Raphael ThysSignificance
We show that for thousands of years, humans have concentrated in a surprisingly narrow subset of Earth’s available climates, characterized by mean annual temperatures around ∼13 °C. This distribution likely reflects a human temperature niche related to fundamental constraints. We demonstrate that depending on scenarios of population growth and warming, over the coming 50 y, 1 to 3 billion people are projected to be left outside the climate conditions that have served humanity well over the past 6,000 y. Absent climate mitigation or migration, a substantial part of humanity will be exposed to mean annual temperatures warmer than nearly anywhere today.
Abstract
All species have an environmental niche, and despite technological advances, humans are unlikely to be an exception. Here, we demonstrate that for millennia, human populations have resided in the same narrow part of the climatic envelope available on the globe, characterized by a major mode around ∼11 °C to 15 °C mean annual temperature (MAT). Supporting the fundamental nature of this temperature niche, current production of crops and livestock is largely limited to the same conditions, and the same optimum has been found for agricultural and nonagricultural economic output of countries through analyses of year-to-year variation. We show that in a business-as-usual climate change scenario, the geographical position of this temperature niche is projected to shift more over the coming 50 y than it has moved since 6000 BP. Populations will not simply track the shifting climate, as adaptation in situ may address some of the challenges, and many other factors affect decisions to migrate. Nevertheless, in the absence of migration, one third of the global population is projected to experience a MAT >29 °C currently found in only 0.8% of the Earth’s land surface, mostly concentrated in the Sahara. As the potentially most affected regions are among the poorest in the world, where adaptive capacity is low, enhancing human development in those areas should be a priority alongside climate mitigation.
The realized human climate niche relative to available combinations of MAT and precipitation. Human populations have historically remained concentrated in a narrow subset (A–C) of the available climatic range (G), which is not explained by soil fertility (H) or potential primary productivity (I). Current production of crops (D) and livestock (E) are largely congruent with the human distribution, whereas gross domestic product peaks at somewhat lower temperatures. Reconstructions of human populations 500 BP are based on the HYDE database, whereas those for 6 Ky BP are based on ArchaeoGlobe (https://doi.org/10.7910/DVN/CQWUBI, Harvard Dataverse, V4). NPP, net primary productivity. See SI Appendix, Methods.
Change in MAT experienced by humans. (A) Current and past human population densities (normalized to sum unity) and modeled human niche (blue dashed curve, a double Gaussian model fitting of current population density) as a function of MAT (°C), contrasted to the projected situation in 2070 (red curve). Bands represent fifth and 95th percentiles of the ensemble of climate and population reconstructions. For the future projection, we take projected populations and climate RCP8.5 and SSP3. (B) Mean temperature experienced by a human being in different periods. Boxplots and data points (gray dots) are shown for the ensemble of climate and population reconstructions. Reconstructions of human populations for 6 Ky BP are based on the HYDE (HY) and ArchaeoGLOBE (AG) (with additional processing) databases.
Expansion of extremely hot regions in a business-as-usual climate scenario. In the current climate, MATs >29 °C are restricted to the small dark areas in the Sahara region. In 2070, such conditions are projected to occur throughout the shaded area following the RCP8.5 scenario. Absent migration, that area would be home to 3.5 billion people in 2070 following the SSP3 scenario of demographic development. Background colors represent the current MATs.
Projected geographical shift of the human temperature niche. (Top) Geographical position of the human temperature niche projected on the current situation (A) and the RCP8.5 projected 2070 climate (B). Those maps represent relative human distributions (summed to unity) for the imaginary situation that humans would be distributed over temperatures following the stylized double Gaussian model fitted to the modern data (the blue dashed curve in Fig. 2A). (C) Difference between the maps, visualizing potential source (orange) and sink (green) areas for the coming decades if humans were to be relocated in a way that would maintain this historically stable distribution with respect to temperature. The dashed line in A and B indicates the 5% percentile of the probability distribution. For an analysis including precipitation effects, see SI Appendix, Fig. S10.
