Air pollution and climate change

Abstract

Concern about climate change is mainly due to the build-up of greenhouses gases in the atmosphere, resulting from human activities such as the burning of fossil fuels and deforestation. The long atmospheric lifetime (decades to centuries) of most greenhouse gases (GHGs) means that they will remain at elevated levels for centuries, even if all emissions were to be dramatically reduced. Air pollutants are another unwanted product of human activities that can also affect the Earth's climate but in a more complex way. Many air pollutants are related to combustion, but some also arise from agriculture and industrial processes. Unlike GHGs most air pollutants have short lifetimes in the atmosphere (days to weeks) and their concentration is largely maintained by current emissions. That means that if emissions were to cease most of their direct effect would be removed in a matter of weeks. Air pollutants are found in the atmosphere as gases, aerosols and particles. The main substances emitted include oxides of sulphur (SOx) from coal and heavy oil, and nitrogen (NOx) from combustion and as ammonia from agriculture and volatile organic compounds (VOCs) including from mining, refuelling and industrial processes. Particles of black carbon and organic carbon from partially burnt fossil fuels and biomass are also important. Air pollutants can react chemically with each other to form secondary pollutants such as low-level ozone, itself a GHG. They can also slightly affect the concentration of long lived GHGs such as carbon dioxide and methane. Such pollutants are generally detrimental to human health and adversely affect vegetation, including crops, and as such are controlled to varying degrees by local and national governments and some regional multilateral agreements. Nevertheless, air pollution remains a serious threat to many millions of people across the world. The World Health Organization estimates that some 4.3 million people worldwide die annually from stroke, heart disease, lung cancer and chronic respiratory diseases due to ambient air pollution. A further 3.7 million are estimated to die from indoor air pollution (World Health Organization, n.d.). Air pollutants affect the climate at both the global scale and more strongly close to sources of pollution. They do this by (a) as aerosols, scattering or absorbing solar radiation, leading to local cooling or warming effects, (b) modifying cloud droplets and rainfall and even in the case of black carbon accelerating snowmelt and (c) in chemical reactions that affect the concentration of long-lived GHGs. On the global scale the IPCC reports (IPCC, 2013) that from the industrial revolution to present, the net warming rate due to all GHGs is about 3.0Wm−2 and that aerosols, including cloud effects, have had a net cooling effect of −0.9Wm−2. As a result, the average surface warming of the Earth of about 1 degC over the last century due to GHGs is less than it would have been without air pollution. More significantly, however, in heavily polluted areas, such as in SE Asia, the level of warming has been further depressed and there is evidence that the hydrological cycle has also been affected (Ramanathan et al., 2005). Although this presents a policy question as to whether priority should be given to dealing with GHGs or air pollution, it is accepted that both warrant action to protect people in the short and long term. Fortunately, many of the actions that are needed to reduce GHGs may also reduce emissions of pollutants. For example, both have major sources in the combustion of fossil fuels for power generation, heating and transport, so decarbonising the world's economy will have a double benefit for the climate, human health and the environment (IPCC, 2014). This paper was developed in collaboration with the Royal Meteorological Society's Climate Science Communications Group.