03/18/2010 05:12 am ET Updated May 25, 2011

Why I'll wear a number on my head tomorrow

I heard Bill McKibben speak last night at the New School, in New York City, and he was predictably gloomy (he always starts these talks with the latest bad climate news). But then he switched to cheerleading for, the climate-action group he founded with seven students. Why does turning out on October 24, the International Day of Climate Action, matter? Because "we are not going to solve this problem one light bulb at a time." We need a bigger change than eighty percent reductions in greenhouse gas emissions by 2050 - that's too far off. "We need big change now, and we need it to be global."

He continued (I paraphrase and I elide): "Scientists and engineers have done their jobs. Writing and speaking about climate change hasn't done any good. We can't blame politicians for not providing until we've asked them to provide." How do we ask? With this massive demonstration of concern - the most widespread day of action in the planet's history.

McKibben is asking citizens to gather together and display the number 350 (the safe upper limit for carbon dioxide, in parts per million, in our atmosphere) in a creative way. Send in photos of your "action" to and they'll "remind our leaders that they need to take physical reality--and not political expediency--into account when they're making decisions about our collective future" in Copenhagen in December. I don't know how persuasive the photos will be: the important thing is that massive numbers of people are present and counted this Saturday. We need to show the world's leaders (ours in the U.S. in particular) that there is, in fact, a movement. A movement gives political cover to those brave enough to speak out and craft protective policies and laws.

McKibben has been traveling the world for the last nine months or so, and though exhausted he seemed to retain a bit of hopefulness. He said, "The planet's immune system is starting to kick in - I really feel that. We're at a pivotal moment." He didn't mean the planet can fend off disease: he meant that people were starting to resist the deadly inertia of politics as usual. Let's hope he's right.

And so I'm going out tomorrow with thousands of others to march over the Brooklyn Bridge, bearing either a 3, a 5, or a 0 on my head (my daughter has first pick; my husband and I will take what's left). It may sound silly, but if there's a shred of hope it will have some impact, I'm in. As McKibben said, you don't want to look back, twenty years from now, and wish you'd done something when you had a chance.