A lot of emphasis when it comes to climate change is placed on reducing carbon dioxide emissions, to prevent future global mean warming. CO2 stays in the atmosphere for a very long time though, and any emission reductions will take decades to produce noticeable results. Consequently the benefits of moving to a low-carbon economy will not be realised for a long time. Perhaps this is why progress towards curbing CO2 has so far been painfully slow. However CO2 is one of many detrimental substances that humans release into the atmosphere, and many other atmospheric pollutants can have more rapid impacts on global temperatures.
Three of the most significant contributors to global warming after CO2 are methane, tropospheric ozone, and black carbon aerosol. Methane is released into the atmosphere from fossil-fuel extraction and processing, leakage from gas pipes, decomposition of waste, and cattle farming, as well as natural sources such as wetlands. Tropospheric ozone forms chemically in the atmosphere from methane and other precursor gases. Methane emissions therefore also help determine ozone levels. Black carbon is produced from incomplete combustion, particularly from diesel vehicles and biomass burning.
Because these three substances get removed from the atmosphere much faster than CO2, lasting from only a few days for black carbon to about 10 years on average for methane, reducing their emission has the potential to significantly reduce global warming on much shorter timescales than CO2 measures. In fact, if we comprehensively implement a relatively small number of existing controls on methane and black carbon emissions (for example, recovering gas from fossil fuel production, and more widely adopting diesel particle filters and clean-burning cooking stoves), we could reduce the global mean temperature rise by about 0.5°C over the next 30-40 years (Shindell et al., 2012).
Unfortunately, the limitation of cutting these short-lived emissions is that the effect of slowing down global warming is also relatively short-lived. We would only benefit from reduced warming rates for about 30 years, after which the impact of cutting these emissions is lost (Fig. 1). In contrast, cutting CO2 emissions would result in a permanent reduction in global warming rates, despite it taking longer before the effect is felt. As a result, how soon CO2 controls are implemented is what ultimately determines how long the climate will continue to warm for. It would therefore be misguided to see controls on short-lived pollutants as being an easy alternative to controlling CO2. If we care about stopping global warming in the long run, there is no substitute for reducing CO2 emissions, and sooner rather than later. The best case scenario would of course be to cut methane, black carbon, and CO2 emissions simultaneously – giving both a short-term and a long-term effect, which when combined will result in the least warming overall. However if you could only pick one set of measures, perhaps because of limited resources or political willpower, then reducing CO2 should be the obvious place to focus your efforts.
It’s not as simple as that though, because a near-term reduction in global mean warming is far from the only benefit of reducing black carbon and methane emissions. These emissions are not just bad for the climate; they are also very bad for people.
Air quality due to pollution is a major public health issue, and also one that people are far more likely to engage with due to its more immediate ramifications. The greatest harm is done by inhaling very fine particulate matter, to which black carbon is a major contributor; inhalation of these fine particles causes heart and lung diseases, including lung cancer. Tropospheric ozone also is toxic and causes respiratory problems. It’s been estimated that cutting our emissions of black carbon and methane using existing control measures could cut premature deaths due to air pollution by around 2.4 million deaths per year globally in 2030 (Shindell et al. 2012). The greatest benefits would be seen in Asia, where both populations and pollution are high (Fig 2.). Inhalation of black carbon particles is responsible for most of these avoidable deaths, and since black carbon has a particularly short lifetime the health benefits of emission reductions would start to be seen very quickly. Even aside from any arguments of climate co-benefits or morality, the economic value of the lives saved would alone justify implementing a wide range of black carbon measures.
Controlling black carbon and methane represents a win-win situation, in terms of benefitting both public health and – at least in the short term – climate. Of course, if we really care about public health (which we should) then we should also be encouraging more widespread controls on other kinds of air pollution. For example sulphur emissions (primarily from fossil fuel burning) again contribute significantly to harmful particulate matter, as well as producing ecologically damaging acid rain. In this case though there can appear to be a potential conflict of interest between air quality and climate. Unlike black carbon and methane, higher levels of sulphate particles actually cool the planet, and have probably offset a certain amount of the global warming that we would otherwise have already experienced. Injection of sulphate aerosol into the upper atmosphere is even one proposed scheme for geo-engineering, to help mitigate global warming. The conflict only arises though if global mean temperature is the climate metric you care about. No-one actually experiences global mean temperature directly though; they experience their local weather. Short-lived pollutants like aerosols and ozone have uneven distributions in the atmosphere, and so they can create regional contrasts in heating. Although irrelevant to the global mean temperature, this can be very important for determining regional patterns of circulation and precipitation. Aerosol pollution has already been linked to weakening of the South Asian monsoon (Bollasina et al., 2011) and drought in the Sahel (Hwang et al., 2013; Booth et al., 2012). Such disruption of climate norms at regional scales is ultimately what makes a difference to people’s lives, and this is not always represented by a metric like global mean temperature (although global temperature changes will of course cause a great deal of regional climate disruption).
Substantial air quality benefits alone undoubtedly justify wider adoption of control measures for a range of short-lived pollutants – many such measures are already common in Europe and the US, but great gains are yet to be made in the emerging economies of Asia. But curbing emissions of short-lived pollutants is made even more attractive by considerable near-term climate benefits, which contrast with the long-term benefits of CO2 controls. Reducing these short-impact emissions alongside those of CO2 would therefore allow us to mitigate global warming more quickly. Consideration of the climate co-benefits should not be limited to global mean temperature though, and better understanding the sensitivity of regional climates to disruption from these pollutants is vital for quantifying the benefits of emission reductions.
Hwang, Y.-T., D. M. W. Frierson, and S. M. Kang, 2013: Anthropogenic sulfate aerosol and the southward shift of tropical precipitation in the late 20th century, Geophys. Res. Lett., 40, doi:10.1002/grl.50502.