Scientists Estimate When Humans Could Become Extinct

The next mass extinction might be driven by extreme heat—and humans could be among the casualties.

Supercomputer climate models have found that in the next 250 million years, nearly all mammals may become extinct as the planet heats to unsurvivable levels, exacerbated by a new supercontinent forecast to form near the equator, according to a new study published in the journal Nature Geoscience.

Humans are also in the line of fire in this extinction scenario. However, we are more likely than other species to survive due to our technological advancements.

"If we only look at humans' natural ability to survive extreme heat (no technology allowed) then there are several heat stress thresholds that can't be crossed in general," Alexander Farnsworth, lead author of the paper and senior research associate at the University of Bristol in the U.K., told Newsweek.

"Exposure to wet-bulb temperatures (which considers heat and moisture) above 35 degrees C [95 degrees F] (this could be lower at 32 degrees C [89.6 F] recent research has suggested) for over six hours would be fatal (this even considers total inactivity, full shade, absence of clothing, and unlimited drinking water). Likewise, dry-bulb temperatures (what you measure of a thermometer) above 40 degrees C [104 F]. and low humidity for a sustained period of time is also lethal."

"If we factor in technology, we can survive thanks to building environmentally controlled shelters with air conditioning. But we would likely have to build other facilities to house food production as well," he said.

These intense temperature extremes—possibly between 104 to 158 degrees F—are predicted to occur due to increased carbon dioxide in the atmosphere, largely as a result of tectonic activity triggering volcanic eruptions, as well as due to the sun itself producing about 2.5 percent more radiation.

"In our study we show that global temperatures could be around 10-15 degrees Celsius [18-27 F] warmer than today and on just the land itself, it could be between 25-30 degrees Celsius [45-54 F] warmer on average than today," Farnsworth said.

The authors predict that this heat issue will become a major problem once the next supercontinent— Pangea Ultima— forms. Between 8 and 16 percent of land will be habitable for mammals. This is because the continent will be situated around the equator of the Earth, where the weather is hottest, as well as the CO2 thrown out by tectonic activity due to the shifting continents.

"We show that there are three main factors that drive to an extreme climate state that will render the Earth in 250 million years inhospitable. [Firstly,] without changing carbon dioxide concentrations in the atmosphere (kept at levels before the industrial revolution) and the sun's brightness (i.e. the amount of energy the sun emits) also at present-day levels and only changing where the continents and re-arranging them into a supercontinent we show that this alone raises land surface temperatures significantly, mainly due to most of the land surface being in the tropics now," Farnsworth explains.

"The sun is around 2.5 percent brighter in 250 million years, adding more energy incident to the Earth further warming the world. [Additionally,] the tectonic assemblage of this supercontinent creates more volcanic degassing that is emitted to the atmosphere (we predict around 600 ppm CO2 being the likeliest)."

Mammals are particularly at threat from these temperature increases, as we have evolved to withstand colder temperatures much better than we can handle warmer temperatures. According to the study, mammals have decreased our lower temperature limit over time, but our upper limit has stayed fairly constant, putting us at risk of hotter temperature extremes.

"As mammals generate their own body temperatures which are in general constant through thermoregulation, for instance for humans this is generally around 37 C [98.6 F]. When it gets hot we cool down by sweating. This is the body's reaction to the body overheating to dissipate the heat. In order for the heat dissipation process to work, the surrounding air must be cooler than the skin, which must be cooler than the core body temperature," Farnsworth said.

"The cooler skin is then able to absorb excess heat from the core and release it into the environment. If the temperature is warmer than the temperature of the skin, metabolic heat cannot be easily released and potentially dangerous overheating can ensue depending on the magnitude and duration of the heat stress. Over sustained periods this overheating can lead to heatstroke that can cause brain tissue or other vital organs to swell, resulting in permanent damage."

Additionally, extreme heat will destroy the habitat that many mammals rely upon, including our food sources.

"Plants, in general, do not like temperatures above 40 (there are some exceptions to this), unfortunately, much of the continent could experience temperatures in excess of this. Plants underpin the food pyramid, if you start to remove them over vast areas you stress the rest of the species (e.g. insects) who rely on them and subsequently the higher trophic level species that in turn rely on them as a food source."

This will likely impact a number of other classes of animals, but this study focused on mammals in particular.

"It is hard to say about the other species as we didn't focus on them in this study. However, what can be said is that this would not be a very hospitable world for many of them either."

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