Efforts to tackle climate change typically focus on boosting renewable energy or using cleaner, more efficient cars, but there’s an entire realm of the economy they’re largely ignoring—one that accounts for about a quarter of global carbon dioxide emissions.

These other sources of climate pollution include industrial processes and long-haul shipping, and cutting their emissions will prove particularly difficult.

Taken together, these hard-to-cut sources accounted for 27 percent of global CO2 emissions from fossil fuels and industrial sources, according to a paper written by more than 30 leading climate scientists and published Thursday in the journal Science. What’s more, as global trade and construction continue to grow, the scientists warn that absolute emissions from these sources could eventually equal the current total level of global emissions.

“If we want to get to a net zero energy system this century, we really need to be scaling up alternatives now,” said Steven J. Davis, an earth systems scientist at the University of California, Davis and the study’s lead author.

The paper focused on three sectors—long-haul shipping and transportation, cement and steel production, and power generation facilities that are turned on only when needed—that won’t be addressed by most current efforts to cut emissions. Each faces its own set of challenges.

About 60 percent of the carbon emissions from cement production, for example, are released by a chemical reaction that’s critical to converting limestone into cement. While some research has looked at replacing limestone with more climate friendly ingredients—including, ironically, coal ash—these efforts remain fledgling. A more likely solution, the authors write, is for cement producers to install equipment to capture the CO2 before it’s released to the air and store it underground.

Ocean shipping accounts for about 3 percent of global CO2 emissions. And again, while nations and the industry have begun to address how to reduce these emissions, the steps so far are modest.

The biggest single source identified by the authors, who used International Energy Agency data from 2014, is the type of flexible power generation used to match the varying demand of the electric grid.

As utilities have added solar and wind capacity, the variable nature of the sun and wind haven’t mattered much because natural gas plants can be turned up or down to match demand. But as renewables become the primary source of electricity, grid operators will need climate-neutral ways to turn up output when demand increases. Batteries are an obvious solution, and their prices are falling, but they’re currently costly at the scale needed.

Avoiding Carbon Lock-In

If we don’t act fast, Davis said, it may soon be too late.

In earlier research, Davis and colleagues explored a concept known as “carbon lock-in.” Once a coal plant—or a steel plant—is constructed, the economics and sociology built around it create an incentive for it to continue operating. Companies have sunk large investments that pay off only over time. Thousands of people may become reliant on the facility for jobs. Entire regional economies can rely on the infrastructure, so shutting it down becomes difficult.

“It’s like if you paid for a car today in cash,” Davis said. “You could stop driving it, but that would be an enormous cost to you.”

With this logic, the emissions locked-in by today’s energy system may already be enough to push us past the goals of the Paris climate agreement, even if the world were to immediately halt building any new fossil fuel infrastructure.

Putting Carbon Solutions to Work

Technologies exist to begin cutting emissions from the hard-to-cut sectors like steel and cement, Davis said, but we need to start thinking about more effective ways of spreading and deploying them to drive down costs.

For example, the authors noted that synthetic fuels could solve two problems at once. When the sun is shining and the wind is blowing, the power grid could send excess energy to facilities that produce fuels by combining hydrogen from water with CO2 from the air. Those fuels would store the excess energy, and could power long-haul ships or even small power plants that could be turned on and off as needed to meet demand.

While there’s been little policy action on these matters, the U.S. Senate adopted a spending bill this week that include language, introduced by Sen. Sheldon Whitehouse (D-R.I.), that encourages the Energy Department to focus on capturing carbon from industrial sources like cement.

Davis thinks there’s still time. “I think we can get there,” he said. “The point of the paper is to hopefully get more people thinking about it.”