Effects of fossil fuel and total anthropogenic emission removal on public health and climate

Possible future impacts of air pollution on mortality have been estimated previously by using the output from chemistry–climate and chemical transport models that applied Representative Concentration Pathway scenarios (RCPs) (27). Apart from the fact that the new GEMM was not available at the time, it differed from our approach by estimating increased and avoided mortality from different RCPs, assuming that economic development drives air pollution control. Here we estimate the attributable effects of both fossil fuel and all anthropogenic air pollution plus greenhouse gases by removing emissions in the model, rather than applying time-dependent scenarios that rely on assumptions about socioeconomic futures. While this may seem a daring assumption, these source categories will need to be phased out to reach the 2 °C target of the Paris Agreement (7). We realize, however, that especially agricultural emissions cannot be fully avoided in a world with growing food demand, although a large fraction (e.g., of ammonia and methane sources) could be effectively controlled. A limitation of our approach is that it refers to emission and demographic data for 2015, while population growth is expected up to about 9 billion by the middle of the century, especially in developing countries where air pollution levels can be very high. Therefore, we performed sensitivity simulations that account for the projected population in 2050, indicating that the total excess mortality may be about 10% higher globally. Most of the increase is due to population growth in South and West Asia, and especially in Africa, while in East Asia the population is expected to decline. In Europe and the Americas (notably South America) projected population changes are comparatively small. Interestingly, the avoidable excess mortality in 2015 and 2050 are practically the same (within 1–2%). Furthermore, we did not take into account that the introduction of clean household energy in low-income countries could substantially reduce excess deaths from household air pollution, estimated at about 2.9–4.3 million per year (3). The phaseout of anthropogenic emissions is expected to be paralleled by a reduction of household air pollution from the introduction of clean “zero carbon” domestic energy sources.

SI Appendix, Fig. S4 presents model results of the direct and total aerosol radiative forcing at the top of the atmosphere (TOA). The direct aerosol forcing is −0.46 ± 0.009 W/m², which closely agrees with the best estimate of −0.45 W/m² of the IPCC (11). Our total direct plus indirect radiative forcing is −0.9 W/m², which also agrees with IPCC (11). However, by also accounting for the chemical aging of aeolian dust by air pollution we obtain a −0.3-W/m² larger effect, about −1.2 ± 0.06 W/m² (28, 29). The dust aging has multiple consequences, such as increased solar radiation scattering from hygroscopic particle growth and decreased lifetime from more efficient rainout, while the climate effect is dominated by the enhanced CCN activity. Since dust particles are globally abundant and relatively large in size, their increased hygroscopicity effectively enhances cloud droplet formation (29). To account for these effects, it is needed to simulate the particle chemistry and thermodynamics of crustal ions, dependent on the composition of aeolian dust from different deserts, which was not included in the IPCC climate models. SI Appendix, Figs. S3 and S4 show the global, multiannual mean geographic distribution of the aerosol direct and total radiative forcing, both at the TOA and the bottom of the atmosphere (BOA), making the distinction between fossil fuel generated and all anthropogenic aerosols. The differences between the TOA and BOA forcings represent heating of the atmosphere through the absorption of sunlight, mostly by black carbon. We find that the negative (cooling) BOA forcings from aerosols are generally largest over South and East Asia, partly in excess of −20 W/m² (annual average), which can regionally dwarf the positive (warming) radiative forcing from greenhouse gases. The strong surface cooling downwind over the ocean can significantly reduce evaporation and precipitation on a regional scale (16).

