[Vision2020] Earth 2300: Too Hot for Humans: Proceedings of the National Academy of Sciences: "An adaptability limit to climate change due to heat stress"

Tue May 11 17:43:53 PDT 2010

>From "New Scientist" website, regarding the article published in 2010 in the
Proceedings of the National Academy of Sciences, which is directly
referenced at the bottom:
http://www.newscientist.com/blogs/shortsharpscience/2010/05/earth-2300-too-hot-for-humans.html#more
Earth,
2300: Too hot for humans
15:26 11 May 2010

*Michael Le Page, biology features editor*

Parts of the planet could start to become too hot and humid for people to
survive in a century or so if we fail to limit global warming. So says a
startling study
<http://www.pnas.org/content/early/2010/04/26/0913352107>published
last week in *PNAS* <http://dx.doi.org/10.1073/pnas.0913352107>, which most
of us journalists seem to have missed
<http://www.pnas.org/content/early/2010/04/26/0913352107>until
now<http://www.google.com/hostednews/afp/article/ALeqM5jTXA_CTHXzXzc-HxwAVLW-nh-eww>
.

Some regions would start to become too hot and humid for human habitation
with a global temperature rise of 7 °C, the paper says. With a rise of 11 °C
or more, most of the human population as currently distributed would either
have to move or rely on air conditioning to avoid dying of heat stress.

"Right now we have air conditioning for comfort. Under these circumstances
you would be using it for survival," lead author
<http://www.pnas.org/content/early/2010/04/26/0913352107>Steven
Sherwood<http://www.ccrc.unsw.edu.au/staff/profiles/sherwood/>of the
University of New South Wales in Sydney
<http://www.pnas.org/content/early/2010/04/26/0913352107>told *Discovery
News* <http://news.discovery.com/human/warming-human-temperature.html>.
"We're not saying it's going to happen soon, but to ignore it seems
foolhardy."

The problem is that we cannot survive if our skin temperature exceeds 35 °C
for more than a few hours. Although many people live and even work in
temperatures of 45 °C or more, sweating keeps their skin cool as long as
it's not too humid.

Put in technical terms, human survival depends on a
<http://www.pnas.org/content/early/2010/04/26/0913352107>wet-bulb
temperature <http://en.wikipedia.org/wiki/Wet-bulb_temperature> of less than
35 °C. This is the temperature recorded by a thermometer covered in a wet
cloth and kept well ventilated.

"The wet-bulb limit is basically the point at which one would overheat even
if they were naked in the shade, soaking wet and standing in front of a
large fan," <http://www.pnas.org/content/early/2010/04/26/0913352107>Sherwood
told *USA Today*<http://content.usatoday.com/communities/sciencefair/post/2010/05/report-climate-change-could-render-much-of-world-uninhabitable/1>
.

At the moment, virtually nowhere on Earth has a wet-bulb temperature of more
than 30 °C. But with a global rise of 11 °C, huge areas would have wet-bulb
temperatures of more than 35 °C for part of the year. According to the
climate model used by the team, these regions would include much of the
eastern US, the <http://www.pnas.org/content/early/2010/04/26/0913352107>entire
Indian subcontinent<http://earlywarn.blogspot.com/2010/05/heat-stress-in-india.html>,
most of Australia and part of China.

[image: Picture-6.jpg]*Projected regional wet-bulb temperatures for an
average global temperature increase of 12 C. Humans cannot survive wet-bulb
temperatures over 35 C (Image: Steven C. Sherwood/Matthew Huberb)*

"If warmings of 10 °C were really to occur in [the] next three centuries,
the area of land likely rendered uninhabitable by heat stress would dwarf
that affected by rising sea level," write Sherwood and co-author
<http://www.pnas.org/content/early/2010/04/26/0913352107>Matthew
Huber<http://web.ics.purdue.edu/~huberm/CDPL.html>of Purdue University
in Indiana. "Heat stress thus deserves more attention
as a climate-change impact."

How likely are we to reach such a point? Well, under business-as-usual
scenarios <http://www.pnas.org/content/early/2010/04/26/0913352107>the
current prediction <http://www.copenhagendiagnosis.com/> is for a 4 °C to 7
°C increase by 2100. In other words, in the worst-case scenario if we carry
on as we are, some of our children might just live to see small parts of the
world start to become too hot for human habitation.

In fact, the same limits would apply to all mammals, including livestock. If
that is true, mammals would have died out in large parts of the world during
past warm periods like the Palaeocene-Eocene Thermal Maximum 55 million
years ago. This may well have happened, say Sherwood and Huber - fossil
records of tropical regions are too poor to tell one way or the other.

The pair did not consider other warm-blooded animals.
<http://www.pnas.org/content/early/2010/04/26/0913352107>Birds have a body
temperature of 40
°C<http://www.newscientist.com/article/mg20126942.000-whats-the-point-of-being-warmblooded.html>(and
presumably the same was true of some dinosaurs), so in theory they
should be able to survive slightly higher wet-bulb
<http://www.pnas.org/content/early/2010/04/26/0913352107>

-----------------------------

http://www.pnas.org/content/early/2010/04/26/0913352107
An adaptability limit to climate change due to heat stress

   1. Steven C.
Sherwood<http://www.pnas.org/search?author1=Steven+C.+Sherwood&sortspec=date&submit=Submit>
   a <http://www.pnas.org/content/early/2010/04/26/0913352107#aff-1>,1<http://www.pnas.org/content/early/2010/04/26/0913352107#corresp-1>and
   2. Matthew Huber<http://www.pnas.org/search?author1=Matthew+Huber&sortspec=date&submit=Submit>
   b <http://www.pnas.org/content/early/2010/04/26/0913352107#aff-2>

+ <http://www.pnas.org/content/early/2010/04/26/0913352107#> Author
Affiliations

   1. aClimate Change Research Centre, University of New South Wales,
   Sydney, New South Wales 2052, Australia; and
   2. bPurdue Climate Change Research Center, Purdue University, West
   Lafayette, IN 47907

   1.

   Edited by Kerry A. Emanuel, Massachusetts Institute of Technology,
   Cambridge, MA, and approved March 24, 2010 (received for review November 19,
   2009)

 Abstract

Despite the uncertainty in future climate-change impacts, it is often
assumed that humans would be able to adapt to any possible warming. Here we
argue that heat stress imposes a robust upper limit to such adaptation. Peak
heat stress, quantified by the wet-bulb temperature *T**W*, is surprisingly
similar across diverse climates today. *T**W* never exceeds 31 °C. Any
exceedence of 35 °C for extended periods should induce hyperthermia in
humans and other mammals, as dissipation of metabolic heat becomes
impossible. While this never happens now, it would begin to occur with
global-mean warming of about 7 °C, calling the habitability of some regions
into question. With 11–12 °C warming, such regions would spread to encompass
the majority of the human population as currently distributed. Eventual
warmings of 12 °C are possible from fossil fuel burning. One implication is
that recent estimates of the costs of unmitigated climate change are too low
unless the range of possible warming can somehow be narrowed. Heat stress
also may help explain trends in the mammalian fossil record.

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