• “It’s hard to avoid the conclusion that we must be looking at hundreds of millions of people being triggered to migrate,” an author of the study said.

A third of the global population — 3.5 billion people — could be living in temperatures inhospitable to human life in the next 50 years because of climate change, a study released Monday found.

The study, conducted by a team of five scientists and published by the National Academy of Sciences, found that most humans have lived in places with an average annual temperature between 51 and 59 degrees F (about 11 C and 15 C). By 2070, billions could be living in a climate currently found only in a select few places, like Mecca in Saudi Arabia, where the average temperature is 86 F (30 C).

If current trends continue, more than 1 billion people in India, 500 million in Nigeria, and 100 million in the Niger and Sudan region will be living with an average annual temperature of 84 F (29 C), according to study co-author Tim Lenton, a professor of climate change and earth systems science at the University of Exeter. That is a temperature range currently rarely seen outside of the Sahara Desert, but could cover 19 percent of the planet in 2070.

The new study does not estimate how many people will leave their homes in search of cooler climates, but rather how many could be forced to live in an increasingly inhospitable world. However, in 1990, the Intergovernmental Panel on Climate Change had stated that human migration could be the greatest impact of climate change.

Human migration is notoriously difficult to predict and responds to many factors other than heat alone, Lenton said. People might not have the money to make a journey, or not be allowed to cross borders. Still, he said his findings show that billions of people will be facing conditions that could push them to leave their homes.

“It would seem a natural human response that when conditions get unpleasant and difficult, to try to move away from those conditions if you have the means,” he said. “It’s hard to avoid the conclusion that we must be looking at hundreds of millions of people being triggered to migrate, whether that’s within a country or between countries.”

And Lenton’s model only considers heat, one of many effects of the climate crisis. Maxine Burkett, a scholar and professor of climate law at the University of Hawaii who was not involved in the study, says the effects of climate change have a tendency to compound. By 2070, a community living in extreme and constant heat could also be dealing with other climate-related stressors such as natural disasters and sea level rise.

“We’ve seen bits of this after Hurricane Maria. This was a major hurricane that followed a major hurricane that was followed by a heat wave. So even in places that are habitable, they’re contending with multiple climate stressors,” Burkett said.

Lenton said his team worked on the study for more than two years, and took a new approach to climate-based prediction. He was surprised with the results. They started with the idea of treating humans like other animals, all of which have an ideal set of climate conditions in which they thrive. While humans live almost everywhere on Earth, we only live in some places in large numbers, he said. Throughout human history, large cities have almost always developed closer to fertile agricultural land and in temperate climates. But those climates are moving.

Nathan Sayre, a geographer studying climate change at University of California, Berkeley, said projections have become increasingly dire as meaningful climate action fails to emerge.

“The idea of a mean annual temperature of 29 degrees celsius (84 F) in significant parts of the world is terrifying,” he said. “Those places are more or less uninhabitable, let alone arable.”

The places facing the most devastating increase in temperatures are also some of the least equipped to adapt to the changing climate with new infrastructure, and whose people rarely have the resources to relocate or afford air conditioning.

“The very people who are in the crosshairs of this extended no-go zone are the same people who bear the least responsibility for the climate crisis,” Burkett said. “It underscores the importance of aggressive mitigation and increasing adaptive capacity, but also what it means to think more compassionately about people crossing borders.”

Source: Joe Purtell, NBC News