Author Bill McKibben never saw this coming. Founder of, an environmental campaign aimed at holding atmospheric carbon dioxide concentrations below 350 parts per million, McKibben sent word that this Saturday would be the day to take to the streets.

The call went viral in ways far beyond anything McKibben and fellow organizers imagined: As of Thursday morning some 4,317 actions and rallies are planned in 171 countries, with 300 events in China, 1500 across the United States, 500-plus in Central and South America.

Organizers credit the increasing inter-connectedness of Web, cellular and social networks for the spread, saying such random and organic growth would have been impossible even two years ago.

The climate crisis is also starting to resonate in a significant way, McKibben added. This is arguably the largest political event ever to take a data point as a rallying cry, he said, and people—particularly the youth behind many of the actions planned for Saturday—get it.

"This is the one most important number in the world right now," McKibben said in an interview. "It's the one number that applies as absolutely in the Maldives as in Manhattan. It somehow has worked its magic."

* On the shores of the dwindling Dead Sea, Israeli activists will make a giant human "3" on their beach, Palestinians a huge "5" on their shore and Jordanians a "0" on theirs.
* In the coup-ridden capital of Honduras, parishioners of the Amor, Fe, y Vida church will host a neighborhood tree-planting while across town activists plan a 5-kilometer march.
* Up in Canada's Yukon Territory, a Whitehorse youth group is planning a group hug—350 people strong—of the territorial legislature.
* With a nod to folk singer Pete Seeger, Greenfield, Mass.' Amandla Chorus has reworked the lyrics to Beethoven's classic Ode to Joy and will perform their version at the town 350 Day festival.
* An energy group is throwing a black-tie gala in Shanghai; in Beijing a few hundred students intend to cycle through downtown; way out in Western China a handful of students plans to hike to a melting glacier.

"We were prepared for a great day in the United States," said Jamie Henn,'s coordinator, who organized China with a visit, some emails, a few calls and a bunch of instant message "chats." "We had no idea it would take off the way it has internationally."

"The great thing about these digits (3-5-0) is that you can recognize them no matter what script you're using," he added. "It goes to show how wired the world is in many ways, and how you can take a real simple and focused bit of information and broadcast it around the world."

The number stems from the amount of planet-warming carbon dioxide scientists believe the atmosphere can safely hold before climate systems start to go haywire.

For the millennia before the industrial revolution, when humans started pumping industrial emissions into the atmosphere, carbon dioxide levels had held fairly steady at about 280 ppm. Carbon dioxide concentrations rose gradually but steadily to the mid-20th century, when they started to skyrocket. Today the level is 387 ppm, with many analysts expecting the globe to hit 450 ppm or even 550 ppm before world economies "decarbonize" sufficiently to radically reduce emissions.

The problem is that data from the past 100 million years suggests the planet was largely ice-free until carbon dioxide levels fell below 450 ppm, plus or minus 100 ppm. Somewhere between 350 ppm and 550 ppm, climatologists suspect, is a critical threshold that triggers irreversible climate change, loss of major ice sheets, abrupt sea-level rise and massive shifts in forests and agriculture.

Until recently the notion of bringing atmospheric carbon dioxide levels back to 350 ppm—voiced most vocally by McKibben and NASA's Jim Hansen—was dismissed as wild-eyed optimism. The Earth last saw 350 ppm in 1987, when President Reagan was in office; the molecule hangs in the atmosphere for centuries; and the world's major industrialized economies so far have shown little ability—to say nothing of inclination—to turn off the tap.

"We know better than anybody exactly how difficult this is and how politically unrealistic it is at the moment," McKibben said. "Our job is to change the political reality, because the physical and chemical reality is not going to change."

Momentum is building for a lower target, particularly as more data and better computer models become available. Last month several of the prominent climatologists and ecologists published a study in the journal Nature calling for the need to set planetary boundaries that must not be transgressed. The 350 ppm threshold was one. "These are rates of change that cannot continue without significantly eroding the resilience of major components of Earth-system functioning," they wrote.

That sense of urgency—that humanity has already stepped out of its safe operating space and is treading in the red zone—is what has propelled the campaign to the far corners of the world, McKibben said. It's as if, after years of admonition, your doctor finds your cholesterol in the danger zone and declares it must come down. "That's the day you go out and buy the running shoes," McKibben said.

That's the message Eveline MacDougall got in Greenfield, Mass. Director of the Amandla Chorus, MacDougall is 45 and doesn't "do computers." Her 78-year-old mother told her about "This is a woman who used to call me up and make sure I'm flossing," she said. "Now she's calling me up and saying we're above 380 and need to get below 350."

MacDougall said she has worked with Pete Seeger over the years, retooling songs for various events. Ode to Joy, she decided, was ripe for 350 Day, and she asked the chorus' 45 singers to draft new lyrics. "I got four phone calls in a row, boom boom boom boom," she said.

"It's a number people can wrap their minds around," MacDougall added. "350 is a limit, and basically over that we're screwed. It's very clear."

The limit's "holistic" concept is what appealed to Matt Koop-Pearce, living a continent away in Whitehorse. Koop-Pearce coordinates events for Bringing Youth Toward Equity, a social justice organization advocating for youth. His boss learned of 350 Day through connections at Canada Youth Climate Coalition; BYTE wanted to participate but didn't like the idea of a march or protest or other action that might be seen as aggressive. Then they heard cyclists in Victoria, British Columbia planned to circle the legislature there, and the idea of a group hug was born.

As of Wednesday, 160 in a town of 26,000 people had signed on.

"There are many, many people ... who don't get why I'm asking them to sign up," he said. "We're not asking them to get behind a political ideology. We're asking them to get behind a fact of the environment."

Koop-Pearce is 24, grew up in the '90s. He remembers thinking "Wow, what a boring time to be alive.... Nothing defines my life historically." The climate crisis, he said, has changed that.

And when McKibben looks around, he finds he's surrounded by faces like Koop-Pearce's: Young, adept at social networking, looking for a cause and possessing intuitive understanding of how to use the tools of the newly wired world to make this happen.

"We couldn't have done this two or three years ago," he said. "We needed not just the Web, but the Web built out over cell phones."

But it's the unplanned stuff—the call from the Hip Hop Caucus, for instance, which came Tuesday—that leaves him floored.

"The dominant metaphor has been a potluck supper," he said. "We said what the date was and what the theme was, and all around the world people have come up with dishes to cook."

350 Day is Saturday 24 October. For information or to find an event near you, visit This article was first published at