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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 51 to 68 out of 68:

  1. johnd wrote : "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." Doesn't that mean extra problems of time, labour and money ?
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  2. JMurphy at 20:09 PM on 14 May, 2010 Yes that's right. To quote from the paper I linked to above:
    "As atmospheric CO2 concentrations rise and NO3 – assimilation diminishes, crops will become depleted of organic N compounds (see Fig. 3), including protein, and food quality wll suffer. Increasing nitrogen fertilization might compensate for slower NO3 – assimilation rates (Fig. 3), but such fertilization rates might not be economically or environmentally feasible."
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  3. chris at 20:43 PM, as mentioned in the findings of the Horsham trials, such real world trials found increased wheat yields for the CO2 enriched samples averaging 31% with protein levels declining from 18% to 17.3%. Food quality in terms of % protein content may be down, but it is the amount of protein produced per hectare that matters when it comes to feeding the worlds population.
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  4. johnd at 21:38 PM on 14 May, 2010 That's not ncessarily a very helpful study johnd. And it hasn't been published. A problem is in the very short period of their study (6 months). Short term enhanced growth under raised [CO2] reduces over time (desentization or aclimitisation). So longer term FACE experiments in several important crop species shows that initial gains aren't sustained. e.g. to quote from the paper I linked to above: (refering to continually rising atmospheric CO2 levels; my highlights):
    "Plants could mitigate these changes through photosynthetic conversion of atmospheric CO2 into carbohydrates and other organic compounds, yet the potential for this mitigation remains uncertain. Photorespiration is the biochemical pathway in which the chloroplast enzyme Rubisco catalyzes the oxidation of the high-energy substrate RuBP rather than catalyzes the carboxylation of RuBP through the C3 carbon-fixation pathway (2). Elevated CO2 (or low O2) atmospheric concentrations decrease rates of photorespiration and initially enhance rates of photosynthesis and growth by as much as 35% in most plants (C3 plants). This enhancement, however, diminishes over time (days to years), a phenomenon known as CO2 acclimation (3, 4). Most studies suggest a strong link between CO2 acclimation and plant nitrogen status [for example, (5)]."
    "In this study, five independent methods affirm that CO2 enrichment inhibits NO3– assimilation in wheat and Arabidopsis plants. The predominant form of N available to plants in most environments is NO3– (6); therefore, CO2 inhibition of NO3– assimilation would lead to lower organic N production. Indeed, this could be responsible for the 7.4 to 11% decrease in wheat grain protein (15, 16) and the 20% decrease in total protein content of A. thaliana (Columbia) (17) observed under CO2 enrichment in FACE (free-air CO2 enrichment) experiments. Because the influence of CO2 enrichment on leaf organic N contents is highly correlated with its influence on photosynthesis and growth (5), it is reasonable to assume that CO2 inhibition of NO3– assimilation and the resultant decline in plant organic N contents play a major role in the phenomenon of CO2 acclimation, the decline of photosynthesis, and growth of C3 plants after long exposures (days to years) to CO2 enrichment."
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  5. chris at 22:04 PM, the length of the Horsham study, 6 months happens to be the life span of wheat from planting to harvesting something that was mentioned, but should have been obvious without it having to be pointed out. I am interested in how the trials you referred to managed to expose wheat to enriched CO2 levels for longer periods, years apparently, and even if they did, what is the relevance to the real world of wheat production? Anyway what is more important protein produced per kilo of wheat or protein produced per hectare? If I am not mistaken, the trials you refer to were conducted with batches of a dozen wheat plants each in sealed plastic boxes all housed in a controlled environment lab and grown hydroponically, thus are hardly comparable to the FACE trial where the plants must interact with the complex soil processes in order to take up the nutrients required for growth.
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  6. chris at 22:04 PM, just to add to my post, one positive outcome from these FACE trials conducted over the last couple of decades, is that varieties that exhibit poor responses to enhanced CO2, such as identified in the trials you referred to, and those that exhibit better responses, perhaps such as those used at Horsham, which interestingly were winter wheat varieties, are able to be identified ahead of time.
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    Moderator Response: As Chris mentioned, these comments on CO2's effects on plant growth really belong in other threads: It’s Not Bad or CO2 Is Not a Pollutant. The valuable info in comments go to waste if other people can't find them. Someone looking for info on CO2's direct effects on agriculture are unlikely to find your comments when they are tucked away in this other thread. And people interested in heat stress (the topic of this thread) must read around your comments on CO2.
  7. Erosion of Lizard Diversity by Climate Change and Altered Thermal Niches I suppose some would say that it doesn't matter or that they will cope somehow ?
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  8. re "Moderator Response: As Chris mentioned"........ I understand, ironically these particular off topic exchanges began in response to Chris himself introducing and following up the subject of these exchanges on this particular thread, and for me to post my initial, or any response in another thread may also have effectively tucked it away. Is there any way such initial posts and any replies can be transferred to an appropriate thread by the moderators so that there is no loss of continuity in the debate? I've seen that practised on other forums.
