Pre-industrial Depopulation, Atmospheric Carbon Dioxide, and Global Climate

The topic addressed by this volume – interactions among human health, global change, and socio-economic factors – is immensely broad and complex. In this paper, we focus on one of many related issues – the link between population size (one of many possible indices of human wellbeing), atmospheric carbon dioxide (CO2), and climate during the preindustrial portion of the historical interval. Specifically, we test a new hypothesis that intervals of significant human depopulation (at the scale of tens of millions of deaths) caused reforestation of abandoned farmland, and thereby reduced atmospheric CO2 concentrations and cooled global climate. In the first section of this paper, we focus on the major multi-regional depopulation intervals identified in historical records. We summarize the likely impacts of famine, war, and disease on depopulation and conclude that disease is the largest factor in most major depopulation intervals. We conclude that the correlation between major pandemics and intervals of decreased CO2 supports a causal link between mass mortality and carbon levels in the atmosphere. In the second section, we outline the methods used to analyze and quantify possible pandemic-climate links. In the third section, we model the response of atmospheric CO2 concentrations to carbon input and removal caused by reforestation, by decreases in rates of deforestation, and by decreases in early coal use. We find that reforestation was probably the major contributor to CO2 decreases during depopulation intervals, while reductions in rates of deforestation and coal use were likely secondary factors.