There are lots of good reasons for buying an electric car, from cutting your gas bill to feeling good for helping out with smog. But climate change? Not really.

Though Priuses and Leafs are associated with greener living, it turns out they do little planetary good, according to a new study. When you quantify the economy-wide impact of electric vehicles, measured in greenhouse gas pollutants, it’s basically a wash. From a climate perspective, you might as well keep your old sedan.

Even assuming electric vehicle adoption rates approaching 50%, the effect in carbon emissions was negligible.

Researchers look at 108 scenarios for adoption of hybrids, plug-in hybrids, and full battery electric vehicles between now and 2050, using combinations of five parameters: oil prices, natural gas prices, vehicle battery costs, government incentives for alternative fuels, and a possible federal cap on CO2 emissions. So in one scenario, for example, they assumed high oil prices, low battery costs, no cap on CO2 in 2030.

Even assuming electric vehicle adoption rates approaching 50%, the effect in carbon emissions was negligible, they found. “The model results do not demonstrate a clear and consistent trend toward lower system-wide emissions as [electric drive vehicle] deployment increases,” the paper says.

The researchers cite two main reasons. One is that lower tailpipe emissions are offset by the impact of increased electricity generation need to charge the cars’ batteries, even assuming a lot of renewable sources of power are used. The second is that any given person’s own vehicle emissions are relatively modest–adding up to no more than 20% of their own total carbon emissions.

The research isn’t the first to conclude that increased power production minimizes the CO2 benefits of EVs, says one of the authors, Joe DeCarolis, an assistant professor at North Carolina State. But the work is unusual in taking a system-wide view, including factors like oil prices.

DeCarolis stresses that the model doesn’t account for improvements like local air quality. But it does have implications for climate policy, suggesting that, if the goal is to cut carbon emissions, the government would be better off introducing a carbon cap–which produces the biggest drop in CO2, sulfur dioxide, and nitrogen oxide according to the model–than encouraging people to buy EVs. New car sales may help Detroit and Toyota, but they don’t do squat for global warming.