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Climate Hustle

Heat stress: setting an upper limit on what we can adapt to

Posted on 11 May 2010 by John Cook

It's widely agreed that warming over 6°C would have disastrous consequences for humankind. Increased drought and rising sea levels are the usual poster boys for climate impacts (and for good reason). However, the direct impact of heat stress on humans gives us a clear climate impact benchmark. Some argue that humans will simply adapt, as we already tolerate a wide range of climates today. But a new paper An adaptability limit to climate change due to heat stress (Sherwood 2010) shows this argument is false. Even modest global warming could expose large fractions of the population to unprecedented heat stress, and severe warming would lead to intolerable conditions over wide regions.

Human skin temperature is regulated at around 35°C. The human body needs to be warmer than it's environment in order to cool. Specifically, it needs to be warmer than the wet-bulb temperature Tw, measured by covering a standard thermometer bulb with a wetted cloth and fully ventilating it. Sherwood 2010 estimates that the survivability limit for peak six-hourly Tw is probably close to 35°C for humans - any longer results in hyperthermia.

Figure 1 depicts temperature over the last decade (1999 to 2008). The black line in Box A is a histogram of annual surface temperature. The blue line is annual maximum temperature. Of particular interest is the red line, showing a histogram of the wet-bulb temperature Tw. Note the vertical dashed line in Box A - this denotes the critical threshold of 35°C. The map also shows the wet-bulb temperature across the globe.

Temperature histogram including wet-bulb temperature
Figure 1: (A) Histograms of temperature (Black), Maximum Temperature (Blue), and Wet-bulb Temperature Tw (Red) during the last decade (1999–2008). (B) Map of Wet-bulb Temperature Tw.

While the distribution of temperature (black) is broad, wet-bulb temperature (red) has a much more narrow range. This means the peak heat stress is surprisingly similar across many regions on Earth. Even though the hottest temperatures occur in subtropical deserts, relative humidity there is so low that Tw is no higher than in the deep tropics. The result is that over the last decade, Tw has never exceeded 31°C.

The effect of global warming is an upward shift of the wet-bulb temperature distribution. Sherwood 2010 concludes that global warming of roughly 7°C would create small zones where metabolic heat dissipation would for the first time become impossible, calling into question their suitability for human habitation. A warming of 11 to 12°C would expand these zones to encompass most of today’s human population. If warmings of 10°C were to occur in next three centuries, the area of land likely rendered uninhabitable by heat stress would dwarf that affected by rising sea level.

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Comments 1 to 50 out of 68:

  1. When working in the Kalahari desert in Namibia I've been in temperatures of 45 degrees C for periods of an hour or two. It's extremely uncomfortable, even in the shade.

    At same time as I've been in discomfort with a camera on my shoulder, wearing shorts and a light cotton shirt, I've been filming locals transplanting, weeding or harvesting crops who are wearing dark trousers, shirts and loose dark-coloured jackets -- some even with woollen hats.

    I guess I learned that, provided there is a supply of clean fresh water, humans can hang on almost anywhere -- speaking geographically -- though I doubt whether it'll be me or thee.

    This comment should in no way be taken to mean that I don't think that efforts to avoid climate change should not be of the highest priority.
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    Response: The wet-bulb temperature is different to air temperature. In a desert environment, the wet-bulb temperature is lower due to the low humidity. This is why desert areas show the same wet-bulb temperature as more humid tropical regions.
  2. Any thoughts on heat-stress effects on animals, domesticated and wild? Even vegetarians need animals in many tropical areas for ploughing and transport.
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    Response: Good point, I forgot to include that they do mention mammals in their study. One of their concluding paragraphs reads (emphasis mine):
    In principle humans can devise protections against the unprecedented heat such as much wider adoption of air conditioning. But the power requirements of air conditioning would soar; it would surely remain unaffordable for billions in the third world and for protection of most livestock; it would not help the biosphere or protect outside workers; it would regularly imprison people in their homes; and power failures would become lifethreatening. Thus it seems improbable that such protections would be satisfying, affordable, and effective for most of humanity. 
    They also mention earlier that "Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible." So obviously heat stress will have a significant impact on livestock which will affect food supply.
  3. It's interesting to notice that the body temperature of mammals, including us humans, falls within a relatively narrow range. We all share the need to dissipate heat and the "last resort" of perspiration. So we also share roughly the same wet-bulb temperature limit.
    This limit can be found in any introductory physics textbook for biomedical sciences but this is the first time I see it applied to humans in their environment on a global scale. Great.
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  4. Would someone explain how temperature at (sub)tropics could increase by as much as 7 °C and more?
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  5. We already see this effect in current weather cycles. The European heat wave of 2003 killed tens of thousands of people. Another in Europe and the US in 2006 killed a few thousand. Then again in 2007 a few thousand more in Asia.

