Heavy rain and heatwaves occur frequently worldwide this summer: Advancing global warming lies behind "extreme weather"

This summer, West Japan experienced record-breaking intense heat, and severe heavy rain struck the Tokyo metropolitan area, the Hokuriku region, and other places. Europe and North America were also affected by intense heat and heatwaves, and a series of forest fires occurred. Extreme weather phenomena occurred in various parts of the world, highlighting the climate crisis. Enormous damage in various parts of the world can no longer be said to be accidental events occurring independently. Global warming is considered to exist in the background, and many people beyond borders have a sense of crisis.

International organizations and experts point out that "on the common foundation of global warming, natural fluctuations such as meanderings of westerlies or El Nino phenomena and atmosphere/topography conditions peculiar to regions overlap further, amplifying natural disasters." An Advisory Panel of the Japan Meteorological Agency (JMA) concluded regarding high temperatures in West Japan from mid-July to early August that "if global warming did not exist, it probably wouldn't have occurred." Familiar preparations against heavy rain or sediment disaster that can occur anywhere nationwide are necessary.

Five consecutive severe heatwaves in west Japan, record-low rainfall in northern Kyushu

The Advisory Panel on Extreme Climatic Events of the JMA (Chair: Professor Emeritus Hisashi Nakamura at the University of Tokyo) held a meeting analyzing in detail extreme weather occurring frequently in various parts of Japan this summer on September 1, announcing results at a press conference after the meeting. It became clear that different atmospheric and meteorological conditions were related intricately to intense heat and low rainfall in West Japan, frequent occurrences of typhoons, and heavy rain in Chiba Prefecture, respectively.

The average temperature of West Japan was the third highest since 1946 when statistics began. Locations observing "severely hot days" with maximum temperature of 40 degrees or higher reached 36 locations in total, becoming the highest since 2010 when the JMA began tabulating. On the other hand, in West Japan, a period without consistent rainfall continued after the end of the rainy season. Precipitation in August in the northern Kyushu region was approximately 12 percent compared with that of normal years, recording the lowest since 1946.

According to analyses of the Advisory Panel, direct factors causing high temperature and low rainfall are large meanderings of westerlies flowing near Japan. Westerlies flowed farther north than normal years from the Eurasian Continent to near Japan, and West Japan was covered by "tall high-pressure system" developing from the sky down to near the ground.

Especially from late July to early August, in addition to strong solar radiation due to clear skies, a high-pressure system covered the area from the upper atmosphere down to the lower atmosphere, and downdrafts were prominent. The temperature of descending air rose markedly through a phenomenon called "adiabatic heating." That warm air flowing around the edge of Pacific high-pressure system also supported temperature rises.

Generally, when soil dries due to low rainfall, heat used when moisture evaporates from land surfaces decreases, increasing heat warming land surfaces and the atmosphere. It can fall into a vicious circle where high temperatures advance drying and drying further raises temperatures.

Global warming pushed up "foundation" of intense heat

It has been pointed out that intense heat in these past few years is related to the meandering of westerlies. However, meandering of westerlies is a meteorological phenomenon that has occurred repeatedly thus far, and that alone cannot explain recent record-breaking intense heat.

"Weather Attribution Center Japan (WAC)," formed by Professor Masahiro Watanabe at the Atmosphere and Ocean Research Institute of The University of Tokyo, researchers at Kyoto University, and other partners, analyzed high temperatures at altitudes of approximately 1,500 meters observed mainly in West Japan from July 11 to 25 using a computer-aided method called "attribution." They compared how much high temperatures in West Japan could occur between climate that actually warmed and climate created on computers assuming that warming via human activity did not exist.

As a result, while it remained at "once in approximately 74 years" under normal meteorological conditions in 1991 to 2020, it became "once in approximately 37 years" under meteorological conditions of 2026 where warming advanced, revealing that occurrence frequency heightened approximately twofold. Furthermore, under climate conditions assuming that warming via human activity did not exist, the occurrence probability of high temperatures at this level became approximately 0.006 percent. WAC published analysis results stating that "without global warming, high temperatures at this level could not have occurred" on August 3.

