Speaking on Wednesday at CERAWeek, the U.S. oil and gas industry conference, the CEOs of French energy company Total, and Spanish renewables giant Iberdrola, said green hydrogen would have to be just one of a wide range of energy sources in order for countries and companies to hit their carbon emissions targets.

“It’s not one thing or another one,” said Ignacio Galán, Iberdrola’s chief executive. “All technologies are needed for achieving the full decarbonization—hydrogen is one of those.”

Only about 70 million metric tons of hydrogen is currently produced, noted Patrick Pouyanné, CEO of Total, the majority of it for use in refineries or to make fertilizers. Hydrogen is not inherently green—almost all of the current supplies are produced with fossil fuels. Hydrogen made with renewables is still just a tiny fraction of overall production and far more expensive than the conventional variety. Creating momentum will require a huge jump in demand, Pouyanné said.

“Obviously the demand will come from policymakers,” said Pouyanné. “The only way to . . . lower the cost down, is to make very large projects. It’s like LNG 40 years ago,” he said, referring to liquid natural gas.

The note of caution comes from two of the fuel’s largest backers. Both have invested in green hydrogen plants in Europe. In January, Total said that along with Engie it would develop France’s fully renewable green hydrogen site in the Côte d’Azur—the hydrogen will fuel a biorefinery and construction will be begin next year. Meanwhile Iberdrola, along with Spanish partner, Firtiberia, says it plans to invest 1.8 billion euros by 2027, expanding capacity to 800 MW. A plant the companies are building in central Spain will be the largest industrial green hydrogen plant in Europe, Iberdrola says.

The momentum behind hydrogen—the most abundant element in the universe—makes sense. Hydrogen produces only water when it’s burned, making it a clean-burning fuel option for infrastructure from cars to power plants. It can also use much of the existing oil and gas infrastructure and potentially be transferred around the world much like LNG, making it a natural entry point for legacy oil and gas producers—like Total.

But there are several complications, as both executives admitted. To create hydrogen, abundant electricity must be used for the electrolysis process, which splits water to create hydrogen.

In order to make no-carbon hydrogen, you either need to make it with gas and then find “very large” CO2 carbon capture and storage, “which might be a real tricky point,” noted Pouyanné. Otherwise, the process must be fueled by renewable electrolyzers, which will require far more renewable power.

“The more you will make electrolyzers, the more you will need renewables,” he said. “People forget that [a] green electrolyzer is a huge multiplier of needs for renewable electricity—a huge multiplier.”