    That is all weather, but in a warming climate the weather conditions for high temperature and humidity occur more frequently. Thus, long before any portion of the Earth would reach the point of regularly experiencing lethal conditions we are going to see large portions of the globe being hit by them every few decades.

    It would be interesting to see what Tw was in decades prior to 1999 or by year rather than decade. If it has been creeping up, as seems likely given the increase in temperatures (and thus perforce also humidity), then it represents a measurable 'fatality index' to global warming which could be correlated against heat wave deaths.
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  6. Berényi Péter, that raised a red flag for me when I first read this paper. However, they are very clear in the introduction that this is (a) a worst-case scenario, and (b) a projection beyond the traditional horizon of 2100.

    If you assume that climate sensitivity is in the upper 5% of the accepted range, that no serious mitigation measures are undertaken, and that population growth and industrialization of developing countries results in the consumption of a larger fraction of coal reserves over the next 200 years, a mean temperature increase of 10C or more is possible (more at NH high latitudes, less in the tropics).

    I think James Annan would probably dispute this approach. Personally I'd like to see any discussion of this paper clearly emphasize that it's based on a "worse" (perhaps not "worst") case scenario rather than on the most probable scenario.

    On the other hand, given the chance of seriously intolerable outcomes for much of the world, I do think it's necessary to consider less-likely but possible negative scenarios. IPCC's forecasts have turned out to be too cautious in some cases, after all.

    In any case, Sherwood and Huber 2010 explain this quite clearly in their first paragraph, with references. Have you looked at the paper yet?
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  7. Berényi Péter:

    If you go to the "4 Degrees and Beyond" conference presentations download page: and download Dr Richard Betts presentation on regional climate change, there are a number of global maps showing model projections for regional temperature anomalies for various warming scenarios. The map for a global anomaly of 5.4C is truly frightening in it's regional implications. 4C is bad enough.
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  8. CBDunkerson writes: Thus, long before any portion of the Earth would reach the point of regularly experiencing lethal conditions we are going to see large portions of the globe being hit by them every few decades.

    This is a very important point. When thinking about the economic and biological impacts of climate change, it's essential to look at the variance, not just the mean. Obviously, it would be disastrous if the global mean temperature increased to the point where Tw exceeded 35C in Texas, Senegal, and New South Wales every year. But it would still be disastrous if temperatures only increased to the point where that happened during the one-sigma heat waves a couple of times per decade, or even the two-sigma heat waves every twenty years or so.
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  9. Other sideeffects of global warming includes asthma and allergies from pollen and fungii, malaria/West Nile virus/dengue fever from spreading mosquitoes, tick borne Lyme disease and perhaps a 30% increase in kidney stones from dehydration.

    Coming global warming could even be the greatest health problem according to experts. Take a minute to contemplate just how radical a statement that really is when it comes from false positive averse experts.
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  10. Thanks, John. I've learnt something.

    Having also been in tropical rain forest in extremely high humidities I'd assumed that I'd also been in equally high temperatures in those conditions; but I guess I haven't or I wouldn't be here to tell the tale.
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    Response: Damn, you've had an interesting life, John. Working from home, it's a good day for me if I manage to get out of the house.
  11. It should be noted that the warming of 6 degrees C (and more), an increase of temperature (mostly) in the cold season. In tropical regions, such as during eemian, could be even colder than it is now ...
    Theory of a tropical thermostat - confirms it: no adjustment to temperature changes in the coral, similar areas inhabited by gorillas (Africa), etc..
    It is also worth noting that even evolutionarily oldest tropical plants are not adapted to higher temperatures than at present.
    Meanwhile - in cold zones: e.g. Dr Jim Provan, from Queen's School of Biological Sciences, describes in his works a huge potential adaptation (for warm) of algae in the Arctic.
    Cod tolerate swings in temperature + / - 8 degrees C ...

    In ancient times the warm (6-12 ° C more than today), underwent extension in the temperate zone of marine version (6-8 degrees), or tropical mild (> 7-9 degrees) - for nearly the entire globe. Do not threaten us, then year-round heat.

