LOW-FREQUENCY OSCILLATIONS IN TEMPERATURE-PROXY RECORDS AND IMPLICATIONS FOR RECENT CLIMATE-CHANGE

The global-mean surface temperature has inherent variability on all ti me scales. Distinguishing natural climatic variability from the observ ed similar to 0.5 degrees C warming since the mid nineteenth century i s crucial to understanding the response of the climate system to the e nhanced greenhouse effect. We perform singular spectrum analysis (SSA) of several long-term (173-1481 years) proxy records of temperature. W e find significant variability on the century time-scale in the record s, with the most predominant modes being oscillations of time period s imilar to 160 and similar to 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 warmi ng trend observed in instrumental records of temperatures from the 185 0s until the 1970s. Our findings suggest that attribution of anthropog enically induced climate change may be more difficult than currently b elieved.