Another global warming worry: Parts of Earth could become uninhabitable

When we think of the coming disasters of global warming, rising sea levels, disruptions to agriculture and disappearing species come readily to mind. We don’t necessarily think of the livability of the Earth’s surface. But if global warming continues to worsen — and every indication is that will be so — there will be places on Earth that could become uninhabitable.

Uninhabitable in the literal meaning of human beings not being able to survive there.

Such places could come into existence during this century, and perhaps sooner than even climate scientists currently fear, given that lethal combinations of heat and humidity have started to occur for brief periods of time. We are not talking about thinly populated or uninhabited desert locations. We are talking here of cities where tens and hundreds of thousands of people currently live.

Yes, one more reason for humanity to tackle global warming.

To understand why survivability could become impossible in small geographic regions in the foreseeable future and, potentially, much larger regions in the more distant future should current trends in global warming continue, we need to turn to an obscure meteorological measurement known as the “wet-bulb temperature.” This is different from the common air temperature, nor is it the same as the various versions of a “heat index” that provide a “feel like” temperature.

^{Dawes Glacier at the head of Endicott Arm in Alaska (photo by Sean White)}

The wet-bulb temperature is a representation of heat and humidity that measures the impact on the ability of human bodies to cool. A discussion of it by the American Association for the Advancement of Science explains it this way:

“It is so named because it is calculated by wrapping the bulb of a thermometer in a wet cloth. In low humidity, water will evaporate from the cloth, carrying away heat and cooling the thermometer in the same way sweat cools the human body. In these conditions, the wet-bulb temperature will be lower than the air temperature. In high humidity — when the air is more saturated with water vapor — the water cannot evaporate as easily so the cloth stays hot. If the wet cloth cannot cool below the air temperature, neither can human skin.”

Because human skin must be cooler than the body’s core in order for metabolic heat to be conducted to the skin, human skin temperature is strongly regulated at 35 degrees Celsius (95 degrees Fahrenheit). Thus a wet-bulb temperature at that level, should it be sustained, represents the upper limit of what a healthy human being can endure without dying from overheating. It is generally believed that six hours in such conditions, even with steadily drinking fluids and sitting in shade, would be fatal for even the healthiest person, and a sustained wet-bulb temperature a couple of degrees lower would be fatal for many, perhaps most, people.

Simply put, at 35 C/95 F, sweat would not evaporate and our bodies would not be able to regulate our internal temperature.

“When wet-bulb temperatures are extremely high, there is so much moisture in the air that sweating becomes ineffective at removing the body’s excess heat, like what happens in a steam room,” said Colin Raymond, the lead author of a 2020 study on the future habitability of the climate, in an interview published by the U.S. National Oceanic and Atmospheric Administration. “At some point, perhaps after six or more hours, this will lead to organ failure and death in the absence of access to artificial cooling.”

It’s the heat and the humidity

Can a combination of heat and humidity become so intense that a wet-bulb temperature of 35 degrees C (95 degrees F) — the point of effective universal lethality — be reached? Such levels have already been reached in a handful of places, albeit for only one or two hours. Wet-bulb temperatures approaching that lethal level are becoming more common — more than 250 occurrences of 33 degrees C (91 degrees F) have been recorded around the world since 1979.

But such high levels don’t have to be reached for death to occur. “Even at lower wet-bulb temperatures, like 79°F (26°C), those with pre-existing health conditions (like respiratory, cardiovascular, and renal disease), the elderly, as well as those performing strenuous outdoor labor and athletic activities, are at a high risk,” said Radley Horton, a co-author with Dr. Raymond of a 2020 academic study published in Science Advances that examined how high wet-bulb temperatures might get. The 2003 European heat wave caused more than 50,000 deaths at wet-bulb temperatures close to 26 degrees C.

The paper, authored by Dr. Raymond of the Jet Propulsion Laboratory, Dr. Horton of Columbia University and Tom Matthews of Loughborough University, found that “Climate models project the first 35°C [wet-bulb temperature] occurrences by the mid-21st century. However, a comprehensive evaluation of weather station data shows that some coastal subtropical locations have already reported a [wet-bulb temperature] of 35°C and that extreme humid heat overall has more than doubled in frequency since 1979.”

