This week marked a grim record for Earth’s climate, as temperatures repeatedly hit — and passed — levels not seen in 125,000 years, according to government scientists.

For context, the last time things were this hot, the first modern humans were just beginning to leave Africa, and chips of sharpened flint represented the bleeding edge of high technology.

Science moves slowly, and it will be years before we learn the impacts of this new era of heat on urban infrastructure, public health, and the natural world.

But as temperatures have consistently climbed over the past decade in a punctuated procession of once-anomalous heat waves, scientists worldwide have worked in tandem to figure out ways to mitigate its impacts.

One overall conclusion of their work is that while heat waves are now inevitable — thanks to decades of human failure to slow the burning of fossil fuels — deaths from heat are not.

And the dramatic — and often tragic — impacts that extreme heat can inflict on the natural world are also largely optional: the result of short-term policy choices, not the implacable movements of the Earth’s natural systems.

Here are five things scientists learned in 2023 about heat waves’ insidious impacts — and how to head them off.

Beachgoers flock to the beach south of the pier in Huntington Beach, Calif., Friday, June 30, 2023, amid a heat wave. (Jeff Gritchen/The Orange County Register via AP)

Rising heat waves are sickening people — and workers in particular

Rising heat has been making people sick for more than a decade. Heat-related illness and injury increased steadily in Arizona, California and Nevada between 2011 and 2019.

This misery will soon have a great deal of company.

A fifth of the world will experience two month-long episodes of combined heat and drought per year by the end of the century if human society doesn’t burn dramatically less fossil fuels.

Some of the risks posed by heat are industry specific. Construction workers laboring in the heat faced higher risks of illness like heat stroke, kidney disease, anxiety, and depression.

To some extent, lifestyle adaptations can help. Acclimation matters a lot: those traveling to hot climates from more temperate ones face “an increasing threat” of heat related illness, which is part of why the federal government advises that new workers in hot climates slowly ease into their new conditions.

That’s not always possible, however. Exposure to pesticides made tropical farmworkers on non-organic farms more susceptible to heat waves, in part by slowing their bodies’ ability to adapt.

Extreme heat raises the risk of dying from stroke and heart disease, and may raise the risks of death from respiratory disease even higher.

Fetuses exposed during the second trimester to temperatures that were unusually high for a given area had lower birth weights.

A crew repairs a roof during a heat wave in Los Angeles, Wednesday, July 5, 2023. (AP Photo/Damian Dovarganes)

City design, not climate change, is the main contributor to dangerous heat

A 1 degree Celsius (1.8 degree Fahrenheit) increase in temperatures would lead to “substantial” impacts on human heat stress across American cities, according to a draft study in Nature Portfolio.

But much of this stress is only indirectly related to rising temperatures.

A study from the sweltering East Asian city of Macau, with a climate similar to that of the U.S. Gulf Coast, found that while the amount of “insolation” — how much solar radiation hits a city — and the amount of water were the biggest contributors to rising temperatures, they had little impact on the populace’s resilience to heat.

A much bigger impact came from a neighborhood’s ability to move water through it — porosity — and its vegetation.

Urban areas tend to run significantly hotter than the surrounding countryside, due to the tendency of asphalt and concrete to soak up, store and radiate heat.

Such heat islands are driving up temperatures even faster than climate change itself. A study of U.S. cities found that between 2001 and 2014, the rise in the forcing power of heat islands — which drove a temperature rise of about 1 to 3.5 degrees Fahrenheit — outpaced the rise in the heat index.

The stifling power of the heat island is dramatic: the cooling “lake effect” breezes that blow off Lake Michigan still aren’t enough to fully counter the heating impacts of the city’s massive heat island.

This greater urban heat dramatically heightens other risks.

A midday blackout during a heat wave (perhaps from a large storm) could double the mortality rate across an urban populace, according to a study in Environmental Science and Technology.

Kayak and canoe outfitter Jessie Fuentes walks along the Rio Grande under a warm sun Thursday, July 6, 2023. (AP Photo/Eric Gay)

Simple fixes can make big changes — even as temperatures rise

There’s a silver lining to the dangerous impacts of heat islands: simple interventions to break them up can do a great deal to make cities — cooler, safer and more pleasant, studies suggest. 

Conversely, weakening the effect of the heat island makes cities more resilient.

A study of 70 parks in Beijing found that they served as “cooling islands” that cut temperatures in surrounding neighborhoods. It also found that the cooling impact of the parks themselves can be amplified by paying attention to the kind of development that surrounded them.

A study of “cool roof” technology in Chicago — which reflects more light back into space, leading to less absorption into the heat island — found that the reflective roofs were the only roofing technology able to lead to significant cuts in street-level temperatures, according to Science of the Total Environment.

An experiment in Phoenix found that reflective pavement absorbed less heat than asphalt-covered roads, leading to lower temperatures — particularly at night when the asphalt would normally radiate much of that absorbed heat back into the air.

That provides a potentially life-saving respite, as night is a crucial time for the human body to recover from the day’s heat, and high nighttime temperatures are a major contributor to heat stress.

