IT’S a lot like one of those math problems that gave you fits in sixth grade: a salesman leaves home in Denver and drives his electric car to a meeting in Boulder. At the same time, a physicist driving the same model electric car sets out from her loft in Los Angeles, heading to an appointment near Anaheim.
For both, the traffic is light, and the cars consume an identical amount of battery power while traveling the same number of miles. Being purely electric, they emit zero tailpipe pollutants during their trips.
The test question: are their carbon footprints also equal?
The answer may be a surprise. According to a report that the Union of Concerned Scientists plans to release on Monday, there would be a considerable difference in the amount of greenhouse gases — primarily carbon dioxide — that result from charging the cars’ battery packs. By trapping heat, greenhouse gases contribute to climate change.
The advocacy group’s report, titled “State of Charge: Electric Vehicles’ Global Warming Emissions and Fuel Cost Savings Across the United States,” uses the electric power requirements of the Nissan Leaf as a basis for comparison. The Leaf, on sale in the United States for more than a year and the most widely available electric model from a major automaker, sets a logical baseline.
The California part of the story is upbeat: a hypothetical Los Angeles Leaf would be accountable for the release of an admirably low level of greenhouse gases into the atmosphere, about the same as a gasoline car getting 79 miles per gallon. But the Denver car would cause as large a load of greenhouse gases to enter the atmosphere as some versions of the gasoline-powered Mazda 3, a compact sedan rated at 33 m.p.g. in combined city and highway driving by the Environmental Protection Agency. In simple terms, the effect of electric vehicles on the amount of greenhouse gases released into the environment can span a wide range, varying with the source of the electricity that charges them. California’s clean power makes the Leaf a hero; the regional mix of coal-dependent utilities serving Denver diminish the car’s benefits as a global-warming fighter.
According to 2010 data from the United States Energy Information Administration, 45 percent of the country’s electricity is generated by burning coal, the dirtiest fuel. Natural gas, a much cleaner fuel, accounts for 24 percent of electricity production, a figure that is shifting rapidly with price swings. Nuclear plants generate 20 percent of the nation’s power, while wind, solar and geothermal sources provide 3 percent.
While the report puts hard numbers on the current situation, it also points out the need for fundamental changes.
“To prevent the worst consequences of global warming,” the report concludes, “the automotive industry must deliver viable alternatives to the oil-fueled internal-combustion engine — i.e., vehicles boasting zero or near-zero emissions.”