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This story was originally published by the bulletin of atomic scientists and is reproduced here as part of the climatic table collaboration.
Last month, during a slow heat wave that smothered much of the United States, the Kansas Department of Health and Environment reported at least 2,000 head of cattle had died from heat stress. In 2021, as the Pacific Northwest sweltered under a dome of heat, More than 650,000 farm animals perished in British Columbia only. And in 2015, a deadly heat wave in India killed more than 17 million chickens.
Hot, humid conditions can cause mass heat deaths in animals, both in livestock and wildlife. These events will become more extensive, longer lasting and more damaging as the world warms, potentially threatening economies and ecosystems. While many studies have shown the impact of single events or gradual trends in heat stress in cattle, there is a Panglossian tendency among many who work in livestock to believe in the nearly infinite ability of modern farming and breeding practices to overcome the challenges induced by heat stress. .
Much of the warming that has occurred in places like the United States or Europe can be addressed by replicating key traits of variants from hot countries like India or North Africa. But as conditions of intense heat move beyond the upper ranges of temperatures experienced recently (over the last few thousand years) in North Africa, India, or South America, there will be limited genetic diversity to draw upon to prepare for these conditions. There are temperature ceilings that humans and mammals (and many other animals) cannot survive, if they are exceeded. What those boundaries are, and what happens when they are crossed, will have profound implications for agriculture and biodiversity in a warming world.
In 2010, Steven Sherwood and me published an article presenting the wet bulb temperature, a measure of damp heat stress, as a way to conceptualize an upper limit of survival of 35 degrees Celsius. Today very small regions cross this threshold for brief intervals. But with every degree of global warming, maximum wet-bulb temperatures will rise by almost 1 degree Celsius. With enough warming, more and more parts of the world will begin to cross this critical threshold of damp heat stress. Since then, this critical surviving wet-bulb temperature concept has spread far beyond its initial scope, and its application to human health appears almost daily in the newspapers and even in science fiction.
Unfortunately, this limit was always understood as an upper limit for perfectly hydrated, sweaty people living in the shade exposed to a strong wind. Real humans don’t behave this way all the time, if ever, and the real-world wet-bulb limit is in all probability substantially lower.
All mammals—indeed, all endotherms, or warm-blooded animals, to a greater or lesser degree—are subject to similar limits on temperature and humidity. Much of the best data exists for livestock, so more attention needs to be paid, but damp heat stress is an equal opportunity killer, and the well-being of entire species and ecosystems needs to be considered. And unlike many humans or even some livestock animals cared for by humans, wildlife do not have access to technologies like air conditioning to adapt to extraordinary temperatures.
When Sherwood and I We began work on this topic inspired by records of past “hothouse” climates over the past 90 million years, in which temperatures and humidity were mostly much higher than they are today. At a conference presenting these paleoclimate records, Sherwood asked, “Given there is substantial evidence that the world used to be much warmer, is there a thermal limit that applies to all warm-blooded animals that could have interbred? in the past? If so, could it intersect in the future, causing mass extinctions?
The standard mammal has a maximum sustainable internal body temperature of around 37 to 38 degrees Celsius, as long as the temperature of blood entering certain critical areas, such as near the base of the brain, is measured. All placental mammals have roughly the same maximum internal temperature, indicating that temperature is shared among mammals with a common ancestor. Internal temperatures can be lower, but not higher without causing injury or death. In fact, there is substantial evidence that placental mammals have the same internal body temperature for tens of millions of years or more.
Birds are a slightly different story. Birds are now widely known to be avian dinosaurs and are derived from a lineage of endothermic avian dinosaurs. Their maximum sustainable internal body temperatures are significantly higher (approximately 43 degrees Celsius), likely a necessary legacy of the hot, humid Mesozoic conditions in which they evolved.
The common maximum internal temperature is about 38 degrees Celsius for placental mammals or 43 degrees Celsius for birds, and is shared among the vast majority of genera and species within these groups, strongly suggesting that these are immutable boundaries. on human time scales. In other words, this is not something that humans or other animals can rapidly evolve from as conditions warrant. The mass death events in recent history described above are excellent examples of this.
In fact, most extant species have experienced strong evolutionary pressure to adapt to cooler “ice house” conditions during the prevailing glacial-interglacial cycles of the last 3 million years. The last time the climate was as warm as it will be in the next 50 to 100 years was 3 million years ago during the “warm” climate of the Pliocene. Projected warming for moderate emissions scenarios beyond 2100 pushes temperatures into a range not seen since the Miocene (23 to 25 million years ago). There is a real danger that in the near future, temperatures and corresponding humidity will increase to such an extent that wet bulb levels will extend beyond the range observed for at least 3 million years, or perhaps 15 million years.
There are good news. In climate model simulations, substantial warming (greater than 6 degrees Celsius) is required to create kill zones for endothermic mammals and birds over substantial areas. That’s not likely to happen even by the year 2300 under the most likely carbon emission scenarios. With 3 degrees Celsius of global warming, which current research indicates is the most likely future, most of the world’s terrestrial biosphere will avoid crossing the 35 degrees Celsius wet bulb limit for significant periods of time.
The bad news is that, as mentioned above, the 35 degree Celsius wet bulb is an upper limit for mammals, not a lower limit for survival, meaning that the limit may actually be reached sooner, with less warming. global. The lower limits of wet bulb temperature can be as low as 31 degrees Celsius for humans and other mammals. But in this lower range, the wet bulb may not be a useful or reliable metric, and less idealized and more case-specific calibrated metrics are likely to be more useful. These include the Wet Bulb Globe Temperature, a measure of heat stress in direct sunlight, or the Universal Thermal Climate Index, which takes into account ambient temperature, as well as humidity, wind and radiation, in the case of humans; or the temperature-humidity index in the case of animals.
Food systems involving livestock do not perform well in simulations of 3 degrees Celsius of global warming when using these livestock-specific metrics calibrated at modern animal tolerances. In the United States, just 3 degrees Celsius of warming conditions in the simulations tend to be warmer, when humidity is taken into account, than heat waves in North Africa today. These future heat waves could devastate US livestock production, if they don’t kill animals outright. Much more work is needed to understand how mammals and birds will fare globally, but more especially in tropical and subtropical regions. As noted above, the animals already tolerate intense heat conditions, but they also have less recent history with warmer and wetter than modern conditions, and thus may have a reduced ability to adapt to warmer conditions.
With 1 degree Celsius of global warming already assured, and several more that appear to be guaranteed by current policies, over the next century scientists will see just how much phenotypic genetic plasticity and adaptability there is. This raises the question of the degree to which humans can and should interfere to aid this process through more selective breeding or genetic modification.
As global warming moves animals out of the environmental envelope that has literally surrounded them for millions of years, they may not have the ability to adapt. The best solution, of course, would be to avoid this experiment altogether. That is within the realm of feasibility. Limiting warming to less than 3 degrees Celsius by drastically reducing fossil fuel use substantially reduces the area of the planet subject to dangerous damp heat stress conditions.
One can rely on natural or man-made shelters, or, in other words, move endotherm populations up or away from hot, humid regions, or build giant air-conditioned enclosures, to allow the animals to continue to survive. But, if humans are in the position to do this in a big way, that already means that the fight to preserve the major ecosystems that make our world beautiful has been lost. For the sake of all animals, including us, I hope we choose a more sensible path.