From penguins to possums, every species on Earth lives within a geographic range that is largely defined by climate – a comfort zone outside of which individual members of the species are rarely encountered.

So, too, with humans who, despite having eked out a living in some of the most inhospitable locales on Earth, are usually found in places that are far milder than those extremes.

Now, a new study shows how the global disposition of humanity’s comfort zone is likely to shift because of climate change, and suggests that vast numbers of people will be negatively affected if carbon emissions due to fossil-fuel consumption continue to rise unchecked.

“It comes down to saying that for each degree of global warming you get about one billion people into trouble,” said Marten Scheffer, an ecologist at Wageningen University in the Netherlands and a co-author of the study.

The study found that areas that have an average annual temperature greater than 29 C – now confined to a few patches of the central Sahara desert and similar areas – will expand dramatically under a business-as-usual emissions scenario. By 2070, those regions, which are among the most hostile on Earth, could encompass most of northern Africa, the Arabian peninsula, India, Southeast Asia, northern Australia and the Amazon basin.

The result adds to the growing body of literature on the effect of a changing climate on human health and well-being, particularly in equatorial regions where temperatures are more frequently pushing the limits of what is considered safe. It also carries implications for higher-latitude countries such as Canada that may become a destination for environmental migrants if the world fails to meet emissions targets.

“At the tail end of the century, Canada is going to be a place of refuge in a climate-disrupted world,” said Robert McLeman, a professor at Wilfrid Laurier University in Waterloo, Ont., who specializes in the human effects of climate change.

Dr. McLeman, who was not involved in the study, cautioned that its conclusions should not be taken as an inevitable prediction for what will happen, but rather as a useful catalyst for policy discussions about how to maintain reasonable living conditions across as much of the planet as possible through a combination of adaptation and mitigation measures. He added that, in addition to climate, economic and political factors are likely to continue to play a large role in how and where people migrate.

The study, which included contributions by researchers in China, Europe and the United States, was published Monday in the Proceedings of the National Academy of Sciences of the United States of America.

Dr. Scheffer said the effort began three years ago as a curiosity-driven exercise to define the climatic conditions that best fit the locations where the majority of humans have lived since the dawn of history. The work involved looking at centuries’ worth of population records and then going back still further with the help of archeological data to help understand where humans have been living on Earth for the past 6,000 years.

While the climate has varied across that time and civilizations have risen and fallen, the authors of the study found that the net result has been remarkably consistent. In general, in whatever era the researchers looked at, most humans at that time seemed to be living in places where the average annual temperature fell between 11 and 15 degrees. And the trend has continued into modern times despite heating and cooling technologies that make it possible for us to live somewhat beyond our natural human comfort zone.

“It was surprising to me to see how much we seem to be sticking to the same temperature conditions,” Dr. Scheffer said.

The very heart of that region today includes places such as California and the mid-Atlantic region of the U.S., northern Italy and the most populous areas of China. But depending on how much the climate warms over the next 50 years, many of those areas will start to slide out of what has historically been the most comfortable and preferred climate niche for humans. Meanwhile, areas that are even warmer today will become increasingly intolerable by late in the century if the targets outlined in the 2015 Paris climate agreement are not met. This raises the possibility of climate-related migrations in numbers greater than anything seen to date.

Dr. Scheffer, who is an affiliated member of the University of Waterloo’s Institute for Complexity and Innovation, added that the COVID-19 crisis highlights the importance of forward thinking and timely international co-operation when facing challenges that affect all of humanity.

“It is possible to get our act together when we think it’s essential,” he said.

The Globe and Mail, May 4, 2020