Great, I love a good fight. What's this one about?

It's about cars' carbon emissions, or to be more precise the right to set standards demanding car manufacturers lower their carbon emissions. Way back in autumn 2004 the California Air Resources Board adopted a package of measures called the Clean Car Program that used powers granted to the state under the US Clean Air Act to impose tough new rules for limiting new cars' carbon emissions. The car companies sued to block the new rules and lost, meaning that all California had to do was gain approval from the Environmental Protection Agency (EPA) to make the new standards legally binding. But late last year the EPA confirmed that it would not approve the legislation.

Why did they reject it? Shouldn't an Environmental Protection Agency want to protect the environment?

It all comes down to how you interpret the Clean Air Act. Under the law, California has the right to set more demanding tail pipe and air pollution standards in order to tackle its exceptional problems with smog. Other states then have the right to adopt California's more demanding standards if they choose to do so, something 12 other states have said they will do. However, to set these more stringent standards California needs a waiver from the EPA and the EPA has decided that it should not grant the waiver for the new emissions rules because, unlike smog, global warming is not solely a problem for California. As administrator Stephen Johnson put it in a statement rejecting the waiver application, "the conditions related to global climate change in California are substantial, [but] they are not sufficiently different from conditions in the nation as a whole to justify separate state standards".

Does he have a point?

Yes and no. The EPA's view is that the fuel economy standards included in the Energy Independence and Security Act passed last year are sufficient and that passing another set of standards for separate states would place an unbearable burden on the car industry, creating a patchwork of standards that firms would struggle to comply with.

I sense a "but" coming.

But, the federal standards really are pretty lax compared with what California is pushing for. The US Energy Independence and Security Act will require automakers to raise fuel economy to an average of 35mpg by 2020, while the California bill aims to increase fuel economy to 36.8mpg by the earlier date of 2016, beginning with the 2009 model year. According to figures from California, that means that by 2016 the Californian standard will have cut emissions by 17.2 million tonnes, more than double the 7.7 million tonnes expected to be curbed by the new US-wide standard.

So where does the fight go from here?

To court. California has filed a suit against the EPA to get the waiver rejection overturned and has been joined by the 12 states that want to adopt its standards and a number of green groups. It is expected to argue that the state's susceptibility to wild fires, heat waves, water scarcity and sea level rises means that it will face exceptional threats from climate change and as such has the right to set separate standards. Meanwhile, both the Senate and Congress have now put forward proposed legislative changes that would force the EPA to approve California's waiver application. California's governor, Arnold Schwarzenegger, has also ratcheted up the political rhetoric claiming it was " unconscionable" that the federal government should ignore "the will of millions of people who want their government to take action in the fight against global warming".

And who's going to win?

The general consensus among environmentalists is that the White House is pulling strings at the EPA and blocking the more stringent standards, so as long as Bush is still president the EPA's stalling tactics are likely to continue. However, according to EPA documents released by Congress in January, EPA staff reckon it will probably lose any legal case challenging the decision and with all the frontrunners in the presidential race all tougher on climate change than Bush there is a good chance the new rules will eventually make it into law.

How big a deal would it be if the standards are passed?

You can tell how serious it is by looking at the massive extent of the US motor industry's lobbying against the rules. It is hard to see them wanting to comply to one set of rules in some states and different sets of rules in other states so they will almost certainly be forced to comply with the Californian rules on a national basis, perhaps reserving just a few gas guzzling models for those states that stick with the more relaxed Federal standards. The net result is that after decades of lobbying against a strengthening of legally binding fuel economy standards, both Europe and the US automakers will soon face highly demanding carbon emissions targets that will force them to finally push green motoring technologies into the mainstream.