The transient response of global-mean temperature to greenhouse-gas forcing between 1765 and 2030 is studied using two types of ocean model, a pure diffusion model and an upwelling--diffusion model. The results are almost the same for both types of model. If the greenhouse-gas contribution to global warming over the period 1880--1985 is Δ, then the contribution to future warming over the period 1985--2030 is shown to be approximately 1.6 Δ independent of model type and model parameters (viz. climate sensitivity and ocean mixing rate). ¿ American Geophysical Union 1987