The human climate niche

Xu et al. 2020

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.

Lenton et al. 2023

The mechanisms behind the niche

The human climate niche is shaped by direct effects of climate on us and indirect effects on the species and resources that sustain or afflict us. Direct climate effects include health impacts and changes in behaviour. Human perceptions of thermal comfort evolved to keep us near optimal conditions of 22–26 °C, with well-being declining above 28 °C. Behavioural changes include altering clothing, changing environment (including to indoor environments) and altering work patterns. These can buffer individual exposure to temperature extremes but still affect collective well-being via effects on work. Sometimes uncomfortable conditions are unavoidable. High temperatures can decrease labour productivity, cognitive performance and learning21, produce adverse pregnancy outcomes, and increase mortality. Exposure to temperatures >40 °C can be lethal, and lethal temperature decreases as humidity increases.

Current situation

9 ± 1% of the global population have been exposed outside the niche due to temperature change alone and 10 ± 1% from temperature plus demographic change. Thus, global warming of 0.7 °C since 1960–1990 has put 624 ± 70 million people in less favourable temperature conditions, with demographic change adding another 77 million.

Future exposure

Hot exposure becomes significant by 2030 at 4 ± 2% or 0.3 ± 0.1 billion as global warming reaches 1.5 °C, and it increases near linearly to 23 ± 9% or 2.1 ± 0.8 billion in 2090 under 2.7 °C global warming. The number of people left outside the niche due to temperature change alone reaches 14 ± 3% or 1.2 ± 0.2 billion by 2030, more than doubling to 29 ± 5% or 2.7 ± 0.5 billion in 2090. The number of people left outside the niche from temperature plus demographic change reaches 25 ± 2% or 2.0 ± 0.2 billion by 2030, and 40 ± 4% or 3.7 ± 0.4 billion by 2090.