Odds are, if you're in the U.S. right now, you're trying to cope with the record heat wave cooking most of the country. You're probably sick of cliches like "Hot enough for you?" or "It's not the heat, it's the humidity." Well, if some informed speculation reported in New Scientist bears up, you ain't seem nothin' yet. In a century, parts of the earth may become too hot for humans to survive without air conditioning.
Titled Thermogeddon: When the Earth gets too hot for humans, the article follows up on a study by Steven Sherwood, an atmospheric scientist at the University of New South Wales in Sydney, Australia and co-author Matthew Huber of Purdue University in Indiana. (* You may need to register for a free account to see the New Scientist article online unless you are a subscriber.) Here's the lead paragraph from the article:
IT IS the late 23rd century. Houston, Tel Aviv, Shanghai and many other once-bustling cities are ghost towns. No one lives in Louisiana or Florida anymore, and vast swathes of Africa, China, Brazil, India and Australia are no-go zones, too. That's because in all of these places it gets hot and humid enough to kill anyone who cannot find an air-conditioned shelter.
(more)
You Don't Just Look Hot, You ARE Hot!
Humans constantly generate heat; being comfortable is a question of controlling how fast you can get rid of it. To quote from Robert A. Heinlein's classic Have Spacesuit - Will Travel:
Three-fourths of your food turns into heat - a lot of heat, enough each day to melt fifty pounds of ice and more. Sounds preposterous, doesn't it? But when you have a roaring fire in the furnace, you are cooling your body; even in the winter you keep a room about thirty degrees cooler than your body. When you turn up a furnace's thermostat, you are picking a more comfortable rate for cooling. Your body makes so much heat you have to get rid of it, exactly as you have to cool a car's engine.
Of course, if you do it too fast, say in a sub-zero wind, you can freeze - but the usual problem in a space suit is to keep from being boiled like a lobster. You've got vacuum all around you and it's hard to get rid of the heat.
That's why one of the critical systems on the International Space Station is the cooling radiators and pumps. When they
lost one pump recently, they had to power down a lot of systems to keep from cooking in their own heat. That's why one of the first items on the checklist for the shuttles on reaching orbit was to open the cargo bay doors - to expose the radiator panels of the cooling system to space. And that's why climate change on Earth is a problem - greenhouse gasses accumulating in the atmosphere mean the Earth isn't radiating heat away into space as fast as it used to.
Now, most concerns about climate change have looked at rising sea levels; droughts, floods and other weather extremes; crop failures; refugees; disruption of biomes on a global scale, and so on. Sherwood and Huber are the first to my knowledge to consider the problem of whether or not humans can actually live in conditions that may prevail on some parts of the Earth - and how much time is left.
Wetter Is NOT Better
It's not just the heat, it IS the humidity. Humans shed heat by sweating - evaporation carries away excess heat. But, climate change isn't just about higher temperatures; it's also about increasing amounts of water vapor in the air. The more humidity in the air, the less the capacity it has to absorb water from a sweating human. Think of it like being trapped in a steam bath - you can be covered with sweat, but you can't cool off because your sweat has no place to go. Stay too long, you'll overheat, pass out, and eventually die. As Hazel Muir summarizes in the article:
What is clear is that to prevent our core temperature rising too high, our skin temperature must not exceed 35 °C [95 °F] for more than a few hours. In dry climates sweating will cool the skin sufficiently even in temperatures of 45 °C or more. But in humid climates where the air is nearly saturated with moisture, sweating makes little difference.
So temperature alone is a very poor guide to what people can survive. A better indicator is the "wet-bulb temperature". This is the temperature that a mercury thermometer wrapped in a wet cloth would record. It is a measure of both heat and humidity, and reflects the temperature you could lower your skin to by sweating.
Even fit and healthy people couldn't survive sustained wet-bulb temperatures above 35 °C, say Sherwood and Huber. This heat-stress limit applies even to people sitting naked in the shade next to a fan. Without air conditioning or access to cooler or less humid places, they will die.
Normally, hot humid air rises and is displaced by cooler air; rising air also tends to lose moisture as it condenses into clouds and possibly storms. The current heat wave in the U.S. is persisting because the hot air is trapped under a layer of higher pressure air, but it should eventually break. As global temperatures rise over time however, the cooler air coming in will not be as cool as it used to be:
To work out how wet-bulb temperatures will change as the world warms, Sherwood and Huber turned to a computer model. The take-home figures: for every 1 °C that the global average temperature rises, maximum wet-bulb temperatures will rise by about 0.75 °C (Proceedings of the National Academy of Sciences, vol 107, p 9552).
