Major corporations like Apple and Disney will be forced to disclose their carbon emissions under a new Californian law approved on Monday.
Governor Gavin Newsom signed bill - passed by the state legislature - requiring companies with more than $1bn (£817m) in annual revenue to report greenhouse gas emissions.
Similar efforts are moving slowly at the federal level.
Mr Newsom praised the law's aims, but questioned how it will be carried out.
"This important policy, once again, demonstrates California's continued leadership with bold responses to the climate crisis," Mr Newsom wrote in a signing statement. "However, the implementation deadlines in this bill are likely infeasible."
He added that he is "concerned about the overall financial impact of this bill on businesses".
The California Air Resources Board must put a system in place for reporting emissions by January 1, 2025, a little more than a year from now, under the law.
As the need to address climate change grows more urgent, pressure is mounting on large corporations to come clean about how much they emit gases that trap heat in the atmosphere, both directly through operations and indirectly, such as purchasing electricity.
California, which has a record of enacting cutting-edge environmental laws, is home to many multibillion dollar companies.
Chevron, Meta, Wells Fargo, Intel, and HP are all headquartered in the state and all pull in more than $50bn (£40bn) a year.
The state also recently passed a law requiring companies with more than $500m (£408m) in annual revenues to report their climate-related financial risks, which Mr Newsom said would encourage companies to work to avoid those risks. But, again, he raised concerns in a signing statement about the costs to businesses.
The US regulator for stocks and markets - the Securities and Exchange Commission (SEC) - has been working on similar federal requirements for companies to report emissions and climate-related risks for more than a year.
Supporters of the commission's proposed rule, such as Senator Elizabeth Warren, say it would prevent "greenwashing", when a company exaggerates its actions on the environment in its marketing, and that it would help investors understand different companies' vulnerabilities.
The Chamber of Commerce, the country's largest business group, though, says the rule could be costly for companies and difficult to follow.
The group has also raised questions about whether emissions and risk reports are material to investment decisions and if a markets regulator should be carrying out environmental policies.
It is not clear when the commission will approve a final version of the rule.
