Despite fuel economy, diesels don't help global warming?

Black carbon: Diesel's dirty little secret?

Diesel vehicles are more fuel efficient that gasoline vehicles, but despite this fuel economy, they offer no benefits in terms of global warming.

"Because of their increased fuel efficiency relative to gasoline-engined vehicles, diesels are seen as an improvement overs gasoline vehicles with respect to global warming issues. However, once soot warming is factored in, the difference between the two platforms is greatly reduced, as diesel emits more soot than gasoline."

(Source: GreenCarCongress)