IN addition to the well-known warming of ~0.5 °C since the middle of the nineteenth century, global-mean surface temperature records1-4display substantial variability on timescales of a century or less. Accurate prediction of future temperature change requires an understanding of the causes of this variability; possibilities include external factors, such as increasing greenhouse-gas concentrations5-7 and anthropogenic sulphate aerosols8-10, and internal factors, both predictable (such as El Niño11) and unpredictable (noise12,13). Here we apply singular spectrum analysis14-20 to four global-mean temperature records1-4, and identify a temperature oscillation with a period of 65-70 years. Singular spectrum analysis of the surface temperature records for 11 geographical regions shows that the 65-70-year oscillation is the statistical result of 50-88-year oscillations for the North Atlantic Ocean and its bounding Northern Hemisphere continents. These oscillations have obscured the greenhouse warming signal in the North Atlantic and North America. Comparison with previous observations and model simulations suggests that the oscillation arises from predictable internal variability of the ocean-atmosphere system.