Right now, the atmospheric concentration of carbon dioxide (CO2) is around 385 parts per million (ppm), up from about 280 ppm before the Industrial Revolution. Scientists, economists, and others disagree about what figure to accept as a ‘safe’ maximum concentration, with uncertainty enduring both about what temperature increase any particular concentration level will mean and what the social, political, physical, and biological consequences would follow.
Probably the most cautious group of all, with regards to what concentration is safe, is Bill McKibben’s 350.org, with their target of a reduction to 350 ppm. One major supporter is top NASA climatologist James Hansen:
If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm.”
By contrast, many environmental organizations have expressed support for targets between 450 ppm and 550 ppm, usually while hoping that such an increase would not generate more than 2°C of additional warming. Whether it would do so or not depends primarily on the relative strength of feedbacks within the climate system.
With extremely aggressive cuts in human greenhouse gas emissions, it could be possible to stabilize concentrations below their current levels. One major reason for this is the oceans. When CO2 is added to the atmosphere, it is akin to adding more CO2 to the area of air at the top of a soda bottle. Left alone, some of that extra CO2 will end up dissolved in the soda. In precisely the same way, if human beings were to stop emitting CO2 today, the levels would gradually begin to decline, until the amount of CO2 dissolving into the ocean became equal to the amount bubbling out of the oceans: a stable equilibrium with constant macroscopic properties. As such, the oceanic acidification that arises from climate change does, to some extent, reduce the amount of warming that would result from any set quantity of CO2 emissions.
Recentlly, Bill McKibben appeared on The Colbert Report, where he did quite a good job of getting his message across despite Colbert’s unique style of interview. (Link for those in Canada)