Deadly climate conditions could soon become the norm in most parts of the world, according to a systematic review.
If we do nothing to curb our greenhouse gas emissions, the authors who conducted the review predict between 44 and 75 percent of the human population will be chronically stressed by heat by the end of the century.
Growing temperatures in southern parts of Asia alone will impact an astonishing number of people. A fifth of the world's population lives around the Ganges and Indus Rivers, and in 80 years' time, these regions are expected to experience frequent deadly heatwaves higher than 35°C (95°F).
Such temperatures can be extremely harmful to humans, especially infants, the elderly, pregnant individuals, and those with comorbidities.
Nor is it just us that will suffer. The authors say livestock, poultry, crops, and other living organisms will experience surprisingly similar levels of heat stress, too.
In 2003, for instance, the European heatwave was estimated to have claimed the lives of 70,000 people across Europe and 24 percent of France's cattle. As it turns out, that was just an omen of what is to come.
"By the end of the century, 45 to 70 percent of the global land area could be affected by climate conditions in which humans cannot survive without technological support, such as air conditioning," says biological engineer Senthold Asseng from the Technical University of Munich (TUM) in Germany.
"Currently, it's 12 percent."
That's a major increase in a relatively short period of time - far too quick for most organisms to adapt to in the long run. Even in the short run, such temperatures can be deadly.
By 2080, cities in Australia could experience at least four times the number of deaths from increasing temperatures alone. In some areas of the tropics, researchers in 2019 predicted heatwaves could one day increase the mortality rate by up to 2,000 percent.
And that's just for humans. When the authors of the current review compared temperature thresholds for humans, livestock, poultry, crops, and some fish, they found surprisingly similar heat limits for all.
When humidity is high, humans are thought to be mildly heat-strained at about 23°C. For cattle and pigs, it's about 24°C.
On the other hand, when humidity is low, mild heat strain arrives at roughly 27°C for humans. While for cattle and pigs, the stress kicks in at 29°C.
"If people are exposed to temperatures above 32 degrees Celsius at extremely high humidity or above 45 degrees Celsius at extremely low humidity for a lengthy period of time, it can be fatal," says Asseng.
"During extreme heat events with temperatures far above 40 degrees Celsius, such as those currently being observed on the US Northwest Coast and in Canada, people require technical support, for example in the form of air-conditioned spaces."
Crops and livestock don't get that luxury. Even if heat exhaustion doesn't cause pigs, chickens, and cattle immediate death, frequent exposure is known to reduce their growth, leading to lower yields and stunted reproduction.
Chickens, for instance, reach severe heat stress at 37°C, at which point they begin to lay significantly fewer eggs.
When experiencing heat strain, dairy cows can actually produce up to 20 percent less milk.
Wildlife could be similarly affected, although that was beyond the scope of this particular review. However, the authors note that Australia's 2018 heatwave killed off a third of one bat population in just two days.
"Continued global warming will gradually become lethal to other species if they cannot avoid, migrate, or otherwise protect themselves from extreme or extended temperature stress," write the authors of the review.
"If the current trajectories towards a so-called hothouse earth continue, most creatures discussed in this Personal View, and many more, could be severely affected or could disappear completely."
The review was published in The Lancet Planetary Health.