Last month , During a slow-moving heat wave that choked much of the United States, the Kansas Department of Health and Environment mentioned At least 2,000 head of cattle died due to heat stress. In 2021, when the Pacific Northwest was rising under a thermal dome, More than 650,000 farm animals have died in British Columbia lonliness. In 2015, a deadly heat wave broke out in India Killed more than 17 million chickens.
Hot and humid conditions can lead to huge losses of heat among animals – in livestock as well as wild animals. These events will become more comprehensive, longer-lasting and more harmful as the world warms, potentially threatening economies and ecosystems. While many studies have shown the effect of individual events or gradual trends in heat stress on livestock, there is a Panglossian tendency among many livestock workers to believe in the nearly unlimited power of modern agricultural and breeding practices to overcome heat stress. .
Much of the warming that has occurred in places like the United States or Europe can be handled by breeding in key traits from variables from warm countries like India or North Africa. But as high-temperature conditions move beyond the upper temperature ranges recently (over the past few thousand years) in North Africa, India or South America, there will be limited genetic diversity that can be relied upon to prepare for these conditions. There are temperature ceilings that humans and mammals (and many other animals) cannot survive, if breached. What these limits are, and what happens when they are exceeded, will have profound implications for agriculture and biodiversity in an increasingly warming world.
In 2010, Stephen Sherwood and me published Paper presenting wet bulb temperature, a measure of wet heat stress, as a way to visualize a 35 °C maximum survival rate. Today very small areas cross this threshold for short periods. But with each degree of global warming, the maximum temperatures of wet bulbs will increase by about 1°C. With sufficient warming, more and more of the world will begin to cross the critical humid heat stress threshold. Since then the concept of viable wet bulb temperature has spread greatly beyond its initial scope and its application to human health appears almost daily in newspapers and even in science fiction.
Unfortunately, this limit has always been understood to be the upper limit for people who are very sweaty, fully watered and who live in the shade exposed to strong winds. Real humans don’t act this way all the time, if ever, and the maximum wet bulbs in the real world would likely be much lower.
All mammals—in fact all endotherms, or warm-blooded animals, to a greater or lesser extent—are subject to similar temperature and humidity limits. There is a lot of the best data for livestock, so more attention needs to be paid there, but humid heat stress is an equal opportunity killer, and the well-being of species and entire ecosystems must be considered. And unlike many humans or even some livestock that humans care for, wildlife does not have access to technologies such as air conditioning to use to adapt to unusual temperatures.
When Sherwood and I Starting to work on this topic, we drew inspiration from records of past “greenhouse” climates over the past 90 million years, where temperatures and humidity were much higher than they are today. At a conference where I was presenting these palaeoclimatic records, Sherwood wondered, “Because there is strong evidence that the world was hotter, is there a temperature limit that applies to all warm-blooded animals that may have crossed in the past? If so, could it be possible that Bypassing it in the future, causing mass extinctions?”
The maximum sustainable internal body temperature for a standard mammal is around 37 to 38 degrees Celsius, as long as one measures the temperature of blood entering certain critical areas, such as near the base of the brain. All placental mammals possess approximately the same maximum internal temperature, which indicates that temperature is shared between mammals with a common ancestor. Indoor temperatures can be lower, but not higher without causing injury and death. In fact, there is strong evidence that placental mammals have had the same internal body temperature for tens of millions of years or more.
Birds are a slightly different story. It is now widely understood that birds are avian dinosaurs and are derived from the endothermic avian dinosaur lineage. Sustained maximum internal body temperatures are much higher (around 43 °C), and are likely a necessary legacy of the hot and humid Mesozoic conditions in which they evolved.
The combined maximum internal temperature is about 38 °C for placental mammals or 43 °C for birds, and is shared across the vast majority of genera and species within these groups, strongly suggesting that this is a firm boundary on human time scales. In other words, this is not something that humans or other animals can evolve from as quickly as circumstances warrant. The mass death events in recent history described above are great examples of this.
In fact, most species today have experienced strong evolutionary pressure to adapt to cooler “icehouse” conditions during the dominant interglacial glacial cycles over the past three 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” Pliocene. Projected warming for moderate emission scenarios beyond 2100 brings temperatures to a range not seen since the Miocene (23-25 million years ago). There is a real danger that the corresponding temperatures and humidity will increase in the near future to the point that wet bulb levels will exceed the range seen for at least 3 million years, or perhaps 15 million years.
But there is good news. In climate model simulations, significant warming (more than 6 °C) is required to create endothermic mammals and birds killer zones in large areas. This is unlikely to happen even by 2300 in the most likely carbon emissions scenarios. With a global warming of 3°C, which current research suggests is the most likely future, most of the world’s terrestrial biosphere will avoid crossing the 35°C wet-bulb limit for extended periods of time.
The bad news, as mentioned earlier, is that a 35°C wet bulb is the upper limit for mammals, not a lower limit for survival, which means that the limit may in fact be reached sooner, as global warming declines. Low limits for wet bulb temperature may be as low as 31°C for humans and other mammals. But in this lower range, a wet bulb may not be a useful or reliable gauge, and less ideal, more condition specific calibrated gauges are likely to be more useful. These include the Wet Globe Temperature, a measure of heat stress in direct sunlight, or the Global Thermal Climate Index, which represents ambient temperature as well as humidity, wind, and radiation, in the case of humans; Or the temperature and humidity indicator in the case of animals.
Diets that include livestock do not do well in simulations of 3°C of global warming when using these livestock measures calibrated to the tolerances of modern animals. In the US, only 3°C of warming conditions in simulations tend to be hotter – when humidity is taken into account – than North African heat waves today. Future heat waves could destroy livestock yields in the United States, if the animals are not killed immediately. There is a lot of work to be done to understand how mammals and birds will do globally, but especially in the tropics and subtropics. As noted above, the animals there are already tolerant of high heat conditions, but they also have a less recent history with warmer, wetter conditions than modern conditions, and so may have less adaptability to warmer conditions.
With one degree Celsius of global warming already confined, and many other factors seemingly guaranteed by current policies, over the next century scientists will see just how much adaptability and genetic plasticity there is. This raises the question of the degree to which humans can and should intervene to assist in this process through more selective breeding or genetic modification.
Since, due to global warming, animals are moving out of the ecosphere that has covered them for literally millions of years, it is quite possible that they will not have the ability to adapt. The best solution, of course, is to avoid this experience altogether. This is within the scope of feasibility. Limiting warming to less than 3°C by cutting fossil fuel use significantly reduces the area of the planet subject to humid heat stress conditions.
One might rely on natural or man-made shelters, or, in other words, move groups of heat sinks either up or away from hot wet areas, or build giant air-conditioned enclosures, in order to allow the animals to continue to survive. But, if humans are in a position to do this in a big way, then it really assumes that the fight to preserve the key ecosystems that make our world beautiful has been lost. For the sake of all animals, including us, I hope we choose a healthier path.
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