The Geologic Record and Climate Change
Earth's climate has varied considerably during the past 2 million years or so as indicated by the more than 33 glacial major advances and retreats that have occurred through this interval. Based on geologic paleoclimatic data it is obvious that climate is and has been very variable. Thus the only real constant about climate is change. It changes continually.
A primary role for climate researchers at present is to try and determine what the magnitude of natural climate variation is, and what sort of variation may be occurring at the present time is due to human induced causes. A major difficulty that we have is that the thermometer record only reaches back to the tail end of the 19th century.