400 ppm May 10, 2013 : 10:25 AM

Yesterday, for the first time in human history, concentrations of carbon dioxide, the primary global warming pollutant, hit 400 parts per million in our planet's atmosphere. This number is a reminder that for the last 150 years -- and especially over the last several decades -- we have been recklessly polluting the protective sheath of atmosphere that surrounds the Earth and protects the conditions that have fostered the flourishing of our civilization. We are altering the composition of our atmosphere at an unprecedented rate. Indeed, every single day we pour an additional 90 million tons of global warming pollution into the sky as if it were an open sewer. As the distinguished climate scientist Jim Hansen has calculated, the accumulated manmade global warming pollution in the atmosphere now traps enough extra heat energy each day to equal the energy that would be released by 400,000 Hiroshima-scale atomic bombs exploding every single day. It's a big planet -- but that is a LOT of energy. And it is having a destructive effect.

Now, more than ever before, we are reaping the consequences of our recklessness. From Superstorm Sandy which crippled New York City and large areas of New Jersey, to a drought which parched more than half of our nation; from a flood that inundated large swaths of Australia to rising seas affecting millions around the world, the reality of the climate crisis is upon us.

Our food systems, our cities, our people and our very way of life developed within a stable range of climatic conditions on Earth. Without immediate and decisive action, these favorable conditions on Earth could become a memory if we continue to make the climate crisis worse day after day after day.

With any great challenge comes great opportunity. We have the rare privilege to rise to an occasion of global magnitude. To do so, our communities, our businesses, our universities, and our governments need to work in harmony to stop the climate crisis. We must summon the very best of the human spirit and draw on our courage, our ingenuity, our intellect, and our determination to confront this crisis. Make no mistake, this crisis will demand no less than our very best. I am optimistic because we have risen to meet the greatest challenges of our past.

So please, take this day and the milestone it represents to reflect on the fragility of our civilization and and the planetary ecosystem on which it depends. Rededicate yourself to the task of saving our future. Talk to your neighbors, call your legislator, let your voice be heard. We must take immediate action to solve this crisis. Not tomorrow, not next week, not next year. Now.