An extreme heat event lasting 367 days touched nearly 85% of Earth’s surface during 2023–24, according to a Reuters investigation published Oct. 20. The news agency calls these sprawling events “heat storms,” a category it created for heat waves covering at least 1% of the planet at their peak.

That 85% figure describes the event’s cumulative reach over its lifetime. It does not mean that much of Earth experienced extreme heat simultaneously. The name is an analytical label, rather than an official meteorological classification.

The underlying danger, however, is well established. Scientific research documents heat events that persist longer, travel farther and expose communities to repeated stress. The consequences reach into hospitals, workplaces, classrooms and ecosystems.

The Intergovernmental Panel on Climate Change’s assessment concludes that hot extremes have become more frequent and intense across most land regions since the 1950s. Human-caused climate change is the main driver. It also finds that human influence has increased the likelihood of concurrent heat waves and droughts.

For communities confronting those conditions, the practical question extends beyond how high the thermometer climbs. It includes how long the heat stays—and whether people have somewhere safe to recover.

Extreme heat is lasting longer

A 2024 study published in Science Advances tracked geographically connected heat waves between 1979 and 2020. Researchers found that these events increasingly persisted longer, traveled greater distances and moved more slowly.

Those findings describe changes in the behavior of heat across space and time. A heat wave can move slowly while eventually reaching a large area because it survives longer.

The researchers identified human influence as the principal driver of several observed changes. They also recognized that natural climate variability and natural forcing contribute to changes in movement.

The study’s warning is consequential: continued greenhouse gas emissions could make these prolonged events increasingly damaging to human health and the environment. Planning around a brief spell of hot weather becomes less adequate as exposure stretches across more days and places.

More recent research examines another part of that problem: the expanding heat season. A study published in Nature Climate Change in September analyzed conditions from 1979 through 2023. It found trends toward earlier seasonal heat-wave onset, later endings and longer heat-wave seasons across much of the world’s land.

The trend toward longer seasons appeared across approximately 92% of global land, although it reached statistical significance across about 34%. That distinction matters: the evidence varies by region.

The authors argue that warning systems and adaptation plans should account for when heat arrives and how quickly it develops. A calendar built around familiar summer conditions can miss exposure arriving earlier or continuing later.

The damage accumulates inside the body

The World Health Organization explains that prolonged high daytime and nighttime temperatures create cumulative physical stress. The body must work to shed excess heat, placing additional strain on the heart and kidneys.

USA Herald’s coverage of dangerous heat domes across the United States examined how prolonged high temperatures threaten people working outdoors or living without reliable cooling.

Heat can worsen cardiovascular and respiratory conditions and contribute to acute kidney injury. Older people, outdoor workers and people with limited access to cooling face particular vulnerabilities.

The burden is substantial. A 2021 study in The Lancet Planetary Health estimated approximately 489,000 heat-related deaths annually worldwide during 2000–2019.

That is a modeled average for a historical period. It measures heat-associated mortality broadly, rather than deaths from named heat waves or deaths attributable exclusively to human-caused warming.

The distinction strengthens the reporting. Heat already carries a major health burden, and climate change is altering the conditions under which that burden develops.

WHO also warns that heat emergencies can undermine the services needed to respond. Power shortages can disrupt health facilities, transportation and water infrastructure. Communities can face rising medical needs while essential systems struggle to function.

Workers cannot always retreat indoors

For millions of people, avoiding the heat would mean abandoning the work that pays their bills.

The International Labour Organization’s assessment of heat at work estimated that roughly 71% of the global workforce experienced excessive heat exposure in 2020. Its findings cover indoor and outdoor working conditions.

The agency also estimated that excessive heat contributes to approximately 22.85 million occupational injuries and 18,970 deaths annually.

Those figures extend beyond officially identified heat waves. The report found that approximately eight in 10 occupational injuries associated with extreme heat occurred outside heat-wave periods.

That finding complicates any response built solely around emergency declarations. Dangerous working conditions can develop before a weather event receives a name or crosses an alert threshold.

The ILO calls for year-round protections and workplace heat plans. Its assessment examines how employers and governments can reduce exposure through occupational safety systems.

The implication is direct: a warning has limited value if a worker lacks the authority, resources or protection needed to act on it.

