Sen. Mary Landrieu, D-La., chair of the Senate Energy Committee, with Sen. Joe Manchin, D-W. Va., a member of the committee, speak to reporters about the new urgency to get congressional approval for the Canada-to-Texas Keystone XL pipeline, at the Capitol in Washington. (AP Photo/J. Scott Applewhite)

For scholars of social movements, the conjoined environmental stories of this week -- the U.S. and China have struck a historic deal to limit greenhouse gas emissions, but Congress may vote to approve the Keystone XL pipeline -- perfectly fit one of their more influential theories.

You see, what may happen here is that the more politically radical climate change grassroots loses out on a symbolic issue (blocking a pipeline that will transfer dirty tar sands oil) but climate moderates win really meaty progress on cutting greenhouse gas emissions. And that aligns nicely with the theory of "radical flank effects" in social movements, as suggested in 1988 by the scholar Herbert Haines, who argued that in the Civil Rights Movement, a "radical flank" -- groups like the Black Panthers -- paved the way for the ultimate success of moderates, like Martin Luther King, Jr.

The Keystone XL pipeline, which would carry crude oil from Alberta to the coast of Texas, has been at the center of a very long running and multifaceted controversy. Because it crosses an international border, the State Department must decide whether the pipeline is in the "national interest" -- a process that has dragged on for years.

Greens have targeted the pipeline on the grounds the oil it would carry comes from tar sands and is very energy intensive to extract. They have therefore decided to make this a major battle in the fight to stop climate change. But environmental reports from the State Department have generally been pretty pro-pipeline, suggesting that the pipeline won't have a big impact on greenhouse gas emissions and that other way of transporting crude from Canada would be even more environmentally detrimental.

So why have environmentalists made such a big deal out of it? As Haines put it, "radicals specialize in generating crises which elites must deal with." The Keystone issue, which came virtually out of nowhere due to environmental activism, would certainly count. It rallied climate advocates dramatically, gave them a cause, gave them energy, got them marching.

Moderates, though, "specialize in offering relatively unthreatening avenues of escape," writes Haines. They're super convenient for politicians to have around when they want to announce some major initiative while simultaneously placating their critics.

This process, of course, doesn't ever leave the radicals happy. Indeed, quite the opposite. Because they are defined by the outside world or the political opposition as being too extreme -- in the Keystone case, as being opposed to jobs and economic progress -- politicians may deem it beneficial or strategic to treat them as a kind of political sacrifice.

In 2013, I interviewed Haines about the Keystone battle and whether it could indirectly advance climate progress through a radical flank effect. One of the most telling things that he said is that the answer very much depends upon political momentum. "For a positive effect to happen," he explained, "what you kind of have to have is things moving in the right direction politically. So around environmentalism, it would have to be that the policy is already moving in a pro-environmentalist direction, like civil rights was, and the radicals come along and give it a boost."

It has been quite clear that momentum has been building, since President Obama's re-election, behind climate change action (partly because of the activists!). At the same time, it has also long been clear that the Keystone issue breaks, politically, in a way that much more favors the GOP than it does the environmental left. A Pew poll from earlier this year, for instance, shows that the public strongly supports Keystone overall, but that the issue divides those on the Democratic side. The center, though, is quite pro-Keystone: Independents favor its construction by 61 percent to 29 percent.

A mainstream U.S. politician, looking at polling like this, would generally see Keystone as a liability. Yet it is clear that the Obama administration has been hesitant to approve Keystone -- it cares what the base thinks, and it cares about climate change. Now, though, with the shift in Congress, the political context changes and Keystone may, simply, have too many supporters. (Although there are still hints that President Obama may veto a Keystone bill.)

So if all of this goes down as it very well might -- Obama lands a gigantically significant climate deal with China, but Congress overwhelmingly supports Keystone and the president feels he cannot fight that consensus -- you are going to see all kinds of pundits slamming greens and talking about how Keystone was always a political loser. But that would be precisely the wrong conclusion to draw.

The real truth would be that greens have won -- just not in the way they wanted or expected. But then, activists rarely get to choose precisely how it is that their actions make the world a better place.