Heat waves and air pollution extremes exert compounding effects on human health and food security and may worsen under future climate change. On the basis of reconstructed daily O3 levels in China and meteorological reanalysis, we found that the interannual variability of the frequency of summertime co-occurrence of heat wave and O3 pollution in China is regulated mainly by a combination of springtime warming in the western Pacific Ocean, western Indian Ocean, and Ross Sea. These sea surface temperature anomalies impose influences on precipitation, radiation, etc., to modulate the co-occurrence, which were also confirmed with coupled chemistry–climate numerical experiments. We thus built a multivariable regression model to predict co-occurrence a season in advance, and correlation coefficient could reach 0.81 (P < 0.01) for the North China Plain. Our results provide useful information for the government to take actions in advance to mitigate damage from these synergistic costressors.

Heat waves and air pollution are two prominent threats, both of which have been reported to cause public health and ecosystem crises, particularly under rapid urbanization and global warming (1, 2). Heat waves, defined as consecutive days of excessively high atmosphere-related heat stress (3, 4), adversely influence human health by impacting respiratory and cardiovascular systems. Heat waves are linked with high O3 episodes that harm human health and vegetation (5–7). In warm seasons, heat waves and extreme O3 events often occur simultaneously due to common driving meteorological conditions, i.e., stagnant high-pressure systems that favor accumulation of heat and O3 precursors (8). Besides, complex chemistry–climate feedbacks through biogenic emissions (source) and uptake by plants (sink) could exacerbate co-occurrence of heat wave and O3 extremes (9). It is imperative to understand driving factors for the co-occurrence of heat and O3 extremes, as accumulating evidence suggests amplified health outcomes beyond the sum of individual effects (10–12). Analitis et al. (13) reported that the number of daily deaths during heat wave episodes was 54% higher on high O3 days compared with low O3 days.

Previous studies have linked occurrences of heat waves or O3 extremes, separately, with large-scale atmospheric circulation or sea surface temperature (SST) anomalies (14–20). For instance, Zhu et al. (17) demonstrated that the frequency and variability of summertime heat waves over North America was closely associated with SST anomalies in the tropical Atlantic and tropical western Pacific in spring and El Niño–Southern Oscillation phase change. Shen and Mickley (21) showed that O3 concentration in Eastern United States was linked with warm tropical Atlantic SST and cold northeast Pacific SST, as well as positive sea-level pressure (SLP) anomalies over central Pacific and negative SLP anomalies over the Atlantic and North America. However, the climate factors modulating the co-occurrence of heat and O3 extremes at a regional level remain unclear and had only been the subject of limited studies (8, 22–24).

With roughly one-sixth of the world’s population and rapid energy-intensive development, China is facing the dual challenge of air pollution and climate change (25, 26). Central and Eastern China, especially the North China Plain (NCP), experienced improved PM2.5 air quality over past years due to the implementation of the most stringent clean air policy, but now suffers from largest increases in summertime O3 exposure (27). O3 concentrations in the NCP enhanced at almost twice the average pace across China (28). An amplified upward trend of the joint occurrences of heat and O3 extremes has been identified in China over 2013 to 2020 (29). Understanding the driving climate factors for its interannual variability would contribute to long-term planning of control of costressors. Characterizing interannual variability also enables prediction which could allow sufficient time for mitigation of the interactive damages from joint exposure (21, 30–33). Previously, we demonstrated the possibility of seasonal prediction of wintertime aerosol pollution in India (34). Considering the strong linkages between O3 level and climate patterns, we argue here that it may also be possible to predict co-occurrence of heat waves and O3 pollution, potentially up to several years in advance, considering the active efforts in developing reliable seasonal (months ahead) and even longer prediction of climate variability (35).

