Diesels are returning to auto showrooms nationwide. Major automakers will offer them from this fall for the 2009 model year, equipped with vastly improved emissions control devices that make them a lot cleaner than the diesel image. But their main attraction — fuel economy — is rapidly fading. The cost of diesel is rising more rapidly than gasoline, while there is increasing skepticism about their impact on climate change.

I explore the environmental implications of diesel in “Dark Clouds Over Clean Diesels”, a web-exclusive news piece on Spectrum.com. The short take: diesels still emit more soot than gasoline-fueled vehicles and the bad news on soot just keeps getting worse. Estimates of mortality from breathing fine particles is still rising, and there are top climate scientists who believe that soot is also a serious contributor to global warming. It could be second only to CO2, even outpacing methane.

Interestingly, while the U.S. and Canada prepares for a new generation of diesel cars and light trucks, European car buyers are moving in the opposite direction. A German automotive research center recently forecast a sharp decline in diesel’s share of the European car market over the coming decades, thanks to the cost of adding tailpipe controls and tough competition from gasoline-powered engine technology. From a current market share of 53% (compared to 3% in the U.S.), the study predicts diesels to drop to 38% of Europe’s car market in 2015 and to 30% in 2020.

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