    For example, heat waves in Europe (2003, 2006), only effects of violent La Nina - CLAW hypothesis - not GW.
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  12. The sharp cutoff of the red Tw plot deserves further explanation. Clearly there is a phenomenon that causes the upper cutoff despite the higher temperature (black) and max temperature (blue) curves.

    Until the Tw curve is explained, it does not necessarily follow that the Tw curve will shift right as temperatures rise.

    One would think it should rise, but one would not expect that sharp upper cutoff.
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  13. From Thermoregulation

    Quote: "The lower limit of temperature that humans can endure depends on many factors, but no one can survive a temperature of 45 °C (113 °F) or above for very long. Mammalian muscle becomes rigid with heat rigor at about 50°C, with the sudden rigidity of the whole body rendering life impossible."

    So remember that the temperature inside your car can easily climb over 50C even with the windows cracked. Guess what happens to Fido. Heat stress for production animals generally diminishes in 24C (75F) and above conditions depending on the animal involved, with onset generally in the 24C to 28C range. There are lots of data on the net to support this.

    Obviously then the amount of land area that would become uninhabitable through dry bulb temperatures exceeding ~44C would also increase.
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  14. Wow, that's something I'd never considered before! I'll certainly look it up.

    @Arkadiusz: interesting point. In the Cretaceous it appears there was a weird equator-pole temperature gradient. Perhaps new mechanisms (increased cyclone activity to redistribute heat perhaps?) could counter some of the dangers raised in this paper. I must read more!

    @factfinder: you're making the common mistake of confusing weather and climate. John did a little article on this. Currently we're going through (globally) the second warmest 12 month period ever recorded, according to satellites, despite a record solar minimum. Europe and the US have been reasonably cool though, for various reasons (winter was thanks to the Arctic Oscillation, f'r'instance).
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  15. It's going to be worse than what these studies indicate. I spent a lot of time in Delhi about 10 years ago, which falls within the pale red zone, due to local humidity. 45C was common, and people were barely able to deal with it. If there was a heat wave, people slept on roofs and sidewalks at night, and avoided exercise in the day.

    I don't think that we have much experience with 48C in a humid climate with few breezes, as in north central India. Mortality is likely to be very high, and there will be more emigrants than from Bangladesh, which has a smaller population that can more easily move to higher ground. The Himalayan foothills are already heavily populated and deforested.

    I've also lived in 50C weather, in the Southern California inland desert, which may be the hottest region on earth by temperature. Delhi was much worse, and the area is obviously far more populated.

    I'm glad you're bringing this up, John- this problem is quite overlooked, since it's simpler to talk about rising tides.
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  16. The difficulty with air conditioning is that most household air conditioners are only rated up to about 41C, some to 43C I think. When the temperature rises to 51C for any length of time, like a summer heat wave, we'll either have to invent better air conditioners and de-humidifiers or find another way to keep cool. (That's only 4C warmer than it got to in the heat wave in Melbourne in southern Australia last year - and the global temperature is still climbing.)

    Maybe building underground like at Coober Pedy will help.
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  17. I found this paper very interesting in the way it establishes a fairly uncontroversial boundary or limit of "adaptability" to global warming. Retreating from areas of the globe where we evolved is adaptation, sure, but could hardly be termed a success story.

    As Ned mentions this won't be a smooth process. Given less extreme conditions than those hypothesized in the paper we'll nonetheless witness unacceptable excursions, presumably with those excursions becoming more frequent and obvious the closer we actually do approach the extreme case.

    Time to begin breeding dogs with longer tongues...
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  18. I remember traveling in the Gangetic plain when it was 120F. It was nice. No mosquittoes or flies. They all died so we didn't have to use a net to sleep. Hot day and night. Just keep a watermelon handy.

    One little appreciated fact is that the global dew point is rising 3x faster than the global temperature. When the two meet the problem is solved.
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  19. Maybe an interesting further step would be comparing map B with the map of future scenarios likethis one:

    Southeastern Asia seems to be already close to the edge, and the few extra degrees of projected warming could make it exceed the tolerable limit. South America and sub-saharian Africa also seem vulnerable.