In this study, we aim to identify leading patterns that control the spatiotemporal variability of occurrence frequency (days in a year) of joint heat wave and O3 pollution events (HWOP). We focus on Central and Eastern China (17.5°N to 47.5°N, 98°E to 125°E), where over 80% Chinese population reside and co-occurrences of HWOP events are prominent. Climate drivers are identified by empirical orthogonal function (EOF), which decomposes historical spatiotemporal variations of HWOP frequency that inferred with atmospheric reanalysis and reconstructed daily O3 datasets. Findings from statistical analyses are further supported by numerical model experiments using the state-of-the-art Community Earth System Model version 2.1.3 (CESM v2.1.3). Encouraged by the robustness of the identified teleconnections between co-occurrence events and SST anomalies, we further build a regression-based statistical model to predict summertime HWOP a season in advance, improving our capability in the management of these important health and vegetation costressors.

 

With over 700 million km2 Siberia is the largest expanse of the northern boreal forest—deciduous-needleleaf larch. Temperatures are increasing across this region, but the consequences to carbon balances are not well understood for larch forests. We present flux measurements from a larch forest near the southern edge of Central-Siberia where permafrost degradation and ecosystem shifts are already observed. Results indicate net carbon exchanges are influenced by the seasonality of permafrost active layers, temperature and humidity, and soil water availability. During periods when surface soils are fully thawed, larch forest is a significant carbon sink. During the spring-thaw and fall-freeze transition, there is a weak signal of carbon uptake at mid-day. Net carbon exchanges are near-zero when the soil is fully frozen from the surface down to the permafrost. We fit an empirical ecosystem functional model to quantify the dependence of larch-forest carbon balance on climatic drivers. The model provides a basis for ecosystem carbon budgets over time and space. Larch differs from boreal evergreens by having higher maximum productivity and lower respiration, leading to an increased carbon sink. Comparison to previous measurements from another northern larch site suggests climate change will result in an increased forest carbon sink if the southern larch subtype replaces the northern subtype. Observations of carbon fluxes in Siberian larch are still too sparse to adequately determine age dependence, inter-annual variability, and spatial heterogeneity though they suggest that boreal larch accounts for a larger fraction of global carbon uptake than has been previously recognized.

Plain Language Summary

Cold, wet soils in boreal forests contain a large amount of carbon. However, warmer temperatures coupled with changes in hydrology could release stored carbon and accelerate its decomposition. The boreal spruce and pine forests in North America and Fennoscandia have been studied extensively, but observations in the Siberian larch forests are limited. Because larch shed their needles in winter their response to changing temperature and moisture may differ from expectations based on evergreen conifers. Our work focuses on a larch forest in northern China that is at the southern edge of the Central-Siberian biome where eco-environmental changes are starting to occur. By studying how the annual growth and carbon balance in this forest respond to variations in weather we will be better able to predict significant changes in the structure and function of the larch ecosystem that could undermine regional ecosystem stability. Larch forest functions differently from evergreen needle-leaf forests and provides the larger carbon sink than had been previously recognized.

Key Points

• Seasonality in permafrost active layer and environmental temperature-humidity dynamics closely regulate boreal larch’ carbon cycle

• Ecosystem functional traits in deciduous larch are distinct from other boreal needleleaf evergreens

• By inadequately accounting for boreal larch's carbon sink, the estimates of global forest carbon budgets will be biased low

Climate actions have focused on CO2 mitigation and only some studies of China consider non-CO2 greenhouse gases (GHGs), which account for nearly 18% of gross GHG emissions. The economy-wide impact of mitigation covering CO2 and non-CO2 GHGs in China, has not been comprehensively studied and we develop a multi-sector dynamic model to compare the impact of CO2-only mitigation with a multi-GHG mitigation policy that also price non-CO2 GHGs. We find that the multi-GHG approach significantly reduces the marginal abatement cost and economic loss to reach the same level of GHG emissions (measures as 100 year global warming potential) compared to a CO2-only scenario. By 2060, multi-gas mitigation can reduce the tax rate by 15.44% and improve real gross domestic product (GDP) by 0.41%. The aggregate gain brought by multi-GHG mitigation are robust to various pathways and but vary across periods and sectors.