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Solar Variability and Climatic Change:Global Temperature Reconstructions |
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Solar Variability and Climatic Change:Global Temperature Reconstructions |
Part of the past history of climate can, of course, be known from the records of meteorological instruments from the time when they first came into use. Unfortunately it is only a small part of the story, though an interesting one, stretching back in just one or two places as much as three hundred years, but over much of the world covering no more than the last few decades. Moreover, a great deal of painstaking work is involved in making sure that the figures derived from readings of the old instruments are rendered truly comparable with those from modern instruments in standard exposures. This task has so far only been thoroughly carried out for a limited selection of observation points in various parts of the world. The barometer and thermometer were invented in Italy, by Torricelli and Galileo respectively, in the first half of the seventeenth century. The wind vane and rain- guage are earlier, but the earliest surviving rain measurements are from the late 1600s. There were already some reliable instruments about by 1700 or earlier, but the problems of exposing the thermometer and rain-guage so as to obtain representative measurements were not solved until much later. The barometer presented fewer problems and was already being used for scientific measurements by Blaise Pascal in France and Robert Boyle in England within a few years of its invention. But thermometers were for many years exposed in an unheated north-facing room (or, at one famous observatory until recently, outside on the north wall of a building); and their calibration gave trouble because of the ageing of the glass. Fahrenheit's real claim to distinction was as the maker of the thermometers with the best glass. Early rain-guages in Europe were sometimes exposed on the roofs of houses, where the catch is reduced by splashing out of the guage and by evaporation in the wind; but this position enabled Richard Towneley - who produced the first guage in England in 1676 - to lead a pipe down through the house so that he could measure the rain in the comfort of his bedroom!Other problems in using early instrument records lie in the miscellany of different units and scales used. (Sir Isaac Newton already in the seventeenth century had a thermometer scale that defined 0 degrees as the freezing point of water, but took as its other fixed point the normal temperature of the human body, labelled as 12 degrees.) A good deal is known of the early thermometers and their scales, for instance from the comparison and calibration tests reported by van Swinden in 1792 and Libri in 1830. Van Swinden listed no fewer than seventy-seven different scales. By the time that anyone seeking to use early instrument records to create long time series of temperature or rainfall measurements, and barometric pressure and wind maps, has worked through such material and coped with inches, feet and miles which had different lengths in every part of Europe (and sometimes even within one country), he is readily convinced of the virtue of the uniformity of the metric system.
Our knowledge of the climatic history of the last two or three hundred years owes a great debt to those who - like Birkeland in Norway, Labrijn in Holland, Manley in England, and Landsberg and his co-workers in the eastern United States - by close study of the instrument records with their occasional changes of site and exposure, entailing endless comparisons of overlapping records at places not too far apart, have produced apparently reliable long series of values. The longest of these is the series of mean temperatures for each month of every year from 1659 to the 1970s at a typical lowland site in central England, produced by professor Gordon Manley. Comparisons with other places in Europe, and with the reported weather and wind patterns, suggests that the values in this series may be reliable to within about 0.2 degrees C from 1720 onwards and to within 1 degree C in the earliest years. ... This work is tedious and time consuming, so that it has not been easy to find willing and suitable people or to get the funds needed to do it, but it is rewarding. Manley emphasized, in a letter to Weather written shortly before his death in 1980, his concern that, because the magnitude of the changes of climate shown by monthly means must be small and 'commonly in decimals of a degree if taken over decades', to produce a reliable record of temperature (or rainfall) using the observations made in early years every detail that can be discovered about the instruments and their exposure, the observer's technique and observation hours, must be painstakingly examined.
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