In the Amazon, the water offered no refuge

The consequences of persistent heat also extend into places that ordinarily provide relief from it.

A 2025 study published in Science documented extraordinary warming in Amazonian lakes during the region’s 2023 drought and heat wave. The event coincided with extensive mortality among fish and river dolphins.

Five of the 10 monitored lakes recorded daytime water temperatures exceeding 37 degrees Celsius, or 98.6 degrees Fahrenheit. One large lake reached approximately 41 C—105.8 F—throughout a water column roughly 2 meters deep.

That measurement matters because the heating extended beneath the surface. Moving deeper within that shallow water column would not necessarily provide a cooler refuge.

Researchers identified several interacting causes: strong solar radiation, reduced water depth, weaker winds and turbid water. The findings show how drought and heat can combine to create conditions more dangerous than either problem considered alone.

The authors warned that temperatures approaching or exceeding aquatic species’ thermal tolerances could become more common as warming continues.

These were measured environmental conditions during an actual disaster. The threat to freshwater ecosystems is already observable.

Scientists can identify the climate contribution

Establishing that an extreme event happened is one task. Determining how human-caused warming changed its likelihood or severity requires additional analysis.

A peer-reviewed study of the April–May 2024 Asian heat, published in Weather and Climate Extremes, examined several regions using observations and climate models.

For the Philippines, the researchers concluded that human influence made the analyzed event approximately 1.2 C hotter. An event of that severity would have been virtually impossible without human-caused climate change, they found. El Niño supplied a smaller additional warming contribution.

For the West Asian region studied, climate change made the event approximately five times more likely and around 1.7 C hotter.

The researchers also assessed conditions shaping the harm: limited cooling access, water scarcity, occupational exposure, agricultural dependence, conflict and displacement.

Those findings help explain why the same temperature does not produce the same consequences everywhere. Exposure and protection influence who becomes ill, whose work becomes unsafe and which essential activities are disrupted.

Climate attribution identifies the additional hazard. Social conditions help determine who bears it.

The definitions matter

Understanding these findings requires distinguishing unusual temperatures from specific health thresholds.

A foundational 2016 paper by Alistair Hobday and colleagues defined marine heat waves using temperatures above the local 90th percentile for at least five days, measured against a historical baseline.

That approach compares conditions with what is normal for a particular place and season. It allows researchers to identify unusual warmth across very different climates.

It does not establish that every location crossing a statistical threshold faces the same danger. A temperature anomaly, a human heat-stress measurement and an ecological tolerance limit answer different questions.

The global temperature record also requires careful interpretation. The European Centre for Medium-Range Weather Forecasts’ ERA5 dataset combines observations with physical modeling to estimate weather conditions consistently across the planet. It provides hourly information, including where direct observations are sparse.

That makes it valuable for studying changes over decades. Its estimates still have uncertainty and finite geographic resolution.

Researchers also scrutinize how the comparisons themselves are constructed. In “Pitfalls in diagnosing temperature extremes,” Lukas Brunner and Aiko Voigt showed that commonly used seasonal calculation methods can bias extreme-temperature estimates unless corrected.

These details determine what a dramatic number actually measures. Careful definitions allow the evidence to carry the story without exaggeration.

Protection can save lives

The research also shows that the death toll is not fixed.

A Nature Medicine study of European heat mortality in 2023 estimated approximately 47,690 heat-related deaths across 35 countries. Researchers analyzed temperature and mortality records from 823 regions.

They also estimated that the toll would have been approximately 80% higher without the societal adaptation achieved since 2000.

That finding should be read precisely. It does not mean adaptation eliminated 80% of deaths or that any single intervention produced the result. It indicates that changing vulnerability substantially influenced the outcome.

Tens of thousands of estimated deaths remained. Protection had improved, but it was still insufficient.

Taken together, the findings create concrete tests for public policy. Can residents access cooling? Take protective breaks? Are hospitals able to keep operating during a power disruption? Do warning systems account for prolonged exposure and dangerous nights?

Governments and employers can measure those protections, fund them and make them accessible before an emergency.

The scientific record establishes that extreme heat is changing. Whether communities can withstand it depends partly on decisions made before the next warning arrives.