The Temperature Threshold We Don’t Want to Reach
What Does 2°C Mean in Terms of Human Survivability?
Climate change models foretell of rising sea levels and large-scale coastal inundations. Many of us have seen maps showing the predicted extent of that future calamity. Thankfully, policymakers and regional leaders in some places have started to discuss possible plans for dealing with these problems.
Climate models also predict that atmospheric anomalies will bring severe storms of greater intensity than normal. This is already beginning to happen. Almost all of us can provide first-hand testimony to this, wherever we live.
Beyond coastal inundations and severe storms, we’ve read about species extinction, loss of biodiversity, the permafrost “methane time bomb”, the age of mega wildfires, and other impending ecological disasters.
But there’s another more direct concern that’s received less attention. It’s the direct question about the temperature rise itself, and its impact on humans. That is, will Earth ever become too hot for human habitation? Should policymakers and regional leaders begin to make plans for that eventuality too?
At first glance, this idea seems far-fetched. After all, even a global temperature rise of 2°C would mean that a pre-industrial (c. 1880) warm summer afternoon temperature of 35°C would only become a slightly warmer 37°C at some point in the future. True, but closer examination reveals the fallacy of that thinking. For starters, the Intergovernmental Panel on Climate Change states that stronger than average warming will occur at mid-latitudes during the warm season.
To explore what a global 2°C temperature increase would mean to human survivability, let’s review the basics of human physiology and heat.
Core Body Temperature
Modern medicine has pegged the human body’s basal temperature to be around 37°C (give or take about 0.5°C). Temperatures higher than this are often triggered by the body itself in response to an infection or illness. Temperatures lower than this are usually in response to prolonged exposure to cold air.
When the body’s core temperature rises, it begins to sweat and the heart begins to beat faster. At just 2°C above normal, the body experiences dizziness and delirium. Anything more than 4°C above normal is dangerous, leading to convulsions, cardio-respiratory collapse, and death. The Centers for Disease Control and Prevention explains the symptoms for heat stroke that should be monitored during extreme temperatures.
In the other direction, as the body’s core temperature drops, it responds with numbness, shivering, slower heartbeats, and hallucinations. Anything more than 5°C below normal causes breathing to become shallow and the brain to lose consciousness. Temperatures more than 7°C below normal put the body into a comatose state, with the heart beating arrhythmically, leading to respiratory arrest and death. The Cleveland Clinic explains the symptoms of hypothermia to watch out for.
All told, the extreme limits of human survivability is a core body temperature between 26°C and 44°C.
Air Temperature
Earth’s air temperature extremes are far outside the human body’s basal range. The lowest temperature ever recorded is -89°C in Antarctica (July 21, 1983). The highest temperature ever recorded is 54°C in Death Valley (July 9, 2021).
School-aged children learn that the polar regions are too cold, and the equatorial desert regions are too hot for humans to live. Every place else humans have adapted their shelters and their use of clothing to their habitat’s climate.
Places that are too cold require well-insulated structures, fuel for heating them, and clothing to stay warm. Places that are too hot require shaded structures, unrestricted movement of air, and water for evaporative cooling.
With these inventions, humans have taught themselves to live beyond the boundaries of naked and nomadic homo sapiens.
It is well understood how humans thrive under favorable weather conditions, and suffer under the extremes.
Wet Bulb Temperature
Physiologically, a skin temperature of 35°C is necessary to allow heat from the human body’s core to move outward. When air temperatures approach 35°C, our bodies respond by sweating. The evaporation of sweat draws heat away from our bodies, helping to keep our core body temperature at 37°C. This only works up to a point. Humans can endure air temperatures greater than 35°C, but only when the air is dry and evaporation can occur. When the air is humid, the effectiveness of sweat diminishes.
The correlation between temperature, humidity, and evaporation is measured using the wet-bulb (WB) temperature.
Here are the approximate air temperatures and relative humidity (RH) levels that correspond to a WB of 35°C under full sunshine and light wind conditions:
50°C — 5% RH
47°C — 10% RH
43°C — 20% RH
40°C — 30% RH
37°C — 40% RH
35°C — 50% RH
33°C — 60% RH
32°C — 70% RH
30°C — 80% RH
29°C — 90% RH
28°C — 100% RH
People enduring extreme heat can reduce their risk of heat-related problems by sitting in the shade and using a fan. But only up to these thresholds.
Dangerous Temperatures
The concern with climate change is that an increase of 2°C will push wet bulb temperatures above the 35°C threshold in parts of the world, making it physiologically impossible to live there. The related concern is that abnormally high temperatures might occur in otherwise habitable regions for short periods of time (say 24 to 96 hours), causing mass fatalities.
The hottest places on Earth are not at the highest risk. So while the African Sahel, the Australian outback, the Chihuahuan Desert of the American Southwest, and the Taklamakan Desert of central Asia may routinely reach temperatures in excess of 40°C, humans can survive there because the relative humidity is low and evaporation of sweat is still effective.
Instead, the places of greatest concern are hot cities where relative humidity is high. Using the Community Climate System Model for the year 2032, developed by the National Center for Atmospheric Research, the 11 cities predicted to have summertime wet-bulb temperatures dangerously close to the threshold are:
- In North America: Houston, Shreveport, Little Rock, and Memphis.
- In the Persian Gulf: Al-Mubarraz (Saudi Arabia) and Doha (Qatar).
- In Pakistan: Sukkur, Faisalabad, Rawalpindi, and Lahore.
- In India: Hyderabad.
We know that climate models can help us predict what will happen, but can’t predict exactly when and where. Regional planners everywhere should make plans for how to provide emergency social services when dangerously high temperatures reach their communities.
