THEGREENGROK    Planetary Watch

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

by Bill Chameides | May 11th, 2010
posted by Erica Rowell (Editor)

Permalink | 18 comments


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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  1. MattN
    May 17, 2010

    Scaremongering nonsense. Just like I said…

  2. MattN
    May 12, 2010

    Your wet bulb scenario is absolutely ludicrous. My rough guestimate for dewpoint in your scenario is ~92-93F, given an air temp of 100F, wetbulb temp of 96F. Can you tell me where outside of the Amazon that this could EVER happen? And I’m not even sure it could ever happen there. Have those conditions existed anywhere at any time on this planet? That is just complete, total, scaremongering nonsense.

    • Bill Chameides
      May 17, 2010

      MattN: No, these do not occur on our planet today. That’s the point.

  3. MattN
    May 12, 2010

    “There is evidence that the warming trend has resulted in more severe heat waves” Based on the poor quality of the surface record data, I do not believe that is an accurate statement. The warmest decade on record for the US was the 1930s (well, at least until GISS/NCAR adjust it into 2nd place, but I digress)

    • Bill Chameides
      May 17, 2010

      MattN: No, the 1930s weren’t even close to being the warmest decade. You are confusing U.S. temperatures and global temperatures.

      • MattN
        May 17, 2010

        Not confusing anything. That’s why I specifically said “for the US” in my post. YOU pointed to a 1995 Chicago heat wave (last time I checked that was in the US), and I rightly pointed out the 1930s are the warmest 10 years in the US on record. check this out: “The temperatures soared to record highs in July with the hottest weather occurring from July 12 to July 16. The high of 106 °F (41 °C) on July 13 was the second warmest July temperature (warmest being 109 °F (43 °C) set on July 23, 1934)” Whaddya know. 1934….

        • Ken Towe
          May 18, 2010

          Actually Matt, in the US, the last decade (2000-2009) might be very slightly warmer than the decade from 1931-1940. A comparison of 36 US cities (only those with complete 10 year records) in H.H. Clayton’s book WORLD WEATHER RECORDS 1931-1940 with those same cities in ’00-’09 shows a decadal average of 13.6°C ± 5.1 in the former and 13.8°C ±5.1 in the latter. A plus 0.2°C warming is hardly a dramatic difference, but technically warmer nevertheless. A third of the cities experienced colder average decadal temperatures in the last decade. Cities included ranged from Eastport, ME to Portland, OR and from Spokane, WA to Key West, FL with an average latitude of 38.28°N and longitude of 96.07°W…close to the geographic US center. Source:

          • MattN
            May 19, 2010
            • Bill Chameides
              May 19, 2010

              MattN: the U.S. is a great and large country, but U.S. temperatures are a very poor indicator of global temperatures. In fact that is a point I made in the post.

              • MattN
                May 21, 2010

                Spin this, Doc: Just sayin’…

              • Ken Towe
                May 24, 2010

                “…small areas around the globe would likely become uninhabitable for the first time…. “…such [uninhabitable] regions would spread to encompass the majority of the human population as currently distributed,” including much of the eastern United States.” It is true that all 10 of the 1998 record highs for the contiguous US were (a) recorded in the eastern US and (b) in a geographically small area: MI, OH, NY, PA, NH, CT, NJ, RI, DE, MD. On the other hand (also according to the 1895-2009 NCDC-NOAA temperature database), it is primarily the western states that have long-term warming trends. The eastern US has experienced long-term downward or level temperature trends in the following states: Downward: ME, MI, TN, AR, AL, SC, GA, MS Unchanged: PA, WV, KY, NC, LA (and TX) All 10 of the record highs in 1934 were recorded in a large contiguous geographical area west of the Mississippi: WA, OR, ID, MT, WY, NE, NV, UT, CO, KS. The 10 record highs of 1921 were recorded in: IL, IN. KY, WV, TN, AR, MS, GA, LA, TX. Not sure how a worldwide warming from CO2 knows when and where to warm and when and where to cool.

