Air pollution causes serious health problems around the world, however, some aerosol particle emissions contribute to a cooling effect on the climate. A recent study has focused on shipping as a source of emissions to explore whether reducing air pollution to improve human health could increase the risk of health problems caused by climate change.
The health effects of exposure to airborne particles emitted from human activities, such as the burning of fossil fuels, is a major cause of global ill health and premature deaths. For example, exposure to particulate matter (PM10) in urban areas is estimated to cause around 800,000 early deaths every year and a further 2 million deaths arise from inhaling indoor smoke from solid fuel fires around the world.
Supported by the European-funded EUCAARI project1, this study investigated some of the complex relationships between air pollution, health problems and climate change by examining the impact of shipping on human health. Ships contribute to aerosol pollution by releasing sulphur oxides, nitrogen oxides and PM to the atmosphere.
Aerosol emissions can also affect the climate by having either a warming or a cooling effect. For example, black carbon (or soot) has an overall warming effect, because it absorbs the sun's radiation. Other particles (e.g. sulphates) produce a net cooling effect by reflecting back the sun´s radiation, altering the reflective properties of clouds or affecting atmospheric circulation patterns. However, on balance, the combined effect of aerosol emissions from shipping is that of cooling.
Although warmer temperatures could have local benefits, such as fewer winter deaths, climate change will create extra health burdens. For example, a 0.4°C increase in temperature in 2000 (compared with the 1961-1990 average) is estimated to have caused 160,000 climate-related deaths from malaria, diarrhoea, malnutrition, heatwaves and floods.
The researchers analysed the 'total health effects' (the combined number of deaths from exposure to air pollutants and climate change) but were not able to determine whether shipping emissions had overall positive or negative effects on human health. For example, one estimate suggests shipping emissions could account for 63,000 deaths a year due to air pollution, but climate cooling from emissions could potentially save 20,000 lives. The 'total health effect' would therefore be 43,000 deaths.
Despite a number of uncertainties about calculating the health effects caused by air pollution and climate change, the researchers suggest there are some short-term benefits from emissions that cause climate cooling. Therefore, it would be preferable to focus mitigation efforts initially on reducing pollutants that cause climate warming, such as black carbon.
Since the major impacts from shipping typically occur within 400 km from the shore and the busiest shipping lanes are found off South East Asia, Europe and North America, the researchers suggest a geographic focus on regulations might be beneficial. Any efforts to reduce air pollution should take into account the impact such reductions would have on warming the climate.