Humankind is releasing CO2 at a rate of about 7 Gton C per year from fossil fuel combustion, with a further 2 Gton C per year from deforestation. Because the atmospheric CO2 concentration is higher than normal, the natural world is absorbing CO2 at a rate of about 2 or 2.5 Gton C per year into the land biosphere and into the oceans, for a total of about 5 Gton C per year. The CO2 concentration of the atmosphere is rising because of the 4 Gton C imbalance. If we were to cut emissions by about half, from a total of 9 down to about 4 Gton C per year, the CO2 concentration of the atmosphere would stop rising for awhile.

This is a bit of a guessing game, but 2°C has been proposed as a reasonable danger limit. A warming of 2° C could be accomplished by raising CO2 to 450 ppm and waiting a century or so, assuming a climate sensitivity of 3 °C for doubling CO2, a . Of the 450 ppm, 170 ppm would be from fossil futypical value from models and diagnosed from paleo-dataels (given an original natural pCO2 of 280 ppm). 170 ppm equals 340 Gton C, which divided by the peak airborne fraction of 60% yields a total emission slug of about 570 Gton C.

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