FIGURE 6-3 Composite isotopic record from low latitudes, including four ice cores from Tibet and three from the Andes. The isotope records have been normalized to mean and standard deviation and averaged. The solid line is a smoothed version of the composite record created using a 50-year triangular filter. SOURCE: Data from Thompson et al. (2006). Reprinted with permission; copyright 2006.

ratios from Kilimanjaro reveal no consistent trend over the last two millennia (Thompson et al. 2002).

The combined isotopic signal from all available ice cores in Tibet and the Andes shows that the climate of the 20th century was unusual with respect to the preceding 1900 years (Figure 6-3).

In Greenland (Figure 6-2) and coastal Antarctica, ice isotopic ratio records clearly shows 20th century warming, a Little Ice Age, and earlier warmth. In Greenland, this earlier warmth is centered at about A.D. 1000, whereas in Antarctica it was much earlier. Borehole temperature analyses yield the same pattern (see Chapter 8). In Greenland, the 20th century warmth is not higher than that during medieval times (11th century). In the Canadian Arctic, ice isotopic ratio records from the Agassiz Ice Cap on Ellesmere Island show warming over the last 150 years, which is unprecedented for the last millennium (Fisher et al. 1995). As a group, the ice cores from interior Antarctica (Figure 6-2) show nothing anomalous about the 20th century (Masson et al. 2000).