CO2 emission trajectory
Nine scenarios combining the characters of socio-economic developments and the ambitions of climate policies are considered in this study, represented respectively by the Shared Socioeconomic Pathways (SSPs) and Representative Concentration Pathways (RCPs). When a specific long-term radiation forcing (RF) target (e.g., 2.6 W/m2 in 2100 under RCP2.6) is imposed as a quantitative constraint on the SSPs (e.g., SSP1, SSP2 or SSP5), the cumulative carbon emissions by 2100 are fixed to meet that RF level for various scenarios, labeled accordingly as SSP1_RCP2.6, SSP2_RCP2.6, and SSP5_RCP2.6 in Table 1. The set of climate policy assumptions (SSPx_RCPx) will thus determine the outcomes of the scenario analysis, including on energy structure and carbon trajectories (see Supplementary Note 1.2).
In China, industry and power generation are the two major sources of CO2 emissions12, contributing more than 80% of the total emissions in 2010 due to the sectors’ intensive use of fossil fuels, especially coal (Supplementary Table 2). Adjustment of the energy structure is the key to mitigating China’s CO2 emissions. For example, to achieve the 1.5 °C goal under the SSP1 pathway (i.e., SSP1_RCP1.9), the fossil fuel share of total primary energy would decline from 91.6% under SSP1_REF to 83.8% under SSP1_RCP1.9 in 2030, and from 85.2% to 42.9% in 2060 (Supplementary Fig. 2 and Note 1.3). And the share of biomass fuels and non-biomass renewable energy (e.g., solar and wind) under SSP1_RCP1.9 would respectively increase to 24.1% and 31.4% in 2060. A strong upscaling of non-fossil fuels (e.g., biomass fuels and nuclear power) also occurs in the SSP2_RCP1.9 and SSP5_RCP1.9 scenarios. Moreover, significant deployment of carbon capture and storage (CCS) seems to be an essential step, especially to achieve more ambitious climate targets in the future. For the SSP2 and SSP5 scenarios, most CO2 emissions from fossil fuels (~98%) and biomass fuels (~92%) used in electricity generation and industry would be captured and stored by 2050 (Supplementary Fig. 3).
The rapid shift away from fossil fuels towards zero- or low-carbon energy sources and increased use of CCS under a strict climate target would lead to substantial reduction in carbon emissions. As illustrated in Fig. 2, under all analyzed mitigation scenarios (i.e., RCP2.6 and RCP1.9), China’s CO2 emissions would peak at 10,423–15,500 Mt CO2 yr−1 in or before 2030, meeting the current NDC target of “around 2030” (Supplementary Table 1). Under the 1.5 °C scenarios (RCP1.9), the peak would occur earlier, around 2020 (10,645~11,901 Mt CO2 yr−1), meeting China’s newer peak year commitment of “before 2030.” Note also that China would be projected to achieve another NDC commitment of cutting CO2 emissions per unit gross domestic product (GDP) to 0.47 t-CO2/1000 USD, a reduction of 60% from 2005 levels, by 2030 in all scenarios (see Fig. 2 and Supplementary Fig. 1).
Co-benefits for ambient air pollution
The implementation of climate policies to mitigate CO2 emissions would yield co-benefits in reduced air pollutant emissions in China (Fig. 3, Supplementary Figs. 4, 5). Emissions of air pollutants, with the exception of ammonia (NH3), decrease with reductions of CO2 emissions as the climate policy becomes more stringent. But the level of mitigation differs by pollutant species, mainly reflecting their specific sectoral sources (Fig. 3). For example, as energy use is the primary source of sulfur dioxide (SO2), nitrogen oxides (NOX) and CO2 emissions12, CO2 mitigation measures such as reduced coal consumption bring great co-reductions in SO2 and NOX emissions. As the major sources of volatile organic compounds (VOCs), black carbon (BC) and organic carbon (OC) emissions do not overlap with major sources of CO2 (see Supplementary Table 3 and Table 5), it makes sense that the co-benefits of CO2 reductions on their emissions are not as significant as those of SO2 and NOX. In China, land use (including agriculture) produces almost all (>99%) of NH3 emissions, which are therefore barely affected by policies to mitigate CO2 in the scenarios. In fact, separate from the growing food demand that accompanies economic development, large amounts of biomass fuels are required in all climate policy scenarios (both RCP2.6 and RCP1.9) (Supplementary Fig. 2 and Note 3.1); both lead to increased ammonia emissions due to increased demand for and application of nitrogenous fertilizer (Fig. 3).