We performed equilibrium climate response computations, following the example of Shindell et al. (15), with the difference that they computed atmospheric composition changes offline with different models, whereas we calculate the processes online with a coupled atmosphere–ocean model. The equilibrium assumption is justified for short-lived climate forcers since most of the climate response that follows phaseout of fossil fuels and other pollution sources happens within a few decades. Although the equilibrium assumption does not apply to CO₂, this is not relevant here because the phaseout of CO₂ emissions does not translate in a near-term temperature decrease (11, 19–22). In the period during which CO₂ concentrations still increase, the warming is tempered by heat transport into the deep oceans. When CO₂ emissions are phased out, the atmospheric concentrations can decrease, but with a delay due to the slow uptake of anthropogenic CO₂ by the oceans (30). These physical climate and carbon cycle effects are of opposite sign and of similar magnitude. Consequently, even if fossil CO₂ emissions stop abruptly, global temperatures remain constant for several centuries, which means that past CO₂ emissions commit the planet to persistent warming on the human timescale (for a discussion, see ref. 11). While the timing of air pollution and greenhouse gas emission phaseout is the subject of scenario studies, here we focus on the climate response magnitude. For example, global warming from increasing CO₂ scales approximately linearly with cumulative emissions (11). It implies that the phaseout may occur over 5 or 50 y, but the integral climate response over these periods is the same. In fact, the key factor is “societal inertia” from the slow phaseout of polluting infrastructure, often constructed to last for many decades (30, 31).

The warming and precipitation pattern changes that result from removing aerosols in our climate simulations (Fig. 3 and SI Appendix, Fig. S7) are comparable to those presented by Samset et al. (32), who analyzed the ensemble results from four climate models. They reported that the removal of anthropogenic aerosols causes a global mean surface warming of 0.5–1.1° and a precipitation increase of 2.0–4.6%, where we find 0.73 ± 0.03 °C and 3.2 ± 0.2%, respectively. Consistent with our results, they showed how the aerosol-related climate response patterns differ markedly from those of greenhouse gases. However, they studied anthropogenic emissions of SO₂ and fossil fuel black and organic carbon, without the distinction between fossil fuel use and other anthropogenic sources, and did not consider pollution impacts on dust. Further, they did not account for the greenhouse gases O₃ and CH₄, and calculated CO₂ and aerosol reductions separately to contrast the cooling and warming patterns. While greenhouse gases act globally, the radiative forcing and consequent net warming from aerosol removal is regional and most pronounced over the continental northern hemisphere (Fig. 3). In agreement with the results of Samset et al., we find that the removal of aerosols creates south–north shifts of precipitation over the Pacific and Atlantic Oceans and relatively strong rainfall increases over Africa, South and Northeast Asia (Fig. 2 and SI Appendix, Fig. S7). Westervelt et al. (33) studied precipitation changes from the removal of SO₂ emissions in the United States. The computed reductions of aerosol sulfate and cloud droplet number concentrations were found to cause increasing precipitation over the United States, and downwind over the Atlantic Ocean, qualitatively consistent with our results. SI Appendix, Figs. S3 and S4 illustrate that the direct radiative effect of aerosols at the BOA typically exceeds that at the TOA by a factor of 2–3, e.g., due to solar radiation absorption within the atmosphere by black carbon, and that the radiative forcing can regionally exceed the global average by a factor of 5–10. This results in strong, although localized influence on surface temperature, evaporation and associated precipitation.

Numerous studies have addressed the causes of observed decreases in precipitation over tropical regions, and analyzed the influence of pollution aerosols on the monsoon circulation over South and East Asia, and to a lesser extent over Africa. An exhaustive review of the South and East Asian studies can be found in Li et al. (34). The basic inference has been that the radiative forcing from increasing greenhouse gases is unlikely to account for the sign and magnitude of negative precipitation trends, associated with a weakening of the monsoon. Dominant factors are trends in sea-surface temperature (SST), particularly the asymmetric warming of the southern tropical oceans relative to the northern oceans. Two principal forcing mechanisms have been proposed. Interdecadal variability of SSTs; and the negative aerosol forcing (from direct and indirect effects) of the ocean surface, which is larger in the northern than the southern hemisphere. Further, the solar dimming from pollution aerosols reduces the land–ocean heating contrast (since pollution levels are highest over land areas), so that winds at the surface weaken and evaporation decreases, hence affecting the dominant drivers of the monsoon. There is compelling evidence that anthropogenic aerosol forcing explains most of the drying tendency in South Asia, also simulated by our model, and supported by observations (35–38). Our results support those of Chen et al. (39), who modeled pollution-induced precipitation reductions over Southeast and Northeast Asia, triggered by aerosol cooling of the midlatitude sea surface. Likewise, in East Asia aerosol cooling over land reduces the thermal contrast with the ocean, and weakens the monsoon circulation (40). Further, it was found that the weakening monsoon contributes to the pollution haze in the Beijing area (41). Negative rainfall trends have been observed over Northeast China, enhanced by the aerosol forcing, which was not captured well by climate models in the past (34, 42), which underscores the need for a comprehensive account of these processes.

