Agriculture commonly is seen as the child of the Holocene, when climates became milder than the preceding ice age; however, the timing of its onset, the number of ways in which it developed and the number of theories to account for it are so various that the role of climate change per se becomes obscure. Importantly, when climatic changes across the Pleistocene-Holocene divide are being considered, comparisons should not be made simply in terms of averages, because if averages were all that matter, then the climate and thus the ecosystems of almost any part of today's world could be found somewhere in the Pleistocene world, even on the same major land-masses as they are found today. However, the fossil record of the Pleistocene contains plant assemblages that lack exact counterparts today, which suggests an influence of factors other than climatic averages. Although reduced atmospheric carbon dioxide may have been partly responsible, another difference between the late Pleistocene and Holocene was the occurrence of large and very rapid shifts of climate every few millenia, at least throughout large regions of the northern hemisphere (these fluctuations were much faster than those explained by the Milankovitch theory of climatic change).
Ethnographic studies suggest that the magnitude and frequency of environmental disturbances affected prehistoric human subsistence and habitat: species diversity tends to be highest at intermediate frequencies of ecosystem disturbance, and human subsistence strategies are broadened by environmental disturbances. This paper considers the possible effects of climatic fluctuations at millenial and shorter scales on ecosystems and human subsistence, as summarised in the figure below, where higher-frequency variability (times scales from interannual to several decades) is represented by the horizontal axis; low-frequency or millenial-scale climatic switches are represented on the vertical axis. It is proposed that the hunter-gatherer (HG) and subsistence agriculture (SA) realms differ in climatic variability, and that Holocene diminuition of variability in some regions favoured the development of agriculture.