Oceanic phytoplankton may exert a warming influence on the planet by decreasing surface albedo. Compared with the case of pure seawater, the globally and annually averaged outgoing radiative flux is decreased by a probable value of 0.25 Wm-2. This value corresponds to about 20% of the combined radiative forcing by greenhouse gases and anthropogenic aerosols since preindustrial times, including indirect effects. The relative importance of phytoplankton is greater on regional and seasonal scales, with forcing values reaching -1.5 Wm-2 in coastal zones and high-latitude regions during summer. The annual amplitude of radiative forcing by phytoplankton is large in subpolar regions, owing to the conjugate action of cloud amount and biomass level. Spatial and temporal variability of the forcing is affected by phytoplankton type, some reflective species increasing the outgoing radiative flux. The effects of space- and time-varying phytoplankton on surface albedo should be taken into account explicitly in the numerical modeling of climate change.