400 PPM Carbon: A Grim Milestone we passed last week, What does it mean?

"Somehow in the last 50 ppm we melted the Arctic," said environmentalist and founder of activist group 350.org Bill McKibben, who called today's news a "grim but predictable milestone" and has long used the symbolic number as a rallying call for climate action. "We'll see what happens in the next 50."

We could find out soon enough: With the East Coast still recovering from Superstorm Sandy and the West gearing up for what promises to be a nasty fire season, University of California ecologist Max Moritz says milestones like these are "an excuse for us to take a good hard look at where we are," especially as the carbon concentration shows no signs of reversing course.

Most scientists, environmentalists, and climate-conscious policymakers agree this will require, at a minimum, slashing the use of fossil fuels, and in the meantime, taking steps to adapt for a world with higher temperatures, higher seas, and more extreme weather. For example, according to Hansen, the world will need to completely stop burning coal by 2030 if returning to 350 ppm is to remain possible. What's the holdup? Texas Tech climatologist Katherine Hayhoe blames "the inertia of our economic system, and the inertia of our political system."

http://www.motherjones.com/blue-marble/2013/05/we-just-passed-climates-grim-milestone