The climatic setting
The strictly environmental influences on human cultures can be considered as comprising two elements: climate and ecology. Both, of course, over a period as long as this chapter covers, registered great differences from time to time as well as from place to place.
In the case of climate, it is possible to make some valid assertions at a level of analysis that considers the whole world conspec- tually. Global climate depends, above all, on the relationship between Earth and the sun. At intervals, the planet tilts on its axis or wobbles in its orbit, increasing the intervening distance or the angle of incidence of the sun’s rays.[369] Very roughly, however - on average, as far as evidence currently available shows, every hundred thousand years over the last million - the result is plain: an Ice Age, which may cool the entire planet, or, according to the extent of the tilt of the Earth, affect one hemisphere more than another. Apparently random variations, unexplained or unsatisfactorily explained (albeit not for want of effort), can buck the prevailing trends for periods of varying length, which, in fairly well documented cases, can extend for a couple of millennia or more. Even in the relatively warm interstices between cooling events, variations in sunspot activity can cause protracted reversions to intense cold. In well- documented periods, these have lasted for as long as a hundred years, interrupting longer spells of global warming.[370]The changes in climate that form the framework of our story can be summarized at once. We can link the first emergence of Homo sapiens in the archaeological record with a cool period at about 150,000-200,000 years ago. Dispersal over an unprecedented swathe of the globe, from about 100,000 years ago, with the accompanying cultural changes, coincided with an Ice Age that, at its most extensive, about 20,000 years ago, spread ice in the northern hemisphere as far south as the present lower courses of the Missouri and Ohio rivers in North America and deep into what are now the British Isles.
Ice covered what is today Scandinavia. Most of the rest of what is now Europe was tundra or taiga. In central Eurasia, tundra reached almost to the present latitudes of the Black Sea. Steppe licked the shores of the Mediterranean. In the New World, tundra and taiga extended to where Virginia is today.Not long after the resumption of warming, between about 18,000 and 15,000 years ago, an extended period of falling temperatures followed. Thereafter, the geological record shows enormous regional fluctuations in temperature. Melting ice meant cooling seas and temporary reversals of warming in affected latitudes. Further periods of perceptibly reduced temperatures worldwide, at intervals of about a millennium, with climaxes at about 13,000 and 12,000 years ago respectively, witnessed (and perhaps stimulated) new forms of cultural divergence, which multiplied when warming resumed and which, in some ways, have been going on ever since. The coincidences between climate change and cultural change do not warrant claims that climate determines culture. They do suggest that climate constitutes part of the framework in which cultural change unfolds and attains the limits of what may be possible from time to time.