However, that neighborhood-level cool came at the cost of reflecting more of the sun’s heat onto passing pedestrians, leading to greater street-level risk during the hottest hours.

A fisherman reels in his catch as the sun rises over the Atlantic Ocean, Wednesday, June 28, 2023, in Bal Harbour, Fla., as a heat dome is spreading eastward from Texas. (AP Photo/Wilfredo Lee)

In unequal world of heat exposure, better planning saves lives

Studies published this year further confirmed that the cost of heat is not spread fairly.

In Africa — a continent that in 2021 released just 4 percent of world fossil fuel emissions, millions of additional people will be exposed to heat stress by the end of the century.

And a study of 911 calls in Austin, Texas, found that calls were most frequent in areas with elderly people, those living alone, Hispanics, and those on some form of government assistance.

Historical examples bear this out: those suffering from schizophrenia, chronic kidney disease and heart disease faced the highest risk of death from the extreme heat wave that hit British Columbia in 2021.

Other risks scientists found were more indirect, though no less serious: in Chicago, heat waves were found to have raised the risks of urban crime.

Because risks are spread so unevenly, some public health experts recommend triage.

A study from the dense, humid metropolis of Hong Kong — another city where a steamy climate intersects with a massive heat island — scientists writing in Science of The Total Environment suggested that planners and first responders should focus on the most impacted areas first.

The Hong Kong experts recommend that city officials first focus on mitigating heat in areas with the most intense heat island effects and vulnerable populations — which means poorer and more elderly communities.

Other studies from this year found an urgent need for such targeted interventions and of better coordination among public officials in meeting the threat of heat waves.

A February study in Public Health Reports found that a large percentage of the populace of 81 U.S. cities lacked access to cooling centers — public buildings where those without air conditioning can go to get their body temperature back to a safe level.

Those numbers ranged from 99.9 percent of the population lacking access in Atlanta to just 37 percent in (comparatively dense) Washington, D.C.

Another study in Applied Geography dug deep into who used cooling centers in Los Angeles during the 2017 heat wave and found that many city cooling centers were misplaced. 

The scientists found that vulnerable residents were less likely to use cooling centers further from public transit, and tended not to visit cooling centers in wealthier neighborhoods. 

People cruise along the path north of the pier in Huntington Beach, Calif., Friday, June 30, 2023. (Jeff Gritchen/The Orange County Register via AP)

Heat is hollowing out the natural world — but ambitious climate action can stop it

Late June featured a staggering week of broken records — including subsequent-day record temperatures measured for average ocean temperature and then for the surface of the North Atlantic specifically, according to Nature.

Evidence from the past decade of heat waves helps paint a scattershot picture of possible marine impacts from the current records.

The heat wave that baked Washington State between 2014 and 2016 disrupted the region’s rich coastal ecosystems, as warmer waters led to a 50 percent die-off in underwater kelp “forests.”

Though the kelp canopy quickly recovered, sea star populations did not, according to the study by scientists the National Atmospheric and Oceanic Administration (NOAA).

Faster heating was a likely predictor of greater “bleaching” among coral reefs, according to a Marine Ecology Progress Series study. Bleaching is a potentially lethal condition that occurs when the algae that give corals their bright colors — and food — “jump ship,” according to NOAA.

The 2014-2016 heat waves also seem to have contributed to declines in the number of Steller sea lion pups to survive to adulthood — likely because of its impact on the sleek predators’ favored prey.

Higher temperatures also led common guillemots — a Northern European seabird — to pant and pace, to spend less time with their breeding partners and even to temporarily abandon their nests.

Some animals have managed to make the heat work for them — like Europe’s reed warbler, which has taken advantage of the shortened winter to kick start the breeding season.

Other animals beat the heat by adapting what, in a human context, might be called their built environments.

The midday gerbils of China’s Mu Us desert countered the searing — and anomalous — heat with their heat-absorbing burrows, which led them to be “barely exposed to heat stress.”

But in general, these disruptions are only the beginning, a study in Nature suggested: rising temperatures will wreak havoc across the natural world if humans don’t keep to their climate commitments.

If human burning of fossil fuels isn’t reduced — leading to about 8 degrees Fahrenheit (4.4 C) in warming — 41 percent of all land vertebrates will face heat extremes above recorded levels over at least half of their ranges.

That breaks down to a quarter of birds, a third of mammals, and roughly half of amphibians and frogs.

This is an avoidable tragedy, the scientists emphasized. 

Intermediate cuts in emissions — enough to keep warming at about 5 degrees Fahrenheit (roughly 3 Celsius) would drop that top-line risk from 41 percent of species to 15 percent.

And dramatic emission cuts — enough to keep the world within the Paris Climate Accord target zone of about 3 degrees Fahrenheit (1.8 degrees Celsius) would mean only 6 percent of land-based vertebrates would face such exposure, and many species would avoid it entirely.

By contrast, a failure to cut our level of fossil fuel use will lead to a drastically impoverished world as “constant severe thermal stress” withers the biosphere, the Nature scientists wrote.

“Deep greenhouse gas emissions cuts are urgently needed to limit species’ exposure to thermal extremes,” they added.