Unstable climate - bad luck for humanity
Whether cultures flourished or declined over thousands of years depended on how much the climate changed. This is suggested by a study in which an international team led by the Helmholtz-Zentrum Hereon compared climate data with tens of thousands of archaeological findings. As the researchers write in the journal “Nature Communications”, the population on the various continents shrank when the climate became unstable over a long period of time.
Archaeological finds such as this ruin provide information about the population on earth many centuries ago. Credit: Nicolas Antunes
A current international study by Hereon together with the Leibniz-Zentrum für Archäologie (LEIZA) and other institutions shows that the increase in the Earth's population was not continuous. The results indicate that periods of strong growth and decline alternated over millennia. These ups and downs appear to be linked to the intensity of climate change, writes the team led by environmental scientist Kai Wirtz from Hereon in the journal “Nature Communications”.
Tens of thousands of archaeological data
The archaeologists, climate researchers and environmental scientists compiled huge amounts of information from various databases, including a new metadatabase on archaeological finds at LEIZA in Mainz. The age of these finds was determined using the so-called 14C method. This is a dating method that uses the decay rates of different carbon isotopes in organic material. In total, the team collected data from around 91,000 archaeological sites in Europe alone, dating from 9,000 to 3,000 years ago. This was supplemented by 14C data from roughly the same number of sites in North and South America, Africa, Australia and Asia. In addition to the accuracy of these data, a major advantage lies in their accessibility - in contrast to archaeological find reports, which are often not integrated for large areas. Using the 14C data, the researchers were able to estimate how many people lived in the respective regions in certain times.
This archaeological-demographic data was then combined with past climate data. This is proxy data from which experts can estimate the past climate in different places on Earth. For example, the chemical isotope composition of ancient stalactites from caves can be used to reconstruct how wet or warm it was.
Stable climate helps population growth
“By comparing the archaeological data with the climate data, we can clearly see that there was large-scale population growth in decades and centuries when the climate was stable,” says Kai Wirtz, first author and head of the study. When it was consistently very cold or warm or when there were alternating dry or wet years, people were able to cope. But not when the climate changed fundamentally over decades. One reason for this could be that hunter-gatherers and farmers were unable to adapt well enough to the changing climatic conditions. Food shortages could then have favored wars and epidemics.
“The question of why societies arise, flourish and collapse has always fascinated archaeology, but lastly all historical sciences,” says Detlef Gronenborn from LEIZA, who co-designed the study. “It has now become apparent that climate change was often a forcing factor on both continental and global levels - superimposing internal social processes such as upheavals. The innovative approach of our study is therefore part of an international and interdisciplinary emergent direction of thinking by which researchers are focusing on the interactions between the planet and humanity.” This direction is called “Planetary Thinking”.
The graph shows the rate of population growth in Europe in relation to climate stability. Credit: Hereon/Kai Wirtz
Kai Wirtz emphasizes that it is not possible to determine the exact number of people in a region with the available data. “However, we can certainly estimate by what percentage the population has decreased or increased.” A strength of the study derives from the very different climate parameters and very many regional population dynamics jointly and systematically evaluated for the first time. Kai Wirtz: “Many studies targeted individual societal collapses attributed to one parameter - such as a drought. Our results show that the influence of climate is more complex.” They make it clear that climate stability was a decisive driver of demographic development worldwide.
In the study, the team also investigated the influence of solar radiation, as its intensity changes over the course of years, decades and centuries. “The comparison with the data suggests that humanity always thrived when solar radiation was lower,” says Kai Wirtz. It is, however, still unclear how the two factors might be connected.
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