Don’t cheer the electric car “revolution” just yet.

This has been a banner week for electric cars. Tesla’s first mass-market electric car, the Model 3, just came off the assembly line. Volvo announced that it will abandon conventional car engines by 2019, and make only electric and hybrid cars. And France, in an attempt to meet its goals under the Paris climate agreement, said Thursday they will phase out the sale of gasoline and diesel vehicles by 2040.

The barrage of news has multiple outlets crying “revolution”—and for good reason. “The Electric Car Revolution Is Accelerating,” Bloomberg Businessweek announced Friday, citing a new forecast that says the widespread adoption of “emission-free” electric vehicles will happen more quickly than previously thought. “The seismic shift will see cars with a plug account for a third of the global auto fleet by 2040 and displace about 8 million barrels a day of oil production—more than the 7 million barrels Saudi Arabia exports today,” the report reads.

But much needs to be accomplished before electric vehicles are truly “emission-free.” The increase in demand for electric-car batteries will increase demand for raw materials like nickel and lithium, which creates a “less than ideal” CO2 footprint in the short term, one analyst told the Financial Times. And although electric vehicles don’t run on gasoline, they do run on electricity—and in many places, much of that electricity is still generated from fossil fuel sources. A 2013 report titled “Shades of Green: Electric Cars’ Carbon Emissions Around the Globe,” showed just how much carbon is emitted by electric vehicles in each country, depending on the power source for the batteries.

In the U.S., battery sources are still fossil-fuel heavy. Their carbon footprint is still better overall than gas-powered cars’, but it shows that until the U.S. switches to a renewable-heavy power grid, electric cars won’t be as green as we think they are.