    Let´s also keep in mind that this map here is the temperature anomaly for the period 2070-2100 (scenarios A2 and B2). Temperature does not stabilize there, though. Warming is expected to continue in the coming centuries.
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  20. One of my biggest issues with living in one of those red zones is that the noise of the air conditioning affects my mental health. That and the cockroaches.

    On the plus side, I'm told that our electricity in South Carolina comes from nuclear power. Think I'll treat myself to another "guilt-free" cold one.
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  21. Ive also spent a bit o time in the jungles in the tropics...

    A decade or so ago, i spent six months kicking around East Timor... And three months of that at Suai, which is basically a big valley/swamp, and even though its further south than Dilli (the capital) Suai is generally approximately 10C warmer(generally fluctuates between lows of 35C and 45C. With extreme humidity(90%+) I dont know how this compares with the wet bulb taken into account?

    Ive seen a fair few people drop with heat stroke(hypothermia???) And some people dont handle it well at all.(might last 10mins o a two week patrol, but this is with 60 odd kgs on yer back.)

    But just too me, it seems 35 is a tad on the low side As far as survivability goes... Is there a calculation showing what temp C vrs humidity is equal to wet bulb temp?
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  22. @Joe Blog: yes, there are calculations, but the easier way is to look up a psychrometric chart. They can be a bit hard to read, though. Wikipedia has some here.

    You can tell from looking at the charts that the conditions to get to a wet bulb temperature of 35 are pretty extraordinary (something like 75% relative humidity at 40ºC). Remember that relative humidity also drops rapidly with increasing temperature unless you put a lot more water vapour into the air (0.02g H20 per gram of dry air = 100% RH at 25ºC, but only ~34% RH at 45ºC).

    There's some more info about apparent & wet bulb temperatures (including formulae) on this Bureau of Meteorology page.
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  23. Further to my last post - I wasn't able to dig up the extremes, but Darwin has a reputation for pretty unbearable weather in summer. Going by BoM data, the mean 3pm wet bulb temperature in January is 26.4ºC.
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  24. #14
    And even well-known popularizer of knowledge Ron Redfern in his book "Origins" (2000), writes that in the tropics during the last glaciation may have been even more tropical than at present (significant reduction - compression THC) and the Northern Siberia, formed stable - static for thousands of years - Anti-cyclones. Strong warming is perhaps the disappearance of cyclones - almost pressure gradient = 0?
    In summer, temperatures in Siberia could be as high as above 45 deg C ... Sherwood'a work is very valuable and interesting but not very useful for prediction of ways to adapt to GW. Thesis GW = greater frequency heat - it is already outside the mainstream of science.
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  25. Arkadiusz Semczyszak writes: Thesis GW = greater frequency heat - it is already outside the mainstream of science.

    Not at all -- quite the opposite, in fact. The largest increase in temperatures is in winter and at night, but there's also an obvious trend of increasing intensity of extreme summer heat waves.

    Just looking at some recent papers, we find:

    Ballester et al. (2009): "... the increasing intensity of the most damaging summer heat waves over Central Europe is mostly due to higher base summer temperatures ... 36% (B2) to 47% (A2) of future Central Europe July and August days at the end of the present century will be warmer than the 1961/1990 99th percentile."

    Beniston et al. (2007): "Heat waves – Regional surface warming causes the frequency, intensity and duration of heat waves to increase over Europe [...] The intensity of extreme temperatures increases more rapidly than the intensity of more moderate temperatures over the continental interior due to increases in temperature variability"

    Meehl and Tebaldi (2004)

    Clark et al. 2006: " [...] Although uncertainty associated with the magnitude of expected changes is large in places, it does not bring into question the sign or nature of the projected changes. Even with the most conservative simulations, hot extreme events are still expected to substantially increase in intensity, duration, and frequency. [...]"

    I'm not sure why you think that increasing frequency of heat waves is "outside the mainstream of science" when it's actually one of the most robust findings of both observational and modeling studies.
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  26. I haven't had time to look into it yet, so I must ask, does this study include the effects of increased temperatures on the amount of water vapor in the air? It would seem that both an increase in water vapor would drive the wet-bulb temperature, unless I'm misremembering something from genchem.
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  27. I notice you’ve added this to the list of global warming impacts. I think it might be worth pointing out that some pretty extreme global warming (at least 7°C) is required for this to happen.
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  28. James Wight has a good point. Unfortunately, the table at global warming impacts doesn't really have a handy way of breaking down impacts as a function of temperature increase.

