The global-mean surface temperature has inherent variability on all time scales. Distinguishing natural climatic variability from the observed ~0.5 ¿C warming since the mid nineteenth century is crucial to understanding the response of the climate system to the enhanced greenhouse effect. We perform singular spectrum analysis (SSA) of several long-term (173--1481 years) proxy records of temperature. We find significant variability on the century time-scale in the records, with the most predominant modes being oscillations of time period ~160 and ~80 years. The findings remain essentially unchanged when the portion of the records after 1900 are excluded to remove any notable anthropogenic influence. These oscillations are of sufficient magnitude to account for a significant portion of the warming trend observed in instrumental records of temperatures from the 1850s until the 1970s. Our findings suggest that attribution of anthropogenically induced climate change may be more difficult than currently believed.¿ 1997 American Geophysical Union