3 emissions while achieving climate targets.

This study provides strong evidence that achieving ambitious climate targets in China, such as peaking CO2 emissions before 2030, generates substantial air quality and health co-benefits that can outweigh the costs of mitigation. The findings underscore the need for integrated policy approaches to maximize the combined benefits of climate change mitigation and air pollution control. Further research is needed to optimize policies and explore synergies and trade-offs between these areas, especially strategies to manage NH3 emissions and adapt to population aging.

The study uses PM2.5 exposure effects to estimate the total health burden from air pollution, potentially underestimating the impact of other pollutants like O3 and NOx. The analysis also simplifies the complexity of PM2.5 composition, sources, and size distribution. Model uncertainties are present, particularly in GCAM projections, due to assumptions about technology selection, parameterization, and base year selection. The study isolates climate policy's effect on air pollution, excluding future air quality control policies (e.g., agricultural nitrogen management). Furthermore, meteorological conditions and baseline mortality rates are held constant across scenarios and years, potentially affecting the accuracy of projections.