You might puzzle over why scientists are so worried over a few degrees of global heating. After all, over the seasons we experience far more temperature variation—from frigid winters to scorching summers. But seemingly small global temperature changes can trigger debilitating, even lethal, local extremes. The effects of such extreme temperatures were succinctly summarized by environmental economists Tamma Carleton and Solomon Hsiang:
Heat induces mortality, has lasting impact on fetuses and infants, and incites aggression and violence while lowering human productivity. High temperatures also damage crops, inflate electricity demand, and may trigger population movements within and across national borders.
Average global temperatures have continued to set modern records. As of this writing, summer 2020 was the hottest year on record for the Northern Hemisphere, according to the US National Oceanic and Atmospheric Administration (NOAA). And the decade 2009-2018 was the warmest on record globally, according to the European Environmental Agency. More broadly, annual global temperatures are the hottest in 12,000 years, researchers found when they reconstructed ancient temperatures from marine sediment cores.
The vicious heat waves spawned by such temperatures are exemplified by those in the summer of 2022, in which records were set in Europe, Africa, and Asia. And record heat waves and high temperatures are continuing.
Mind-blowing future heat
Multiple studies have predicted an enormous future rise in lethal heating—“not only more intense extremes but also events that break previous records by much larger margins,” found one study. The study predicted many-fold increases—compared with the last three decades—in the probability of record extremes over the next five decades.
By 2050, even if the 2°C limit is met, dangerous heat index levels will increase up to 100% across much of the tropics, found another study. The heat index measures the combined effects of temperature and humidity and reflects heat’s impact on the body. Such heating will increase by 3 to 10 times in the temperate zones, such as the US and Europe. By 2100, tropical and subtropical countries will experience dangerous heating during most days, and “the kinds of deadly heat waves that have been rarities in the midlatitudes will become annual occurrences,” found the study.
By 2070, up to 3.5 billion people could live in heat zones so extreme as to be unsuitable for habitation, found a study that analyzed data on demographics, land use, and climate.
Even with drastic reductions in greenhouse gas emissions, by 2100 48% of the global population might be exposed to lethal heat for at least 20 days a year, versus about 30% today, concluded a study led by biogeographer Camilo Mora. With increasing emissions, the percentage of lethal heat days might reach 74%. 
“Finding so many cases of heat-related deaths was mind blowing, especially as they often don’t get much attention because they last for just a few days and then people moved on,” Mora told The Guardian.
In the US, a 3°C warming would produce about 15 times as many high-temperature records as low records by century’s end, researchers have calculated. By contrast, the current average ratio is about twice as many highs as lows, they found.
An “extreme heat belt” will emerge in the US, stretching from Northern Texas to Illinois, found one modeling study. By 2053, that region will experience days with a heat index above 125°F. Overall, in the US, the population exposed to extreme heat will rise from 8 million today to 107 million over the next three decades, found the report.
Lethal heat waves will have broad economic impacts, said Elfatih Eltahir, co-author of a paper forecasting heat waves in South Asia:
“With the disruption to the agricultural production, it doesn’t need to be the heat wave itself that kills people. Production will go down, so potentially everyone will suffer,” he said (see “Heat robs labor” below).
Some regions, such as parts of Africa, will experience lethal heat waves sooner than others. African heat waves have become more frequent over the past decades, according to one analysis of temperature records. A team of geophysicists projected that at the current rate of temperature rise, by 2040, African heat waves that are unusual today could occur every season and that heat and drought will render some regions uninhabitable.
Another analysis emphasized that while all regions will face new temperature extremes, “the impacts, in terms of frequency of heat extremes, will become significantly worse for poorer nations when compared with their wealthier counterparts.”
Summers worldwide will become steadily hotter if the current pace of CO2 emission continues unabated, noted an analysis by climatologists. Their modeling revealed that, with unabated heating, global summer temperatures would have an 80% chance of breaking a record each year between 2061 and 2080. 
Lethal heat + humidity
The combination of high temperatures and high humidity will be particularly deadly. High humidity can render otherwise tolerable temperatures lethal because the body can no longer cool itself through perspiration, reaching what’s called “wet-bulb temperatures.” In his interview with The Guardian cited above, Mora described the physiological impact of high heat and humidity:
Your sweat doesn’t evaporate if it is very humid, so heat accumulates in your body instead. People can then suffer heat toxicity, which is like sunburn on the inside of your body. The blood rushes to the skin to cool you down so there’s less blood going to the organs. A common killer is when the lining of your gut breaks down and leaks toxins into the rest of your body.
Populations exposed to dangerous wet-bulb temperatures increase enormously with higher global temperatures. One analysis found that the global population exposed to at least one day of lethal heat/humidity:
Brief waves of potentially lethal heat/humidity are already occurring in areas of South Asia, the coastal Middle East, and the coastal Mexican southwest, found an analysis of weather station data. The number of such events more than doubled between 1979 and 2017, found the analysis.
