The UK, Japan, and South Korea Experience the "Hottest Summer on Record" ── The Reasons Behind Simultaneous Extreme Heat Due to the Climate Crisis and How We Can Prepare

1. Why the "Scorching Summer" of 2025 Occurred Simultaneously

The summer of 2025 saw record-breaking temperatures in the UK, Japan, and South Korea simultaneously. The UK's seasonal average of 16.10°C surpassed the 15.76°C of 2018, marking the "hottest on record," with a deviation of +1.51°C from the average. Within the season, four heatwaves occurred, attributed to dry soil, high sea temperatures, and persistent high-pressure systems. Additionally, an attribution assessment indicated that **"a summer of this scale is 70 times more likely due to natural climate", highlighting its significance (Met Office) Met Office.

On a global scale, July 2025 was the third hottest July on record, with a 12-month moving average reaching +1.53°C (compared to pre-industrial levels), according to objective indicators (C3S/Reuters, WMO) ReutersWorld Meteorological OrganizationCopernicus Climate Explorer. Superimposed on the long-term upward trend of global warming, factors such as high ocean temperatures in the North Atlantic and North Pacific and widespread blocking high-pressure systems

contributed to persistent heat in different regions.

2. "Records" and the "Quality of Heat" by Country

2-1. UK: Average 16.10°C, the Design Philosophy of a "Cool Country" Backfires

• Record: The seasonal average of **16.10°C (+1.51°C)** was the highest on record, updating the series since 1884. The overlap of four heatwaves, low rainfall, and high sunshine was observed (Met Office) Met Office.

• Vulnerability in Daily Life: Homes prioritize winter insulation, and many public transport systems (e.g., London Underground), schools, and workplaces lack air conditioning, exposing a **"design gap"** vulnerable to sustained high temperatures. Water shortage declarations and impacts on agriculture were reported (AFP/IBTimes, The Guardian) International Business Times AustraliaThe Guardian.

• Significance: Even without reaching the dramatic peak temperatures (40.3°C in 2022), this summer demonstrated that **"an abnormal elevation of average values"** can have long-lasting effects on health and social infrastructure (Reuters) Reuters.

2-2. Japan: +2.36°C, National Record for Highest Temperature Also Broken

• Record: The seasonal average was **+2.36°C above normal**, the highest since statistics began (1898). **A "record-breaking summer" for the third consecutive year** (reported by various agencies based on JMA announcements) XinhuaAl Jazeera.

• Extreme Phenomena: The domestic highest temperature was 41.6°C (August, Isesaki), resetting the national record. The previous week, 41.2°C was observed in Tanba, Hyogo (Reuters) Reuters.

• Health Impact: From May 1 to August 24, there were 84,521 heatstroke hospitalizations (preliminary figures from the Fire and Disaster Management Agency, reported by various agencies), with consecutive tropical nights and peak power demand measures becoming issues in urban areas. Monthly detailed reports also confirmed a surge in hospitalizations (Fire Department materials) International Business Times AustraliaFire Department.

2-3. South Korea: Average 25.7°C, Highest Since 1973, Tropical Nights and Drought

• Record: The average from June to August was 25.7°C, the highest since 1973. It surpassed last year's (25.6°C) record (reported by MBC and others based on KMA announcements) MBC NEWS.

• Quality of Heat: The continuation of tropical nights and persistent high temperatures day and night were notable, leading to national disaster declarations and water supply restrictions in regions like Gangneung City (AFP/IBTimes) International Business Times Australia.

• Implications: **"Significant increases in minimum daily temperatures"** raise health risk levels, leading to a nighttime bias in air conditioning demand and changes in peak power structures.

3. What Created the "Persistent High Temperatures" — Key Physical Mechanisms

1. Warming Base Increase
   On the 12-month moving average of +1.53°C (compared to pre-industrial levels) globally, regional extremes have become more likely (C3S/Reuters). This indicates that **"it is no longer an outlier."** Reuters
   
   

2. Abnormal Ocean Temperatures and Humid Atmosphere
   High sea temperatures around the UK and warm areas in the North Pacific supplied lower-level warm and humid air. This amplified nighttime heat where radiative cooling is less effective, leading to frequent tropical nights (Met Office, WMO/C3S overview) Met OfficeWorld Meteorological Organization.
   
   

3. Persistence of Blocking High-Pressure Systems
   With the stabilization of high pressure, clear skies, low winds, and low rainfall persisted, activating a positive feedback loop of soil drying→temperature rise (Met Office) Met Office.

4. Impact Assessment: Health, Power, Agriculture, Tourism

• Health: In Japan, over 80,000 heatstroke hospitalizations were confirmed, with an increase in emergency dispatches regardless of time in major cities like Tokyo. Severe cases were notable among elderly households not using indoor air conditioning (Fire Department and various agencies) Fire DepartmentTV Asahi NEWS.

• Power: Increased demand for nighttime cooling is altering the load curve. Consecutive tropical nights in Japan and South Korea are creating new bottlenecks in power supply and demand (MBC, Guardian reports on tropical nights) MBC NEWSThe Guardian.

• Agriculture: In the UK, concerns about water shortage declarations and poor yields are growing, while in Japan's fruit and livestock sectors and South Korea's rice farming, **high-temperature damage (white immature grains, pollination failure, reduced milk yield)** is increasingly being watched (AFP/IBTimes) International Business Times Australia.

• Tourism and Urban Life: In tourist cities like Kyoto, Seoul, and London, the danger threshold for outdoor stays (WBGT) is exceeded more frequently, prompting a shift towards indoor activities and nighttime tourism (local reports by The