With electric passenger vehicles setting new sales records, regulators worldwide are now focusing on removing emissions from heavy trucks—a process known as decarbonizing.

Big trucks—those weighing 10,000 lbs or more—account for 25% of carbon dioxide emissions from U.S. transportation, even though they are only 10% of the vehicles on the road. About 13 million are in use across the U.S., including about 3 million semi tractor-trailers.

While federal efforts to push truck decarbonization are underway in Washington, D.C., California is leading in taking aggressive steps to shift these vehicles from fossil fuels to green power.

By 2045, the state wants to achieve carbon neutrality. Its plan, unveiled in late 2022, strives to cut air pollution by 71%, and slash greenhouse gas emissions 85%, including a 40% reduction by 2030. As part of this, the state last year mandated that half of all heavy-duty trucks sold in California must be zero-emission vehicles by 2035. Ultimately, the California Climate Commitment means fossil fuel consumption would be one-tenth its current levels. “This plan is a comprehensive roadmap to achieve a pollution-free future,” California’s Gov. Gavin Newsom said when the proposal was unveiled.

While there might be a roadmap, there’s no agreement across industry on the best way to achieve it. “It’s pretty easy for policy to do a job when the technology is clear,” said Samantha Gross, director of the energy security and climate initiative at the Brookings Institution. “We’re still in the ‘let different kinds of flowers bloom’ stage.”

But because California is the largest state economy in the U.S., its decisions have huge influence over what businesses and other states do. In this case, its climate plan is pushing companies to innovate. And other states have already signaled intentions to follow California’s lead in electrifying trucking.

Read more: We Test Drove The First Wireless EV Charging Road in the U.S.

The two most-frequently discussed options to decarbonize trucking are electric power and hydrogen, with each having advantages and drawbacks. For now, electric gets the most attention because it’s already had success with passenger vehicles.

Amazon has rolled out its first 10,000 U.S. electric delivery vans from Rivian, part of a fleet of 100,000 it plans to put on the road by 2030. Walmart says it remains committed to plans announced two years ago to decarbonize its fleet by 2040. Thus far, it has taken delivery of five eCascadia trucks from Freightliner and 13 electric terminal tractors from Autocar.

But turning diesel-powered trucks into electric vehicles poses three significant problems, says Lew Fulton, director of the energy futures program at the Institute of Transportation Studies at the University of California-Davis. First, “they don’t get enough driving range,” he says. While automobiles can travel several hundred miles on a single charge, some trucks that travel cross country need to be in operation for 1,000 miles. To achieve that distance, says Fulton, “You have to recharge them a couple times. It’s lost time.” Rivian’s electric trucks for example, have a range of between 350-400 miles depending on the battery type, and with a fast charger trucks can regain 140 miles in 20 minutes. Plugging in overnight meanwhile recharges half the battery’s range.

Additionally, the batteries needed to power heavy trucks can weigh 5,000 to 10,000 pounds each, meaning trucks couldn’t carry a full payload of goods. Federal regulations bar the biggest trucks from transporting more than 80,000 pounds on the interstate highway system. Moreover, charging up these big batteries would be a significant draw on electrical grids across the country.

However, manufacturers are teaming up to kickstart charging development. In January, Daimler Truck North America, Navistar, and Volvo Group North America, which together comprise around 70% of the medium and heavy-duty truck sales in the United States, announced plans to accelerate a charging network. That’s a tall order: to serve the companies’ existing fleet, experts estimate that at least 600,000 big truck chargers will be needed by 2030. (TIME CO2 Futures is presented in part by Volvo Group.)

Read more: The Strategic Ambiguity of Toyota’s EV Play

Fulton believes hydrogen fuel cells, which produce fuel from electricity, water, and heat, are another option. When in use, the cells emit only water, do not create air pollution, and are quiet because they don’t have many moving parts. They can easily be refilled. “In theory, you’d be able to refuel a big truck in 10 minutes,” he says.

While there are a handful of hydrogen-powered cars, there currently aren’t any hydrogen-powered trucks. But San Francisco-based startup Verne, is one company trying to change that. It says its technology doubles the density of conventional hydrogen in each tank, thereby increasing the energy available and allowing trucks to travel farther. “Our goal is to make sure trucks can make sure they maintain diesel parity [with] a full range and a full payload and the same refueling time,” says Ted McKlveen, Verne’s co-founder, and chief executive.

Ultimately, it might not be an either-or situation. McKlveen thinks there’s room for electric-powered trucks and hydrogen-fueled ones to coexist. Electric might be the better choice for short trips, while hydrogen could power vehicles that travel cross country.

While the California standard seems idealistic for now, he believes it is spurring alternatives that might not otherwise happen. “It’s been incredibly helpful for the industry to have these aggressive targets in place.”

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