Climate simulations for the period 1880-2003 driven by a multitude of natural and anthropogenic climate forcing mechanisms paint a reasonably convincing picture of the causes of global warming of the past century. For most of that century more than half of the warming effect of greenhouse gases was counterbalanced by the cooling effect of anthropogenic aerosols. Hansen and Lacis in 1990 described this aerosol counterbalancing as a “Faustian bargain”, because it could be maintained only via continued exponential growth of the atmospheric aerosol load. Beginning in the 1970s, due in substantial degree to the shift from a 4%/year to 1%/year growth rate of fossil fuel use, the warming shielded by aerosols began to be partially unmasked. Current policies and practices concerning aerosol and greenhouse gas emissions promise to bring the full force of the greenhouse effect to bear during coming decades. Avoidance of accelerating anthropogenic climate impacts will require addressing the full range of climate forcing agents.