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Surface Temperature Reconstructions for the last 2,000 Years
FIGURE 5-6 Relationship between the concentration of ice-rafted debris (as inferred from iron [Fe] concentration) and the flux of cosmogenic 14C. SOURCE: Bond et al. (2001). Reprinted with permission from AAAS; copyright 2001.
Jones et al. (2006) showed increased (i.e., more positive) δ18O values corresponding to the Little Ice Age along with decreased (i.e., more negative) δ18O values before about 1400 (Figure 5-7). However, the primary control on this and similar lake records was not temperature per se but the intensity of drought—linked in this case to the intensity of the summer monsoon over South Asia—and precipitation, determined here by winter atmospheric circulation over the North Atlantic. Although the East Mediterranean was relatively dry during the Little Ice Age, low lake levels in East Africa and North America indicate that droughts in these regions were more extreme in medieval times than during the 20th century, possibly linked to changes in solar activity (Hodell et al. 2001, Verschuren et al. 2000).
Fossil remains from terrestrial sediments also offer indications of past surface temperatures. Nonbiting midge larvae (chironomids) and some species of beetles are highly sensitive to temperature, and the hard parts of both organisms are preserved in lake and peat sediments. However, the main application of these records to date has been on timescales longer than the last 2,000 years. In peat bogs, testate-forming amoebae species are sensitive to water table depths, which in turn are generally controlled by both rainfall and temperature. Charman et al. (2006) have used reconstructed water table changes to build up decadal-resolution climate histories for the last 4,000 years in northern Britain. Pollen analysis is a key technique for longer term climatic and vegetation history, but does not provide climatic data of sufficient time precision or reliability to assist directly in temperature reconstruction for the last two