Global Warming iN Context The Past Millennium

When discussing global warming, a fair question to ask is, "How does the temperature increase of the past century compare with other periods in the past?" It is a difficult question to answer precisely because we only have direct measurements of surface temperature for the past 140 years or so, although a few individual records go back further; there is a record of measured temperatures in central England from 1659, for example. However, the use of various proxies enables the construction of the temperature record for the past millennium, as shown in figure 7.3.

How are these records constructed? They use proxy data, such as the width of tree rings and the isotopic composition of ice cores, corals, and stalactites. Even more indirect records, such as the time of year of crop harvests and the altitude of the tree line, may also be used where available. These records all provide some measure of temperature. However, to a much greater extent than when dealing with temperatures of the past 100 years, the data coverage is spotty and may reflect only the local temperature and in an indirect way, so the difficulty lies in constructing a fair record of globally averaged temperature. To construct this record, multiple proxies that are available over the past 100 years or so are combined and calibrated against the instrumental record in such a way that the proxies yield a good approximation of the globally averaged temperature of that period. The multi-proxy record may then be used to reconstruct a globally averaged temperature going back many centuries. The method is accurate to the extent that the proxies have the same relationship to temperature in the past as they do in the present, and to the extent that they can be

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Figure 7.3. Global mean surface temperatures of the past 1,800 years. The lighter solid curve extending from about 1850 to 2000 shows the instrumental record. The longer solid curve is an estimate of temperature over the entire period using proxy reconstructions, and the gray shading is an error estimate (the 95 percent confidence interval). The series are smoothed to remove fluctuations of periods shorter than 40 years, and the temperatures represent anomalies in °C from a late twentieth century value. Source: Adapted from Jones and Mann (2004).

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Figure 7.3. Global mean surface temperatures of the past 1,800 years. The lighter solid curve extending from about 1850 to 2000 shows the instrumental record. The longer solid curve is an estimate of temperature over the entire period using proxy reconstructions, and the gray shading is an error estimate (the 95 percent confidence interval). The series are smoothed to remove fluctuations of periods shorter than 40 years, and the temperatures represent anomalies in °C from a late twentieth century value. Source: Adapted from Jones and Mann (2004).

combined in such a way as to provide a reconstruction of global temperature even though the proxies themselves do not have a uniform global coverage.

Figure 7.3 suggests that the temperatures of the past millennium until about 1900 were relatively uniform compared to the rapid increase in the twentieth century. Note, though, the hint of a "Medieval Warm Period" from about a.d. 900 to a.d. 1200, when temperatures were a little higher than the millennium average, and a Little Ice Age from about a.d. 1400 to a.d. 1800, when temperatures were a little lower. Evidently, though, the rapidity and sustained nature of the warming of the twentieth century was much greater than in any of the previous

Year dozen centuries. The characteristic shape of the curve illustrated in figure 7.3—relatively flat for several centuries and then rising rapidly—has led to it being known as the hockey stick. Although the methodology and accuracy of the calculations leading to the hockey stick have been criticized and continue to be discussed, there remains a consensus among the scientific community, at least insofar as it is represented by Intergovernmental Panel on Climate Change (IPCC) reports, that the temperature reconstructions shown in figure 7.3 are broadly correct. That is, the sustained rate of increase of temperature over the past century is unprecedented, and the current temperature levels are the highest of the past millennium.4

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