By Jan TenBruggencate
Advertiser Columnist

Owners of diesel-powered cars often tout their exceptional gas mileage. Diesel engines can also produce less carbon dioxide than gasoline engines of similar capacity — suggesting that they might be less of a factor in global warming.

But there's a downside.

A recent Stanford University study, cited in the Journal of Geophysical Research-Atmospheres, says diesel engines produce dramatically more soot, and that soot can be a bigger factor in promoting global warming than the carbon dioxide.

Such is the science of warming and changes in atmospheric science that the role of soot was not recognized in the Kyoto Protocols just five years ago, said Mark Z. Jacobson, associate professor of Civil and Environmental Engineering at Stanford.

Some governments — notably in Europe — are focusing entirely on diesel's gas mileage and lower carbon dioxide output in developing policies to combat global climate change. Some countries use tax policies to promote diesel engines in cars.

The climate issue is a big one for Europe, where one-quarter of the light vehicles run on diesel. In the United States, only one-tenth of 1 percent of cars do. Recent news suggests that hybrid vehicles, which combine gasoline engines with electric drive components, may be the future for U.S. drivers.

"Gasoline/battery hybrid vehicles now available not only get better mileage than the newest diesels but also emit less black carbon," Jacobson said.

The hybrid cars, such as the Toyota Prius and the Honda Insight, are available in Hawai'i and, within the next couple of years, they'll be joined by more hybrid cars from these and other auto manufacturers.

In part, that's because some car makers are less enchanted than they were with purely electric vehicles, and also because Toyota — the industry leader in hybrids — has made a huge commitment to the technology.

Toyota announced in October that it will convert its entire fleet to gas-electric hybrid engines. But it's not doing it all at once. The firm says it will be switched over by 2012, and it anticipates that with volume production, it will be able to reduce the premium that hybrid technology has cost.

The Toyota technology is good enough that Nissan has agreed to buy Toyota hybrid engines for its vehicles, and General Motors is said to be discussing purchase of the technology. General Motors is promising hybrid systems in trucks in 2004. Meanwhile, Ford is promising a hybrid engine in its Escape SUV next year. Chrysler has produced several developmental hybrid vehicles.

Jan TenBruggencate is The Advertiser's Kaua'i bureau chief and its science and environment writer. Reach him at (808) 245-3074 or jant@honoluluadvertiser.com.