    That would be a long-term project -- it's basically what Mark Lynas does in his book Six Degrees.

    There's a handy version of this kind of chart in the IPCC AR4 SPM:

    Figure SPM.7. Examples of impacts associated with projected global average surface warming. 

    But it's entirely qualitative, unfortunately. I wonder if anyone ever tried to produce a similar graphic with quantitative metrics? That could get ugly fast, but it's an interesting information visualization challenge.
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  29. James Wright,
    It was pointed out earlier in the thread that this was a worst case analysis. The temperature change required depends on what you consider bad. If you are only worried about large areas of Earth being completely uninhabitable it requires 7C. On the other hand, CB Dunkerson at #5 pointed out that several thousand people have died already in heat waves throughout Europe. If climate sensitivity is really 6C per doubling (long term) we have currently committed the Earth to substantial warming. How bad does it have to be to be considered bad?
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  30. Ned #28

    I find it important to stress that those ranges in your bottom box is just the projection for the end of the century.

    The warming (particularly the slow albedo feedback) is projected to go beyond that.
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  31. Right, IPCC always gives figures for warming by the end of the century (or 2095, in this case from AR4) but the warming won't stop there.

    2095 is actually not that far away. Go back the same amount of time and you're in 1925 ... approximately when my parents-in-law were born.

    The average girl born in Japan this year will still be alive in 2095, the end point for the IPCC AR4 projections.

    That's pretty amazing when you think about it.
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  32. #5 CBDunkerson at 21:38 PM on 11 May, 2010
    The European heat wave of 2003 killed tens of thousands of people.

    The heat wave of 2003 has nothing to do with high wet bulb temperatures. It was worst in mid France. However, if you had a look at the weather history of Lyon, August 2003, you could see humidity was rather low on the hottest days. Dew point high there has never exceeded 21°C during August and when it was really hot (40°C on 12-13 August), relative humidity decreased to astonishingly low values (down to 12%).

    The high death toll is due to something else. Where people (and the government) are prepared for such a weather event, they can handle it pretty well. Unfortunately it was not the case in France. Most lonely elders died simply because they have not drunk enough water and relatives were out on holidays, not able to help them.
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  33. According to Wiki mammals evolved 200+ million years ago. What you might call modern mammals have been around for over 100 million years. Here's a reconstruction of temperatures. Looks like mammals have lived through periods with global temps 10oC greater than now. According to Wiki upper ocean temps may have even touched 17oC above todays average during the Cretaceous.

    Do people not think there is a hint of alarmism about all this? Life isn't hell now so lets imagine a hell for future generations. It also ignores the possibility that humanity would do something about this.

    15.mike roddy
    Things might get sticky in Mumbai but it seems nobody told the indians they should be dropping dead of heat exhaustion. Life expectancy in India has almost doubled in the past 50 years.
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  34. Bern, thanks for the reply, unfortunatly there is no comparison between Darwin and Suai(it really is a hell hole).. but according to those tables i must have been in temperatures above this 35... there were no real weather stations mind. This was going of the battalions station they had set up on the edge o the main base, basically in jungle/grassland. So dont really know how accurate it was, hottest day tipped the mercury at 52 though.

    But to put this wet bulb in context... basically this paper is saying that someone submersed in 35c water for an extended period will die from heat stroke? i suppose i can see yer bodies metabolism easily enough throwing in the extra 5-7c for it to be fatal.(when yah core hits 40, its like yer on acid.. and aint long after that yah twitchin on the ground)

    But i do have to say, i have big doubts as to the accuracy o this claim. Speaking for myself, in those upper temps, i was drinking 15-20liters a day, which was warm, but still cooler than yer body temp. Im not going to pay to read the paper. But how exactly did he determine this survivability level?
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  35. @Humanity Rules at #33:

    Isn't current global average temp more like 15ºC? That would but the entire Cretaceous at less than 7ºC higher than current.

    But just because some species of mammals have survived through that time-frame doesn't mean they weren't highly restricted in their range - mountain tops, high latitudes, other areas with cooler-than-average climates.

    Re the Mumbai comment - I rather suspect increases in Indian life expectancy have more to do with hygiene, nutrition, and modern medicine than temperatures. If we see this sort of nearly-worst-case warming, then life expectancy for India would start to plummet again in a few hundred years.

    @Berényi Péter at #32: you're right, 41ºC with 13% RH is only a wet-bulb temperature of 20ºC. But the same issues that resulted in so many deaths (heat stress, dehydration) under those conditions will, of course, only get worse as temperatures rise further.

