The last thousand years

The detailed systematic record of weather parameters such as temperature, rainfall, cloudiness and the like presented above, covering a good proportion of the globe over the last 140 years, is not available for earlier periods. Further back, the record is more sparse and doubt arises over the consistency of the instruments

Year

Figure 4.5 Northern hemisphere temperature anomalies (relative to the 1961-90 mean) during the last 1300 years from ten published overlapping reconstructions from proxy records - e.g. tree rings, corals, ice cores and historical records - (shadings), and from 1860 from instrumental data (black line). For the percentage shading scale, temperatures within ± 1 standard error (SE) of a reconstruction score 10% and regions within the 5-95% uncertainty range score 5%. The maximum of 100% is obtained only for temperatures that fall within ± 1 SE of all ten reconstructions.

Year

Figure 4.5 Northern hemisphere temperature anomalies (relative to the 1961-90 mean) during the last 1300 years from ten published overlapping reconstructions from proxy records - e.g. tree rings, corals, ice cores and historical records - (shadings), and from 1860 from instrumental data (black line). For the percentage shading scale, temperatures within ± 1 standard error (SE) of a reconstruction score 10% and regions within the 5-95% uncertainty range score 5%. The maximum of 100% is obtained only for temperatures that fall within ± 1 SE of all ten reconstructions.

used for observation. Most thermometers in use 200 years ago were not well calibrated or carefully exposed. However, many diarists and writers kept records at different times; from a wide variety of sources weather and climate information can be pieced together. Indirect sources, such as are provided by ice cores, tree rings and records of lake levels, of glacier advance and retreat, and of pollen distribution (found in sediments in lakes for instance), can also yield information to assist in building up the whole climatic story. From a variety of sources, for instance, it has been possible to put together for China a systematic atlas of weather patterns covering the last 500 years.

Similarly, from direct and indirect sources, it has been possible to deduce the average temperature over the northern hemisphere for the last millennium (Figure 4.5). Sufficient data are not available for the same reconstruction to be carried out over the southern hemisphere. Because of the uncertainties underlying the precise interpretation of proxy data and because of the sparsity of data and coverage especially for earlier periods, there are large uncertainties associated with the reconstructions shown in Figure 4.5 - as is illustrated also by the range within the ten different reconstructions. However, it is just possible to identify the 'Medieval Warm Period' associated with the eleventh to fourteenth centuries and a relatively cool period, the 'Little Ice Age', associated with the fifteenth to nineteenth centuries. There has been much debate about the extent of these particular periods that only affected part of the northern hemisphere and are therefore more prominent in local records, for instance those from central England. The increase in temperature over the twentieth century is particularly striking and the 1990s are likely to have been the warmest decade of the millennium in the northern hemisphere.

Although there is as yet no complete explanation for the variations that occurred between 1000 and 1900, it is clear that greenhouse gases such as carbon dioxide and methane cannot have been the cause of change. For the millennium before 1800 their concentration in the atmosphere was rather stable, the carbon dioxide concentration, for instance, varying by less than 3%. However, the influence of variations in volcanic activity can be identified especially in some of the downturns of temperature in the record of Figure 4.3. For instance, one of the largest eruptions during the period was that of Tambora in Indonesia in April 1815, which was followed in many places by two exceptionally cold years; and 1816 was described in New England and Canada as the 'year without a summer'. Although the effect on the climate even of an eruption of the magnitude of Tambora only lasts a few years, variations in average volcanic activity have a longer-term effect. It is likely also variations in the output of energy from the Sun provide some part of the explanation.3 Although accurate direct measurements of total solar radiation are not available (apart from those made during the last two decades from satellite instruments), other evidence suggests that the solar output could have varied significantly in the past. For instance, compared with its value today it may have been somewhat lower (by a few tenths of a watt per square metre) during the Maunder Minimum in the seventeenth century (a period when almost no sunspots were recorded; see also box on page 166). There is no need, however, to invoke volcanoes or variations in solar output as the cause of all the climate variations over this period. As with the shorter-term changes mentioned earlier, such variations of climate can arise naturally from internal variations within the atmosphere and the ocean and in the two-way relationship - coupling - between them.

The millennial record of Figure 4.5 is particularly important because it provides an indication of the range and character of climate variability that arises from natural causes. As we shall see in the next chapter, climate models also provide some information on natural climate variability. Careful assessments of these observational and model results confirm that natural variability (the combination of internal variability and naturally forced, e.g. by volcanoes or change in solar output) is very unlikely to explain the warming in the latter half of the twentieth century.

Scientists in Antarctica use a hand drill to take 10-metre ice cores. Chemical analyses of the cores will reveal changes in climate and the composition of the atmosphere.
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