As expected, the EPA officially issued its finding today that greenhouse-gas emissions endanger public health and welfare. (See the full text here.) A few days ago, I rehashed why this ruling was so important—it basically means that regulations for carbon-dioxide emissions are now on the way whether Congress passed a global-warming bill or not. Kate Sheppard also has a terrific report over at Grist.
For a variety of reasons, it'd be more effective to have Congress set climate policy and regulate greenhouse-gas emissions rather than the EPA. (The big Waxman-Markey energy bill in the House would, among other things, set up a new cap-and-trade system for emissions and then repeal the EPA's authority to devise new CO2 regulations.) Most obviously, taking the legislative route would make the regulatory regime much more stable. If it's all up to the EPA, then Sarah Palin could waltz into the White House in 2016 and undermine the rules, only to have a green-minded president undo all her work four years later.... That sort of erratic back and forth won't help anyone.
There's also a case to be made that the existing Clean Air Act isn't a particularly efficient device for curbing greenhouse-gas emissions. For one, existing polluters would presumably get grandfathered in under existing rules, which could give utilities incentives to keep, say, open old dirty coal plants running for as long as possible without modifications. A congressional bill could also address broader swathes of the economy than just vehicles and power plants. (The Waxman-Markey cap-and-trade system covers industry, electricity generation, transportation, and some residential and commercial—though it notably excludes forestry and agriculture.) Not to mention that an EPA-led regulatory approach could get knotted up in the courts for years.
Still, at this point, it's entirely up to Congress. If Senate Republicans and conservative Democrats plan to filibuster a climate bill for all eternity, then they'll have to watch the EPA do its thing. (Then again, many Democrats might prefer this approach, letting the EPA take the blame for whatever consequences ensue, rather than passing a controversial bill themselves.) Meanwhile, James Inhofe is already threatening to introduce a "simple, narrowly targeted bill that stops EPA in its tracks," although that'll surely get blocked by the Dem leadership unless it comes alongside some alternative way of dealing with greenhouse-gas pollution.