More than a third of recent heat-related deaths can already be attributed to global heating, found an analysis of records during 1991-2018. The study covered 43 countries and found dozens to hundreds of deaths each year evident on every continent.
Future deadly heat waves affecting billions of people will occur throughout Pakistan, Nepal, India, Bangladesh, and Sri Lanka—constituting one-fifth of the world’s population—predicted another study of heat/humidity.
Future temperatures, future heat waves
In a future world of a 4°C increase, summers in China and the eastern US would include extreme humid heat waves of 55°C (131°F), found an analysis by ecologists. These high-humidity heat waves would trigger heat strokes in people exposed to them. Such heat waves would be expected as often as every other year in a 4°C world, the scientists said.
In a future 7°C world, frequent heat waves would cripple humans’ very ability to survive and render some regions uninhabitable.
An 11-12°C warming—possible if all fossil fuels are burned—would spread lethal temperatures to most human-populated regions, concluded researchers Steven Sherwood and Matthew Huber. 
They pointed out that their assumptions likely overestimate survivability: “Our limit applies to a person out of the sun, in gale-force winds, doused with water, wearing no clothing, and not working,” they wrote.
In the US, more than 1,300 people die every year from heat waves today, according to a statistical estimate cited by the report The Impacts of Climate Change on Human Health in the United States. Climate models project that the number will rise to about 27,000 a year by 2100 for 209 US cities, according to the report. In addition, increased heat is projected to worsen respiratory, hormonal, urinary, genital, and renal problems. Heat can also kill indirectly. Extreme heat will cause more wildfires and airborne allergens, which will contribute to premature deaths, the report pointed out.
Climate is the culprit
Extreme temperatures have clearly been linked to climate disruption. Record global temperature increases and Asian and Alaskan heat waves would not have happened in a preindustrial climate, according to an American Meteorological Society report on extreme events. The report went beyond previous assessments that climate disruption had only increased the odds and intensity of such events.
The report noted that climate scientists had long predicted that global heating “would at some point become sufficiently strong and emergent to push an extreme event beyond the bounds of natural variability alone . . . . It is striking how quickly we are now starting to see such results.”
Both heat extremes and heavy rainfall have increased due to even the relatively small global heating since the Industrial Revolution, found a study by climatologists Erich Fischer and Reto Knutti. Their modeling compared the chances of extreme temperatures from preindustrial times to today—the difference being attributable to global heating. Their analysis indicated that 75% of extremely hot days could be attributed to global heating.
“People can argue that we had these kinds of extremes well before human influence on the climate . . . And that’s correct. But the odds have changed, and we get more of them,” Fischer told The New York Times.
Ominously, their analysis indicated that the effect of global heating on hot extremes is “nonlinear.” That is, as global temperature rises linearly, hot extremes will soar. For example, they forecast that a 5°C global heating would cause a 62-fold increase in extreme temperatures.
Global heating’s influence on record heat waves, downpours, and droughts was also explored using historical data and computer models by climatologist Noah Diffenbaugh and colleagues. Their analysis revealed a clear signature of global heating in more than 80% of record heat events, as well as increasing the probability of the driest and wettest events.
“This suggests that the world isn’t yet at a place where every single record-setting hot event has a human fingerprint, but we are getting close to that point,” Diffenbaugh told The Washington Post.
Heat bakes cities
Heat stress will be higher in cities—home to more than half the world’s population—due to the “urban heat island” effect, in which pavement and buildings soak up more heat than does the countryside.
One analysis of temperature data found an “alarming” trend in heat wave temperatures in large cities since 1966. Analyzing temperature data from about 9,000 stations globally, they found an accelerating rise during heat waves over the last three decades, with the largest rise in megacities such as Houston, Moscow, Paris, and Beijing.
Another study of temperatures in 13,115 cities found a nearly 200% increase in extreme heat exposure from 1983 to 2016. The heat conditions affected some 1.7 billion people, found the researchers, who said their results “suggest that previous research underestimates extreme heat exposure” in cities. Climatologist Robert Wilby explained this underestimation:
Temperature projections from global climate models are typically for wild or agricultural landscapes . . . . These projections are far removed from the conditions that will be encountered on city streets, inside workplaces, public spaces, and our homes. But these are the places where health, comfort, and productivity will be decided during the more intense heat waves that climate change will bring.
Wilby’s research on the urban impacts of a 4°C global warming found that “high indoor temperatures with high humidity could become unbearable—even deadly—for millions. . . . By 2050, 68% of humanity may live in urban areas and populations in the tropics will be most exposed to extreme humid heat.”
A forecast of future US heat trends found that by mid-century cities will suffer more than double the number of days with heat index temperatures above 100°F. The number of days per year above 105 degrees will triple. Another US analysis by the Union of Concerned Scientists concluded that “failing to reduce heat-trapping emissions would lead to a staggering expansion of dangerous heat.”
