Saturday round-up: Humanity’s upper warming limit

This map shows the maximum wet-bulb temperatures reached in a climate model from a high carbon dioxide emissions future climate scenario with a global-mean temperature 12 degrees Celsius warmer than 2007. The white land areas exceed the wet-bulb limit at which researchers calculated humans would experience a potentially lethal level of heat stress. Credit: Purdue University graphic/Matthew Huber

This map shows the maximum wet-bulb temperatures reached in a climate model from a high carbon dioxide emissions future climate scenario with a global-mean temperature 12 degrees Celsius warmer than 2007. The white land areas exceed the wet-bulb limit at which researchers calculated humans would experience a potentially lethal level of heat stress. Credit: Purdue University graphic/Matthew Huber

Global warming of 12°C would make most areas of the world uninhabitable, and burning all the fossil fuels on the planet could get us to this level. That’s what Steven Sherwood of the University of New South Wales, Australia, and Matthew Huber of Purdue University, USA, claim.

Writing in a paper published online in the Proceedings of the National Academy of Science this week, the scientists examine the heat stress that humans and mammals can bear. This is assessed through a measure known as wet bulb temperature, which combines the effect of heat and humidity. The world’s hottest areas normally have low humidity, referred to as “dry heat”, Huber explained. “When it is dry, we are able to cool our bodies through perspiration and can remain fairly comfortable,” he said.

They say that currently wet bulb temperatures on the planet never exceed 31°C, while humans and other mammals would die if exposed to levels above 35°C for extended periods. They predict that conventional temperature rises of 7°C would begin to see wet bulb temperatures above 35°C in some regions, while 11-12°C rises would force humans out of most areas where they currently live .

Such rises are not impossible if emissions of CO2 and other greenhouse gases that trap heat in the atmosphere continue. “Each doubling of carbon dioxide is expected to produce 1.9–4.5°C of warming”, Huber and Sherwood write, but note that even these estimated increases could be on the low side. “Because combustion of all available fossil fuels could produce 2.75 doublings of CO2 by 2300, even a 4.5°C sensitivity could eventually produce 12°C of warming.”

Conventional temperatures rises above 6°C are normally seen as potentially disastrous, but specific consequences can be hard to come by, Huber and Sherwood write. Heat stress is one such clear outcome, they say. “It is often assumed that humans would be able to adapt to any possible warming,” the scientists write. “We argue that heat stress imposes a robust upper limit to such adaptation.”

Although these findings are clear, are they trustworthy? Many people would question that, as shown by the survey highlighted by Sheila Jasanoff in this week’s issue of Science. The survey reports a 30% drop over one year in the percentage of British adults who believe climate change is “definitely” real. To help tackle this, the Harvard academic argues, climate scientists must look beyond the familiar questions of establishing trust between themselves and work harder on gaining acceptance from the wider community.

Scientific progress has relied on researchers being able to take findings at face value, Jasanoff says. To do this, scientific papers use “peer review”, where a few typically anonymous and independent scientists check the reliability of the research before it is published. “In earlier times, it was enough to build trust within a researcher’s community of scientific peers,” Jasanoff writes. “Today, the circle of stakeholders in science has grown incomparably larger. It is no longer enough to establish what counts as good science; it is equally important to address what science is good for and whom it benefits.”

Finally, this week Nature published a paper marking the 25th anniversary of the discovery of the ozone hole over Antarctica. Jonathan Shanklin, an author of the original 1985 Nature paper notes how the Montreal Protocol to phase out ozone-depleting CFCs was delivered just two years later.

It is unclear how the success of the Montreal Protocol could be duplicated in bringing a climate treaty into force,” he writes. “We now face a problem that requires difficult change, and so requires a new approach to convince people to take action.”

Worst case scenario sees combined effect of humidity and temperature making much of the world uninhabitable, plus where scientists should focus their efforts on gaining trust, and lessons to be learnt from the ozone hole 25 years on.

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4 Responses to “Saturday round-up: Humanity’s upper warming limit”

  1. What can we get out of the ozone hole, 25 years on? « Simple Climate Says:

    […] Saturday round-up: Humanity’s upper warming limit […]

  2. What price global warming? « Simple Climate Says:

    […] Saturday round-up: Humanity’s upper warming limit […]

  3. Air-stilling temperatures create heat, wind power threats « Simple Climate Says:

    […] On one hand, this is good news because it means the frequency of hurricanes and tropical storms will not rise. But on the other it will pose problems for humanity as it means we will be exposed to more dangerous combinations of heat and humidity, a threat underlined by Steven Sherwood from the University of New South Wales, Australia,  earlier this year on this blog. […]

  4. A picture of climate change is worth 1,000 words « Simple Climate Says:

    […] This image was first published in this blog entry. The importance of this kind of warming is demonstrated by a diagram in this blog entry explaining why coral bleaches with higher temperatures. Other adverse effects of temperature include the staggering number of budgies that died in a heatwave shown in this entry, and how lizard populations are changing shown in this entry, and areas where researchers predict humans will eventually not be able to live due to the heat, shown here. […]


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