The effects of decade-to-century (Dec-Cen) variations in total solar irradiance on global mean surface temperature Ts during the pre-Pinatubo instrumental era (1854--1991) are studied by using two different proxies for the irradiance and an energy-balance climate model. Irradiance anomalies based on solar-cycle length (CL) and solar-cycle decay rate (CD) proxies can account for most of the warming observed up to 1976, but anthropogenic forcing is needed to explain the subsequent sharp increase in Ts. The time series of CL-solar and anthropogenic radiative forcing resemble each other; this similarity makes it difficult to separate their effects in the instrumental Ts record. The CD-based irradiance values reflect heuristically intensity variations in photospheric turbulence; this proxy allows one to obtain more tightly constrained values of both solar variability and terrestrial climate sensitivity from the instrumental Ts record. ¿ 1999 American Geophysical Union