Due to limits of human heat tolerance, much of Earth’s surface may not be habitable by 2300 if we continue to emit greenhouse gases at the current rate. Sherwood states that well-known threats like rising oceans and economic depressions are not nearly as serious as the potential heat that might make the world, thermally, partly or completely uninhabitable by humans.
Most heat on the planet is dry, and we can handle that, but we’re not adapted to surviving very humid heat — a wet-bulb temperature of over 95 F — for more than 6 hours, even if we’re resting in well-ventilated shade. Hot, humid heat leads to hyperthermia, heat stress, and eventually death. Heat stress is already a leading cause of fatalities.
What will happen when temperatures rise this much:
- 4 °C: would subject over half the world’s people to unprecedented heat
- 7 °C: some regions may become uninhabitable
- 10 °C: the amount of land that would become uninhabitable from heat stress is far more that what we’ll lose from rising sea levels
- 11-12 °C: would expand these regions to include most of today’s human population
It’s unlikely we’ll adapt with air-conditioning due to limited fossil fuels, nor would AC protect livestock or outside workers, and power failures would be life-threatening.
More information, summaries of Sherwood’s paper:
4 May 2010. Global Warming: Future Temperatures Could Exceed Livable Limits, Researchers Find. ScienceDaily.
McMichael, A., et al. 24 Mar 2010. Climate change: Heat, health, and longer horizons. Proceedings of the National Academies of Science.
Sherwood, S, et al. 24 Mar 2010.An adaptability limit to climate change due to heat stress. Proceedings of the National Academies of Science.
Details: why heat kills
The reason crowded indoor theaters get so hot is because everyone is radiating heat like a 100 Watt light-bulb. Normally this heat is carried away by sweating, heat conduction, and other radiative cooling. But when the air is very moist and hot, the second law of thermodynamics does not allow us to lose heat when the wet bulb temperature (TW) exceeds 95 °F for a long period.
We all have core body temperatures around 98.6 °F regardless of climate, and our skin is lightly cooler, about 95, so that metabolic heat is conducted to the skin. If our skin sustains temperatures above 98, then our core temperatures will rise even more, and once our core reaches about 108 F for any length of time, we’re likely to die of hyperthermia, no matter how acclimated and fit a person is.