Future Temperatures Could Exceed Human Livability
All the focus on reducing climate change is based on what will happen this century, but what happens the century after if we fail?
According to new research findings, reasonable worse-cast scenarios for global warming if rising greenhouse gas emissions are not stamped out immediately could see Earth’s temperature exceed that which humans can tolerate in coming centuries.
Steven Sherwood, professor at the Climate Change research Centre at the University of New South Wales, and Matthew Huber, professor of earth and atmospheric science at Purdue University, will publish their findings in the May 6 issue of the Proceedings of the National Academy of Sciences.
Their research shows for the first time the highest tolerable “wet-bulb” temperature that humans can withstand and the fact that this temperature could be exceeded for the first time in human history if greenhouse gas emissions continue at their current rate of increase. The research shows that humans and most mammals will suffer a likely lethal level of heat stress at wet-bulb temperatures above 95 degrees Fahrenheit if sustained for six hours or more.
What is Wet-Bulb Temperature?
A wet-bulb temperature is a measurement of temperature that describes the lowest temperature that can be reached by the evaporation of water only. It is the equivalent to what is felt when wet skin is exposed to moving air and includes temperature and atmospheric humidity. The temperature is found by covering a standard temperature bulb with a wetted cloth and fully ventilating it.
“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,” said Sherwood. “Although we are very unlikely to reach such temperatures this century, they could happen in the next.”
The wet-bulb temperature estimates offer the upper limits on the ability of the human body to cool itself by sweating or other means. In order for the heat dissipation process to work effectively the surrounding air must be cooler than the skin, which itself must be cooler than the core body temperature. If the wet-bulb temperature is warmer than the skin temperature, metabolic heat cannot be released and can lead to potentially dangerous overheating depending on the magnitude and duration of the heat stress.
“Although areas of the world regularly see temperatures above 100 degrees, really high wet-bulb temperatures are rare,” said Huber. “This is because the hottest areas normally have low humidity, like the ‘dry heat’ referred to in Arizona. When it is dry, we are able to cool our bodies through perspiration and can remain fairly comfortable. The highest wet-bulb temperatures ever recorded were in places like Saudi Arabia near the coast where winds occasionally bring extremely hot, humid ocean air over hot land leading to unbearably stifling conditions, which fortunately are short-lived today.”
“Whole countries would intermittently be subject to severe heat stress requiring large-scale adaptation efforts,” Huber said. “One can imagine that such efforts, for example the wider adoption of air conditioning, would cause the power requirements to soar, and the affordability of such approaches is in question for much of the Third World that would bear the brunt of these impacts. In addition, the livestock on which we rely would still be exposed, and it would make any form of outside work hazardous.”
Future Global Warming and Wet-Bulb Temperature Limits
Sherwood and Huber’s work related to possible future warming beyond the end of the 21st century. The Intergovernmental Panel on Climate Change has estimated that warming by the end of this century could reach seven degrees Fahrenheit, Huber believes that an eventual warming of 25 degrees is feasible.
“We found that a warming of 12 degrees Fahrenheit would cause some areas of the world to surpass the wet-bulb temperature limit, and a 21-degree warming would put half of the world’s population in an uninhabitable environment,” Huber said. “When it comes to evaluating the risk of carbon emissions, such worst-case scenarios need to be taken into account. It’s the difference between a game of roulette and playing Russian roulette with a pistol. Sometimes the stakes are too high, even if there is only a small chance of losing.”
The National Science Foundation-funded research investigated the long-term implications of sustained greenhouse gas emissions on climate extremes. The team used climate models to compare the peak wet-bulb temperatures to the global temperatures for various climate simulations and found that the peak wet-bulb temperature rises approximately 1 degree Centigrade for every degree Centigrade increase in tropical mean temperature.
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