This graph shows the global average temperature for the past 65 million years. The graph has been reconstructed from “proxy data” that are indicators of climate variability and stand-ins for temperature (e.g., isotope ratios in ice cores, fossil pollen, ocean sediments, and coral data). Because of the diverse sources of proxy data, statistical methods are being used extensively to reconstruct paleoclimate temperature records like the one above.
Data from ftp://ftp.ncdc.noaa.gov/pub/data/paleo/contributions_by_author/zachos2001/zachos2001.txt