Future ozone-related acute excess mortality under climate and population change scenarios in China: A modeling study

BackgroundClimate change is likely to further worsen ozone pollution in already heavily polluted areas, leading to increased ozone-related health burdens. However, little evidence exists in China, the world’s largest greenhouse gas emitter and most populated country. As China is embracing an aging population with changing population size and falling age-standardized mortality rates, the potential impact of population change on ozone-related health burdens is unclear. Moreover, little is known about the seasonal variation of ozone-related health burdens under climate change. We aimed to assess near-term (mid-21st century) future annual and seasonal excess mortality from short-term exposure to ambient ozone in 104 Chinese cities under 2 climate and emission change scenarios and 6 population change scenarios.Methods and findingsWe collected historical ambient ozone observations, population change projections, and baseline mortality rates in 104 cities across China during April 27, 2013, to October 31, 2015 (2013–2015), which included approximately 13% of the total population of mainland China. Using historical ozone monitoring data, we performed bias correction and spatially downscaled future ozone projections at a coarse spatial resolution (2.0° × 2.5°) for the period April 27, 2053, to October 31, 2055 (2053–2055), from a global chemistry–climate model to a fine spatial resolution (0.25° × 0.25°) under 2 Intergovernmental Panel on Climate Change Representative Concentration Pathways (RCPs): RCP4.5, a moderate global warming and emission scenario where global warming is between 1.5°C and 2.0°C, and RCP8.5, a high global warming and emission scenario where global warming exceeds 2.0°C. We then estimated the future annual and seasonal ozone-related acute excess mortality attributable to both climate and population changes using cause-specific, age-group-specific, and season-specific concentration–response functions (CRFs). We used Monte Carlo simulations to obtain empirical confidence intervals (eCIs), quantifying the uncertainty in CRFs and the variability across ensemble members (i.e., 3 predictions of future climate and air quality from slightly different starting conditions) of the global model. Estimates of future changes in annual ozone-related mortality are sensitive to the choice of global warming and emission scenario, decreasing under RCP4.5 (−24.0%) due to declining ozone precursor emissions but increasing under RCP8.5 (10.7%) due to warming climate in 2053–2055 relative to 2013–2015. Higher ambient ozone occurs under the high global warming and emission scenario (RCP8.5), leading to an excess 1,476 (95% eCI: 898 to 2,977) non-accidental deaths per year in 2053–2055 relative to 2013–2015. Future ozone-related acute excess mortality from cardiovascular diseases was 5–8 times greater than that from respiratory diseases. Ozone concentrations increase by 15.1 parts per billion (10−9) in colder months (November to April), contributing to a net yearly increase of 22.3% (95% eCI: 7.7% to 35.4%) in ozone-related mortality under RCP8.5. An aging population, with the proportion of the population aged 65 years and above increased from 8% in 2010 to 24%–33% in 2050, will substantially amplify future ozone-related mortality, leading to a net increase of 23,838 to 78,560 deaths (110% to 363%). Our analysis was mainly limited by using a single global chemistry–climate model and the statistical downscaling approach to project ozone changes under climate change.ConclusionsOur analysis shows increased future ozone-related acute excess mortality under the high global warming and emission scenario RCP8.5 for an aging population in China. Comparison with the lower global warming and emission scenario RCP4.5 suggests that climate change mitigation measures are needed to prevent a rising health burden from exposure to ambient ozone pollution in China.

研究背景 气候变化或进一步加剧本已污染严重区域的臭氧污染，进而增加与臭氧相关的健康负担。然而，作为全球最大温室气体排放国和人口最多国家的中国，相关研究证据仍较为匮乏。当前中国正面临人口规模变动、年龄标准化死亡率下降以及人口老龄化加剧的局面，人口变化对臭氧相关健康负担的潜在影响尚不明确。此外，气候变化背景下臭氧相关健康负担的季节变化特征也鲜有关注。本研究旨在评估两种气候与排放变化情景、六种人口变化情景下，中国104个城市在近期（21世纪中期）因短期暴露于环境臭氧所导致的年度及季节超额死亡人数。

研究方法与结果 我们收集了2013年4月27日至2015年10月31日（2013–2015年）期间中国104个城市的历史环境臭氧观测数据、人口变化预测数据及基线死亡率数据，覆盖中国大陆总人口约13%。基于历史臭氧监测数据，我们针对2053年4月27日至2055年10月31日（2053–2055年）的未来臭氧预测结果开展偏差校正与空间降尺度：将全球化学-气候模型（global chemistry–climate model）粗空间分辨率（2.0°×2.5°）的输出，降尺度至0.25°×0.25°的精细空间分辨率，所用情景为政府间气候变化专门委员会（Intergovernmental Panel on Climate Change, IPCC）提出的两种典型浓度路径（Representative Concentration Pathways, RCPs）：RCP4.5（中等全球变暖与排放情景，全球升温幅度介于1.5℃至2.0℃之间）与RCP8.5（高全球变暖与排放情景，全球升温幅度超过2.0℃）。随后，我们通过病因特异性、年龄组特异性及季节特异性的浓度-响应函数（concentration–response functions, CRFs），估算了由气候与人口变化共同导致的未来年度及季节性臭氧相关急性超额死亡率。我们采用蒙特卡洛模拟（Monte Carlo simulations）获取经验置信区间（empirical confidence intervals, eCIs），以量化浓度-响应函数的不确定性以及全球模型不同集合成员（即基于略微不同初始条件得到的3组未来气候与空气质量预测结果）间的差异。相对于2013–2015年，2053–2055年年度臭氧相关死亡率的预测结果对全球变暖与排放情景的选择较为敏感：在RCP4.5情景下，由于臭氧前体物排放下降，死亡率下降24.0%；而在RCP8.5情景下，由于气候变暖，死亡率上升10.7%。在高全球变暖与排放情景（RCP8.5）下，环境臭氧浓度更高，2053–2055年每年较2013–2015年新增1476例（95%经验置信区间：898至2977）非意外死亡。与呼吸系统疾病相比，心血管疾病相关的未来臭氧急性超额死亡率是其5至8倍。在较寒冷月份（11月至次年4月），臭氧浓度升高15.1十亿分之一（parts per billion, 10⁻⁹），导致RCP8.5情景下臭氧相关死亡率全年净增长22.3%（95%经验置信区间：7.7%至35.4%）。人口老龄化将显著加剧未来臭氧相关死亡率：65岁及以上人口占比将从2010年的8%升至2050年的24%至33%，由此导致的死亡人数净增长达23838至78560例（增幅110%至363%）。本研究的主要局限性在于仅使用了单一全球化学-气候模型，且采用统计降尺度方法预估气候变化背景下的臭氧变化。

研究结论 本研究表明，在高全球变暖与排放情景RCP8.5下，中国老龄化人口面临的臭氧相关急性超额死亡率将上升。与低全球变暖与排放情景RCP4.5的对比结果显示，中国需采取气候变化减缓措施，以避免环境臭氧污染暴露带来的健康负担进一步加重。