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The US Environmental Protection Agency (EPA) on Wednesday shot down the state of California’s proposed greenhouse gas standards for automobile emissions. The decision sets up yet another legal battle over global-warming regulations in the United States. Nature News takes a look at what's at stake.

What would California’s standards do?

Under the federal Clean Air Act, California can apply for a waiver from EPA and set its own air pollution standards, so long as they are at least as stringent as federal law. The regulations in question, which are part of a broader program to address global warming in California, would require automobile manufacturers to reduce greenhouse gas emissions by 30 percent by 2016.

Since California proposed the standards in 2004, some 17 other states have indicated they will follow suit, according to Environmental Defense, an environmental group based in New York. All told, these states account for roughly 45 percent of automobile sales in the US each year.

How do these regulations compare with the rest of the world’s?

The European Commission took similar steps this week, proposing greenhouse gas regulations for vehicles beginning in 2012. The legislation could reduce vehicle emissions by 19 percent if the proposal is adopted by the European Union.

On average, experts estimate that vehicles in Europe currently travel roughly 50 percent further on a given gallon of fuel than in the United States, while the Japanese fleet does almost double the distance.

Why did EPA deny the waiver?

Calling climate change a problem for the nation and the globe, EPA administrator Stephen Johnson said that California failed to make a compelling case that it should be allowed to address the problem separately at the state level. The announcement came shortly after President George W. Bush signed into law legislation requiring automobile manufacturers to boost fuel efficiency by 40 percent, to 35 miles per gallon, by 2020. Because burning less gasoline naturally translates to fewer greenhouse gas emissions, Johnson said the new law provides a national solution to the problem while avoiding “a confusing patchwork of state rules.”

In a letter to California governor Arnold Schwarzenegger, Johnson also said the new federal law would be stronger than the state regulations, which he said translate to 33.8 miles per gallon. California officials, however, maintain that their standards are in fact stronger and go into effect sooner than the new federal law; the state is also looking at follow-up regulations that would go into effect after 2016.

Why regulate greenhouse gas emissions in addition to fuel efficiency?

Automobile manufacturers, as well as the White House, have described California’s standards as a back-door attempt at setting automobile fuel efficiency standards, an authority exclusively granted to the US Department of Transportation. But the industry lost this argument in a series of lawsuits stemming back to a Supreme Court decision in April 2007, in which the justices ruled that the EPA — and by extension California — has clear authority under the Clean Air Act to regulate greenhouse gas emissions from vehicles.

Although fuel efficiency generally correlates with greenhouse gas emissions, advocates say California has taken a more comprehensive approach to combating pollution, an approach that could lay the groundwork for a similar program at the national level.

What happens next?

The question now heads back to the courts. California attorney general Edmund Brown Jr called the EPA’s decision “completely absurd” and promised to follow up with a lawsuit. Democrats in Congress might well decide to look into the matter as well, given reports that Johnson ignored staff recommendations to approve the waiver.

Meanwhile, the EPA announced plans earlier this year to look at greenhouse gas regulations in response to the Supreme Court ruling. It is unclear how the newly enacted law might play into that process.