ABSTRACT
The ‘forcing factors’ which have operated in the past to produce climatic changes will continue to affect climate in the future. Reconstructing past climates helps to place contemporary climates in perspective, making it easier to identify trends and periodicities. Climates from the recent and more distant past provide yardsticks against which people can assess the scale of possible future climatic changes, especially those resulting from greenhouse gas increases. The thermometer, rain gauge and barometer were invented in the seventeenth century but very few surviving sets of measurements are as old as this. The interpretation of past meteorological records is far from straightforward. Changes in the types of instruments used mean that figures within any time series, or from different stations, may not be strictly comparable without some adjustment. The term ‘mean global surface temperature’ is used throughout this book in relation to past, present and future climate.
