While other climate-change initiatives spearheaded by President Obama — like the EPA strategy for reducing emissions from power plants, called the Clean Power Plan — received more scrutiny from industry and conservative critics, emissions standards for cars are just as consequential for curbing the buildup of atmosphere-warming gases, analysts said.

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“The benefits from those rules are comparable to the Clean Power Plan in terms of greenhouse gas reductions,” said Dave Cooke, a senior vehicles analyst at the Union of Concerned Scientists. “The program as a whole is quite significant.”

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The notice begins a 45-day public comment period on a potential relaxation of the rules for cars and light trucks.

“We are moving forward with an open and robust review of emissions standards, consistent with the timeframe provided in our regulations,” EPA Administrator Scott Pruitt said in a statement. As recently as Wednesday, Pruitt in a radio interview brushed off the large body of research establishing that human activity is warming the Earth as “so-called settled science.”

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In March, the EPA announced that it wished to withdraw final determination on the stricter fuel-efficiency standards. But critics of the administration were surprised how expansive the review announced this week will be.

The EPA stretched the review period to include 2021 model vehicles, instead of starting at 2022 model cars as the agency had initially indicated.

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The agencies also added 10 additional factors, such as consumer preference, to be considered when finalizing the new standards.

“It was really surprising to see that,” said Shannon Baker-Branstetter of Consumers Union, the public policy division of Consumer Reports, adding that the decision to include 2021 vehicles in the review was a sign of “aggression on the part of the administration.”

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In 2012, the EPA and the National Highway Traffic Safety Administration, a part of the Transportation Department, adopted rules requiring the nation’s car and light trucks to average 54.5 miles per gallon by 2025.

Automakers objected, arguing that the standards were too stringent to meet. After Trump’s surprise election victory, industry groups, including the Alliance of Automobile Manufacturers and the Association of Global Automakers, pressed the new administration to reconsider the rule.

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“In my opinion, these additional factors reflect direct input from the manufacturers,” said Margo Oge, a former director of the EPA’s Office of Transportation and Air Quality, in an email.

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The review begins amid signs that car companies are moving toward greater efficiency, with or without the federal government’s guidance.

In July, Swedish automaker Volvo vowed to make all its vehicles hybrids or electrics starting in 2019. Earlier this month, Japan-based Honda introduced a new all-electric sedan in California and Oregon to compete with Tesla’s new line of more affordable electric vehicles.

“It is clear that the Asian manufacturers in the United States are fully capable of meeting this standard,” said Jack Gillis, director of the Consumer Federation of America and author of “The Car Book.” Noting consumer surveys that indicate car buyers prefer fuel-efficient vehicles that save them money at the gas pump, he added: “The first loser will be the domestic car companies.”

And climate policies set by nations such as France and the United Kingdom, which both plan to ban all diesel and gasoline vehicles by 2040, will continue to shape international auto markets and drive car companies to continue lowering emissions.