Climatic constraints on maximum levels of human metabolic activity and their relation to human evolution and global change

No matter what humans do, their levels of metabolic activity are linked to the climatic conditions of the land surface. On the one hand, the productivity of the terrestrial biosphere provides the source of chemical free energy to drive human metabolic activity. On the other hand, human metabolic activity results in the generation of heat within the body. The release of that heat to the surrounding environment is potentially constrained by the climatic conditions at the land surface. Both of these factors are intimately linked to climate: Climatic constraints act upon the productivity of the terrestrial biosphere and thereby the source of free energy, and the climatic conditions near the surface constrain the loss of heat from the human body to its surrounding environment. These two constraints are associated with a fundamental trade-off, which should result in a distinct maximum in possible levels of human metabolic activity for certain climatic conditions. For present-day conditions, tropical regions are highly productive and provide a high supply rate of free energy. But the tropics are also generally warm and humid, resulting in a low ability to loose heat, especially during daylight. Contrary, polar regions are much less productive, but allow for much higher levels of heat loss to the environment. This trade-off should therefore result in an optimum latitude (and altitude) at which the climatic environment allows humans to be metabolically most active and perform maximum levels of physical work. Both of these constraints are affected by the concentration of atmospheric carbon dioxide pCO ₂, but in contrary ways, so that I further hypothesize that an optimum concentration of pCO ₂ exists and that the optimum latitude shifts with pCO ₂. I evaluate these three hypotheses with model simulations of an Earth system model of intermediate complexity which includes expressions for the two constraints on maximum possible levels of human metabolic activity. This model is used to perform model simulations for the present-day and sensitivity experiments to different levels of pCO ₂. The model simulations support the three hypotheses and quantify the conditions under which these apply. Although the quantification of these constraints on human metabolic activity is grossly simplified in the approach taken here, the predictions following from this approach are consistent with the geographic locations of where higher civilizations first emerged. Applied to past climatic changes, this perspective can explain why major evolutionary events in human evolutionary history took place at times of global cooling. I conclude that the quantification of these constraints on human metabolic activity is a meaningful and quantitative measure of the “human habitability” of the Earth's climate. When anthropogenic climate change is viewed from this perspective, an important implication is that global warming is likely to lead to environmental conditions less suitable for human metabolic activity in their natural environment (and for large mammals in general) due to a lower ability to loose heat.