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I would like to how it was possible to establish what the temperature and climates were of earth in the past. For example, how was it discovered what the Earth's temperature / climate was during the Holocene Maximum period 10,000 ago and the mini ice age in the seventeenth century?
Tim Johnson, Christchurch
Professor Mitchell: There are a large number of techniques which have been used to estimate past climates and there is only space for a brief overview here.
Instrumental temperature time series extend in Europe back to the middle of the seventeenth century (e.g. the Central England Temperature series) and so include the little ice age. On longer timescales, proxy indicators such as those based on width of tree rings, relative frequency of pollen types in land cores and marine organisms such as diatoms in ocean floor cores have been used. The proxy data are calibrated against their variation with temperature and rainfall during the instrumental period. Ice cores provide a record of atmospheric composition and, through oxygen isotope ratios form ice cores, it's possible to derive information on sea level and temperature. All these approaches have uncertainties, including their calibration and representativeness of large scale climate: they are inherently less certain than direct instrumental measurements.
