Hot Planet

Is it so bad if the World gets a little bit hotter? You bet!

Many of us share some dim apprehension that the world is flying out of control, that the centre 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 pre-industrial 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 civilisation 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.

The author met the 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.

“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 civilisation 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 civilisation, and Huber says that mass extinction or not, it’s our tenuous reliance on an ageing and inadequate infrastructure—perhaps, most ominously, on power grids—coupled with the limits of human physiology that may well bring down our world.

“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.”

By the year 2050, according to a 2014 MIT study, there will also be 5 billion people living in water-stressed areas. And it doesn’t get any better after 2050. But forecasts about the disintegration of society are social and political speculations and have nothing to do with mass extinctions. Huber is more interested in the hard limits of biology. He wants to know when humans themselves will actually start to disintegrate.

“We ran a whole set of model results, and it was rather alarming to us.” Sherwood and Huber calculated their temperature thresholds using the so-called wet-bulb temperature, which basically measures how much you can cool off at a given temperature. If humidity is high, for instance, things like sweat and wind are less effective at cooling you down, and the wet-bulb temperature accounts for this.

“If you take a meteorology class, the wet-bulb temperature is calculated by basically taking a glass thermometer, putting it in a tight wet sock, and swinging it around your head,” he said. “So when you assume that this temperature limit applies to a human, you’re really kind of imagining a gale force wind, blowing on a naked human being, who’s doused in water, and there’s no sunlight, and they’re immobile, and actually not doing anything other than basal metabolism.”

Today the most common maximums for wet-bulb temperatures around the world are 26 to 27 degrees Celsius. Wet-bulb temperatures of 35 degrees Celsius or higher are lethal to humanity.

Above this limit, it is impossible for humans to dissipate the heat they generate indefinitely and they die of overheating in a matter of hours, no matter how hard they try to cool off. “So we were trying to get across the point that physiology and adaptation and these other things will have nothing to do with this limit. It’s the E-Z Bake Oven limit,” he said. “You cook yourself, very slowly.”

What that means is that this limit is likely far too generous for human survivability.

“When you do real modeling, you hit a limit much sooner, because human beings aren’t wet socks,” he said. According to Huber and Sherwood’s modeling, 7 degrees Celsius of warming would begin to render large parts of the globe lethally hot to mammals.

Already in today’s world, heated less than 1 degree Celsius above preindustrial times, heat waves have assumed a new deadly demeanor. In 2003, two hot weeks killed 35,000 people in Europe. It was called a once-in-500-year event. It happened again three years later (497 years ahead of schedule). In 2010, a heat wave killed 15,000 people in Russia. In 2015, nearly 700 people died in Karachi alone from a heat wave that struck Pakistan while many were fasting for Ramadan. But these tragic episodes are barely a shade of what’s projected.

But for the very worst-case emissions scenarios, heat waves would not merely be a public health crisis, or a “threat multiplier,” as the US Pentagon calls global warming. Humanity would have to abandon most of the earth it now inhabits. In their paper, Huber and Sherwood write: “If warmings of 10 degrees C were really to occur in the next three centuries, the area of land likely rendered uninhabitable by heat stress would dwarf that affected by rising sea level.”

So, yes. It is bad if the world gets a bit hotter. Especially if it keeps getting hotter.