The Kerry-Lieberman climate bill is out now, and with it comes a fateful decision for the political left in the U.S.

If the left’s institutions and messaging infrastructure succumb to internal squabbling or simple indifference; if the public is not actively won over and fired up; if President Obama and Senate Majority Leader Harry Reid (D-Nev.) stick their fingers in the wind to see which way it’s blowing … the bill will fail. The default outcome now is failure. Very few people in Washington, D.C., today believe the bill has a chance of passing.

The odds are long, but the bill could be saved if the left — and I mean the whole left, not just environmentalists — pulled together and fought like hell. What’s needed is concrete political pressure. That means tracking who’s for it and against it; relentlessly pressing for commitments; actively organizing in a few key Republican and centrist Democratic states; pressing establishment pundits and media figures to cover it; calling out those who stand in the way of progress; and never, ever letting Obama and Reid have a moment’s peace until they fulfill their promises.

The left hasn’t shown itself particularly capable of that kind of single-minded campaign. And there’s no guarantee it would succeed even if attempted. Without it, the bill’s failure is all but inevitable.

So is it worth doing? Is the bill worth fighting for with the kind of passion that was brought to health care or even the presidential election?

I believe the answer to that question is an absolute, unqualified, overwhelming yes. However flawed and inadequate, Kerry’s bill would represent a sea change in American life. It would lend desperately needed momentum to the global fight against climate change. Failure would be a tragedy and passage a huge, vital victory.

I know many of my fellow travelers on the left disagree. Some have convinced themselves that not only is the bill flawed, it’s worse than passing nothing at all; many others view it with distaste or resignation. Both left and right have attacked the bill relentlessly since its inception in the House, and for the vast muddled middle the lesson has been simple: if both sides hate it, it must not be worth supporting. A climate bill has come to Congress and it has almost no passionate supporters.

Nevertheless, the fact remains: It’s overwhelmingly important to pass the damn thing. I’ll argue as much in my next few posts, but to begin with I want to emphasize two reasons we ought to have an overwhelming bias toward immediate action, even compromised, inadequate action. One is physical, one political.

The physical argument in favor of immediate action

Geographically, CO2 reductions are fungible — from the climate perspective, a reduction here is as good as a reduction there; the source is irrelevant. However, the same is not true temporally. Present and future CO2 reductions do not have equal value. A ton of reduction today is worth more than a ton of reduction 10 years from now.

The reason is simple: For every molecule of CO2 added to the atmosphere today, future emission rates must be slashed more to return to safe levels in time. (This is the point of the famous bathtub analogy.) Every bit of delay makes the ultimate task more abrupt, difficult, and expensive. Neither the public nor policymakers seem to understand this ineluctable fact of atmospheric physics, but it is absolutely central to climate policy. Here’s a visual representation:

emission reduction scenarios“A slow start leads to a crash finish.”Science: Doniger, Herzog, Lashof

The longer action is postponed as we wait for a sufficiently ambitious climate bill, the more ambitious it needs to be — the target recedes. Getting started quickly, even with less force than most climate campaigners would like, makes the hill less steep and every future battle easier.

The political argument in favor of immediate action

By almost all projections, Republicans are going to clean up in 2010. Democrats’ current large majorities are anomalous and unlikely to return any time soon. (They couldn’t even hold on to 60 in the Senate for a full session.) Meanwhile, the remaining Republican moderates are being vigorously purged from the party by the teabaggers. It’s hard to see Republicans getting sensible on climate any time soon, when every internal dynamic is pushing the other way. If this bill doesn’t pass this year (and the filibuster remains in place), it could be another four to eight years before it comes up again, likely in weaker form. That’s 10 to 20 percent of the time left between now and 2050, at which point emissions in the U.S. ought to be getting close to zero. Meanwhile the bathtub keeps filling up.

If the American Power Act dies, state cap-and-trade programs will still proceed. The administration will do what it can through executive branch action at the Department of Energy and elsewhere. The EPA will wade into greenhouse-gas regulations (and a fog of lawsuits). But without a declining carbon cap in place, the market won’t get the 20-to-40-year predictability sought by large energy investors. There won’t be the massive shift in private capital needed to kickstart a green economy. It won’t be enough.

Meanwhile, the international climate process, which has effectively been idling for 12 years as it waits for the U.S. to get its act together, could well fall apart. Maybe it can limp along if the U.S. is allowed to count non-carbon-market reductions toward its Copenhagen commitments — Obama could probably hit America’s tepid 17 percent by 2020 target through executive action alone. But it will send an unmistakable signal to other countries. If you thought Copenhagen was difficult, with the U.S. insisting it might pass legislation, wait until Cancun after it’s clear the U.S. won’t. We can say goodbye to leverage, or good faith, or the ability to look Tuvalu’s representative in the eye.

Leaning forward

Donald Rumsfeld was wrong about the problem but right about the posture: When it comes to greenhouse-gas reductions, we should be “leaning forward.” Our bias should be toward action, even if it means making unpleasant policy or political concessions in the short term. As I said earlier:

Right now, policy is being made out of fear: fear by the private sector that decarbonization will be a crushing burden; fear by consumers that their energy prices will skyrocket; fear by politicians that the project will prove electorally unpopular. Campaigners can organize marches, think tanks can put out reports, scientists can issue dire warnings, but ultimately, that fear simply can’t be overcome in advance. The only way to overcome it is through experience.

Does the American Power Act get us started? Yes: it’s got mandatory targets. In my mind, that alone gives it an overwhelming presumption of support. It would have to contain a lot of extremely bad stuff to overcome that presumption, and while there’s certainly some lamentable provisions, I don’t think any of them are bad enough to meet that threshold. More on that soon.