I was reading a new paper titled "Impacts of Climate Change on Marine Organisms and Ecosystems" when I came across this statement: It has been suggested that a CO2[atm] of 450 ppm is a critical threshold beyond which catastrophic and irreversible change might occur  this would bring a global mean temperature rise of 2C above pre-industrial values. At present rates, this threshold will be passed by 2040, but climate-related systems are notoriously non-linear . By 2040, some particularly sensitive marine ecosystems such as coral reefs and ice-covered polar seas could already have been lost, and other unexpected consequences may arise . http://www.cell.com/current-biology/fulltext/S0960-9822(09)01181-6#sec1 This caused me to wonder what level of annual CO2 emissions would we have to reach in order to stabilize at a level under 450PPM as opposed to reaching the 450PPM level in 2050 or 2060 or 2100? Even if we reached a 80% reduction in CO2 emissions, would it cause the CO2 levels to stabilize or only increase more slowly? We have good measurements on CO2 going back to 1959 and it has been increasing during the entire period. Unless there is some evidence to suggest that the capacity of the BioSphere to absorb CO2 has increased during that period, then it isn't obvious what level of emissions other than zero would cause it to stabilize.