Opponents of the Keystone XL pipeline celebrate President Obama’s veto of the legislation at a rally outside the White House on Tuesday. (AP Photo/Jacquelyn Martin)

The seemingly unending Keystone XL saga hit the spotlight again Tuesday — when President Obama vetoed legislation that would have approved the pipeline. The bill sent to him, wrote the president, “conflicts with established executive branch procedures and cuts short thorough consideration of issues that could bear on our national interest.”

Obama could still approve the pipeline itself after the executive processes that he describes, centered at the State Department, run their course. But for now, the significance of this move is worth considering. For what it means, above all, is that a relatively novel environmentalist strategy — aimed at deliberately blocking certain kinds of fossil fuel production and extraction — has now forced the hand of no less than the president himself.

“Most actions that have been taken on climate change have been about smokestacks and tail pipes,” says Michael Brune, executive director of the Sierra Club. Keystone, he said, “has been the first major public fight to argue that we have to begin to curtail production.”

President Obama vetoed a bill authorizing the controversial Keystone XL pipeline, carrying out a threat to reject one of the first measures the Republican-led Congress sent to the president's desk. (Reuters)

The entire environmental movement has been built around demanding clean air and clean water — that polluters stop polluting and conduct their businesses more cleanly. Traditional means of curbing carbon pollution are similar. They don’t tell you not to manufacture cars any more or not to run power plants any longer. Rather, they simply require that you do so with fewer emissions.

But the Keystone fight is truly different. For environmentalists, it’s fundamentally about keeping fossil fuels in the ground, unburned, a stance that causes a more polarizing clash with industry.

So what justifies this change? Science itself, suggests the Sierra Club’s Brune. In particular, if there’s one scientific paper that you need to consult to understand his thinking — and that of many other environmental leaders — it may be a study that just came out in Nature last month that calculates what the world must do to have “at least a 50 percent chance” of staying below a temperature rise of 2 degrees Celsius, globally averaged. That’s widely considered the threshold beyond which truly dangerous global warming kicks in.

The paper, by Christophe McGlade and Paul Ekins of University College London, calculates that to stay under 2 degrees we can emit only about 1,100 gigatonnes of carbon dioxide (a gigatonne is 1 billion metric tons) between now and 2050. But current fossil fuel reserves, they say, imply emissions equaling three times that amount, were they all to be burned.

That would mean that two-thirds of fossil fuels must go unburned and that we simply can’t exploit all the resources that we may have, or think we have. “Our results show that policy makers’ instincts to exploit rapidly and completely their territorial fossil fuels are, in aggregate, inconsistent with their commitments to this temperature limit,” notes the paper.

The study also examined the fossil fuel reserves of different countries, finding that when it comes to Canada, 74 percent to 75 percent of its oil reserves would need to remain unburned in order to be consistent with the 2 degrees Celsius target. The vast majority of the world’s coal reserves would also have to stay unburned, as would any fossil fuel resources contained in the Arctic.

“These results demonstrate that a stark transformation in our understanding of fossil fuel availability is necessary,” the study concludes.

This is why environmentalists are increasingly adopting what Brune calls a “supply side” strategy — trying to stop fossil fuels from ever being burned or even coming out of the ground. “Over the course of the last six years,” he says, “concerns about climate change have grown to the extent that we now have pressure campaigns against every form of extraction, every operation on the supply side — from drilling and mining, to pipeline fights, to refineries, all the way to the tailpipe.”

And that in turn means that, although it is overwhelmingly framed this way, the Keystone XL fight is not actually about the Keystone XL pipeline or what its environmental consequences would be. Rather, it’s about drawing a line in the sand and forcing the world to really act as though it respects the 2 degrees C limit.  “This entire fight has never been about the pipeline; it’s about the oil that it would transport,” says Brune, who notes that activists are also trying to block other pipeline projects that might also carry tar sands oil.

It’s a strategy that many will not agree with. To take on industrial operations so directly is highly polarizing, and most Americans don’t agree with the idea of stopping the building of Keystone XL. (Indeed, it’s still not clear whether the president himself does.)

Nonetheless, this way of thinking — of making everything about keeping the world below 2 degrees C — is now generating dramatic new conflicts over the exploitation of resources and the building of infrastructure. You may not agree with it, but you have to reckon with it.

The president certainly has.

Also in Energy & Environment:

The remote Alaskan village that needs to be relocated due to climate change

Solar energy’s new best friend is … the Christian Coalition

The dirty dozen: 12 of the most destructive invasive animals in the United States