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 °C, is suggested as a pragmatic position on climate change due to the emission of "greenhouse gases" (CO 2 , CH 4 , N 2 O, and halocarbons) by human activities. CO 2 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 °C rather than -18 °C is due to atmospheric water vapor in various forms and is, therefore, benign. It is also shown that roughly doubling pre-industrial CO 2 -equivalent greenhouse gas concentrations would theoretically increase the effective global emission temperature from the troposphere by only 1.2 °C, but the effect on the average global surface temperature is obscured by other poorly defined factors.