News Release

Other short-lived greenhouse pollutants need to be controlled too

Peer-Reviewed Publication

The Lancet_DELETED

Carbon dioxide is not the only greenhouse pollutant which needs to be controlled. The short-lived greenhouse pollutants —such as black carbon and ozone —need to be controlled also, because they collectively create a substantial proportion of all human-contributed global warming and also directly damage health. Importantly, unlike control of carbon dioxide, control of some short-lived greenhouse pollutants can lead to quick reductions in global warming as well as improvements in health. However, control of others, sulphates for example, while having health benefits actually exacerbate climate change because they act as cooling agents. The complex relationships between the health and climate implications of these short-lived greenhouse pollutants, which are produced across a range of economic sectors, are addressed in the fifth paper in the Lancet Series on Health and Climate Change, written by Professor Kirk R Smith, School of Public Health, University of California, Berkeley, CA, USA, and colleagues.

A major question for health co-benefits policy is whether the short-lived greenhouse pollutants are more or less dangerous for health compared to other pollutants. The answer would affect the relative weight given to health versus climate benefits among different control strategies. The authors present new meta-analyses of existing health-effects studies and the results of the first long-term health-effects study of black carbon ever published, which was based on a cohort of 352,000 people in 66 US cities during 18 years of follow-up.

The authors say that there is good evidence that sulphates are somewhat more hazardous to health than undifferentiated fine particles, which means that there is unfortunately no health justification for slowing the control of sulphates globally even if there may be a climate benefit. On the other hand, they also find strong evidence that ozone pollution exerts an effect on cardiovascular mortality that is independent of other pollutants. Thus, there can be expected to be both significant climate and health benefits to controlling the precursors to ozone creation globally. Of particular importance globally is methane, which not only adds to ozone creation but is a powerful greenhouse gas in its own right.

Furthermore, the authors find that there is some indication that black carbon particles are more dangerous than undifferentiated fine particles, which would imply an enhanced health benefit as well as significant climate benefit to their control. The evidence, however, is not as clear as for sulphates and ozone and is also complicated because black carbon controls usually also control lighter coloured particles that have a cooling effect. Thus the climate, if not the health, benefits of control depend on the ratio of black to light-coloured particles produced by each source.

The complexity of these health and climate effects needs to be recognised in mitigation policies. Substantial lowering of health risks along with climate protection can be achieved by choosing wisely, but will depend on the relative emissions of each sector. Black carbon, for example, is emitted from incomplete combustion in the household transport, and industrial sectors, while sulphates are produced mainly by power plants and transport globally. Substantial methane, on the other hand, is produced by the agriculture and waste-management sectors.

The authors point out that one of the recurring suggestions to slow climate change is the deliberate injection of sulphates into the atmosphere because of their climate-cooling properties and low toxic effects. They say: "Any such so-called geoengineering needs to be analysed carefully and implemented cautiously. The present epidemiological evidence for mortality effects of sulphate is not conclusive because of the strong association with other pollutants from the same combustion sources."

They add: "Another widely discussed option for mitigation of climate change is to use biofuels instead of fossil fuels. Research suggests that a switch from fossil fuels to ethanol could increase emission of volatile organic compounds (aldehydes), which are known precursors of tropospheric ozone. Thus, the health effects could potentially rival those of the fossil fuels that ethanol is intended to replace."

In conclusion, the authors suggest that separate climate change agreements may be necessary for these short-lived pollutants, in the same vein as those in place for long-lived pollutants such as carbon dioxide.

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Professor Kirk R Smith, School of Public Health, University of California, Berkeley, CA, USA. T) +1 510-914-062 E) krksmith@berkeley.edu

For full Series paper 5, see: http://press.thelancet.com/tlhacc5.pdf


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