California wants you to consider greenhouse gas emissions when you buy your next new car. And to help you do that, they’ve revised their mandatory Environmental Performance label for all 2009 model-year cars. All new vehicles must display the modified sticker by January 1, but some could appear on cars as early as this month.

Next to the traditional “Smog Score,” there’s now a “Global Warming Score.” Both show scales of 1 to 10 and display where a particular vehicle stands on the scale. The higher the score, the cleaner the vehicle—and the scales adjust so that the average across all new cars each year is exactly 5. The scales incorporate all classes of consumer vehicles, from zero-emission electric cars to the heaviest SUVs and vans.

The new Global Warming Score reflects the emissions of greenhouse gases from the vehicle’s operation and the production of fuel to power it. The older Smog Score looks at smog-forming emissions just from operating the vehicle, including the three “criteria emissions” that have been regulated by law for 35 years: carbon monoxide (CO), hydrocarbons (HC), and nitrous oxides (NOx).

A car’s Global Warming Score might well differ from its Smog Score. One that’s exceptionally economical with fuel—emitting relatively little CO2, the main greenhouse gas—but produces close to the maximum permitted criteria pollutants, for example, would be relatively “dirtier” than its Global Warming Score would show. That said, a Toyota Prius is still likely to do better than average on both scores, while a Hummer H2 will probably do worse than average.