Lucrative new tax subsidies for companies that catch the planet-warming gas carbon dioxide and store it deep underground were one of the few aspects of President Biden’s 2022 climate legislation that the oil industry embraced.

The potential tax benefits spurred the industry, one of the largest contributors to the current climate crisis, to invest billions of dollars in the process, called carbon capture and sequestration.

Now some Democratic lawmakers, tax watchdogs and climate activists are raising concerns that the Internal Revenue Service, tasked with verifying fossil-fuel industry claims on stored carbon, lacks adequate safeguards to ensure that no companies are taking more taxpayer dollars than they qualify for. And they are equally frustrated that the I.R.S. and the Environmental Protection Agency rely on the companies’ own reported data.

The agencies do not “go out into the real world and track CO2 emissions from carbon capture facilities,” said Maggie Coulter, a senior attorney at the Center for Biological Diversity. “They’re just accepting these reports as they come in.”

The tax subsidies were boosted to reward companies that embrace carbon capture to help mitigate global warming. They also helped secure a critical vote in Congress to pass the Inflation Reduction Act of 2022, President Biden’s signature climate legislation that is designed to reduce the nation’s carbon emissions by 40 percent by 2030. While subsidies for carbon capture already existed, the law increased their value.

The Treasury Department, which oversees the I.R.S., estimates the subsidies will cost the federal government more than $36 billion in revenue over 10 years. That is nearly as much as the cost of the tax program that allows some Americans to claim tax credits for their child-care expenses.

Fossil fuel producers say the process lets them continue pumping out oil while reducing their emissions. It is also one of the few ways that cement, steel, chemical and aviation industries can reduce their carbon footprint because their energy-intensive operations depend heavily on burning fossil fuel.

Some climate advocates, and even some lawmakers who voted for the legislation, remain skeptical of the process. They are wary of the risk of leaks as well as the tax program that supports it.

The I.R.S. cannot disclose the amount of tax subsidies that each carbon capture facility receives, because of taxpayer confidentiality laws. Neither can the E.P.A. publish the amount of carbon that individual capture sites are declaring, as such data is considered confidential business information.

“It’s in the industry’s interests to have the number as high as possible, if nobody’s going to check up on them,” said David Schlissel, the director of resource planning analysis at the Institute for Energy Economics and Financial Analysis.

When the Treasury’s inspector general checked the paperwork, he found that nearly $900 million in carbon capture tax credits from 2010 to 2019 had been wrongfully claimed, as they lacked the required E.P.A. monitoring plans for carbon sequestration sites. Most of the credit claims were subsequently denied.

Tax watchdogs say the E.P.A. has data the I.R.S. could use to scan for possible tax cheats. All carbon capture facilities are required to obtain the environmental agency’s permit to drill a well for carbon storage and to report the amount of carbon they sequester.

But the agency, in response to a letter from Food & Water Watch, an environmental group, said in August that it “has no role in verifying the individual tax claims submitted to the I.R.S.”

Some lawmakers want that to change.

“To ensure actual carbon sequestration takes place, we urge your agencies to develop a set of strong guardrails,” wrote Senator Elizabeth Warren, Democrat from Massachusetts, in a letter issued Friday. She was joined by six lawmakers including Senator Angus King, independent of Maine, and Representative Ro Khanna, Democrat of California.