The study, published in the Proceedings of the National Academy of Sciences on Monday, breaks new ground by quantifying the temperature range society is most adapted to and projecting how climate change will push people outside it.
“What we have looked for is humanity’s sensitivity to warming, and that is about 1 billion people in trouble per degree [Celsius] of warming,” said study co-author and Dutch research ecologist Marten Scheffer of the Santa Fe Institute and Wageningen University.
Scheffer and his colleagues examined the history of global temperature, human population and land-use estimates from the mid-Holocene period, starting about 6,000 years ago, to 2015.
They found that people, crops and livestock have heavily concentrated in a narrow band of relatively constrained climate conditions. This range, referred to in the study as the human “climate niche,” has remained largely unchanged since 6,000 years ago.
Projecting into the future using a scenario with high emissions of heat-trapping greenhouse gases, the researchers found that the position of the human climate niche is projected to change more in the next 50 years than it has during the past 6,000. Such a shift would leave 1 billion to 3 billion people outside the climate conditions that have nurtured human society to date.
Because of recent trends and projections in greenhouse gas emissions, which are slightly less extreme, the shifts may not reach the highest estimates by 2070, but even a less aggressive emissions scenario would still bring about radical change.
The study, from a group of anthropologists, climate scientists and ecologists, is the first of its kind to show what the optimum climatic conditions for human society have been across the past millennia, and then to project how they will shift under various global warming scenarios.
According to the study, the optimum conditions for human society to flourish have a mean annual temperature of between 51.8 to 59 degrees Fahrenheit (11 to 15 degrees Celsius). For reference, the average annual temperature in Washington is 58.2 degrees.
The study also finds a second band of temperatures that coincides with the region that benefits from the Indian monsoon, which helps support billions of people in South Asia. The average annual temperature in that region is between 68 and 77 degrees (20 to 25 degrees Celsius), the study found.
Study co-author Tim Kohler, an archaeologist at Washington State University, says that while our technological progress has allowed us to settle virtually everywhere on Earth, and even in space, the study shows, “Our preferences (as opposed to our abilities) have long been for a rather narrow band of temperatures in which we typically have our densest numbers and greatest economic success.”
In the past, when climate conditions have been pushed outside of these optimum ranges, upheaval resulted, including mass migration, famine, conflict and other disruptions that the study’s authors say we need to plan for in coming decades.
Already, about 0.8 percent of the Earth’s surface experiences mean annual temperatures above 84.2 degrees (29 degrees Celsius), mostly in the Sahara region of Africa. However, with projected increases in global average surface temperatures, this area is expected to expand significantly to cover about 19 percent of the global land, home to 3.5 billion people, in 2070.
This would not be a huge problem if few people lived in that part of the world, but it happens to be one of the fastest-growing regions on the planet.
“It’s a bit unfortunate that most population growth happens to be in the place that will be hardest to live in,” Scheffer said.
According to the study, the ideal temperature range for human society is expected to expand toward the poles “in unprecedented ways,” while population growth mainly occurs in developing countries in the tropics and sub-Saharan region, thereby exacerbating the disconnect between how humans will be distributed and the new, much warmer and more extreme climate.
The study points to the likelihood of increased climate-induced migration. If a few billion get in trouble because of climate change, Scheffer says, “The people won’t stay where the trouble is.”
The study notes multiple uncertainties, mainly concerning decisions made about reducing emissions of carbon dioxide and other planet-warming greenhouse gases, as well as our ability to adapt to a warmer climate.
“Populations have avoided stifling hot or freezing temperatures in large numbers, concentrating instead on the goldilocks zones,” said Neil Adger, professor of human geography at the University of Exeter. Adger reviewed an early copy of the study but was not involved in the research itself.
“It is likely climatic changes will in effect move large cities and whole countries into temperature niches that present inhabitants would find unimaginable,” he said in an email.
“So will cities move? Unlikely. But will they become less attractive destinations for people to move to? Definitely. And ultimately some present cities will stop growing and ossify,” he said.
Adger points out that questions remain about economic output in areas that are pushed outside their climate niche, as it is known.
“A key unknown is whether labor productivity for outdoor work can adapt to these new niches. Even if plant breeding technologies can solve the temperature tolerance of crops, can technologies help farming practices, which are always labor intensive, to be tolerable?” he said.
“Ultimately, we will witness imperceptible shifts that over decades will represent a profound shift in the economic geography of the human world.”
John Schellnhuber, director emeritus at the Potsdam Institute for Climate Impact Research in Germany, said the study is useful because it quantitatively confirms “the rather small historic habitat of humankind on Earth.” It also shows the coming, dramatic shifts to this habitat due to global warming, he said via email.
Schellnhuber, who has advised German Chancellor Angela Merkel on climate change and was not involved in the study, said the new research lends support to his view that “large-scale (managed, facilitated) migration needs to be part of a global [climate] adaptation strategy.”