The separate impacts on the ocean of increases in greenhouse gases and aerosols are investigated through ensembles of experiments with a state of the art climate model (GFDL CM2.1). It is shown that aerosols have had a profound impact on the ocean by temporarily offsetting a substantial fraction of the changes induced by increasing greenhouse gases. For example, while simulations that include only increasing greenhouse gases show a significant decline in the Atlantic thermohaline circulation (THC) in the 20th century, additional simulations that include the effects of anthropogenic aerosols show no significant change. The increasing aerosols tend to stabilize the THC by cooling the North Atlantic and reducing poleward atmospheric moisture fluxes, thereby increasing sea surface salnity at higher latitudes. Aerosols also play an important role in modulating sea surface temperature (SST) in the Atlantic. Ensembles of simulations are used to assess the impact on Atlantic SST attributable to these differing forcing agents. The combined effect of changing aerosols and increasing greenhouse gases has a major impact on tropical Atlantic SSTs, which may be important for the observed multidecadal changes in Atlantic hurricanes, as well as Sahelian drought.