In 2015, the population-weighted concentration of PM2.5 (PWC-PM2.5) was 53.0 μg/m3 in China. The implementation of different climate policies will result in different levels of PM2.5 mitigation relative to the REF scenario (see Fig. 4a, Supplementary Table 6 and Fig. 7), and the climate policies will yield greater improvements to air quality in the long run, as they become more stringent. For example, the PWC-PM2.5 in the SSP1_RCP1.9 scenario is 50.9 μg/m3 in 2030 and 2.1 μg/m3 lower than that of the SSP1_REF scenario (Fig. 4b), while in 2050 it will decrease to 43.4 μg/m3 and 5.1 μg/m3 lower (Fig. 4c). Under the same socio-economic path (SSP), the RCP1.9 scenario has a lower PWC-PM2.5 than the RCP2.6 scenario. This implies that stricter climate policies would not only mitigate more CO2 emissions but also lead to greater air quality improvement. In 2050, SSP1_RCP1.9’s PWC-PM2.5 is 3.5 μg/m3 lower than that in the RCP2.6 scenario.
Nevertheless, the co-benefits of climate policies alone on air pollution mitigation are limited. With the most ambitious assumptions of the SSP1_RCP1.9 scenario, about 778 million people in China will still be exposed to ambient air with annual average PM2.5 concentrations above 35 μg/m3 in 2050. This will not meet the latest air quality objective declared in the country’s “Beautiful China” strategy, which aims to reduce PM2.5 concentrations in all Chinese cities to at most 35 μg/m3 by 2035. The co-benefits of climate policies will not be enough to realize China’s air quality goals, which will also require strengthening of dedicated air pollution control policies10.
It is notable that the PWC-PM2.5 of SSP1_RCP2.6 is slightly higher (0.4 μg/m3) than that of SSP1_REF in 2030 (Supplementary Fig. 8a). Especially for provinces with large agricultural outputs like Henan, Shandong, and Jiangsu13, their PWC-PM2.5 levels in the SSP1_RCP2.6_2030 scenario are higher than those in the SSP1_REF_2030 scenario, by 0.7, 0.5, and 0.5 μg/m3, respectively in these three provinces (see Supplementary Note 3.1). Because emissions of the acidic gases NOX and SO2 do not decline enough by 2030 under SSP1_RCP2.6, increasing NH3 emissions (Fig. 3) discussed above may hinder PM2.5 mitigation by enhancing formation of ammonium nitrate and sulfate14,15. These key forms of secondary PM2.5 comprise nearly 30% of China’s annual average concentration of PM2.516,17. See details in Supplementary Note 3.
In 2015, the deaths attributable to ambient PM2.5 exposures in China are estimated at 2,441,900 (95% CI, 2,052,000–2,809,100), recognizing that in reality the mortality impacts of chronic exposure to air pollution may be distributed over years or even decades after exposure18,19. In 2030, they grow to 3.6–3.9 million across all scenarios and increase to 6.4–7.5 million in 2050, with the elderly (>65 yr) particularly at risk (see Fig. 5, Supplementary Table 7). We find that aging of the population is the leading causal factor of such growth (see Supplementary Note 4), similar to previous studies20. This reinforces that relying on the co-benefits of climate policy alone will be insufficient to prevent the growth of the PM2.5-related health burden in China before 2050, especially given aging of the population.
Nevertheless, implementation of climate policies would yield considerable health co-benefits by reducing PM2.5-related mortality across all provinces in the analysis years of 2030 and 2050 (Fig. 6, Supplementary Table 8 and Fig. 9) compared to scenarios with no climate policy (SSPx_REF), except for SSP1_RCP2.6 in 2030 as discussed in the last section. And the more stringent the climate policy is, the greater the health benefits it would bring. For instance, the PM2.5-related deaths in SSP1_RCP1.9 are 3.7% and 6.1% lower than those in SSP1_RCP2.6 by 2030 and 2050, respectively (Fig. 6c and e). In addition, the health co-benefits of climate policies tend to increase in the longer run. For example, the PM2.5-related deaths in SSP1_RCP1.9 are 3.2% (118,000 deaths) and 8.7% (614,000 deaths) less than those in SSP1_REF, in 2030 and 2050, respectively (Fig. 6b and d). In addition to larger reductions in the long-run PM2.5 concentrations due to the reasons discussed above, sustained aging of the population exacerbates the baseline mortality risk of PM2.5 exposure (see Supplementary Note 4), both of which yield greater total reductions in PM2.5-related mortality in 2050 as a result of climate policies.