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  9. I do have a question about the effect of CO2 doubling. According to this speaker the greenhouse effect is 34.5C. CO2 accounts for 10% of this effect, which is 3.45%. Doubling the amount of CO2, according to the speaker, would cause further warming of 0.345C. This sounds far to simple to be true. According to the speaker, we do not have to worry about the heat stress. What is the presenter's reasoning for such simple maths?
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  10. correction: 3.45% = 3.45C
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  11. Monckhausen, if you do a bit of digging around this site, you'll find some figures for climate sensitivity. As the numbers on that page point out, various estimates range from 0.9ºC per doubling of CO2, right up to 7.7ºC per doubling. The average (and most likely value) seems to be around the 3 to 3.5ºC mark. I can't comment on the other numbers given, but the fact that everything neatly lines up with factor-of-ten differences immediately rings alarm bells to me. Nature is rarely that neat.
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  12. This story about the current heat wave & forest fires in Russia perhaps gives an example of what a significant increase in global temperatures would bring. Long before we got to the point where areas became uninhabitable, we'd see these sorts of results in heatwaves become more and more frequent. The current problems in & around Moscow are likely due to the smog more than just the heat, but increasing temperatures brings an increasing risk of forest fires, too (and peat bog fires? That's a lot of carbon being released that was tied up for a few thousand years...)
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  13. Well before a disastrous 'upper limit' is reached, stress fractures occur: Search some variation of 'high electricity demand blackouts' in Google News and you find US electricity blackouts skyrocketing A hot month and a record power demand IEC, private producers pull out stops to avoid blackouts Electricity consumption reaching record highs Saudi Arabia needs $80B to spend on power by 2018 and so on. There was a popular saying in the late 70s-early 80s US oil patch: Let the bas_ards freeze in the dark (referring to folks in the northern US who complained about the high price of heating oil during those cold, cold winters). The times, they are a-changin'!
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  14. Canada and Russia are almost empty countries and seem likely to become more hospitable. The northern half of the USA, and its west coast, could possibly warm 6 degrees and still be more habitable than Florida (hot, humid) today. While the USA's not "almost empty", it has <10% the population density of many European countries. It needs a change in mentality. People might have to move country, and countries that believe they have a right to sprawl might have to accept European-style small-scale, space-efficient living - whether this be narrow streets, public transport or row houses. As far as I am aware, the entire world could move to Canada (3 people per km2) and it would still be ... about as densely-populated than England or the Netherlands are today (500 people per km2). I'm not proposing this, but matters might not be as bad as we think, if northern countries with space are willing to let some people in. Europe has some very underpopulated countries; e.g., France, Sweden, Finland. Surely there's space here for 50-100 million climate refugees? I live in the UK which is contemplating a rise in population from 60 million to 70 million by 2050. If this is possible, I fail to understand objections from other cool countries to admitting more people.
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  15. D, all very interesting speculations leaving a myriad of important details unaddressed. Have we got a grip on providing decent living standards for people situated where they live today? If we don't, why would we believe we can relocate hundreds of millions of people and end up with an improvement? Is something about human nature going to change in way that makes such an enormous upheaval and migration feasible, desirable? Anyway, your remarks leave a bigger question hanging in the air. Why set such a course? Why would we do such a thing if we don't have to?
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  16. D (and others) -- Yes, ideally we could arrange people neatly in rows in Manitoba and make it work. But let's take a hard look at the economic reality of the private property system. We don't have a world government that can quickly plan and implement the kinds of population movements that have been discussed. We have hundreds of small and large nations (and global entities) in competition with each other for resources, and many of these nations are themselves made up of millions of plots of private property. Consider the current U.S. response to immigration (illegal or otherwise). Consider the U.K. response to the Empire machine being shifted into reverse. How long will it take for >ordered< mass immigration to take place, even on the realistic scale (5-10% of world pop)? And will the people who have to migrate also have to sell their futures to real estate opportunists? Worse yet, if people are to move, how much energy (and from what source) will be needed to build new homes (or are we thinking tent farms?), create new transportation, food, water, energy, communication networks, etc.? And who is going to pay for it? (don't mention raising taxes in the U.S. or, I'm guessing, Canada) And how much ill will and resistance will occur among the "natives" of these immigration destinations (an example)? We do need a change in mentality, but such changes occur very slowly, and how much frustration, misery, and despair will occur in the meantime?
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  17. Is the wet bulb temperature the same thing as the heat index? Heat indices above 98 degrees Fahrenheit are common around the world.
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  18. Karamanski they're loosely similar in what they both consider humidity. But the heat index is related to human perception while the wet bulb temperature is a physical property.
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