    If increasing global temperatures result in more water vapour in the atmosphere, then the RH is likely to be the same or higher. The point of this posting, though, is that with sufficient increases in global temperatures, we might find areas where the heat waves are not survivable by humans, irrespective of how well hydrated you are. The frail, elderly, and sick would be the first to succumb, most likely well before we reached that cut-off point, even with the best of care.

    All this is assuming the absence of air-conditioning, of course, but not many people outside the US have that (although it's becoming much more common here in Australia). And even if you did, you're only a power failure away from big trouble.

    Moving underground (or using ground-loop heat exchange with extremely well-insulated buildings) might be the only way to cope in the affected areas. It's still most likely centuries away, though, so there's time for technological adaptation to cope with such extreme heat waves.
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  36. @HumanityRules... If I'm not mistaken late cretaceous mammals were limited to early marsupials and other rodent-like creatures. Not a huge mammal population.
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  37. @Joe Blog at #34: I don't know, some quick google-fu didn't help me out either. Can someone with access to the full article check for a reference for that figure?

    But this NASA page has lots of information about the heat conductivity & capacity of a typical human body (and they're talking about astronauts, so people in good physical condition). Presumably, it's possible to calculate from this (and lots of other thermodynamics) what the maximum survivable temperature is. Note, also, that there's a time-frame associated with it - you can endure high wet bulb temperatures for some time, with appropriate precautions (hydration, shade, rest), but if it goes on for too long your core temperature will rise too high and you'll be in trouble, as you mentioned.
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  38. Well I doubt even centuries away really. Surely with even our most destructive impulses, we are going to have job maintaining a BAU CO2 emission strategy beyond 2100 even if run it till there.
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  39. HummanityRules: "It also ignores the possibility that humanity would do something about this." I guess that's what we're talking about, right? Trying to assess whether theres a justification for doing something about "it."

    Re possibility of extinction, looking at the fossil record and comparing to the current situation is tricky. Climate changes over previous eons - even after glaciation - were relatively slow compared to what we're seeing now even if the final temps were higher in some instances. A "modest" shift globally of 3C or more in a little over 100 years is quite a large rate of change relative to what species can adapt to evolutionarily, especially long lived species. Species will have to shift their ranges much faster than occured during the last deglaciation over landscapes that we have made far more fragmented.

    I'm not sure about the danger immedaitely posed to human survival, but conservation of other species has always been based on a model of protecting crital spaces or preserves and assuming those spaces would remain good habitat. Now the game has shifted and park managers are and conservation organizations struggling to deal with it. Low dispersal species, and those endemic to mountains and poleward ends of continents, are especially sensitive. The concern of those managers is definitely real and not based on alarmism.

    There is a huge and still burgeoning literature on struggling to deal with this problem too. It's true there's a lot we don't know, but that doesn't make me feel more secure.
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  40. #33 HumanityRules

    "Do people not think there is a hint of alarmism about all this?"

    I think the only thing that's relevant here is whether the science is sound. If it is, then it's kind of silly to call it "alarmist." If it isn't, then it'd be better to point out exactly where the mistake is, instead of casting moral aspersions or making arguments from incredulity.

    I'm not sure what the Cretaceous has to do with anything. How many cities did we have back then?

    As far as the observation that "life isn't hell now" goes, I'd point out that while this may be true for you and me, a lot of people around the world are currently living in pretty hellish conditions, and there's good reason to believe that they'll be especially vulnerable to AGW.

    Even if alarmism were a serious problem, complacency wouldn't be the solution.
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  41. Joe Blog, we need to be able to evaporate water at a certain rate in order to shed metabolic waste heat resulting from a certain level of activity.

    Our skin surface area remains the same even as our liberation of waste heat and the ambient wet bulb temperature may vary. Realizing that at a certain wet bulb temperature we can evaporate water and thus remove heat at a certain rate from this fixed surface area, it's then not hard to see that as wet bulb temperature increases, the rate at which we may shed heat decreases. Knowing that, we can predict what levels of physical activity are sustainable at a given wet bulb temperature; for a given activity level, if the wet bulb temperature is high enough we can't evaporate a sufficient mass of water rapidly enough to maintain a stable temperature. If the rate we're able to evaporate does not keep up with the waste heat we're trying to shed, our temperature climbs.

