But there is good news. In climate model simulations, significant warming (greater than 6 degrees Celsius) is required to create deadly zones for endothermic mammals and birds over wide areas. That won’t even happen until the year 2300 in the most likely CO2 emissions scenarios. With global warming reaching 3 degrees Celsius, which current research suggests is the most likely future, most of the Earth’s terrestrial biosphere will avoid exceeding the wet-bulb limit of 35 degrees Celsius for an extended period of time.
The bad news, as mentioned earlier, is that the 35 degree wet bulb is an upper limit for mammals, not a lower limit for survival, meaning that the limit could actually be reached sooner, with less global warming. Lower limits for wet-bulb temperature could be as low as 31 degrees Celsius for humans and other mammals. But at that low end, the wet bulb may not be a useful or reliable metric, and less idealized and more case-specifically calibrated metrics are likely to be more useful. These include the Wet Bulb Globe Temperature, a measure of heat stress when exposed to direct sunlight, or the Universal Thermal Climate Index, which in humans takes into account the ambient temperature as well as humidity, wind and radiation; or the temperature-humidity index in animals.
Livestock food systems do not perform well in 3 degree Celsius global warming simulations using these animal-specific metrics calibrated to modern animal tolerances. In the United States, just 3 degrees of warming in simulations tends to be hotter — when humidity is accounted for — than heat waves in North Africa today. These future heatwaves could devastate US livestock if they don’t kill the animals immediately. Much more work needs to be done to understand how mammals and birds will fare worldwide, but especially in tropical and subtropical regions. As mentioned above, animals there already tolerate high heat conditions, but they also have the least recent history of having warmer and wetter conditions than modern conditions and may therefore have a reduced ability to adapt to warmer conditions.