Will Climate Change Cause Human Extinction?

In the last few days an alarming article making the rounds on social media has revived the idea that our species might be doing itself in by changing the global climate. The article, by vice.com's Nathan Curry, minces no words in its title: "Humanity Is Getting Verrrrrrry Close to Extinction," with the extra rs in the original.

Curry cites sources ranging from climatologist James Hansen to admitted doomsayer Guy McPherson to advance a startling notion: human society has triggered enough irrevocable climate change mechanisms that we're locked in to warming temperatures sufficient to kill us off.

It's alarming, but is it alarmist? Could we really heat ourselves to extinction? Maybe, but that's asking the wrong question.

All species eventually go extinct, and things can always go badly wrong when you're futzing with your life support system. But if a few things happen that aren't accounted for in climate forecasts -- like a wholesale release of methane from permafrost and seabed deposits, there's a significantly larger chance that most of what we humans currently think of as good places to live will become literally uninhabitable for at least part of the year.

Doomsday scenarios have their fans, the aforementioned McPherson among them. In a January post from his blog Nature Bats Last, McPherson suggested that Obama had turned his back on a 2009 global climate conference because he had inside information that it was all pointless:

To be fair, McPherson does allow in that same post that a colleague's prediction that all life on Earth will die before mid-century "appears premature."

But running through articles like Curry's and websites like McPherson's is a startling scientific claim that has merit: there's a chance that large parts of the world will get hot enough to kill humans outright.

Climate change threatens to alter the way we live our lives in a whole lot of ways, from changing the frequency and severity of storms and droughts to causing crop failures to promoting increases in pest animals and diseases. Clever animals like us may well be able to come up with ways to mitigate such horrors.

But as global temperature warms, some reserachers have said that some places run the risk of getting too hot and humid for humans to survive in for more than a few days.

Humans, like other mammals, generate heat just by being alive, and we rely on our surroundings as sinks for that heat. Even if the air is considerably hotter than our body temperature, we can cool ourselves by sweating: water evaporating from our bodies takes a lot of heat with it.

At 100 percent humidity, a temperature of 35°C -- 95°F -- proves fatal within days or hours to people in good health in ideal theoretical conditions -- as James Hansen puts it in a passage quoted by Curry, "even a person lying quietly naked in hurricane force winds would be unable to survive" such temperatures. If we can't shed our waste heat, our organs fail and death results.

And that's for a healthy person tring to get as cool as possible while relaxing. People who aren't in top physical condition or who must continue to work can drop dead from less extreme conditions.

That "temperature at 100 percent relative humidity" is referred to as a "wet bulb temperature," a slightly confusing mathematical concept named for the easiest way to measure it: draping a thermometer bulb with a wet, non-insulating cloth. As almost everyone reading this will know from personal experience, temperatures of 35°C by themselves aren't uniformly deadly: it is, as they say, not so much the heat as the humidity. That world record air temperature set a century ago in Death Valley of 132°F would have needed an accompanying atmospheric humidity of 26 percent to work out to that fatal 35° wet bulb level. As I write this in Joshua Tree on August 27, in the days after a series of tropical storms dumped water into the desert, the relative humidity is 52 percent, it's 86°F, which works out to a slightly muggy but still comfortable wet bulb temperature of 22.5°C. Los Angeles, at 83° and 44 percent humidity, is a comfortable 19.7°C wet bulb.

Very few places on the planet ever exceed a wet bulb temperature of 30° or 31°C. But a study released in 2010 by researchers Steven C. Sherwood and Matthew Huber of the University of New South Wales and Purdue University suggests that if the global climate increases by an average of about 10°C, which is not completely implausible by the year 2200, vast areas of the planet could regularly exceed that deadly 35°C wet bulb temperature, making them essentially uninhabitable, at least for part of the year for anyone without abundant resources.

Increases in wet bulb temperature don't track directly to increases in air temperature for a couple reasons. For one thing, wet bulb temperature is also influenced by barometric pressure. More importantly, if you heat a mass of air, its relative humidity will decrease unless you add more water to it. What that means is that even with global temperature increase, the parts of the planet that are now deserts generally won't see much of an increase in wet bulb temperatures.

