One million down, another billion or so to go.

In a new report, the International Energy Agency estimates that 1.26 million electric cars hit the world's roads in 2015, passing a nifty (if symbolic) milestone. Here's a chart showing the very rapid growth of both battery electric vehicles (BEVs) and plug-in hybrids (PHEV):

The United States now has 400,000 electric vehicles on the road — a massive increase since 2010, though well short of Obama's goal of 1 million by 2015. Meanwhile, China has become the world's largest market, overtaking the US in annual sales last year.

Is 1 million a lot? It depends how you look at it. It's jaw-dropping growth given that there were only a few hundred electric vehicles on the entire planet back in 2005. And the total number of electric vehicles worldwide has tripled just since 2013.

But to put this in perspective, there are more than 1 billion gasoline- and diesel-powered cars on the world's roads — and demand will keep soaring in the decades ahead as China and India's middle classes expand. So we have a long, long way to go before electric cars take over the world.

In order to avoid more than 2°C of global warming, the IEA calculates, we'd likely need to see about 150 million electric cars on the road by 2030 and 1 billion by 2050 as part of a broader climate strategy. The good news, the agency says, is that this ambitious electric vehicle target seems much more feasible than it did just a few years ago.

Two big reasons for the rapid growth of EVs: public subsidies and falling battery prices

For starters, more and more countries are enacting policies to build up charging infrastructure and incentivize vehicle purchases. The table below details some of those policies, which include everything from tax breaks to tailpipe emission standards (which favor cleaner electric cars) to HOV lane access:

"Ambitious targets and policy support have lowered vehicle costs, extended vehicle range and reduced consumer barriers in a number of countries," the report says.

As a result, electric vehicles now make up more than 1 percent of sales in China, France, Denmark, and Sweden. They make up 9.7 percent of sales in the Netherlands, and 23 percent of sales in Norway, which offers some of the most generous tax incentives around, worth about $13,500 per car.

The other huge driver here is falling battery costs, which have fallen by a factor of four since 2008. Since batteries make up around one-third of the price of electric vehicles, getting this number down even further is crucial for widespread adoption.

The Department of Energy estimates that battery costs need to fall to $125 per kilowatt-hour by 2022 to achieve cost-competitiveness with conventional vehicles. The IEA says this "seems realistic" given current rates of technological improvement, and points out that manufacturers like Tesla and GM have set even more ambitious cost targets.

The amount of energy that batteries can hold (known as energy density) has also improved significantly. The IEA cites various reports that electric cars will soon be able to travel more than 180 miles on a single charge — also critical for boosting consumer adoption and alleviating "range anxiety."

Meanwhile, electric cars get all the attention, but the IEA points out that the electrification of other modes of transport, including motorcycles and buses, is just as important. Particularly in countries where these vehicles are widespread:

The electrification of road transport modes other than cars, namely 2-wheelers, buses and freight delivery vehicles, is currently ongoing in a few localised areas. With an estimated stock exceeding 200 million units, China is the global leader in the electric 2-wheelers market and almost the only relevant player globally, primarily because of the restriction on the use of conventional 2- wheelers in several cities to reduce local pollution. China is also leading the global deployment of electric bus fleets, with more than 170 000 buses already circulating today.

To help solve climate change, electric cars need to do much, much more

Here's one last chart from the IEA, showing what electric car deployment would have to look like to help meet the emissions-reduction promises put forth at the Paris climate talks last year (around 100 million electric cars by 2030). It also shows the even faster deployment that would be needed to keep global warming below 2 degrees Celsius (about 150 million):

We're on track now, but it's early. In fact, as the IEA points out elsewhere, electric vehicle deployment is basically the only area where the world's nations are on track to hit the targets needed to stay below 2 degrees Celsius.

One final caveat: As David Biello recently explained at Scientific American, electric cars aren't inherently greener than their gasoline-powered counterparts. If you use coal-fired power plants to charge all those electric cars, the climate benefits are minimal (or, worse, negative). The IEA is well aware of this, although it also notes that electric car deployment can help support the rollout of cleaner renewable energy, too:

The climate change-related benefits of EVs can be fully harvested under the condition that their use is coupled with a decarbonised grid, an additional challenge for countries that are largely dependent on fossil fuels for power generation. Investment in EV roll-out can support this transition, e.g. increasing the opportunities available to integrate variable renewable energy.

Cleaning up the grid is a sine qua non for electric cars to help ameliorate climate change, although this hardly seems like a deal breaker for the technology. Think of it this way: If we don't clean up the world's electric grid, we have little chance of stopping global warming either way. The two have to go hand in hand.

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