Could more fuel-efficient engines lead to more global warming?

Auto industry experts predict that more than 50 percent of cars on the road by 2020 will use a relatively new type of fuel-efficient engine. This transition, however, has raised questions about its ultimate effect on the climate. A new study has found that because the newer engines emit higher levels of the climate-warming pollutant black carbon than traditional engines, their impact on the climate is uncertain.

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