Forget cars. We need electric airplanes.

The race is on to build batteries big enough for planes to fly on clean electricity.

If we’re going to limit climate change this century, we need to electrify everything. That includes air travel, a large and rapidly growing source of some of the most potent greenhouse gas emissions. Yes, high-speed rail could displace some flights, but for longer journeys, travelers will clearly still have to take to the skies.

Which means we’re going to need electric airplanes. And while it might sound far-fetched, we may actually have a path to them. Norway was optimistic enough to announce last year that the country wants all domestic flights to be electric by 2040.

Aircraft engineers are jazzed about the challenge. “I believe it’s one of the hottest topics at the moment in aircraft engineering,” said Andreas Schäfer, a professor of energy and transport at University College London.

It may still be a few decades before buzzing motors will replace the roar of jet engines in the skies. We’ll need much more powerful and cheap batteries, and we likely won’t get them until the middle of the century. But when we do, electrification will radically change the design of aircraft and likely the business of aviation altogether while shrinking the environmental footprint of air travel.

Why we need electric airplanes

Air travel remains one of the most difficult challenges for climate change. Aviation is responsible for 2 percent of the world’s carbon dioxide emissions. And nitrogen oxides and particulates spewed by aircraft at cruising altitudes also have a warming effect.

According to the Air Transport Action Group, an industry association, aviation contributes $2.7 trillion to the global economy and supports 63 million jobs. And as the global economy keeps growing, aviation’s contribution to climate change will rise.

By the middle of the century, demand for flying could increase aviation sector greenhouse gas emissions by upward of 700 percent compared to 2005 levels, according to the International Civil Aviation Organization. This linkage between air travel, the economy, and emissions is a key reason US greenhouse gas emissions rose last year after years of decline.

This looming surge in flying makes it critical to decarbonize aviation, though there are no binding emission caps for aviation in the Paris climate agreement. Most of the low-hanging fruit airline companies can tackle — fuel efficiency, better aerodynamics, improved route mapping — have been picked. Fuel is the single largest expense for most airlines, so they already have a strong incentive to use it judiciously.

“If you look at the opportunities for reducing aviation CO2 that have been looked at for a long time, then you’re running out of options,” said Schäfer. “The potential for reducing CO2 emissions is not sufficiently strong in comparison to the growth rate of air transport.”

What options are left? Some airlines are experimenting with biofuels, which in theory could be carbon-neutral. However, biofuels are still struggling with costs and scale. The other major strategy is electrification.

Where do electric aircraft stand now?

As with electric cars, electric aircraft have the potential for emissions-free travel. They also unlock a whole new suite of airplane design and even new business models for air transport. Some critical engineering challenges remain, but researchers, and some in the industry, do expect electric planes to take off.

We’ve already seen electric aircraft pull off some impressive feats. In 2016, the Solar Impulse 2 aircraft completed an around-the-world journey powered only by sunlight. Granted, the aircraft cost $170 million, carried only one passenger, and topped out around 45 miles per hour, but it showed what’s possible. Recall that the time between the Wright brothers’ first 120-foot flight and John Alcock and Arthur Brown’s first nonstop trans-Atlantic flight was just 16 years.

In fact, there are already production electric aircraft like Pipistrel’s Alpha Electro, a two-seat trainer.

But electric motors also let you do things that you can’t do with a jet or piston engine, so engineers are experimenting with radical and bizarre new designs. NASA is trying out some of these ideas with its X-57 Maxwell, an all-electric test plane. Take a look:

You may have noticed a slight difference from most propeller-driven planes. Sean Clarke, the principal investigator for the X-57 at NASA, explained that the design helps resolve a key trade-off that plagues many aircraft.

“For this scale aircraft, one of the driving design considerations is the landing and takeoff performance,” Clarke said.

Taking off from the ground and getting to cruising altitude requires a lot of energy. It also needs a large amount of lift. For most aircraft, that means they need a larger wing surface area to provide adequate lift at lower speeds. But that larger surface area adds drag and makes cruising less efficient at high speeds.

