As George Zornick reports, at least 35,000 attended the rally in Washington, DC, on the National Mall last Sunday for the Forward on Climate march, a project of 350.org, the Sierra Club and the Hip Hop Caucus. “This is the last minute in the last quarter of the biggest, most important game humanity has ever played,” Van Jones told the crowd gathered in opposition to the Keystone XL pipeline. “President Obama, all the good that you have done, all the good you can imagine doing, will be wiped out by floods, by fires, by superstorms if you fail to act now to deal with this crisis.”

Prior to the rally Bill McKibben—author, longtime activist and president of 350.org—told reporters that building a movement gives the president the support he needs to say no to the pipeline. And the movement showed up, making the rally the largest of its kind in US history—and the message of activists was clear. “Their audience was really just one man, the only one with the power to stop the project: Barack Obama,” writes Zornick.

“Humanity is staring down the barrel of a calamity beyond measure, and the president is one of the few people on earth who on his own authority can do something about it,” we write in this week’s issue of The Nation. While President Obama mentioned climate change in his State of the Union address, he hasn’t committed to any of the big steps needed to avert catastrophe. The Keystone XL, a 1,700-mile pipeline that would transport tar sands oil from Canada across the United States to the Gulf Coast, which the president failed to mention in his address, is the most urgent and obvious example of an executive action he could take.

“The stakes in this battle could not be higher. If Keystone XL fails to win the president’s approval, the industry will certainly grow at a far slower pace than forecast and possibly witness the failure of costly ventures, resulting in an industry-wide contraction,” writes Nation correspondent Michael T. Klare. “If approved, however, production will soar and global warming will occur at an even faster rate than previously projected.”

President Obama must choose, not only what his climate legacy will be, but what kind of planet he leaves behind. Find out what you can do to take action and stop the Keystone XL. Through grassroots organizing, the climate movement’s energy has never been higher. Join the fight to push the president to use his second term to protect our precious resources and fight catastrophic climate change.