The pre-modern Holocene spans the present interglacial from its onset 11,500 years ago to the time when a reasonable amount of observational data became available, about 1000 years ago. The pre-modern Holocene is regarded as probably the best source of information for understanding how natural radiative forcing agents have changed on timescales of millennia to multimillennia within a modern-like interglacial climate. New evidence suggests that at least regionally the entire ~10,000-year interglacial period contains relatively large oscillations of millennial to multimillennial duration (Hodell et al., 1991; Bianchi and McCave; 1999; deMenocal et al., 2000; Bond et al., 2001; Haug et al., 2001; Thompson et al., 2002; Friddell et al., 2003; Hu et al., 2003; Poore et al., 2003; Niggemann et al., 2003; Magny and Bégeot, 2004).

Orbital-Forced Solar Insolation

Solar insolation changed during the course of the pre-modern Holocene by about 10 to 20 W m−2, in opposite phase for winter and summer. The peak high-latitude summer insolation between about 7,000 and 11,000 years ago likely favored warmer high-latitude summers (sometimes referred to as the Holocene optimum), but cooler high latitude winters and slightly cooler tropical summers, with any net hemispheric or global-scale changes representing a subtle competition between these seasonally and spatially heterogeneous changes (Hewitt, 1998; Kitoh and Murakami, 2002; Liu et al., 2003) and seasonally specific (e.g., vegetation-albedo) feedbacks (e.g., Ganopolski et al., 1998). Recent modeling studies suggest that mid-Holocene global mean surface temperatures may actually have been cooler than those of the mid-twentieth century, even though extratropical summers were likely somewhat warmer (Kitoh and Murakami, 2002).

Extratropical summer temperatures appear to have cooled (Figure 3-3) over the subsequent four millennia (e.g., Matthes, 1939; Porter and Denton, 1967). This period is sometimes referred to as the “Neoglacial” because it was punctuated with periods of glacial advance and retreat of extratropical and tropical mountain glaciers (Grove, 1988). It is reminiscent of, although far more modest than, a full glacial period of the Pleistocene epoch. In many cases, glacial advances appear to have culminated in extensive valley glaciers during the so-called Little Ice Age between the seventeenth and nineteenth centuries.

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