Muir did some checking to see how Sherwood and Huber's numbers hold up:
Other researchers see no problem with this prediction. "The climate modelling here is very solid," says Peter Stott of the Met Office Hadley Centre in Exeter, UK, which researches climate change. "Having chatted to a couple of colleagues about this, we think what they are saying seems entirely reasonable and consistent with what we would expect."
The upshot is that if global average temperatures rise by 7 °C, the maximum wet-bulb temperatures in a few places will start to exceed the 35 °C survival barrier for periods of hours or days. Of course, heat stress won't suddenly start to be a problem only at this point. Rather, as heat and humidity slowly rise from today's levels, heatwaves will kill more and more people. Their economic impact will also climb as physical labour outdoors or in buildings with poor air-conditioning becomes increasingly difficult.
A global map shows predicted changes in wet bulb temperatures as the earth heats up. This is the classic '
boiled frog' scenario on a planetary scale. The only uncertainty at this time is how quickly this scenario will play out.
Slow Cooker or Towering Inferno?
What's the time frame here? Again from the New Scientist article:
How much the world will warm depends on two things: how much more carbon dioxide we pump into the atmosphere and how much warming that CO2 produces, also known as climate sensitivity. According to the Intergovernmental Panel on Climate Change (IPCC), every doubling of CO2 in the atmosphere will raise the temperature by between 1.9 and 4.5 °C, with 3 °C being the most likely value.
If climate sensitivity is as low as 1.9 °C, then it would take centuries for the planet to warm by 7 °C even if we continue pumping out lots of CO2. On the other hand, if climate sensitivity is as high as 4.5 °C, we could hit the 7 °C point within a century if we carry on as we are, although this could still be avoided as long as we slash emissions soon enough.
Tricky thing, that, slashing emissions. We're already seeing
record demands on the power grid. As demands for cooling increase, power generated by fossils fuels will only make the problem worse. Some people are starting to get a clue
at long last. Some
cities and
towns are making plans now. There are
promising technologies that can help,
new tricks to put into practice, ways to
increase efficiency. But, there are also people bound and determined to
keep turning up the thermostat, even to the point of
violence.
And there are other considerations. Predictions based on climate change models are only as good as the models and the information that goes into them. If I am tracking climate change news reports correctly, it appears the trends are actually picking up speed faster than predicted only a few years ago. In addition, there are threshold effects that may not be apparent until some equilibrium shifts, such as the ability of the oceans to absorb rather than give off CO2, changes in ocean currents, spiking methane emissions from permafrost,
pollution in China, or changes in solar activity. Climate change is not a linear process; it has a lot of moving parts, so to speak, and we're still figuring out how those parts interact - but our understanding continues to improve. Even if we stopped producing greenhouse gasses instantly, unfortunately, there would still be a long lag effect while the gasses already out there continue to keep the planet warming up. We've passed the point at which we can keep pretending there won't be some consequences from our actions.
Still, contemplating the idea that we may be well on the way to making parts of Earth lethal to unprotected humans for at least part of the year should give any rational person pause. Every so often there are news stories about
mass die-offs of fish. What will it be like the first time we get a perfect storm of heat and humidity that exceeds that lethal 35 °C threshold in a population center long enough to cause mass casualties? It took events like
this and
this to get people to clean up visible air pollution - and that was relatively easy compared to cleaning up greenhouse gasses.
Interesting times ahead.
4:04 PM PT: UPDATE: a Noel Coward classic that seem apropos. It may not be so funny a hundred years from now.
http://youtu.be/...
8:02 PM PT: H/T to Just Bob for finding a link to what appears to be the full version of the article without registration. I urge everyone to take a look at the entire piece - there's a lot of interesting stuff in there, including a discussion of how this relates to the fossil record and previous warm periods on Earth.
http://barringtonstewart.wordpress.com/...
Sat Jul 23, 2011 at 3:08 PM PT: While it's a bit after the fact, just ran across a piece at the NY TImes that fits right in with this diary, someone who'd been living with Global Warming for a long, long time. http://dotearth.blogs.nytimes.com/...