The once-distant promise of clean, affordable hydrogen-powered cars is starting to become a reality.

Several major automakers, including Toyota, Honda and Hyundai, have started or will soon start selling these cars, which will be more expensive than comparable gasoline models but a lot cheaper than they were just a few years ago.

Executives at Toyota say that the cost of making the critical components of hydrogen vehicles has fallen 95 percent since 2008. That is why the company plans to market its first mass-produced hydrogen car, the Mirai, in the United States next year. Other companies, like General Motors, Ford and Audi, are working on similar cars.

The broad adoption of hydrogen-powered cars, which emit only water and heat, could play an important role, along with electric vehicles, in lowering emissions of carbon dioxide and other pollutants responsible for climate change. Cars and other modes of transportation account for about 28 percent of greenhouse gas emissions in the United States, second only to power plants, according to the Environmental Protection Agency.