What parts of the world will approach that fatal 35°C? Places where there's a lot of water laying around for that heated air to suck up. Places like India. West Africa. The Amazon. Coastal China and southeast Asia. And essentially all of what is now the eastern United States.

As Sherwood and Huber said in 2010, our technology may not offer those trying to live in these affected areas much help:

If temperatures increase by 11° or 12°C, said Sherwoood and Huber, we'd lose most of our species' habitat.

How likely is an increase of 10°C by 2200? The Intergovernmental Panel on Climate Change (IPCC) offered a number of scenarios in its Fourth Assessment Report in 2007, one of which was based on the assumption of "a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies." If that world was primarily focused on fossil fuel use, said that report, the road ours seems to be headed down, temperatures could be as much as 6.4° higher by 2100. In a renewable-centered world, that increase could be kept to 4°C or lower. But few expect temperatures to stop rising in 2100, regardless of the scenario.

The website Skeptical Science offered a look at best- and worst-case climate scenarios in February, with the best-case scenario modeled on the assumption that doubling our atmospheric CO₂ will cause 2°C of global temperature increase, and the worst case scenario making that 4°C for the same input of CO₂. Their best-case scenario showed between 2° and 3°C increase by 2150 for the two most likely industrial strategies for emitting carbon, and their worst-case scenario pumped that up to a range of 5° to 7°C.

Skeptical Science offers two sobering caveats for this data:

The total eventual effect of those complicating factors Skeptical Science omitted from its calculations is unknown, but it's near certain that at least some will occur, and it's also near certain that their cumulative impact on global temperatures will be unpleasant. For instance: one of the more likely effects of a very optimistic 2°C global increase in temperature will be that polar regions increase by 4°C. That means that huge areas of permafrost soild will be altered. Permafrost is thought to hold about twice as much carbon as the atmosphere does at present: when it melts, that carbon enters the atmosphere either as methane, a devastatingly potent greenhosue gas, or as carbon dioxide. Methane once sequestered in permafrost is already entering the atmosphere. Even if our greenhouse gas emissions stopped today, the UN Environment Programme estimates it would take a century for the resulting permafrost emissions to peak, and another century to drop back down to anywhere close to their current levels.

The IPCC will be releasing its Fifth Assessment Report in stages coming next month, so we'll see how the world's leading climate science body has updated what it thought in 2007. A 10°C increase in global temperatures by 2200 would seem far more likely than any of us would like. Which means that, according to Sherwood and Huber's projections, 58 percent of the world's current population now lives in areas with a good chance of being uninhabitable in 190 years.

Loss of habitat is the major cause of extinction, and a species that loses 58 percent of its habitat is in trouble. The Sumatran elephant is now considered critically endangered after losing 70 percent of its original habitat. We're better than any other species at shifting from one habitat to another, but even those skills have limits. How well will our descendants on the U.S. Pacific Coast fare when they're faced with accomodating migrants from all of North America east of the Missouri River? Or from the likely Death Zone in Asia?

Conflict over resources and space leads to wars and other conflict, and it's very likely that if Sherwood and Huber's projections come to pass, life will be excruciatingly bad, even in places where the wet bulb temperatures never get close to 35°C.

Will we go extinct? There's some thought that humans have already passed through a near-extinction event, related to the Toba eruption about 70,000 years ago, that may have reduced our total global population down to 15,000 people or fewer in southern Africa. That theory is questioned by some who cite the possibility of other survivng bands of humans. Either interpretation offers us hope for our species: humans can survive horrible catastrophes and rebuild.

Which means the question may not be so much "will we die out" as "will we wish we had." If humans were to go extinct, the earth would at least have a chance to repair itself, evolve new biodiversity, and move on. But having the globe sprinkled with scattered bands of a few hundred survivors, each desperately scraping whatever sustenance might come from the planet their ancestors ruined? That's a much more frightening prospect for both the planet and our great great grandchildren.