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Obama administration rejects Keystone pipeline proposal

The decision marks a historic win for activists and the first time a world leader has stepped in to deny a fossil fuel project out of concern for the planet.

After more than a half-decade of controversy, the Obama administration on Friday rejected a permit to build the Keystone XL pipeline, an $8 billion steel straw that would have doubled the flow of oil from Canada to the Gulf Coast.

The president tried to downplay the decision, claiming that the pipeline had taken on an "overinflated role" in American politics and environmental activism alike. "This pipeline would neither be a silver bullet for the economy, as was promised by some, nor the express lane to climate disaster proclaimed by others," Obama said in an announcement at the White House.

In an earlier speech, however, he called the pipeline's net effect on the climate "absolutely critical to determining whether this project is allowed to go forward." And in his remarks on Friday, he again summoned the specter of global warming.  

WATCH: State Department rejects delay for Keystone permit review

"If we're going to prevent large parts of this Earth from becoming not just inhospitable, but uninhabitable," the president said, flanked by Vice President Biden and Secretary of State John Kerry, "we must act not later, not someday, but right here, right now."

Kerry, whose department actually issued the rejection, echoed Obama's sentiments in a statement. "The critical factor in my determination was this: Moving forward with this project would significantly undermine our ability to continue leading the world in combating climate change."

"If we're going to prevent large parts of this Earth from becoming not just inhospitable, but uninhabitable, we must act not later, not someday, but right here, right now."'

Obama was already staking his remaining time in office on an all-out push to address the issue. But the decision to block the Keystone XL pipeline is his most significant yet, a historic win for activists and a major milestone in the politics of climate change. It marks the first time that a world leader has denied a fossil fuel project after publicly considering its effect on the Earth. 

A decision to slow the expansion of oil is in line with mainstream science, which holds that if the planet is to avoid irreversible damage -- and an unmanageable explosion of storms, drought, and rising tides -- at least 70% of the known oil, coal and gas reserves in the world will need be left in the ground. The value of these “stranded assets” is about $100 trillion, according to a recent report by Citigroup.

But until Friday's announcement, no world leader had ever acted on that science. The president's decision will set off a difficult, multi-decade debate about who gets to burn what fossil fuel, and how the companies involved should be compensated or penalized along the way. Some of that discussion will get underway on Nov. 30, the first day of the United Nation's climate talks in Paris. 

Obama's decision on Friday adds momentum to the talks, which may yield the first global agreement to reduce emissions. To make his case and help America do its part, the president has already traveled to the melting edge of the Alaskan Arctic, rallied alongside climate allies like Pope Francis, and -- through an executive order -- issued the first U.S. limits on power plant emissions. 

Those efforts have been sharply attacked by Republicans, who generally question the severity of climate change and -- in the minority among the world's political parties -- doubt the need to act.

RELATED: The nine Senate Democrats who support the Keystone XL pipeline

“Barack Obama and Hillary Clinton caved to extreme special interest groups and rejected good American jobs," said Republican Party Chairman Reince Priebus. "It’s time for new leadership in the White House.”

Many of the people vying for that position took their attacks to Twitter on Friday.

Who is standing up to the Keystone Pipeline?In a series of tweets, Florida senator and Republican presidential hopeful Marco Rubio called the president's Keystone decision "a huge mistake." 

"When I'm president, Keystone will be approved and President Obama's backwards energy policies will come to an end," he wrote. 

Former Florida Gov. Jeb Bush also commented, tweeting: "The Obama Admin's politically motivated rejection of the Keystone XL pipeline is a self-inflicted attack on the U.S. economy and jobs."

About those jobs: A 2014 State Department report on the pipeline calculated that it would create the equivalent of 3,900 year-long construction jobs. The multi-billion price tag on the project would also support another 16,000 jobs related to materials, including the pipe itself, which is already built. And the tide of money would indirectly support about 30,000 more jobs, the government estimated. 

But activists successfully presented the pipeline as a planet-killer. It’s designed to slurp the tar sands of Alberta, a repository of heat-trapping carbon so large it could tip the Earth toward catastrophic climate change, they say. Tap it, in the words of James Hansen, the former NASA climate scientist, and "it's game over for the planet.” 

The tar sands may still be tapped in their entirety, despite the president's decision. But the work would be slower and more costly without Keystone, potentially sending investment dollars toward alternative fuels. 

Climate change has already become a defining concern of every Democratic presidential hopeful. All three candidates -- Hillary Clinton, Martin O'Malley, and Bernie Sanders -- have come out against the pipeline. They have called it an urgent moment for the planet, a chance to cut emissions while unleashing the power of wind, solar, and other alternative sources of energy.

Activists, meanwhile, declared victory on Friday. "We just made history together," said Bill McKibben, the founder of 350.0rg and leader of the anti-Keystone movement. "Four years to the day after we surrounded the White House, President Obama has rejected the Presidential Permit for the Keystone XL tar sands pipeline! This is huge."