Electric motors, however, are cheaper and easily scaled up and down. They’re also less mechanically complicated since they don’t need fuel lines, valves, and exhaust systems, so they fit in a smaller package.

“Our wing has the cruise propellers at the wingtip, and that reduces the drag of the wingtip,” Clarke said. “We also have 12 small propellers that are distributed across the leading edge of the wing. That increases the lift at low speed.”

For takeoff and landing, the small propellers are switched on, which means the wing can be much smaller than in a comparable conventional aircraft, which saves energy in flight.

Tom Tschida/NASA

Beyond the hardware, electric aircraft stand to change airline business operations. Electric motors can allow for very short — even vertical — takeoff and landing, which means they don’t necessarily need an airport with huge runways. So rather than airlines, these aircraft could operate as air taxis.

“There’s a lot of urban air mobility targets where different air framers and operators are advertising a future where you can take an air taxi from somewhere in a metropolitan area to somewhere 10 miles away and fly over all the rush hour traffic,” Clarke said. “It is really exciting because it’s starting to grow pretty quickly around the technologies that we’re already working with.”

One company, Lilium, has already tested such a prototype.

But these technologies still need incentives and investment. Clarke said that batteries also introduce new safety concerns to aircraft that have to be managed. In 2014, Boeing had to ground its entire fleet of 787 aircraft due to fires in its lithium-ion batteries. Nonetheless, electric aviation is gathering momentum, and it could take off sooner than we might expect.

How long until we have a fully electric airliner?

Schäfer co-authored a study in the journal Nature Energy late last year that tried to get at this very question.

The key limitation for aircraft is the energy density of its fuel: When space and weight are at a premium, you want to cram as much energy into as small a space as possible. Right now, some of the best lithium-ion batteries have a specific energy of 250 watt-hours per kilogram, which has already proved viable in cars. But to compete on air routes up to 600 nautical miles in a Boeing 737- or Airbus A320-size airliner, Schäfer estimated that a battery would need to have a specific energy of 800 watt-hours per kilogram. Jet fuel, by comparison, has a specific energy of 11,890 watt-hours per kilogram.

While it would take a significantly more powerful battery to compete with a transcontinental airliner, the shorter routes are still a promising target. Sub-600 nautical mile flights represent about half of global departures, and they have an outsize environmental impact.

Nature Energy

“At very low distances, the dominating determinant of pollution is takeoff and climb,” Schäfer said.

The energy required to get to altitude means airliners are less fuel-efficient in short flights. The efficiency per passenger gradually increases with the distance traveled, but it decreases again on long-haul trips since the aircraft also has to expend more energy to move the requisite fuel for the flight. That’s why most of the fuel use in aviation is still in long journeys. If all aircraft on short routes were electrified, it would only reduce aviation fuel use by 15 percent, according to the study.

Schäfer estimated that battery energy densities have increased by 3 to 4 percent per year in recent years. If this trend continues, we’ll have an 800 watt-hour per kilogram battery by roughly the middle of the century, barring a breakthrough.

“It is certainly a long way, but because the time scales of aviation are so long, [airliners] tend to live for 20 to 30 years, we need to start looking at these technologies now so they’re available in 2050,” he said. One stepping stone in this direction could be hybrid-electric aircraft, but those designs still produce greenhouse gases and also depend on cheap, powerful batteries.

The other key variable, of course, is cost. Jet fuel is cheap right now and batteries are expensive. If jet fuel prices go up and battery prices come down, then it will be easier for electrics to compete. However, you also have to factor in the cost of electricity, as this chart illustrates:

Nature Energy

The y-axis also shows what would happen to fuel prices if a $100-per-ton carbon tax were imposed. Electric airliners would only be as clean as the electricity that charges them, so pricing carbon would be one way to ensure that they aren’t trading emissions from jet wash for emission from a smokestack.

Taken together, this means it will likely be decades before you can book a flight powered solely by electrons. But it’s still worth hoping it takes off.