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Introduction
Understanding present and future climates requires that we first understand some characteristics
of past climates. Past climates can be divided into two periods: (1) the recent past when we have had
instruments to measure and record climate variables over a large portion of the globe, and (2) all of the
history of planet Earth before the era of instrumental measurements.
Instrumental records of sufficient spatial coverage to represent global observations began in
about 1880 (some would say 1850 is a better starting date). Although we have scattered
observations before this time, they were not sufficiently widespread to represent global values.
It could be argued that even current measurements do not include some areas of the globe, say
Africa, to be considered to be truly global in scope. There is a period prior to the instrumental
record when we have anecdotal evidence of climate conditions, but these observations were regional
in scope and lacked a quantitative base. Examples include locations of glacier termini, date of
lake freeze-up, and memorable floods and droughts.