Many of us share some dim apprehension that the world is flying out of control, that the center cannot hold. Raging wildfires, once-in-1,000-year storms, and lethal heat waves have become fixtures of the evening news—and all this after the planet has warmed by less than 1 degree Celsius above preindustrial temperatures. But here’s where it gets really scary.

If humanity burns through all its fossil fuel reserves, there is the potential to warm the planet by perhaps more than 10 degrees Celsius and raise sea levels by hundreds of feet. This is a warming spike comparable in magnitude to that so far measured for the End-Permian mass extinction. If the worst-case scenarios come to pass, today’s modestly menacing ocean-climate system will seem quaint. Even warming to half of that amount would create a planet that would have nothing to do with the one on which humans evolved, or on which civilization has been built. The last time it was 4 degrees warmer there was no ice at either pole and sea level was hundreds of feet higher than it is today.

I met University of New Hampshire paleoclimatologist Matthew Huber at a diner near campus in Durham, New Hampshire. Huber has spent a sizable portion of his research career studying the hothouse of the early mammals, and he thinks that in the coming centuries it’s not impossible that we might be headed back to the Eocene climate of 50 million years ago, when there were Alaskan palm trees and alligators splashed in the Arctic Circle.

Harper Collins Excerpted from The Ends of the World: Volcanic Apocalypses, Lethal Oceans, and Our Quest to Understand Earth's Past Mass Extinctions, by Peter Brannen

“The modern world will be much more of a killing field than the Paleocene–Eocene Thermal Maximum was,” he said. “Habitat fragmentation today will make it much more difficult to migrate. But if we limit it below 10 degrees of warming, at least you don’t have widespread heat death.” In 2010, Huber and coauthor Steven Sherwood published one of the most ominous science papers in recent memory: “An Adaptability Limit to Climate Change Due to Heat Stress.”

“Lizards will be fine, birds will be fine,” Huber said, noting that life has thrived in hotter climates than even the most catastrophic projections for anthropogenic global warming. This is one reason to suspect that the collapse of civilization might come long before we reach a proper biological mass extinction. Life has endured conditions that would be unthinkable for a highly networked global society partitioned by political borders. Of course, we’re understandably concerned about the fate of civilization, and Huber says that, mass extinction or not, it’s our tenuous reliance on an aging and inadequate infrastructure---perhaps, most ominously, on power grids—coupled with the limits of human physiology that may well bring down our world.

In 1977, when power went out for only one summer day in New York, whole swaths of the city devolved into something like Hobbes’s man in a state of nature. Riots swept across the city, thousands of businesses were destroyed by looters, and arsonists lit more than a thousand fires. In 2012, when the monsoon failed in India (as it’s expected to do in a warmer world), 670 million people—that is, 10 percent of the global population—lost access to power when the grid was crippled by unusually high demand from farmers struggling to irrigate their fields, while the high temperatures sent many Indians seeking kilowatt-chugging air conditioning.

“The problem is that humans can’t even handle a hot week today without the power grid failing on a regular basis,” he said, noting that the aging patchwork power grid in the United States is built with components that are allowed to languish for more than a century before being replaced. “What makes people think it’s going to be any better when the [average summer temperature] will be what, today, is the hottest week of the year in a five-year period, and the hottest temperatures will be in the range that no one has ever experienced before in the United States? That’s 2050.”

Wet-bulb temperatures of 35 degrees Celsius or higher are lethal to humanity.

By the year 2050, according to a 2014 MIT study, there will also be 5 billion people living in water-stressed areas. “Thirty to fifty years from now, more or less, the water wars are going to start,” Huber said. In their book Dire Predictions, Penn State’s Lee Kump and Michael Mann describe just one local example of how drought, sea level rise, and overpopulation may combine to pop the rivets of civilization: Increasingly severe drought in West Africa will generate a mass migration from the highly populous interior of Nigeria to its coastal mega-city, Lagos. Already threatened by rising sea levels, Lagos will be unable to accommodate this massive influx of people. Squabbling over the dwindling oil reserves in the Niger River Delta combined with potential for state corruption will add to the factors contributing to massive social unrest.

about the author About Peter Brannen is an award-winning science journalist whose work has appeared in the New York Times, The Atlantic, Wired, Slate, and the Boston Globe, among other publications. This is his first book.

“Massive social unrest” here being, of course, a rather bloodless phrase masking the utter chaos coming to a country already riven by corruption and religious violence. “It’s sort of the nightmare scenario,” said Huber. “None of the economists are modeling what happens to a country’s GDP if 10 percent of the population is refugees sitting in refugee camps. But look at the real world. What happens if one person who was doing labor in China has to move to Kazakhstan, where they aren’t working? In an economic model, they’d be immediately put to work. But in the real world they’d just sit there and get pissed. If people don’t have economic hope and they’re displaced, they tend to get mad and blow things up. It’s the kind of world in which the major institutions, including nations as a whole, have their existence threatened by mass migration. That’s where I see things heading by midcentury.”