The three climate scientists believe that, under the “business-as-usual RCP8.5 emissions scenario” (a worst-case model in which fossil fuel use continues to increase in a world with ongoing high emissions), wet-bulb temperatures could regularly exceed 35 degrees C in parts of South Asia and the Middle East by the third quarter of the 21st century. They cite three other studies to back up this prediction. They write:

“Our findings indicate that reported occurrences of extreme [wet-bulb temperatures] have increased rapidly at weather stations and in reanalysis data over the last four decades and that parts of the subtropics are very close to the 35°C survivability limit, which has likely already been reached over both sea and land. These trends highlight the magnitude of the changes that have taken place as a result of the global warming to date. At the spatial scale of reanalysis, we project that [wet-bulb temperatures] will regularly exceed 35°C at land grid points with less than 2.5°C of [global] warming since preindustrial—a level that may be reached in the next several decades. According to our weather station analysis, emphasizing land grid points underplays the true risks of extreme [wet-bulb temperatures] along coastlines, which tends to occur when marine air masses are advected even slightly onshore. The southern Persian Gulf shoreline and northern South Asia are home to millions of people, situating them on the front lines of exposure to [wet-bulb temperatures] extremes at the edge of and outside the range of natural variability in which our physiology evolved.”

The limits to the human ability to withstand heat stress

A 2010 study published in PNAS (Proceedings of the National Academy of Sciences) in 2010 by climate scientists Steven C. Sherwood and Matthew Huber warned that the areas that may someday be subject to wet-bulb temperatures are currently inhabited by billions of people, in a worst-case scenario. Dr. Sherwood and Dr. Huber were writing before the Paris Climate Accord, and although the Accord remains inadequate to constrain global warming to 2 degrees C, much less the pact’s 1.5 C goal, it renders the worst-case scenarios less likely. But not impossible, given that a global temperature rise of more than 2 C would set off a cascade of events and feedback loops that are not possible to reasonably forecast.

Even if now somewhat less of a possibility than at the time of their writing, the potential disaster sketched out by Dr. Sherwood and Dr. Huber is frightening. Noting that “heat stress imposes a robust upper limit to adaptation,” they wrote:

“[E]xcedence of 35 °C … would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed. … If warmings of 10 °C were really to occur in the next three centuries, the area of land likely rendered uninhabitable by heat stress would dwarf that affected by rising sea level. Heat stress thus deserves more attention as a climate-change impact.”

Adding together the Paris Climate Accord goals, if fully implemented, and the efforts by institutions around the world to reduce carbon footprints, it might appear that humanity will avoid the worst-case scenarios. With further effort, those scenarios can be avoided. Nonetheless, it is far too early to breathe a sigh of relief. The emergence of large areas of Earth’s surface that become uninhabitable remains a possibility. The PNAS study said:

“Warming will not stop in 2100 if emissions continue. Each doubling of carbon dioxide is expected to produce 1.9–4.5 °C of warming at equilibrium, but this is poorly constrained on the high side and according to one new estimate has a 5% chance of exceeding 7.1 °C per doubling. Because combustion of all available fossil fuels could produce 2.75 doublings of CO₂ by 2300, even a 4.5 °C sensitivity could eventually produce 12 °C of warming. Degassing of various natural stores of methane and/or CO₂ in a warmer climate could increase warming further. Thus while central estimates of business-as-usual warming by 2100 are 3–4 °C, eventual warmings of 10 °C are quite feasible and even 20 °C is theoretically possible.”

Record heat around the world

The record heat reported around the world in recent months, even if not yet deadly in the absence of sufficiently high humidity, portends trouble. On January 13, the highest temperature ever recorded in the ocean-dominated Southern Hemisphere was reached in Onslow, Western Australia, at 50.7 degrees C (123.3 F). Three stations in Western Australia exceeded 50 degrees C that day; before that week, the entire nation of Australia had recorded only four 50 degree C days in recorded history, according to the Eye on the Storm blog. That same week, multiple stations in Argentina, Brazil and Uruguay neared or beat their all-time high temperatures.

The summer 2021 heat wave in British Columbia, Washington state and Oregon is said by some climate scientists to have been without precedent in meteorological records. The village of Lytton, British Columbia, set an all-time heat record for all of Canada three days in a row and then was destroyed by a wildfire on the fourth day. Portland set its all-time high temperature three days in a row. Seattle reached an all-time high on consecutive days and broke 100 degrees F (37.8 C) three days in a row; there had only been two 100-degree days in its history prior.

