Internet Archaeology 11: Ray & Adams 2. The Last Glacial Maximum (LGM).

2.0 The Last Glacial Maximum (LGM)

Although time is, of course, a continuum, it has been necessary here to concentrate the reconstructed scenarios around a particular slice of time, the Last Glacial Maximum (LGM), that seems particularly significant in relation to the processes taking place in the global system. We feel that the particular choice of this time-slice is likely to coincide with the interests of many others in archaeology, anthropology and palaeoecology. Other time-slices, such as the Younger Dryas (approx. 10,800-10,200 14C years ago), are of similar interest, and we plan on producing updated maps in electronic format for different time-slices in the near future (earlier maps are available as images at the QEN website).

The LGM time-slice is placed at around 18,000 years ago in radiocarbon years, now thought to correspond to about 20,000-21,000 years ago in calendar years (Crowley and North 1991). We can consider, however, that the LGM sensu stricto is representative of the broader interval from 25,000 to 15,000 BP, during which period the climate was very similar in most areas. In this paper, our LGM maps refer to this slightly broader time interval, the LGM sensu lato.

The LGM is seen as the stage during the last glacial cycle at which the greatest mass of ice was present on Earth, showing up in ice cores and carbonates as a peak of 18O (Crowley and North 1991). It is also thought of as being the time at which other components of the ocean-atmosphere system were at their most 'glacial' (e.g. lowest global temperatures, lowest atmospheric C0₂ concentration, and apparently greatest aridity in many continental regions). In fact, there are numerous signs that not all attributes and processes reached their peak of 'glaciality' (in the sense of maximum cold, maximum ice extent, and maximum difference in water balance relative to the present) at exactly the same time during the last glacial phase. Colinvaux (1987), for example, suggests that the lowest temperatures and maximum glacier extensions in tropical uplands may have occurred several thousand years before those at higher latitudes. In contrast, in the mountains of Africa and in the equatorial region of the Far East, at least some glaciers reached their maximum size well after 18,000 years ago, at around 15,000 14C years ago (Crowley and North 1991; Markgraf 1993; Street-Perrott and Perrott 1994).

It is important to bear in mind that in some areas for which there is continuous well-dated evidence, climates only a few thousand years before or after 18,000 radiocarbon years ago can be shown to have been quite different from the LGM itself, often being much moister. The LGM phase was relatively brief (lasting for perhaps 2000 calendar years, and depending on the precise definition that one is using) and represents only the most extreme part of a globally cold and dry phase.