From Tokyo to Dublin: A Summer That Rewrote the Record Books

TLDR: From Japan to Ireland, chronic and acute heat are rewriting the risk map — and challenging assumptions about who’s prepared. Summer 2025 shattered global temperature records and exposed vulnerabilities in health, energy, and infrastructure systems not built for sustained extremes. No geography is immune, and resilience now depends on adapting faster than the heat is rising.

Summer 2025 set records around the world. Japan, South Korea, Ireland, and the United Kingdom all recorded their hottest summers ever, while the Northern Hemisphere overall experienced its third-hottest summer on record. The season wasn’t defined only by isolated heatwaves, but by an unrelenting persistence of high temperatures that turned whole months into endurance tests.

Health impacts were immediate: thousands of excess deaths and tens of thousands of hospitalizations occurred across Asia and Europe, while workplace heat stress became an urgent focus for both the World Health Organization and the World Meteorological Organization. Economic and energy systems strained under conditions that pushed the limits of what infrastructure and people were built to tolerate.

Chronic Heat: When Every Day Feels Like the Hottest

In Japan, South Korea, Ireland, and the United Kingdom, the story of 2025 wasn’t one of a single blistering week. It was months of oppressive heat. Compared with the 1991–2020 average, Ireland and the UK were 1–2°C (1.8–3.6°F) hotter on average. In Japan and South Korea, the increase was closer to 2°C (3.6°F).

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In Hokkaido, Japan’s northernmost and typically cool prefecture, temperatures soared to nearly 40°C (104°F), averaging 4°C (7.2°F) warmer than normal. The nation also recorded a new all-time high of 41.8°C (107.2°F). The result: more than 90,000 hospitalizations for heat-related illness. In Seoul, residents endured over 30 tropical nights (minimums above 25°C/77°F), while Tokyo experienced nearly 40.

Persistent high-pressure systems, clear skies, and warm seas locked in the heat, producing conditions the UK Met Office described as an environment “where heat builds quickly and lingers”. Even nights offered no respite, a trend that dramatically increases both physiological stress and grid strain alike.

By 2050, hot days (above 35°C/95°F) are projected to increase by 1.5 to 3 times in Japan and South Korea; in the UK and Ireland, hot days (over the 99th percentile of temperature) are projected to double.

Acute Heat: When Extremes Strike

While chronic heat wears on the body and the grid, acute heatwaves trigger immediate crises. The summer of 2025 brought multiple waves of severe, region-spanning heat, particularly in nations unaccustomed to it.

Scandinavia’s Long, Hot July

In July, Sweden, Norway, and Finland endured an exceptional heatwave. For 13 consecutive days, Sweden’s national daily high topped 25°C (77°F), and Finland remained above that level for 22 straight days. While absolute highs didn’t break 2018 records, the duration was unprecedented. Jupiter’s data projects that heatwaves in Sweden and Finland will become four times more likely by 2050. A seemingly small 0.2°C (0.36°F) of additional warming since 2018 has already doubled the likelihood of heatwaves like this, underscoring how nonlinear these impacts can be.

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In the far north, the impact ripples beyond human health. For the Sámi people, whose livelihoods depend on reindeer herding, hotter summers disrupt migration and grazing. Meanwhile, warming lakes trigger algal blooms that threaten water quality and aquatic ecosystems.

Western Europe’s Early Heatwave

By late June, temperatures soared across western Europe. Highs surpassed 35°C (95°F) across much of the continent, and nighttime lows stayed above 20°C (68°F). These temperatures aligned with the upper range of Jupiter’s projections for 2025.

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Attribution studies suggest that climate change tripled the death toll from this heatwave, exacerbated by its early timing and persistently warm nights. Simultaneous heat in North America exposed over 250 million people to extreme temperatures.

The ripple effect on energy was immediate and profound. Jupiter’s projections indicate that annual cooling-related energy demand in western European cities will rise 10–28% by 2035, and as much as 73% by 2050. Northern cities such as London and Frankfurt, where air conditioning is still rare, face the steepest relative increases.

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When Cool Destinations Become Hot Zones

Extreme heat is no longer confined to tropical or desert nations. The steepest percent increases in heat-related losses are occurring in the north: places like Ireland, the UK, and Finland. Europe is now the fastest-warming continent, with 2025 alone responsible for roughly 25,000 heat-related deaths, two-thirds of which are directly attributed to climate change.

By mid-century, cooling losses could double in many northern European cities, while cooling degree days (a measure of how much and how often outdoor temperatures exceed levels that typically require air conditioning) are expected to rise even more sharply. 

In Japan’s Hokkaido region, cooling-related energy losses may increase 55%, compared with 25% nationally, reflecting how relative vulnerability to heat can outpace absolute heat itself.

While southern Europe, Japan’s main islands, and the U.S. Sun Belt will remain the hottest, cooler nations are now the fastest-changing. Their infrastructure, economies, and health systems are less prepared for heat stress, amplifying their vulnerability to exposure.

The Cost of Inaction

The 2025 heat season was a preview of a new reality: one where small shifts in temperature translate into exponential changes in impact. Whether we frame it as chronic heat, acute events, or rapidly increasing relative risk, the message is the same: heat risk is growing faster than our ability to adapt.

The World Meteorological Organization and World Health Organization have found that worker productivity declines 2–3% for every degree above 20°C (68°F). As global temperatures rise, that loss compounds across industries, sectors, and economies.

The Imperative for Preparedness

In this era of compounding extremes, adaptation plans can no longer rely on historical averages. Risk and ESG professionals must plan for the tails of the distribution — the extremes that define tomorrow’s losses.

Jupiter’s forward-looking data equips decision-makers with probabilistic projections of heat, flood, and other perils that reflect not only average changes but also the behavior of extremes. Our data can then model multiple adaptation scenarios using local construction costs, so our customers can compare their adapted and unadapted losses, and even see the break-even point and ultimate ROI of their resilience plans. With a science-first approach, globally trusted data and transparent methodology, Jupiter helps organizations prepare for the world as it is becoming, not as it was.

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