    Long story shorter, no surprise: as the wet bulb temperature increases, the rate of sustained physical activity we can support drops since we know our surface area is fixed even as the rate at which we can shed waste heat via evaporation drops.

    So to experience hyperthermia we need not be "submersed in 35c water." Moist, warm (high wet bulb temperature) air will do the job. This is not a matter of doubt or speculation, though it may seem counterintuitive to you.

    Also helps to bear in mind, it's not possible to drink enough chilled water to directly cool a human body performing sustained physical activity. We stay cool by sweating, not urinating.
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  42. 35.Bern

    I'm not sure it's even a worst case scenario. I haven't read the paper. But what John describes is the possible upper limit for human life. There's no sense in John's post that these numbers will ever be reached or approached. In that sense it's an intellectual fancy derived from a curious mind.

    I still contend that does suggest a fairly passive role for human society. As though we would quickly approach extra-ordinary conditions and do nothing about it.
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  43. @HumanityRules at 42: You're right, this may never come to pass. But while we may never quite get to a Tw of 35ºC, even getting a few degrees closer to that mark will result in enormous death rates in heat waves, particularly in the very young, the elderly, and the sick.

    You're also right, it suggests a passive role for humans. The question is, if we get a century down the track with 'Business as usual', will there in fact be anything we *can* do about it?

    Is there anything we can do about it now? Well, yes, we can drastically cut our rate of carbon emission, to give nature time to soak up the excess before it gets too hot.

    Perhaps in 50 years we'll have sufficient non-carbon sources of power & massive CO2 capture plants that do nothing but liquefy CO2 from the atmosphere and pump it deep underground. It might take that kind of effort, but it's going to be a job and a half to sequester a few hundred billion tonnes of CO2.
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  44. I have over a ton of hydrated CO2 stored in my garage. To prevent the hydrated CO2 from decomposing, pure CO2 was injected into the #10 cans in which it is contained. That is how I store my wheat. I highly recommend this form of storing excess CO2.
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  45. HumanityRules at 42:
    I am not sure what country you live in, but I live in the USA. There are a lot of people, approaching a majority, who are adamently opposed to taking any action about AGW. My government has taken virtually no action about this topic in spite of all current scientific findings. Other countries have taken more action, but the overall global response is small. Why are you so convinced that human society will respond strongly when the current response is so small? What makes you think people will be wiser in the future? When will this time come?

    In the meantime, we need to work to convince politicians to take substantial action.
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  46. HumanityRules, following up on michael sweet's response, there is even a post about inaction: Are We Too Stupid?
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  47. About earlier hot periods:

    One important thing to remember is the speed of change. Even very fast warming events like the PETM were something like 6ºC in 10,000 years, or even slower. The world did not reach the high temperature of the Cretaceous as fast as that.

    Give us a couple million years to genetically adapt to global warming and new ecosystems will flourish - provided we do enough conservation in the meantime.

    But we´re talking about some 6ºC of warming in a century or two. That´s too fast even for many species´ moving capabilities, let alone genetic evolution.
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  48. Of course, while these analysis relate to extreme temperatures that would likely only be achieved with another few hundred years of unlimited fossil fuel burning, the effects of relatively small temperature increases can have very large effects on species survival.....even in lizards.

    A paper describing warmth-induced extinctions in lizard species is described in today's Science (also an accompanying commentary).

    These papers also relevant to arguments about adverse effects of global warming and effects on species survival.
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  49. Likewise, increasing [CO2] is increasingly identified as a a problem for agricultural production. A paper in today's Science shows that increased [CO2] (of itself, and not heat related) results in reduced nitrate assimilation in wheat and Arabidopsis.

    (This also could be moved to arguments about effects of increased [CO2] and global warming on agricultural prodution).
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  50. chris at 07:37 AM, this FACE wheat trial conducted at Horsham Victoria in 2008 do not support your assertion of increasing problems.

    Nitrogen uptake increased 20-30%.
    Limitations due more to soil processes than plant inability to uptake.
    Higher plant yields but lower protein content should be benchmarked against area planted, not solely plant biomass.

    As happens in everyday cropping, yields can vary enormously within short distances and slight changes in conditions. Wheat yields and protein are often the inverse of each other meaning these problems have always existed.
    The conclusion of the abstract from the paper you linked states the obvious, and what has always been known, that being, if you want plants to produce more then you must feed them more.
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