The Sunny Side of the Ice Age: Solar Insolation as a Potential Long-Term Pacemaker for Demographic Developments in Europe Between 43 and 15 ka Ago

After a decade of research under the auspices of the project ‘Population dynamics: Land use patterns of populations between the Upper Pleistocene and Middle Holocene in Europe and the Middle East,’ a consistent sequence of high-resolution paleodemographic datasets has been compiled, spanning the entire Upper Paleolithic from roughly 43 to 15 ka. When viewed in a diachronic perspective, long-term trends of increasing and decreasing population sizes and densities, as well as expanding and contracting areas of settlement activities (Core Areas), become evident. An environmental parameter with potentially strong impact on hunter-gatherer societies is solar insolation. The sun’s energy available at any given time and place is one of the main factors influencing plant growth. The amount of plant biomass, in turn, largely determines the amount of animal biomass in a landscape. The latter was the most important source of energy for European Upper Paleolithic hunter-gatherers. Here, we aim to assess the potential influence of changes in solar insolation on paleodemographic development in Western and Central Europe between 43 and 15 ka. To this end, we present estimates on the number, density, and spatial distribution of hunter-gatherers for five consecutive Upper Paleolithic periods in Europe. Based on regional climate model data for the Last Glacial Maximum and solar insolation data, we calculate: 1) differences in the amount of Megajoule per square meter (MJm-²), 2) start, end, and length of the growing season, as well as 3) summed temperatures during the entire duration and during the first 30 days of the growing season. A comparison shows that a moderate, steady increase of population size and an extension of the Core Areas between 43 and 29 ka coincides with an increase in the summed temperature, particularly during the first 30 days of the growing season. The period between 29 and 25 ka shows a pronounced population decline, a strong contraction of Core Areas and a withdrawal from higher latitudes. This coincides with a markedly delayed growing season, a decrease in summed temperatures, and a marked reduction in solar insolation during the early part of the growing season. Between 25 and 20 ka, we see consolidation and renewed growth in both numbers and densities of people and an expansion and merging of Core Areas in Western Europe. There is a slight gain in the energy available during the first half of the year. The growing season starts earlier and is of increasingly longer duration, coupled with rising summed temperatures. Between 20 and 15 ka, the meta-population grows strongly, Core Areas expand, and the higher latitudes become repopulated. This coincides with further-increasing summed temperatures and an ever-earlier start to the growing season. Additionally, the gain in available solar energy during the early phase of the growing season is particularly pronounced. These findings indicate that solar insolation and its effects on an ecosystem’s phenological configuration over different trophic levels is indeed an important factor in the long-term demographic development of Paleolithic hunter-gatherers. This special issue is guest-edited by William Davies (Centre for the Archaeology of Human Origins, University of Southampton) and Philip R. Nigst (Department of Prehistoric and Historical Archaeology, University of Vienna).