Separately from this, a joint team of a research program of the Ministry of Education, Culture, Sports, Science and Technology and the Meteorological Research Institute announced from analysis results via the attribution method on September 1 that "global warming contributed to record-breaking high temperatures in West Japan from mid-July to early August." Nakamura of the JMA's Advisory Panel explained at the press conference: "When situations where westerlies meander north near Japan and the region is covered by tall, warm high-pressure systems, temperatures rise further than before compounded by the effects of global warming."

Chiba's heavy rain was an "extremely rare phenomenon"

The Advisory Panel of the JMA also analyzed typhoons in this summer and Chiba's heavy rain in August. In Chiba Prefecture, which serves as a commuting and schooling area of the Tokyo metropolitan area, record-breaking heavy rain occurred from the evening of August 13 to early morning of August 14. In Chiba City, 24-hour precipitation reached 367 millimeters and 1-hour precipitation reached 115 millimeters, both ranking first in observation history. In addition to causing widespread inundation damage, there was also a major impact on transportation.

According to analyses of the Advisory Panel, near the Boso Peninsula, atmospheric conditions were extremely unstable due to influences of warm moist air flowing in from the east and strong cold air in the upper atmosphere. On the other hand, in inland areas of the Kanto region, cold air layers expanded due to precipitation accompanied by daytime convective activity. Near the Boso Peninsula corresponding to the eastern end, a local front (shear line) was formed between the aforementioned cold layers and warm moist air flowing in from the east. As a result, cumulonimbus clouds generated one after another, forming linear precipitation zones, and some clouds developed up to an altitude of 15 kilometers or higher. Nakamura stated: "This is an extremely rare phenomenon, and calling it extreme weather causes no issue."

Regarding typhoons, 17 typhoons generated between June and August this year. This exceeded 11 typhoons in normal years, tying with the third highest record since statistics began in 1951. Regarding this cause, the Advisory Panel cited that atmospheric flow called an "easterly wave" moving west inside easterly winds blowing from east to west in tropical regions was active. They pointed out that possibilities exist where easterly waves occurred due to influences of remarkable El Nino phenomena.

Among the typhoons that occurred, Typhoon 15 took a path with few past precedents, making landfall in Ibaraki Prefecture from the sea east of Japan and traversing Honshu westward. Beyond this a large-scale low pressure system called a "monsoon gyre" formed over the sea south of Japan and a strong high-pressure system lingering over the sea east of Japan existed. Typhoon 15 took an unusual westward path near Japan between the two systems. In addition, that westerlies separated into north and south over the sea east of Japan from near Honshu, and several upper-layer cold lows moved south near Japan through meandering of the southern westerlies among them influenced the path of Typhoon 15 as well.

Nakamura highlighted: "Past experiences or common sense are becoming hard to apply," citing possibilities where increases in water vapor amounts due to rises in sea surface temperatures via warming influenced rainfall amounts or typhoon intensities.

This summer, it was demonstrated clearly that linear precipitation zones generate anywhere nationwide. Nakamura and officials of the JMA stated at the press conference: "Challenges still exist in accurately predicting occurrence positions of linear precipitation zones immediately before, and we will work on improvements of accuracy in the future." Similarly to earthquake measures, we wish to hasten familiar preparations against heavy rain or sediment disaster.

"Hottest July in observation history" in various parts of world

"Summer with extreme weather" is not limited to Japan. World Meteorological Organization (WMO) published a report on August 12 stating that record-breaking high temperatures, drought, forest fires, and floods occur successively over wide areas of the Northern Hemisphere. According to analyses of the National Oceanic and Atmospheric Administration (NOAA) of the United States, July this year became the hottest July in observation history, tying with 2024. Among regions, in North America, Africa, and Asia, it became the hottest July respectively.

According to the Copernicus Climate Change Service, a weather information organization of the European Union (EU), Western Europe experienced the hottest June to July in observation history. In an announcement on August 10, the average temperature of Western Europe during this period was 21.62 degrees, becoming the highest in observation history as an average of both months. In Europe, maximum temperatures exceeded 40 degrees in late June in Germany and the Czech Republic, surpassing national maximum temperature records.

