Global population densities, climate change, and the maximum monthly temperature threshold as a potential tipping point for high urban densities

Abstract

Heat waves lasting days or weeks are among potential hazards expected to increase with climate change, but beyond heat waves, sustained temperatures may be incompatible with high urban population densities. I examined patterns of population density classes and exposure to maximum monthly temperature to identify where high urban population density classes were more at risk for prolonged heat than other density classes and if heat may have influenced high urban population densities during the recent past (1970–2000). Then, I considered exposure of population density classes to maximum monthly temperature during the near future (2021–2040, under the sustainability shared socio-economic pathway ssp1-2.6) and the long-range future (2081–2100, under fossil fuel-driven ssp5-8.5) for three general circulation models. Population densities generally increased with increasing maximum monthly temperature by country; the highest density urban class had the greatest exposure to heat for 82 out of 199 countries. By temperature zone, the hypertropical zone inverted the order such that population densities increased with decreasing maximum monthly temperature, indicating that concentrated populations generally did not develop at high maximum monthly temperatures ≥ 36 °C. The 36 °C maximum monthly temperature threshold corresponds to heat wave warnings, which typically consist of a few consecutive days. Therefore, 36 °C maximum monthly temperature may indicate the start of an inhospitable tipping point beyond which human civilization may not flourish. Most land area will exceed 36 °C maximum monthly temperature under the CanESM5 ssp5-8.5 scenario by 2081–2100. Adaptation is not sufficient to moderate sustained unrelenting heat and migration of human populations would be less feasible than mitigation to reduce carbon emissions.