^{Lytton, British Columbia, before the wildfire (screen grab from Google maps)}

In his research, Bob Henson, a meteorologist then writing for Weather Underground, reported that 14 examples of 35 degree C wet-bulb readings that have already occurred since 1987 in Pakistan, Saudi Arabia and the United Arab Emirates. Ten of these have occurred since 2000. Six of the 14 occurrences were in one city, Jacobabad, Pakistan; five of these since 2005. Separately, my own study of an interactive map provided by the Columbia University Climate School found six locations where a 35 C/95 F wet-bulb reading had been recorded on at least one occasion. These are Sindh and Khyber Pakhtunkhwa provinces in Pakistan (specific cities not given, but Jacobabad is in Sindh); Hisar, India; Mecca, Saudi Arabia; Ras Al Khaimai, United Arab Emirates; and Yannarie, Western Australia.

There are locations in North America that have approached that level — Palm Springs, California, and multiple locations in Mexico along the Gulf of California have recorded wet-bulb readings of 33 C/91.4 F.

To give an idea of what conditions would achieve a 35 C/95 F wet bulb temperature, these combinations would be required:
• 105 F (40.6 C) & 67% humidity
• 110 F (43.3 C) & 56% humidity
• 115 F (46.1 C) & 46% humidity

Alarm bells continue to get louder, if we want to hear

Unfortunately, the possibility of future areas of uninhabitability isn’t an abstraction or alarmist. Even if all post-Paris promises made at the yearly global climate summits, including last November’s in Glasgow, were fulfilled, global warming would almost certainly go beyond 2 degrees C, and as we have been forced to repeatedly note, there are no enforcement mechanisms to ensure these pledges are met. Following the Glasgow summit (the 26th Conference of the Parties to the United Nations Framework Convention on Climate Change or COP26), Climate Action Tracker reported that full implementation of the goals set for 2030 would be enough for the world’s temperature to rise by 1.9 to 3 degrees by 2100. Worse, what the Tracker calls “real world action based on current polices” would result in a temperature increase of 2 to 3.7 degrees by 2100.

Not that any of this is somehow unknown. The Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), summarizing the knowledge of the world’s climate scientists, issued last summer, states, “many of the changes observed since the 1950s are unprecedented over decades to millennia. Updated paleoclimate evidence strengthens this assessment; over the past several decades, key indicators of the climate system are increasingly at levels unseen in centuries to millennia and are changing at rates unprecedented in at least the last 2000 years.”

The latest report from IPCC climate scientists, released to the public on February 27, said there is a “very high confidence” that global warming of 1.5 degrees C in the near term “would cause unavoidable increases in multiple climate hazards and present multiple risks to ecosystems and humans.” For the mid to long term (2041 to 2100), there is “high confidence” that “climate change will lead to numerous risks to natural and human systems” and “the magnitude and rate of climate change and associated risks depend strongly on near-term mitigation and adaptation actions.”

There are plenty of other warnings out there. For example, a widely cited 2015 study by the Stockholm Resilience Center, prepared by 18 scientists, found that the Earth is crossing several “planetary boundaries” that together will render the planet much less hospitable. Or that two scientific studies issued in 2015 suggest that so much carbon dioxide already has been thrown into the air that humanity may have already committed itself to a six-meter rise in sea level. Or that the oceans can’t continue to act as shock absorbers — heat accumulated in them is not permanently stored, but can be released back into the atmosphere, potentially providing significant feedback that would accelerate global warming.

Lurking in the background, and not often something that many wish to notice, is the role our world economic system plays in all this. Economic incentives under capitalism are for producing and consuming more, and capitalism can’t function without growth. As has been said so many times, you can’t have infinite growth on a finite planet, and even if taking resources from the rest of the solar system were to become financially viable — something unlikely to happen anytime soon no matter how much we might enjoy watching Star Trek — the solar system is finite as well. We can create a sustainable world economy and society, or nature will impose it on us. And the harshness of the latter will only be magnified by the vast number of refugees that runaway global warming will surely impose. We are part of nature, whether or not we wish to acknowledge that.

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