Short-lived greenhouse pollutants emitted largely from fuel combustion account directly or indirectly for a large proportion of present global warming. They also account for most of the direct damage to human health from energy use worldwide. These pollutants include two important health-damaging agents—sulphates and organic-carbon aerosols—which generally have global-cooling characteristics. Another aerosol, black carbon, is also health damaging, but is a warming agent. Other short-lived greenhouse pollutants include warming agents in the form of health-damaging gases such as ozone, a secondary pollutant formed after complex photochemical reactions, and other gases that contribute to ozone formation such as carbon monoxide, non-methane volatile organic compounds, methane, and nitrogen oxides. Most of these precursors to ozone also exert direct effects on human health.
Key messages
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Short-lived greenhouse pollutants need to be controlled in addition to regulating carbon dioxide emissions because they collectively create a substantial proportion of all human-contributed global warming and directly damage health. Importantly, control of some short-lived greenhouse pollutants may lead to quick reductions in global warming.
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Short-lived greenhouse pollutants include gases such as the directly health-damaging carbon monoxide and non-methane volatile organic compounds, and others responsible for ozone creation in the lower atmosphere such as methane. Aerosols of short-lived greenhouse pollutants include sulphate, organic carbon, and black carbon particles, which have differing climate implications: the first two cooling, but the third strongly warming.
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The toxicology of sulphate and black carbon in pure form does not adequately indicate their health effects in ambient conditions where they are closely associated with other pollutants. The epidemiological effects of atmospheric sulphate and black carbon therefore should be interpreted as representing mixtures.
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Meta-analyses of time-series studies of short-term exposure suggest larger mortality effects per unit mass of sulphate than of black smoke, an optical measure correlated with black carbon. Although measurements of black smoke correlate well with estimates of black carbon in some studies, black smoke measurements do not provide a reliable quantitative indicator of black carbon concentrations because of large variations by site, season, and year.
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Our analysis of a 66-city, 18-year nationwide US cohort provides estimates of the mortality effects of long-term exposure to elemental carbon, the best available measure of black carbon. This analysis shows stronger effects for elemental carbon than for undifferentiated fine particles (PM2·5), but the model estimates are unstable with respect to inclusion of other pollutants.
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Differential mortality effects between various components of PM2·5 are difficult to assess. Our analysis, however, does not lend support to the view that sulphate has smaller mortality effects than does undifferentiated PM2·5 and provides new evidence that long-term exposures to sulphates and ozone exert adverse effects on mortality that are independent of other constituents.
Conversely, carbon dioxide, the most important greenhouse gas, and nitrous oxide and halocarbons, the other long-lived greenhouse gases, have little direct effect on health. Nitrous oxide and halocarbons arise mainly from sources outside the energy supply system.
All short-lived greenhouse pollutants, whether warming or cooling, have effects on health when people are exposed to them or, in the case of methane, their atmospheric byproduct, ozone. Patterns of emissions and exposures vary greatly, and because the pollutants are short-lived, their health effects depend on the location of sources in relation to local and regional factors such as weather patterns, geography, and population distribution. Localised concentrations of ozone and black carbon have been identified as agents increasing the urban heat island effect by trapping heat and interacting with urban carbon dioxide concentrations.1 In turn, these short-lived greenhouse pollutants might increase health burdens from heat waves in urban cores.1
Several of the short-lived greenhouse pollutants have substantial effects on the human-managed and natural biosphere through acid precipitation (sulphate and nitrate),2 eutrophication (nitrate), direct damage to organisms (ozone), and in the case of black carbon deposition, accelerated melting of ice and snow. Increased forest growth through eutrophication could lead to interactions with the global carbon cycle through, for example, enhanced growth and carbon dioxide uptake, suggesting a cooling effect on climate,3 which might in turn be partly offset by ozone's negative effect on carbon uptake in ecosystems.4
Many important sources emit more than one short-lived greenhouse pollutant, and in some cases, control of one and not another is difficult—eg, black carbon and organic carbon from combustion of biomass, coal, and diesel fuel. Additionally, ozone and sulphate are not emitted directly, but are secondary products from transformations of precursor emissions in the atmosphere. The net effect of control measures on climate warming can thus be difficult to estimate because of simultaneous changes in both warming and cooling agents. This challenge is amplified by the scientific and policy complexities related to the widely different temporal patterns that characterise the burden of such pollutants compared with carbon dioxide control, which is usually the major consideration in climate negotiations and discussions.
Aggressive policies directed towards carbon dioxide reduction, although necessary for the long term, are by themselves insufficient to reduce the rate of warming in the next few decades because of the long atmospheric lifetime of this gas.5 Thus, governments will need to reduce warming from short-lived greenhouse gas pollutants when considering climate change mitigation policies.6, 7 Choice of policies with positive health and ecosystem effects provides the opportunity for substantial co-benefits (eg, reductions in ozone concentrations will diminish warming while also providing substantial health benefits to human populations and ecosystems) and climate protection. Alternatively, poor choices could result in major net additional risks to human health and ecosystems.
The short-lived greenhouse pollutants climate primer (panel) summarises the issues related to these pollutants. We examine the present state of health evidence for three important short-lived greenhouse pollutants: black carbon, sulphate, and ozone. The figure shows the relative importance of different sectors that are thought to cause most anthropogenic emissions for black carbon and the precursor of sulphate, sulphur dioxide. The relative emissions differ greatly by sector, which has important policy implications for control measures. We focus on the health effects of ambient pollution and not on exposures from indoor sources, such as household fuels, which also substantially affect health worldwide.12
We discuss the present state of knowledge of health effects on the basis of toxicological evidence in controlled settings and from observational epidemiological studies. This review includes new meta-analyses of time-series studies and new evidence for relative mortality effects of long-term exposures to sulphates, elemental carbon, and ozone from a national US cohort study. We conclude by discussing cross-cutting issues such as the benefits of removing remaining uncertainties and the need for analyses that incorporate both climate and health implications of control policies.