Too Darn Hot: Climate Change, Heat Stress, Human Life

When it comes to surviving global warming, it’s the heat and the humidity.

Global temperatures are on the rise: since the Industrial Revolution, the increase has been about 0.7 degrees Celsius or about 1.5 degrees Fahrenheit.

There is evidence that the warming trend has resulted in more severe heat waves (possibly such as the deadly ones that struck Chicago in 1995 and Europe in 2003), but establishing a direct link between such meteorological events and a climatic trend like global warming is problematic. Nevertheless, it‘s reasonable to expect that global warming will lead to higher peak temperatures during the summer, including more extreme heat waves.

Could Global-Warming Temperatures Climb So High as to Threaten Human Life?

That would be a drag. No one wants higher summertime temperatures or more oppressive heat waves. But is there any chance it could get so hot that just standing outside for any period of time could be deadly? A recent paper appearing in the Proceedings of the National Academy of Sciences by Steven Sherwood of the University of New South Wales, Australia, and Matthew Huber of Purdue University suggests that’s a definite possibility.

The authors begin by considering the human body’s heat budget. Have you ever placed your hand near a 100-watt bulb? Turns out that amount of heat is about what your body generates while resting; that’s right, just like your Mom says — even when you’re chilling out, you can light up a room.

To keep things operating properly, your body must maintain its core temperature of about 98.6 degrees Fahrenheit. That means it must be able to continuously dissipate the 100 watts it generates. If it can’t, your core temperature rises, hyperthermia sets in, and after a few hours of hyperthermia, you’re basically toast.

Fortunately, your body has an efficient mechanism for getting rid of those 100 watts of heat: by keeping your skin at about 95 degrees Fahrenheit, slightly below your core temperature, your body conducts its internally-generated heat to the skin, thus cooling itself; evaporative cooling — better known as sweating — also helps lower body temperature by dissipating heat in the escaping water vapor.

But there’s a catch. It must be both cool enough and dry enough for your skin to transfer the excess heat to the atmosphere. Technically that means that your skin’s temperature must be higher than the atmosphere’s so-called wet-bulb temperature. What’s the wet-bulb temperature? It is defined as the “lowest temperature an object [like your body] may be cooled to by the process of evaporation.” Does that not help? Let me give you an example.

Sweating Through Wet-Bulb Temperatures

Imagine the temperature outside is 100 degrees Fahrenheit. That’s hot. But as long as the humidity is low enough to allow you to perspire, you will be okay. For example, suppose the relative humidity is only 70 percent. Then the wet-bulb temperature is only 90 degrees — a full 10 degrees below the actual temperature and 5 degrees below your skin temperature. You may be hot, but you will survive as long as you have access to water.

But now suppose that the relative humidity is 85 percent instead of 70 percent. Now the wet-bulb temperature would be 96 degrees — one degree above your skin temperature, making it impossible for you to perspire without raising your skin temperature, which in turn would require you to raise you core temperature. Spend too much time under these conditions and you would be in trouble. (If you’d like to play with your own temperature and humidity, check out this page or this one.)

You know how they say “it’s not the heat, it’s the humidity”? Whoever “they” are, they’re wrong — it’s the heat and the humidity. (More on wet-bulb temperature.)

Could Global Warming Make Some Parts of the Globe Uninhabitable?

Sherwood and Huber argue that wherever and whenever global warming causes the wet-bulb temperature to exceed your skin temperature (of 95 degrees Fahrenheit), you will find yourself in a life-threatening situation just by being outdoors. The danger occurs with an increase in ambient temperature or humidity or both.

Today, wet-bulb temperatures around the globe rarely exceed 86 degrees Fahrenheit — no problem there.

But could global warming cause atmospheric wet-bulb temperatures to exceed 95 degrees Fahrenheit?

To answer that question, the authors used a combination of current weather patterns and climate models to assess the likely wet-bulb temperatures under various global warming scenarios.

They found that small areas around the globe would likely become uninhabitable for the first time because of heat and humidity if global temperatures increase by about 7 degrees Celsius (12.6 degrees Fahrenheit) above preindustrial levels. And, “with 11–12 °C [about 20 °F] warming, such [uninhabitable] regions would spread to encompass the majority of the human population as currently distributed,” including much of the eastern United States.

But global warming so far has led to a modest increase. Are such large temperature increases even feasible? Unfortunately yes.

The climate’s sensitivity to increases in carbon dioxide is not known perfectly, but using data from past climate changes, we’ve been able to constrain its range. Based on this range, it’s quite possible that global warming will reach 7 degrees Celsius by the end of the century if greenhouse emissions continue to grow unchecked. And global temperature increases of as much as 12 degrees Celsius could occur eventually.

The prospect that taking a stroll outside could become life threatening is not exactly cause for celebration. We could avoid such a possibility by resolving to cut greenhouse gas emissions. But barring that, I suggest the following strategies:

  1. Ostrich Tactic: Keep telling yourself climate scientists are evil hoaxters who don’t know anything about the climate.
  2. “Don’t Worry, Be Happy” Tactic: No problem, man, you won’t be around 100 years from now anyway.
  3. “Invest in Trane” Tactic: Hope your kids and grandkids are among the fortunate of the 9+ billion who have air conditioning.

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