Scientists say a mix of long-term warming and regional weather quirks is turning ordinary summer heat into dangerous, record-breaking extremes
Extreme heat has become a near-annual headline on both sides of the Atlantic. The Northeastern United States, eastern Canada, and Western Europe have each endured punishing, prolonged heat waves in recent months — the kind that once would have been considered once-in-a-generation events. Climate researchers say that’s no accident, and that a specific combination of forces is converging to make these heat waves more frequent, more intense, and more dangerous than anything in the historical record.
The findings from World Weather Attribution (WWA) suggest Europe is entering a period in which exceptional heat is becoming less exceptional. The continent already records more deaths from heatwaves than from floods, storms, wildfires and all other weather-related hazards combined. An ageing population, rising rates of chronic illness and unequal access to cooling are leaving millions more vulnerable as periods of extreme heat become more frequent.
The study estimates that the June 2026 heatwave would have been about 3.5 degrees Celsius cooler during the day had it occurred under the climate conditions of 1976, one of Europe’s and US benchmark hot summers. Even compared with the notorious 2003 heatwave, daytime temperatures were roughly 2 degrees higher than they would have been in the climate that existed at the time.
Yet, North America and Europe not insulated against what happens in the rest of the world.
Globally, several blistering heatwaves are likely across the world now and in the coming months, linked to strengthening El Niño conditions in the tropical Pacific, the UN World Meteorological Organization said on Friday.
“El Niño will also give an extra boost to global temperatures,” said WMO scientist Alvaro Silva. “We know that during El Niño years, the global temperatures normally reach record levels.” The WMO classifies El Niño and La Niña events as weak, moderate, strong or very strong.
“The intensity of El Niño is important because it increases the likelihood of extreme weather and climate events in different parts of the world…on top of long-term climate change due to human activities,” Silva noted.
Based on findings of several recent studies, main drivers behind this weather pattern can be explained this way:
The Baseline: More Greenhouse Gases, More Trapped Heat
The foundational cause is simple physics. For more than two centuries, burning coal, oil, and natural gas has released carbon dioxide and other gases into the atmosphere. These gases trap heat that would otherwise radiate out into space, gradually raising the planet’s average temperature.
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That might sound like a modest shift, but it changes the entire range of what’s possible on any given day. A warmer atmospheric baseline means that when a heat wave does form, it starts from a higher floor and can reach far higher peaks than it would have a century ago. Scientists who study this call it “loading the dice” — the same weather pattern that once produced a hot week now produces a dangerous one.
The researchers also assessed heat stress using the Wet Bulb Globe Temperature (WBGT), an internationally recognised measure that combines heat and humidity. During the June heatwave, nearly 45 per cent of the European cities included in the study exceeded indoor WBGT thresholds associated with potentially hazardous conditions, even for healthy people exposed over long periods.
A Weakening, Wobblier Jet Stream
The jet stream — the fast-moving band of air that steers weather systems across North America and Europe — depends on the temperature difference between the Arctic and the equator. Because the Arctic is warming roughly three to four times faster than the rest of the planet, that temperature gradient is shrinking.
Many atmospheric scientists believe this is making the jet stream slower and wavier. Instead of pushing weather systems through in a couple of days, a weakened jet stream can allow high-pressure systems — often called “heat domes” — to stall in place for a week or more, trapping hot air over the same region day after day. This mechanism has been linked to several of the most severe heat waves in both Europe and North America over the past decade.
Dried-Out Soil Feeds the Heat
Drought and heat reinforce each other in a feedback loop. When soil is moist, some of the sun’s energy goes into evaporating that moisture, which has a cooling effect on the surrounding air. When soil is dry — as it increasingly is across parts of the U.S. Southwest, the Great Plains, and southern and central Europe — there’s no moisture left to evaporate, so nearly all of that solar energy goes directly into heating the air instead.
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This means regions already dealing with drought are primed for even more extreme heat spikes, and the heat itself further dries out the soil, extending and worsening the drought. Researchers have identified this land-atmosphere feedback as a major amplifier in several of Europe’s worst recent heat waves.
Oceans Are Storing — and Releasing — Enormous Amounts of Heat
More than 90 percent of the extra heat trapped by greenhouse gases over the past several decades has gone into the oceans. That stored energy doesn’t stay put. Marine heat waves — unusually warm patches of ocean water — have become more frequent in the North Atlantic and Mediterranean, and they influence the air masses that move over nearby land.
Warmer oceans mean more moisture evaporates into the atmosphere, which raises humidity levels during heat waves. That’s a critical detail: humid heat is far more dangerous to the human body than dry heat, because it interferes with the body’s ability to cool itself through sweating. Several recent heat waves in the eastern U.S. have been unusually severe specifically because of this added humidity.
Cities Make Their Own Heat Worse
Urban areas experience heat differently than rural ones. Asphalt, concrete, and rooftops absorb solar radiation throughout the day and release it slowly after sunset — a well-documented phenomenon known as the urban heat island effect. In large cities, this can keep nighttime temperatures 5 to 10 degrees Fahrenheit higher than in nearby rural areas.
That matters enormously for public health. The human body needs a break from heat stress to recover, and that recovery mostly happens overnight. When nights stay hot, the cumulative strain on the cardiovascular system builds day after day, which is why urban heat waves are consistently associated with the sharpest spikes in heat-related illness and death.
Natural Climate Cycles Can Pile On
Long-term warming from greenhouse gases isn’t the only thing shifting temperatures from year to year. Natural ocean-atmosphere cycles — most notably El Niño and La Niña — shift global weather patterns and can push a given summer toward being hotter or cooler than the underlying trend would suggest. In years when these natural cycles align with human-caused warming, the result can be a heat wave that shatters records by a wide margin, rather than by a fraction of a degree.
Aging Infrastructure Wasn’t Built for This Climate
Much of the power grid, housing stock, and cooling infrastructure across the U.S. and Europe was built decades ago, for a climate that no longer exists. This is a particularly acute problem in Europe, where residential air conditioning has historically been far less common than in the U.S., leaving millions of homes with little defense against extreme heat.
Even where air conditioning is available, extreme heat drives electricity demand to its highest points of the year, straining grids that were not designed for such sustained peak loads. Heat waves are now among the leading causes of large-scale power failures — failures that, in turn, can cut off air conditioning and refrigeration precisely when they’re needed most.
More People Are Vulnerable Than in the Past
Finally, demographic and social trends mean that a heat wave of a given intensity now affects more people more severely than it would have a few decades ago.
Populations in both the U.S. and Europe are aging, and older adults are significantly more vulnerable to heat stress. Urbanization has concentrated more people in the hardest-hit urban heat islands. And outdoor workers — in agriculture, construction, and delivery services — face direct occupational exposure that has drawn growing attention from public health researchers and regulators alike.
The Bottom Line
No single factor fully explains why heat waves in the U.S. and Europe have become so severe. Instead, researchers describe it as a stacking effect: a warmer atmospheric baseline from greenhouse gas emissions sets the stage, while a wavier jet stream, dried-out soils, warmer oceans, urban heat retention, natural climate variability, and vulnerable populations and infrastructure all interact to turn an ordinary hot spell into a record-breaking, and often deadly, event.
Climate scientists broadly agree that as long as greenhouse gas emissions continue to rise, this stacking effect will keep intensifying — meaning today’s record heat waves may look ordinary within a generation.
Kiran Asim is a writer who covers the U.S., Europe, South Asia, and the Middle East. Her reports and analyses look at the geostrategic issues and contemporary developments and their economic and political implications. She also writes about society and education.
