You can’t build a movement without numbers. If anyone understands that, it’s 350.org co-founder Bill McKibben.

Standing in front of an estimated crowd of 50,000 people gathered for the Forward on Climate rally yesterday on the National Mall in Washington, D.C. he said, “All I ever wanted to see was a movement of people to stop climate change, and now I’ve seen it.”

Billed as “the largest climate rally in U.S. history,” the event was intended as one final push to convince President Obama that his environmental legacy hinges on whether he rejects the Keystone XL pipeline — a conduit to what has been called by NASA scientist James Hansen “the world’s largest carbon bomb.” To underscore this point, 350.org has consistently made an effort to quantify its achievements into superlatives, ready-made for headlines.

Yet, had they not put so much effort into creating the perception of a powerful movement, they might not have ever built one. According to political scientist Erica Chenoweth, co-author of Why Civil Resistance Works, “There is power in numbers, and the more people participate, the more likely the movement is to effect real change. Interestingly, this may lead more people to participate because they want to join a movement that will ultimately be successful.”

Patrick Reinsborough of the Center for Story-Based Strategy (formerly smartMeme), which trains activists to use narrative as a tool, agrees. “The most important thing to communicate is that this movement is growing, and that everyday citizens are willing to step out of their comfort zone in order to be seen and heard,” he said.

For more than six years, McKibben has been at the forefront of efforts to create a broad-based movement that can create the pressure for policies that would bring carbon emissions to a safe upper limit. According to James Hansen, that limit, which was long ago surpassed, is 350 parts per million — a number so important to McKibben, he named his group after it.

While this decision has led some to criticize 350.org for having a name that’s too ambiguous or scientific for the average person, McKibben has long argued, “Arabic numerals are the one thing that cross globally.” This fact seems to be guiding his broader belief in the power of numbers as well.

“The hardest thing about climate change is the sense that one is too small to make a difference,” McKibben told Waging Nonviolence. “So we’ve helped people to understand that they’re part of something large, maybe large enough to matter. That helps them feel engaged, I think, and has the advantage of being the truth.” McKibben’s feature article for Rolling Stone last summer — one of the most-read in the magazine’s history — and his recent 21-city sold-out speaking tour had the word “math” in the title.

Even before the debate over its name, when 350.org was just six students and a professor at Middlebury College in Vermont, the focus was on numbers — numbers that set records, showed the scale of an action or quantified an achievement.

For instance, in 2006, the group successfully pressured Middlebury to commit to carbon neutrality by 2015. Soon after that, it organized a five-day march across Vermont to demand action on global warming. Nearly a thousand people took part, and many newspapers called it the largest climate change demonstration in America. Then, in 2007, with a campaign called Step It Up, which sought to visually depict the concept of an 80 percent carbon reduction by 2050, 350.org organized a day of action that netted 1,400 demonstrations across all 50 states, calling it, “the first open source, web-based day of action dedicated to stopping climate change.”

Since becoming 350.org a year later, the group has had a string of even more impressive achievements. In 2009, it organized 5,200 actions in 181 countries for “the most widespread day of political action in the planet’s history.” The following year saw two other landmark actions: the Global Work Party and 350 EARTH. The former generated more than 7,000 climate solutions projects in 188 countries and has been called the most widespread day of climate action in history. Meanwhile, 350 EARTH, which took place a month later, managed to gather tens of thousands of people for several of the biggest art projects ever seen — so big they could only be seen from space.

If there was any criticism of 350.org at this point, it was that the organizers were having too much fun. During those two years of dramatic actions, Congress and the United Nations failed to pass binding climate legislation. Many activists were beginning to wonder whether the impressive showing by 350.org was anything more than just a show.

Leading voices within the climate movement, such as Tim DeChristopher — who famously disrupted an oil and gas lease auction in 2008 and spent the last two years in prison as a result — wanted to see the group leverage the power of its growing base by engaging in civil disobedience. McKibben eventually heeded the call and in August and September of 2011, 350.org — under the guise of Tar Sands Action — held two weeks of sit-ins outside the White House, calling on President Obama to reject the Keystone XL pipeline. Despite some initial uncertainty about whether arrests would scare people away, the campaign proved to be yet another historic moment for the climate movement. Over 1,200 people were arrested and McKibben called it “the largest civil disobedience action on any issue in 30 years.”

Since then, there has been a boom in civil disobedience and nonviolent direct actions against the pipeline, from grassroots activists in Texas and Oklahoma to mainstream environmentalists like Sierra Club executive director Michael Brune. McKibben has also recently hinted at another mass civil disobedience, possibly this summer, telling a crowd of students in New York City a couple weeks ago to “keep an eye on 350.org and save up bail money.”

In order to get to this point, 350.org has had to slowly build upon action after action, finding the right way to frame its accomplishments for maximum effect. Other successful movements have done the same, such as the Serbian student movement Otpor!, which started with just 11 people and used graffiti and small, clever actions that never revealed their numbers until they had grown enough to topple dictator Slobodan Milosevic.

More recently, in Egypt, says Erica Chenoweth, “groups of activists would deliberately make their way down small alleyways to give the impression that there were many more people participating. It created something of an optical illusion — a small number in a small space looks bigger than a small number in a big space.”

While the climate movement may be close to toppling a pipeline, it’s far from toppling the dictatorship of the fossil-fuels industry. Chenoweth has a number of her own for what major systemic change requires. “If you buy the 5 percent rule — that if 5 percent of the population mobilizes, it’s impossible for the government to ignore them — then in the U.S. context it would mean mobilizing well over 15 million people in a sustained way,” she surmises.

When asked what he thought winning would require, McKibben said, “I’ve got no idea. It will take more than any of us can imagine.” That might be surprising coming from a man so concerned with numbers and so good at making them compelling. But right now, the only math that seems to matter to him is how long it has taken to get to this point. And for that reason, he’s savoring the moment.

“I waited a quarter century since I wrote the first book about all this stuff to see if we were going to fight,” McKibben told yesterday’s crowd. “And today, I know we are going to fight. The most fateful battle in human history is finally joined, and we will fight it together.”