In his climate speech on Tuesday — long since swept away by the news cycle, as I predicted — Obama delivered one big surprise: a passage on the Keystone XL pipeline. Not just that, but an utterly inscrutable passage, one that has proven a kind of Rorschach blot for the energy world.

Here’s what he said:

I put forward in the past an all-of-the-above energy strategy, but our energy strategy must be about more than just producing more oil. And, by the way, it’s certainly got to be about more than just building one pipeline. (Applause.)

Now, I know there’s been, for example, a lot of controversy surrounding the proposal to build a pipeline, the Keystone pipeline, that would carry oil from Canadian tar sands down to refineries in the Gulf. And the State Department is going through the final stages of evaluating the proposal. That’s how it’s always been done. But I do want to be clear: Allowing the Keystone pipeline to be built requires a finding that doing so would be in our nation’s interest. And our national interest will be served only if this project does not significantly exacerbate the problem of carbon pollution. (Applause.) The net effects of the pipeline’s impact on our climate will be absolutely critical to determining whether this project is allowed to go forward. It’s relevant.

Amidst these gnomic utterances, it is possible to find support for any number of interpretations. Naturally I have my own.

First off, contrary to some of my more confident counterparts on both sides, I don’t think it tells us much of anything about whether Obama will approve the pipeline. After all, Keystone’s carbon impact has always been, and remains, a point of contention.

A lot of people have pointed to the word “significantly” as providing Obama wiggle room, but it’s worth noting that an administration official told Huffington Post reporter Sam Stein point-blank that “the State Department should approve the pipeline only if it will not lead to a net increase in overall greenhouse gas emissions.”

“Significantly exacerbate the problem of carbon pollution” is vague and toothless. But “will not lead to a net increase in overall greenhouse gas emissions” is quite specific, and powerful.

Of course, even that more stringent test doesn’t necessarily settle the Keystone question. Pipeline advocates are fond of saying that tar-sands oil will be extracted and burned whether Keystone is built or not. If tar-sands producers can’t pipe it through Keystone, they’ll pipe it through some other pipeline, or by road and rail. One way or another the oil will find a way to market, which means building Keystone won’t raise overall greenhouse gas emissions at all relative to not building it. That’s basically the position of the State Department in its draft Environmental Impact Statement [PDF], which says Keystone is “unlikely to significantly impact the rate of extraction in the oil sands.”

All sorts of people, including EPA, have called BS on this. They say State is using clunky models with outdated assumptions that ignore real-world frictions. This Oil Change report [PDF] summarizes the argument pretty well. An in-depth Reuters analysis found that rail may not be a viable substitute for pipelines. The Globe & Mail says, “Without Keystone XL, oil sands face choke point.” Numerous oil industry analysts and executives have said similar things [PDF].

For a comprehensive, fair-minded, and deep-in-the-weeds look at this question, see this post by Jesse Jenkins. He makes the key point that I would reiterate here: Your assessment of the net carbon impact of Keystone will depend entirely on the assumptions and counterfactuals you bring to bear. If you’re keen to get one answer or another, you can.

All of which is to say, Obama’s declaration that net carbon impact will decide Keystone’s fate says nothing about which way he’ll decide. It all depends on whose assumptions and counterfactuals he relies on. If the State Department sticks with its assessment, Obama will have plenty of cover to approve the pipeline if he wants. If he wants to reject it, he can ask State to update its assumptions. We still don’t know what he will do.

So. The significance of his comments is not that he showed his hand. But what is the significance? On that score, I have one question and one point to make.

The question is: Why say anything about Keystone at all? No one expected him to. It’s a politically fraught topic that was bound to distract from the rest of the climate plan. Even the day before the speech, “senior administration officials” (sigh) were strongly indicating that he wouldn’t mention it. It’s obviously something that got added pretty late in the process. But … why?

It seems absolutely insane to me that he would bring it up in a climate speech meant to mollify environmentalists, highlight the salience of climate in the determination, and then … go ahead and approve it. That would be such an own goal. If he plans to approve it, why not just keep quiet about it in a speech targeted to greens?

Does that mean he’s planning to reject it? Oh, hell if I know. Even if he were, there’s still no obvious political logic in dredging it up for the dippy Beltway media and Republicans to focus on, overshadowing his other proposals.

There’s one reason I can think of why Obama would say what he said, and it’s not political, or at least not narrowly tactical. Rather, it’s a statement of principle. This is the key line: “our national interest will be served only if this project does not significantly exacerbate the problem of carbon pollution.”

That is, if you stop to think about it, a radical thing to say. To see why, you have to think beyond Keystone. Imagine the same criterion applied to all infrastructure projects, all roads, bridges, airports, trains, electrical grids, sewer systems: If it raises carbon pollution, it is not in our national interest. If that test were taken seriously by the entire federal government (military included) … well, we’d be living in a very different world.

It’s worth noting here that the administration is, in fact, implementing something like this, making climate a key part of federal project assessment and approval under the National Environmental Policy Act (NEPA). But even that falls short of the kind of categorical proclamation in the speech.

So anyway, maybe that’s what Obama was up to, doing exactly what his critics on left accuse him of never doing: trying to shift the Overton window, trying to change the long-term frame, trying to push climate pollution to the center of national decisionmaking.

“Our national interest will be served only if this project does not significantly exacerbate the problem of carbon pollution.” If Obama can disseminate that perspective throughout the fabric of the federal government, it would be a climate legacy bigger than any single project, policy, or speech.