Global heating will shift cities’ climates to make them resemble those in warmer regions. By 2050, cities in the Northern Hemisphere will resemble those some 1,000 kilometers (600 miles) closer to the equator, the researchers found. So, London will resemble Barcelona; Portland will resemble San Antonio; Seattle will resemble San Francisco; and San Francisco will resemble Lisbon. In fact, found the study, one-fifth of the cities, mostly in the tropics, will experience climates that do not even exist on Earth today.
In the US, combined higher temperatures and increased population will produce a huge rise in the number of “person-days” of heat exposure, one study suggested. 
“We find that US population exposure to extreme heat increases four- to six-fold over observed levels in the late twentieth century, and that changes in population are as important as changes in climate in driving this outcome,” they concluded.
Heat robs labor
Heat cripples the economy by reducing labor capacity, since outdoor work becomes difficult or even life-threatening. Losses in global labor capacity will double from 10% to 20% by 2050, predicted researchers who used occupational health and safety thresholds and climate projects to arrive at their estimate. Such workers as firefighters, farmers, construction workers, and factory workers will be forced to slow their pace due to heat stress. Said co-author John Dunne:
Most studies of the direct impact of global warming on humans have focused on mortality under either extreme weather events or theoretical physiological limits. We wanted instead to describe climate warming in practical terms that people commonly experience already.
Excess heat will cause global heat-related productivity losses of $2.5 trillion by 2030, concluded a report by the UN Development Programme.
In a review of research on temperature and human performance, environmental epidemiologists led by Tord Kjellstrom concluded that:
Later this century, many among the four billion people who live in hot areas worldwide will experience significantly reduced work capacity owing to climate change. In some areas, 30–40% of annual daylight hours will become too hot for work to be carried out. The social and economic impacts will be considerable, with Gross Domestic Product product (GDP) losses greater than 20% by 2100.
Researchers in another study of productivity and temperature wrote that “unmitigated warming is expected to reshape the global economy by reducing average global incomes roughly 23% by 2100 and widening global income inequality, relative to scenarios without climate change.”
The poorest countries will suffer more, the researchers concluded, with their average income plummeting by 75% by 2100. The figure is likely low, because the analysis included only temperature effects, and not sources of climate-related economic loss such as hurricanes and sea-level rise.
For the US, each degree Celsius of higher temperature between 1981 and 2010 cost about 1.2% of gross domestic product, found another study. And, this cost was geographically unequal, with a transfer of value northward and westward. By late in this century, the researchers calculated, the poorest third of US counties would experience economic damage between 2% and 20% of income.
Heat erodes mental health
Mental stresses due to heat will also threaten survival. Extreme heat can worsen psychiatric symptoms and, according to an American Psychiatric Association statement, has been linked to:
. . . increases in irritability and symptoms of depression and with an increase in suicide. It can also affect behavior, contributing to increased aggression, incidence of domestic violence, and increased use of alcohol or other substances to cope with stress.
Statistics also seem to bear out the theory that high temperatures make people more hostile, which can lead to violence. When sociologists analyzed the crime rate in 57 countries, they discovered that each degree Celsius of temperature increase was associated with an approximately 6% average increase in homicides.
And when environmental economist Matthew Ranson analyzed 30 years of crime and temperature data from nearly 3,000 US counties, he came up with a sobering forecast of the effects of higher temperatures:
Between 2010 and 2099, climate change will cause an additional 22,000 murders, 180,000 cases of rape, 1.2 million aggravated assaults, 2.3 million simple assaults, 260,000 robberies, 1.3 million burglaries, 2.2 million cases of larceny, and 580,000 cases of vehicle theft in the United States.
So, heat not only kills. It may make us killers.
Heat begets more CO2
In a vicious circle, rising temperatures also increase air conditioning demand, requiring more electricity, which generates more CO2.
And in a human vicious circle, more air conditioning makes people less heat tolerant, increasing demand for air conditioning. What’s more, air conditioners become less efficient as temperatures rise because they are less able to shed heat to the outdoors. Thus, hotter temperatures will feed back to increase cooling and electricity load.
The impact of such increasing demand will be stunning. Future global energy demand for air conditioning could dwarf that of the US, which now expends more energy for air conditioning than all other countries combined. Countries outside the US have the potential to use 50 times more energy for cooling than the US because of rising temperatures and incomes, calculated researcher Michael Sivak.
In some countries extreme heat will require a massive increase in air conditioning, just for people to survive, found one analysis. By 2050, the electricity needed for cooling in countries such as India and Indonesia could reach 75% of the total demand, overwhelming power grids.
An International Energy Agency analysis found that “without action to address energy efficiency, energy demand for space cooling will more than triple by 2050—consuming as much electricity as all of China and India today.”
In the US, electricity demand for air conditioning could require additional peak generating capacity costing up to $180 billion, found one analysis by environmental economists.
Higher demand could increase global energy needs by up to 58% by 2050, found one computer modeling study. Decreased heating energy demand would be far more than offset by increased cooling energy demand with predicted global temperature rise, the study found.
All these impacts of rising temperatures will continue unabated, driven by the unconstrained momentum of the carbon contagion.
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