The Sahara desert is an unforgiving place. The mercury regularly climbs above 120 degrees Fahrenheit, and the average temperature—that’s including nighttime and winter temps—is 86 degrees. The desert’s extreme heat is one of the main reasons why vanishingly few people call it home. While 2.5 Million people live in the Sahara, they are spread over 3.5 million square miles, one of the lowest population densities of any region on earth.

We may want to pay close attention to how those nomadic peoples survive out there. According to a new study, without major action to mitigate climate change, a third of the world’s population could live in a climate similar to the Sahara in just 50 years. That would be 3.5 billion people living in a scorching hot landscape with a mean average temperature in the mid-80s.

It’s a harrowing and unprecedented prospect. As part of  the research recently published in the Proceedings of the National Academy of Sciences, an international team of climatologists, anthropologists, and ecologists examined temperature conditions and human migration to answer a pair of important questions: Is there an ideal climate for humans? And, if so, which parts of the earth’s landmass will remain in that climatic range in the future?

As for the ideal climate for human well-being, there is remarkable consistency in the data. Over the past 6,000 years, human populations have clustered in regions where the mean annual temperature is between 52 and 59 degrees Fahrenheit—a window that the researchers call the “human climate niche.” But here’s the problem: In a business-as-usual scenario for carbon emissions, the regions that offer that climate niche will shift dramatically toward the poles in the next half century. In other words, many of the areas in which human communities are thriving today could become nearly unlivable in less than one lifetime.

For example, according to the study’s projections, much of Africa would be climatically challenging for human survival. The southern United States and the northern half of South America would become unbearably hot. India, currently home to one in every six people on earth, would also experience major problems. Separate research has suggested that heat waves in India could become  1oo times more common in the next 50 years.

One of two things will have to happen in response to this shift. Either humans will have to adapt to living in a significantly different climate, or a mass migration will occur at a scale unprecedented in human history.

Adaptation will be challenging. Agriculture appears to be the driving force behind humanity’s preferred climate. Crop productivity and livestock have both historically peaked in the high 50s. Engineering advancements have improved the heat resistance of certain crops and food can be taken into cities from distant farms, but these changes cannot solve the problems alone. Heat resistance has limits, and someone would still have to grow that food in those hotter conditions . (Today, nearly one in three labors world wide currently work in agriculture.) Since food availability is a pretty important part of human existence, the agricultural advantages trickle down. Overall economic production—manufacturing, construction, and other sectors—peaks in the same temperature range.

Farms are key to prosperity, and they are most productive within a tight temperature range. Adapting to a massively different climate won’t be as easy as turning on an air conditioner because air conditioning a field of soybeans is currently not a feasible approach to agriculture. As farms decline in productivity, overall economic activity will follow. People forced outside the climate niche by global warming would face a steadily declining quality of life.

All right then, can we just move with the climate? Well, the scale of the migration would be mind-blowing. Today, there are  70 Million refugees worldwide, and finding suitable homes for them has caused monumental political strife and human rights issues. Imagine what would happen if 3.5 billion people had to migrate. Which countries are going to graciously welcome them?

“This paper’s findings about the disruption to human societies that can be expected from projected climate change are quite important. They point to the absolutely pivotal importance of helping human society to adapt more effectively, especially in the hottest climates where many people live in poverty and could use economic development assistance to adapt effectively,” says  Kim Knowlton, senior scientist at NRDC. “We had better start talking ASAP about global and local responses to that scale of disruption.”

Consider this study in the context of the current debate over climate change mitigation strategies. Those who favor business as usual  argue  that reducing carbon emissions is too expensive. But taking all the potential associated costs of disruption and migration into account, those losses would still be the smallest of potatoes compared to the long-term costs of the emissions status quo.

The study suggests that, for every one degree of increase in global mean annual temperature, one billion more people will be left outside the human climate niche. We are currently on course for at least four degrees of temperature shift inside of the current century, under the most conservative of forecasts. There is virtually no price we should be unwilling to pay to prevent the displacement of nearly 4 billion people.

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