The best thing about the Prius is that it achieves its high fuel economy without sacrificing size or performance and, most importantly for global warming, without being a diesel. There seems to be a lot of confusion on this point, so let me elaborate.

Bottom Line: If you care about global warming, don’t buy a diesel car (certainly not in this country), and if you must buy a diesel, only get a new one with a very good particle trap. [Does this mean that Europe’s massive switch to diesel was not good for the climate? In a word,"probably."]

First, diesel fuel has a considerably higher carbon content than gasoline, so burning a gallon of diesel emits 22.2 pounds of CO2 vs. 19.4 for gasoline (see here). A diesel car with the same mpg as a gasoline car would have considerably higher carbon dioxide emissions per mile. [This is offset one third by the fact that diesel has fewer upstream emissions, which, if I did the math right, takes total life-cycle CO2 emissions from a gallon of diesel to 25.8 pounds vs. 24.2 for gasoline (see here).]

Second, and more importantly, we have known for a number of years that black carbon (BC) or small soot particles are a major greenhouse gas — and that diesel engines are a major source of BC. A March 2008 review article published in Nature Geoscience, (subs. req’d, abstract below), "Global and regional climate changes due to black carbon," found that BC may be as much as 55% as potent in total greenhouse warming as CO2.

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In October, the House held "a hearing to examine the climate change and other impacts of black carbon emissions" (testimony and transcript here). Dr. Mark Jacobson, Co-founder and Director of the Atmospheric Energy Program at Stanford University’s Department of Civil and Environmental Engineering, testified directly on how BC emissions significantly reduce the climate benefits from diesel cars (here):

It is generally thought that diesel vehicles obtain better gas mileage and emit less carbon dioxide than equivalent-class gasoline vehicles and, therefore, using more diesel vehicles will address the climate problem. However, this concept ignores the larger emissions of fossil-fuel soot from diesel than gasoline vehicles and the resulting climate effects. It also ignores the fact that the addition of control devices to diesel vehicles to reduce their soot and nitrogen oxide emissions, required to meet California and EPA Tier 2 Permanent Bin emission standards and to address the climate problem of soot, reduces the gas mileage of the diesel vehicles. Finally, it does not consider that, in the United States, the lowest-carbon-emitting vehicles in 2006 were gasoline and gasoline-electric hybrid vehicles, not diesel vehicles [See Table 7 in testimony]. The addition of particle traps to the best diesels sold in 2006 in the U.S. would reduce the standing of the diesels further.

[Note: Jacobson explains "the addition of a trap decreases the mileage, thus increases the carbon dioxide emissions from such vehicles by 3.5-8.5%."]

Also, the addition of a particle trap to diesel increases the NO2:NO ratio in diesel exhaust increases, exacerbating photochemical smog. Finally, even with a particle trap, diesel vehicles still emit more particles than do gasoline vehicles.

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Jacobson provides analysis and figures to show that

when diesel vehicles have 30% better mileage than gasoline vehicles, diesel vehicles emitting particles continuously at a particulate matter emission standard of 40 mg/mi or 80 mg/mi may warm climate more than gasoline vehicles for more than 100 yr for a CO2 lifetime of 30 years…. However, diesel emitting at 10 mg/mi (Tier 2, bins 2-6 emission standard) may
warm climate relative to gasoline for about 10 yr at 30% higher mileage.

However, because no diesel vehicle available in the U.S. in 2006, 2005, or 2004 emitted less CO2 than did the best gasoline vehicle available, the 30% scenario in not applicable to the best available vehicles in the United States. As such, 2006 and earlier diesel vehicles sold in the U.S. all caused more global warming than did the best gasoline cars available, over a 100-year period.

And Jacobson also explains …

… when diesel vehicles have 15% better mileage than gasoline vehicles, the diesel vehicles cause more global warming over 100 years, regardless of whether they are emitting fossil-fuel soot at a particulate matter emission standard of 10 milligrams per mile (mg/mi), 40 mg/mi, or 80 mg/mi and regardless of the atmospheric lifetime of carbon dioxide (30 or 50 years). This conclusion applies to diesel vehicles having 0-15% better mileage as well.

I did not see anything in the testimony of the other experts that called into question these conclusions.
To repeat the bottom line: If you care about global warming, don’t buy a diesel car (certainly not in this country), and if you must buy a diesel, only get a new one with a very good particle trap. And the corollary — unless you need a much bigger car, you just can’t beat the Prius for total greenhouse gas emissions in this country.

Here is the abstract of "Global and regional climate changes due to black carbon":

Black carbon in soot is the dominant absorber of visible solar radiation in the atmosphere. Anthropogenic sources of black carbon, although distributed globally, are most concentrated in the tropics where solar irradiance is highest. Black carbon is often transported over long distances, mixing with other aerosols along the way. The aerosol mix can form transcontinental plumes of atmospheric brown clouds, with vertical extents of 3 to 5 km. Because of the combination of high absorption, a regional distribution roughly aligned with solar irradiance, and the capacity to form widespread atmospheric brown clouds in a mixture with other aerosols, emissions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions. In the Himalayan region, solar heating from black carbon at high elevations may be just as important as carbon dioxide in the melting of snowpacks and glaciers. The interception of solar radiation by atmospheric brown clouds leads to dimming at the Earth’s surface with important implications for the hydrological cycle, and the deposition of black carbon darkens snow and ice surfaces, which can contribute to melting, in particular of Arctic sea ice.