                • Bill Chameides
                  Jun 1, 2010

                  Ken, Some places have warmed, some maybe not. But overall the global trend is upward. I guess you just have to hope you live in one of those places that doesn’t warm — assuming that such a place exists in the future — and then everything will be hunky dory, right? But what if you don’t?

        • Bill Chameides
          Jun 7, 2010

          Which had the warmer average global temperatures: the 1920s and 1930s or the 1990s and 2000s? Whaddya know — the 1990s and 2000s.

          • MattN
            Jun 7, 2010

            Based on the condition of the surface record (I could make a list a mile long of all the problems), that statement, in reality, may or may not be true, Doc. I mean, CRU doesn’t even have the raw data anymore, so we can’t actually verify their temp garphs anymore…

            • Bill Chameides
              Jun 14, 2010

              MattN: We’ve been over this ground before. I will let our previous discussion stand.

  4. Ken Towe
    May 11, 2010

    A global temperature is the average of two hemispheres. Today’s ‘normal’: NH = 14.6°C, SH = 13.4, GLOBE = 14.0. Adding your 0.7°C to today’s globe yields 14.7°C. Adding it to 14.6 yields NH 15.2°C. In 1975 the ‘normal’ NH average was 15.0°C and an increase of 0.6°C was reached around 1938 at 15.6°C., making it 0.4°C warmer THEN than NOW. But these values are only accurate to +/- 0.5 anyhow. “WEATHER OF 1938 IN THE UNITED STATES” EXCERPTS: “The year 1938 averaged much warmer and slightly wetter than normal. Summations of temperatures over the country reveal that 1938 was one of the warmest years of record. The departure from normal temperature chart shows that every first-order station in the United States had above-normal temperature for the year — a condition which probably is unparalleled in the Bureau’s records.” “Recent studies indicate that the tendency to higher temperatures which set in about 1900 has been definitely outstanding in the last decade. Temperature records for the past 20 years show 1929 as the only year appreciably cooler than normal, for the country as a whole; 1919 and 1924 were slightly below normal. The remaining years had above-normal warmth.” According to NCDC-NOAA drd964x.tmpst.txt: since 1895 the RECORD average ANNUAL temperatures for the states of IL,IN,KY,TN,WV,GA,MS,AR,LA and TX each took place 89 years ago in 1921. These records still stand. In 1921 the city of New York experienced one of its hottest years. North Carolina has not warmed on average in 115 years. The warmest MONTHS on record in the contiguous US were ALL recorded before 1955. Only the November record is later… 1986.

  5. Ecocampaigner
    May 11, 2010

    Bill, what exactly do you think people will take away from announcements like this? It won’t encourage activism, just fatalism. You seem to acknowledge this with your three part reaction choices. 1. Most people don’t think climate scientists are “hoaxters”, but many think they’re just mistaken and somewhat inclined to make doomsday scenarios to attract funding. 2. People may not be around in 100 years to experience your predictions, but they also won’t be around to see if they actually come true, which will work against you. The problem with these predictions, is that they are neither provable or unprovable, yet seem highly alarmist. 3. I hope you realize that if these predictions actually did come true, it would devastate the industrial world, causing a collapse in CO2 output. This would infact reverse the trend. Reports like this do nothing to help the cause. They simply alienate those who we need on our side if we’re ever to have a chance of fixing the problem.

    • Jim
      May 17, 2010

      On #3: Actually the trend would not reverse right away. If all CO2 production stopped immediately, temperatures would continue to rise for some time before cooling. It takes a while before the CO2 can be reabsorbed. I think Dr. Chameides could have toned it down with his 3 part actions, but it does not invalidate the paper and we do need to know such things, it helps to frame future legislation on greenhouse gas. It tells us we need to start sooner, not later and act more aggressively.

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