Chi Xu, Timothy A. Kohler, Timothy M. Lenton, Jens-Christian Svenning, and Marten Scheffer
a School of Life Sciences, Nanjing University, Nanjing 210023, China;
b Department of Anthropology, Washington State University, Pullman, WA 99164;
c Santa Fe Institute, Santa Fe, NM 87501;
d Crow Canyon Archaeological Center, Cortez, CO 81321;
e Research Institute for Humanity and Nature, Kyoto 603-8047, Japan;
f Global Systems Institute, University of Exeter, Exeter, EX4 4QE, United Kingdom;
g Center for Biodiversity Dynamics in a Changing World, Department of Bioscience, Aarhus University, DK-8000 Aarhus C, Denmark;
h Wageningen University, NL-6700 AA, Wageningen, The Netherlands;
i SARAS (South American Institute for Resilience and Sustainability Studies), 10302 Bella Vista, Maldonado, Uruguay
PNAS May 26, 2020 117 (21) 11350-11355; first published May 4, 2020; https://doi.org/10.1073/pnas.1910114117
Contributed by Marten Scheffer, October 27, 2019 (sent for review June 12, 2019; reviewed by Victor Galaz and Luke Kemp)
Significance
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.
Figure 2. 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.
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.
Global warming will affect ecosystems as well as human health, livelihoods, food security, water supply, and economic growth in many ways (1, 2). The impacts are projected to increase steeply with the degree of warming. For instance, warming to 2 °C, compared with 1.5 °C, is estimated to increase the number of people exposed to climate-related risks and poverty by up to several hundred million by 2050. It remains difficult, however, to foresee the human impacts of the complex interplay of mechanisms driven by warming (1, 3). Much of the impact on human well-being will depend on societal responses. There are often options for local adaptations that could ameliorate effects, given enough resources (4). At the same time, while some regions may face declining conditions for human thriving, conditions in other places will improve. Therefore, despite the formidable psychological, social, and political barriers to migration, a change in the geographical distribution of human populations and agricultural production is another likely part of the spontaneous or managed adaptive response of humanity to a changing climate (5). Clearly there is a need to understand the climatic conditions needed for human thriving. Despite a long and turbulent history of studies on the role of climate, and environment at large, on society in geography and beyond (6), causal links have remained difficult to establish, and deterministic claims largely refuted, given the complexities of the relationships in question (7). Rather than reentering the murky waters of environmental determinism (8, 9), here we take a fresh look at this complex and contentious issue. We mine the massive sets of demographic, land use, and climate information that have become available in recent years to ask what the climatic conditions for human life have been across the past millennia, and then examine where those conditions are projected to occur in the future.