For the Sahel, drying during the latter part of the 20th century has been largely attributed to changes in SST patterns, influenced by external greenhouse gas and aerosol forcings as well as internal climate variability (43–45). By comparing climate model simulations with increasing greenhouse gases to those that also account for pollution aerosols it was shown that the latter cause a meridional SST gradient, associated with a reduced West African monsoon flow (46). Hence, the reduction or removal of anthropogenic aerosol forcing can be expected to increase the North Atlantic SST, leading to a moistening of the Sahel (47, 48). In addition, warming of the Mediterranean, through a decrease in solar dimming from European pollution, contributes to evaporation and moisture transport into the eastern Sahel (49, 50). Our results demonstrate that the asymmetric aerosol radiative forcing creates interhemispheric heating gradients and meridional rainfall shifts over all tropical oceans, including the Pacific (Fig. 2), which could be restored by removing the particulate pollutants. While Central America has been affected by droughts throughout its history, including El Niño influence on summer precipitation, our model simulations suggest that recent dryness, e.g., in Mexico, has been aggravated by air pollution. More work will be needed to substantiate these findings. Generally, it is a major challenge to attribute the contributions of greenhouse gas and aerosol forcings to decadal precipitation trends, and distinguish them from natural variability of the atmosphere–ocean system.

Our model results show that the phaseout of air pollution emissions leads to substantial precipitation increases in these four regions. Recovery from aerosol-perturbed rainfall patterns could be relevant for countries with rapidly growing populations, e.g., in the Sahel, South Asia, and West Africa, which depend on monsoon rainfall for water availability and food production. While the surface temperature and hydrologic cycle respond rapidly to the radiative forcings of short-lived climate pollutants such as aerosols, the response to CO₂ emissions is irreversible for centuries (11, 19–22, 30). On the other hand, the simultaneous reduction of short-lived greenhouse gases, such as O₃, counteracts the warming from removing aerosols on short timescales. We find that the fossil-fuel-related and the total anthropogenic O₃ cooling could compensate about 0.12 and 0.19 °C of the aerosol warming, respectively (i.e., counteracting the 0.51 and 0.73 °C warming from fossil and total anthropogenic aerosols). The reduction of fossil methane, on the other hand, has limited impact, as it accounts for less than 20% of the total source, while an additional 40% of the emissions are related to other anthropogenic activities such as agriculture (rice paddies, domesticated animals), landfills, biomass burning, and biofuel use. Phasing out fossil as well as nonfossil CH₄ emissions additionally moderates the warming from removing aerosols by about 0.21 °C.

The importance of reducing methane emissions to mitigate climate change, in part through the impact on O₃, and together with black carbon, was emphasized by Shindell et al. (15), who calculated the cobenefits for human health and food security. They developed CH₄ and black carbon scenarios to optimally decrease the rate of climate warming (up to about 0.5 °C), and considered improved air quality and food security as cobenefits. Here we stress that a complete phaseout of fossil fuels, and accompanying reductions of other anthropogenic emissions, will be needed to reverse the major impacts on public health, regional climate, water supply, and food production. The prospect of preventing millions of excess deaths attributable to air pollution, and restoring perturbations of the hydrologic cycle that have contributed to regional drying, with the cobenefit of limiting climate warming to below 2 °C, is compelling and underscores the urgency of acting on global environmental change.