Abstract :
A least-cost pathway to cap cumulative anthropogenic carbon emissions at 1000 gigatonnes (Gt) between 1991 and 2100, thereby limiting further average global surface temperature increases to 1.6-2.8 (degree)C, is suggested as a pragmatic position on climate change due to the emission of "greenhouse gases" (CO2, CH4, N2O, and halocarbons) by human activities. CO2 produced in fossil fuel combustion and cement production is the major anthropogenic greenhouse gas, but represents only 4% of its natural circulation. However, at least 97% of the "greenhouse effect" that keeps the average global surface temperature at +15 (degree)C rather than -18 (degree)C is due to atmospheric water vapor in various forms and is, therefore, benign. It is also shown that roughly doubling pre-industrial CO2-equivalent greenhouse gas concentrations would theoretically increase the effective global emission temperature from the troposphere by only 1.2 (degree)C, but the effect on the average global surface temperature is obscured by other poorly defined factors.
