Okay, start with an easier question: Is McCain sincere about tackling global warming? As the story goes, he was first asked about the subject eight years ago in New Hampshire, and, after pleading ignorance on the matter, studied up and became a convert. He's reportedly close to Fred Krupp, the head of Environmental Defense, who has a history of reaching out to Republicans with green leanings (and, occasionally, trusting them long after they've ceased to deserve it—as with George W. Bush).
Plus, let's face it, going into Michigan and talking up fuel-efficiency, as McCain is now doing, takes some chutzpah, even if he is just doing it to woo independents. (Jon Cohn is writing more on this—I'll add, however, that McCain has been silent on the fight over new coal plants in Michigan, which speaks poorly for his green bona fides.) He's also taking his lumps from Mitt Romney and National Review on the issue. So, yeah, I think he's fairly sincere. One might even wonder if a McCain presidency, combined with a Democratic Congress, offers the best chance for a bipartisan-yet-still-decent emissions-reduction bill to get enacted and stay enacted. (Think Schwarzenegger and health care in California.) I'm skeptical, but it's not an outlandish argument.
That said, McCain's policy proposals are... far, far weaker than anything put forward by the Democratic front-runners. Whereas the Dems have all put out cap-and-trade plans with emission targets in line with what climate scientists are urging, McCain's website is vague on details. The cap-and-trade legislation he sponsored with Joe Lieberman in 2003 was a fine (if modest) gesture for its time, but, this year, McCain has opposed its successor, the watered-down Lieberman-Warner bill in the Senate, because it doesn't lavish enough money on the nuclear industry. We can debate the merits of nuclear handouts all day, but it's a lame reason to oppose the biggest cap-and-trade bill going, and is enough to make one question how "sincere" McCain will be when it actually matters. (Much like how, in 2005, he talked a big game on habeas corpus but, in the end, folded like a lawn chair.)
So, substantively, there's less than meets the eye. One thing I do like about McCain, though, is the way he frames the issue—and here Dems could take lessons. Barack Obama, for instance, loves to emphasize the "sacrifice" required to avert global warming, which seems Carter-esque in its tone-deafness. Here, by contrast, is McCain's preferred delivery:
Suppose that climate change is not real, and all we do adopt green technologies, which our economy and our technology is perfectly capable of. Then all we've done is given our kids a cleaner world. But suppose they are wrong. Suppose they are wrong, and climate change is real, and we've done nothing. What kind of a planet are we going to pass on to the next generation of Americans? It's real. We've got to address it. We can do it with technology, with cap-and- trade, with capitalist and free enterprise motivation. And I'm confident that we can pass on to our children and grandchildren a cleaner, better world.
That sort of optimism would make Nordhaus and Shellenberger proud. Now, if McCain snags the nomination, he'd likely neutralize the Democrats' traditional advantage on the environment, as Bush did in 2000 by pretending he cared. But if there are still concrete differences between McCain and his opponent, then, as Dave Roberts points out, Al Gore could potentially step in by endorsing the Democratic nominee—unless, of course, McCain puts out an equally effective climate change proposal, etc. Gore's stature here is presumably big enough that he could unblur whatever differences exist.