Diesel engines may contribute more to global warming than petrol-driven engines, a new study has suggested, writes Steve Farrar.
The finding suggests that green motoring regulations drawn up to encourage more use of diesel may be having the opposite effect to that intended.
While diesel cars emit less carbon dioxide and have a better mileage than petrol-driven vehicles, they also produce 25 to 400 times more soot per kilometre.
Research by Mark Jacobson, associate professor of civil and environmental engineering at Stanford University, suggests that this may more than offset the cooling due to reduced greenhouse gas emissions over several decades.
Professor Jacobson collectively quantified 12 identifiable effects of airborne particles on climate using a computer model.
This revealed the potential impact that soot particles - a mixture of particulate black carbon and associated organic matter - could have on global warming.
"Whereas carbon dioxide clearly causes most global warming, control of shorter-lived warming constituents, such as black carbon, should have a faster effect on slowing warming," Professor Jacobson said.
Soot not only warms the air to a much greater extent per unit mass than carbon dioxide, it also persists for a much briefer time, leaving the air in a matter of months as opposed to 50 to 200 years.
Professor Jacobson said this meant that taking action to control fossil-fuel black carbon and organic matter may be the most effective way of slowing global warming.
He warned that European Union environmental strategies to promote the use of diesel vehicles could ultimately backfire.
Even with the new particle traps being introduced by some European car manufacturers that cut the emission of soot to just 0.003 grams per kilometre, Professor Jacobson said these emissions could still warm the climate more than petrol over a ten to 50-year period.
The findings are published in the Journal of Geophysical Research-Atmospheres .