Applying estimates of mitigation costs and the value of statistical life (VSL) discussed in “Methods”, our analysis shows that the health co-benefits brought by CO2 emission reduction policies could fully offset the mitigation costs in the long-term, to 2050 (Table 2). Compared to the REF scenarios, SSP1_RCP2.6 and SSP5_RCP2.6 will produce net benefits of 393 billion and 3017 billion (in 2017 USD value) in 2050, respectively, equivalent to 0.45% and 2.77% of China’s GDP in 2050. Note that the mitigation cost mentioned here is from the Global Change Analysis Model (GCAM), which is an approximation of the welfare loss from emission mitigation efforts based on theoretical carbon tax revenue21,22. It refers to direct abatement costs such as those of new power plants and CCS technologies, but not the full macroeconomic costs in terms of GDP or consumption losses23. Co-reductions in air pollutant emissions under climate policies could also induce significant cost savings in pollution control, an additional co-benefit of climate policies that is not included in this study (see Supplementary Note 5.3).
The peak of China’s CO2 emissions is almost certain to occur at some point during 2020–2030. China could realize its NDC commitment by peaking CO2 emissions around 2030 under the 2 °C scenarios, and earlier, before 2030, under the 1.5 °C scenarios. The stricter the climate policy and the cleaner the socio-economic pathway, the earlier the peak of CO2 emissions will occur and the larger the air quality and public health co-benefits achieved in the long run. Such health co-benefits could substantially if not fully offset the costs of more stringent climate policies and bring significant net economic benefits in 2050.
China’s Ministry of Ecology and Environment has set up a Department of Climate Change to promote synergies between climate change mitigation and air pollution control policies. Our results imply that the path of sustainable, low-carbon social and economic development (SSP1) will bring great environmental and health benefits for China at an affordable cost, especially in the long run. Implementing climate policies as soon as possible, especially decarbonizing the energy structure and applying CCS in industry and power sectors, can not only ensure achievement of the emission reduction target set in China’s NDC, but also bring more net co-benefits in air quality, public health and the economy. However, relying on the air pollution co-benefits of such climate policies alone is insufficient to prevent the growth of PM2.5-related mortality in China, and it is also necessary to adopt other measures, including further tightened conventional pollution controls, greening infrastructure with alternative building materials, and designing cities to encourage low-emission lifestyles.
A climate policy that increases the share of biomass fuels will likely combine with growing food demand to increase use of chemical fertilizer and, therefore, NH3 emissions, potentially exacerbating PM2.5 pollution and related health burdens in some regions. Measures should be taken to effectively control NH3 emissions. These could include enhancing the nitrogen use efficiency of fertilizer application and optimizing the scale of agricultural operations24,25, which may also reduce emissions of N2O, another greenhouse gas26. Informed by improved understanding of complex air pollution formation mechanisms (including emissions, atmospheric transport, and secondary chemistry), future policy analyses should also attach greater importance to both synergies and trade-offs between climate change mitigation and air pollution control. Only through policies that coordinate control of air pollution and carbon emissions can policymakers maximize their total benefits.
We used effects of PM2.5 exposures to estimate the total health burden caused by air pollution. While ignoring health effects of exposure to other air pollutants (e.g., O3 and NOX) may lead to underestimation, it is likely to be small because the health burden of PM2.5 is estimated to be around 6.2-fold greater than that of O3 on average27 and controversy exists about whether health effects of NOX double-count those of nitrate PM2.528. Also, the common approach of using mass concentrations of PM2.5 was used, disregarding composition, sources and particle size distribution. All of these characteristics may have different impacts on health29, but are commonly ignored in such studies as the epidemiological evidence is currently limited and inconsistent30,31.
In addition, uncertainties may be introduced by various factors in the integrated assessment model, GCAM, including methods of model application, selection of representative technologies and parameterization, and choice of the base year32. As we attempted to isolate the impact of climate policy on air pollution, we also did not consider special policies of future air quality control in our scenario analysis. For example, air quality could be further improved if stronger agricultural nitrogen management is implemented in the future, for example increasing the efficiency of nitrogen use during fertilizer application and agricultural waste management24,33. Nevertheless, the GCAM results for different scenarios here are reasonable, showing consistent trends in emissions of CO2 and air pollutants with the results of the other five integrated assessment models selected by the IPCC23.
Finally, since we mainly focus on the air pollution and health co-benefits of carbon reduction, the meteorological fields (from 2015) were assumed to remain the same for all scenarios and years, to eliminate possible confounding effects of climate change. Similarly, due to the lack of projected baseline mortality data for 2030 and 2050, we assume that the baseline mortality rates will remain the same as 2015 in the future.