According to reports from Europe, precipitation and soil moisture decreased remarkably in France, Spain, the United Kingdom, Germany, and other countries, and discharge of the Seine River, the Rhine River, the Danube River, and other rivers declined. In the Rhine River and the Danube River, vessel operations were restricted due to large decreases in water levels, influencing logistics and power generation as well. In France and Spain, large-scale forest fires occurred. High temperatures and drying are considered to have created conditions where large-scale forest fires occur easily, exerting serious influences on ecosystems as well.

The UK Met Office announced that it recorded 37.7 degrees in eastern England on June 26, updating the record of 1976. In addition, the UK Health Security Agency published preliminary estimates on July 30 stating that the death toll related to heatwaves from May to June was 2,877 people in England.

The United States was also struck by severe heatwaves from early July, recording record-breaking heat in cities such as New York and Boston on the East Coast. Some events for Independence Day, celebrating 250 years of nation-founding on July 4, were canceled. In addition, large-scale forest fires occurred in Oregon, Washington, Nevada, and Colorado this summer.

Debris flow occurred via glacier collapse in Nepal

Influences of warming appear not only in temperature and rainfall but also in glaciers in mountain regions. On the border area between Nepal and China on August 26, a large-scale slope collapse including glaciers and bedrock occurred. Debris flow where large amounts of ice, rock, soil, and mud mixed struck settlements, roads, and hydroelectric power generation facilities with tremendous force. According to reports from the site as of September 10, at least 1,380 people died and approximately 5,500 people are missing in Nepal and the Tibet Autonomous Region of China. Searches and identity confirmations of bodies continue in disaster-stricken areas, and the possibility exists where the number of victims increases in the future.

Differing from typical "glacial lake outburst floods" where dams damming glacial lakes collapse, it is believed to be a chain compound disaster starting from collapses of glaciers and bedrock.

According to WMO, over 270,000 glaciers exist in the world, and the total area is approximately 700,000 square kilometers, corresponding to approximately twice the land area of Japan. It has been stated that if glaciers become thin due to temperature rises and permafrost thaws, forces fixing bedrock weaken. How much warming was related to this slope collapse remains unclarified. However, WMO, the United Nations, and research institutions in the Himalayan region have warned repeatedly that risks of glacier collapses or glacial lake outburst floods increase through global warming.

In northern India near the border with China in February 2021, avalanches and debris flows due to large-scale collapses of bedrock and ice/snow occurred. It was reported that dead and missing people exceeded 200 people and hydroelectric power generation facilities received major damage as well. In debris flows in Nepal and China this time as well, numerous hydroelectric power stations including those under construction suffered damage. It is a situation where hydroelectric power generation facilities, indispensable to combatting global warming, were damaged by glacier-related disasters increasing as a result of global warming.

Extreme weather is becoming the "new normal"

In heatwaves, heavy rain, typhoons, and floods occurring frequently this summer in Japan and various other parts of the world, inherent meteorological conditions are intertwined. Factors such as meanderings of westerlies, El Nino phenomena, high-pressure system, cold lows, topography, and sea surface temperatures are combining in a complex way.

For this reason, many weather experts point out: "Simplifying that extreme weather occurred solely through warming is not accurate." Various factors are intertwined in atmospheric phenomena affecting weather, and suggesting that warming influenced them in pushing up the "foundation" of extreme weather is correct.

Global warming is certainly changing the background conditions under which meteorological phenomena occur. It raises temperatures, causes oceans to store heat, and increases water vapor in the atmosphere. On land, it dries soil and destabilizes glaciers and permafrost. Overlapping of "natural fluctuations" with these environmental changes can be said to increase the intensity and frequency of occurrence of extreme weather more than ever before.

That we face an era of "climate crisis" is clear, and "mitigation measures" reducing emissions of greenhouse gases such as carbon dioxide to suppress the progress of global warming even slightly must not be delayed. At the same time, "adaptation measures" such as city planning to combat intense heat, flood warning networks, forest fire measures, monitoring of glacial lakes, and disaster-resilient infrastructure are urgent challenges.

The world in the summer of 2026 demonstrates in a cruel form that extreme weather is becoming a new normal rather than a temporary exception.