Oooh, it rhymes.
Anyway, that’s due to a nearly undetectable substance called “black carbon,” which sounds every bit as mysterious and science-y as dark matter. It’s basically the stuff that’s in soot, but because it’s so hard to measure, black carbon is not factored into carbon dioxide emissions tests.
According to experts even reduced emissions from soot could add up to additional gram of carbon emissions per kilometer (slightly under that per mile). However, many automakers live right on the edge when it comes to emission standards, so if the powers that be ever started testing and measuring black carbon, it could just barely push some models over the limit.
There’s also the issue of climate effects. Climate scientists are saying that black carbon contributes to global warming as well, which is pretty much a given, because almost everything nowadays either contributes to global warming or cancer. Fortunately a Pew Research study indicates that black carbon doesn’t hang around nearly as long as CO2 does, so reducing black carbon emissions could dramatically improve air quality in BC-heavy cities—and the results would be almost instantaneous.
There doesn’t seem to be a huge push to do so in the U.S. Given the relative scarcity of diesel vehicles, that’s somewhat understandable. Europe, China, and India, however, might be wise to study this issue a little more.