In January of 2008, nobody had heard of the number 350–not until scientists on the cutting edge of climate research declared that it represented the safe upper bound for carbon in the atmosphere measured in parts per million.

Now, just 18 months later, nearly half the world’s countries have come together to state, unequivocally, that it represents the bottom line for effective action on climate.  Today, at the UN Climate Talks in Bonn, the Least Developed Countries (LDCs) joined forces with the Alliance of Small Islands States (AOSIS) to make a powerful statement to demand that a climate agreement coming out of Copenhagen must actually be bold enough to fix the problem.  For AOSIS and the LDCs (and our best and brightest climate scientists) that means a peak in global emissions by 2015 and deep short-term reductions.  According to the press-release, these reductions are necessary to make it possible to "return atmospheric greenhouse gas concentrations to 350."

While this may sound like just a numbers game, the human impacts are all too real: climate impacts in a world at 450 or higher "have the potential to threaten social and political stability, and in some cases, the very survival of low-lying island states.”

The human impacts of climate change–many of them felt here and now–are precisely why the stakes are so high, and why more than 80 countries are endorsing a 350 target.  Remember, 350 is a target that was literally not even part of the discussion 18 months ago.  That’s remarkably quick action in what’s been a very slow-moving climate debate, but it represents the growing realization that global warming is already out of control.

We’re at 390 parts per million CO2 right now, which is why the Arctic is melting, seas are rising, and drought is spreading. It’s past time to get to work reducing that number–and now that work has been spurred by the forthright demand from the world’s most vulnerable countries.