350 is the most important number in the world. 350 parts per million (ppm) is the safe upper limit of carbon dioxide concentration in the atmosphere. It’s the number agreed upon by many of the world’s leading scientists and recently endorsed by 80 countries, but it’s not the number in the current version of the climate and energy bill under debate in Congress or the target that seems likely to be set at the international climate negotiations in Copenhagen this December.

350 is where we need to be “if humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted,” as James Hansen, NASA’s top climate scientist so dryly puts it.  The bad news is that we’re already at 390 ppm and climbing.  So, is it too late?

Is it too late for the obese man to quit junk food and start exercising? Is it too late for him to lower his cholesterol and prevent a heart attack? Absolutely not. But until he changes his lifestyle, he remains at a higher risk. And until we change our lifestyle, the Earth will remain in the danger zone. There is still time to bring carbon dioxide levels back down, but it’s going to take a major transformation in how we think and act. Getting back to 350 means developing a thousand different solutions. It means building wind farms not coal plants. And it requires that world leaders recognize our interdependence and work together like never before.

Eight years ago, I felt a swirl of emotions. I was scared for my family and friends in New York City, where I was born and raised. I was angry at the people who had done this to us. I was hurting for the victims and their families, especially those from Hook and Ladder Company 25, the firehouse where I used to play when I was a child. And I radiated with the patriotism that swept America, reveling in our shared sense of purpose. That night, I gathered with friends in my Yale dorm to mourn together and mark the immensity of the day. We knew our world had fundamentally changed and that that day marked a turning point for our nation.

Six weeks from today, on October 24, I hope for a similar turning point. The largest ever global grassroots action on climate change will take place, calling on world leaders to make 350 ppm the target in the global climate treaty to be negotiated in Copenhagen. I’ll be in Flagstaff, AZ, where I live, spreading the word about 350 and joining with over 1,400 groups in 110 countries (so far), from the Great Barrier Reef to the Taj Mahal, who are organizing on behalf of our planet.  Anyone can join a group or start their own by going to 350.org.

While October 24 is a day of hope, America is still being threatened by a politics of fear, hatred, and division. Witness Glenn Beck’s vicious smear campaign that led to the resignation of Van Jones, my friend and one of the most visionary leaders in the nation. We need fewer Glenn Becks and more Van Joneses. People, ideas, and events that inspire hope, justice, and collective action.

That’s why I love 350. 350 is a bright line to which we must return. It doesn’t belong to one group or one nation—it belongs to all of us alive today and those yet to be born.

350 slices through all the confusion and misinformation around the climate crisis. It’s about being prepared. Eight years ago, we were caught off guard. This time there is no secret memo. Everything we need to know is for all to see, out in the open.

I can’t wait to live in a post-350 world where the disastrous affects of climate change have been averted, and a thriving clean energy economy unites the planet. I hope some day my now one-and-a-half year old daughter looks back on my work with pride, and that she and her generation are up to the finishing the job. This is an intergenerational challenge and the stakes couldn’t be higher.

This entry is cross-posted at The Huffington Post.