Bill McKibben has been the clarion voice of the climate movement for 20 years, long before it identified itself as a movement. His 1989 The End of Nature was the first popular book to outline the basic reality of the global warming problem. His current passion is to raise the awareness that science is now telling us that we must reduce atmospheric concentrations below their current levels of 390ppm. Today, at Yale360, Bill McKibben writes "A Timely Reminder of the Real Limits to Growth."

This is the most inconvenient truth of all: not only do we need to massively reduce greenhouse gas  emissions globally, we also need more and better methods to reabsorb CO2 already in Earth's atmosphere.

A decade ago, I recall sitting in a room of eminent policy voices and being the only one in the room saying that stabilization at 550ppm was too dangerous, and that 450ppm or lower  was needed. Of course, science since then shows that I was wrong, and stabilizing at  even 450ppm will not "avoid dangerous anthropogenic changes to the climate" as the nations of the world agreed in Rio in 1992.

Now Bill McKibben has popularized the 350ppm  mantra around the globe, and the youth-run network 350.org is making this truth understood by millions. Watch for actions around the globe on October 24 to raise this awareness, or better yet, join an action or initiate one at http://www.350.org/.