Japan records its first ‘cruelly hot day’ as cities swelter in 40-degree heat

Japan’s meteorological agency coined the term kokushobi – or “cruelly hot day” to warn the public of dangers tied to extreme heat. Photograph: Nicolas Datiche/SIPA/Shutterstock

by  Gavin Blair in Tokyo

 In April Japan coined new term kokushobi for days above 40C as country advises workers to wear shorts to the office to cope with stifling heat

 

Japan logged its first official kokushobi or “cruelly hot day” on Tuesday since the new term for when temperatures top 40 degrees Celsius was introduced in April. The Japan Meteorological Agency coined the new designation to warn the public of the heat dangers of days that were once freak weather events but have now become more common.

The central Japan cities of Gujo and Tajimi in Gifu prefecture, along with Toyota in Aichi prefecture, all hit 40C. Gujo’s 40C and Toyota’s 40.1C were all-time highs recorded at their respective weather stations.

 

Across 914 weather stations nationwide, 278 registered temperatures exceeding 35C or moshobi – “extremely hot days,” the highest level of warning until the recent introduction of the new terminology. Days over 25C are called natsubi or “summer days”, while those exceeding 30C are known as manatsubi – “true summer days.”

On Monday, a man in his 40s was found dead on a street in Sanjo, Niigata prefecture, while another in his 70s died in Kanagawa, southwest of Tokyo. Both are believed to have succumbed to the heat.

Heatstroke deaths topped 2,000 for the first time in 2024, while more than 100,000 people were taken to hospital last year between the beginning of May and late September, according to the fire and disaster management agency.

 

The rising heat is affecting many aspects of Japanese life – including yields and the quality of rice, a staple. In response, farmers are planting more heat-tolerant varieties, while some are switching to crops such as avocado that have not traditionally been grown at scale in Japan.

Impacts are also being seen in sport, with the most high-profile being the restructuring of the hugely popular koshien national high school baseball tournament. Played in the sweltering heat of August, the games have been split into morning and evening sessions since 2024 to reduce the chances of heatstroke for both players and spectators.

 

Meanwhile, office workers are being encouraged to wear shorts on hot summer days, the latest relaxing of Japan’s buttoned-down corporate culture as part of “Cool Biz” measures to reduce electricity consumption through lower use of airconditioning. Previous initiatives have encouraged “salarymen” to shed jackets and ties, and wear short-sleeved shirts but this year’s expected scorching summer has prompted calls for another level of casualness.

Tokyo governor Yuriko Koike announced the new clothing recommendations for office wear which also include polo shirts, T-shirts and trainers where appropriate, against the backdrop of expected high temperatures and a “challenging outlook for electricity supply”.

A study by the National Institute for Environmental Studies (NIES) and Tokyo’s Waseda University in 2025 found that without drastic cuts to greenhouse gas emissions, school sports will become unplayable outdoors by the 2060s.

Average summer temperatures in Japan have risen about 1.3C over the past 100 years, according to the Japan Meteorological Agency (JMA). Both 2023 and 2024 were the hottest years since records began in 1898, but last year saw those levels surpassed again. The summer of 2025 was 2.4C above the average of the previous three decades, with 132 weather stations logging their highest ever average heat and a new all-time Japanese high of 41.8C at Isesaki, Gunma, north of Tokyo, on 5 August.

Tokyo workers asked to swap suits for shorts to combat energy costs and heat

 

People on the Shibuya crossroad during an intense Tokyo heatwave in June last year. Photograph: Anadolu/Getty Images

by Justin McCurry in Tokyo

 

Public servants working for the Tokyo metropolitan government are being encouraged to swap their suits for shorts this summer to combat sweltering heat and rising energy costs caused by the US-Israel war on Iran.

Inspired by the country’s Cool Biz energy-saving initiative, Tokyo officials hope the measure will cut dependence on air conditioning.

Cool Biz, launched by the environment ministry in 2005, initially encouraged bureaucrats to dispense with ties and jackets, but has so far stopped short of allowing them to display their bare legs in front of colleagues.

Japan and other countries in Asia are growing anxious about the economic effects of the conflict in the Middle East, amid rising oil prices and shortages of petroleum products such as jet fuel.

 

Resource-poor Japan is particularly vulnerable to a prolonged war as it depends on the Middle East for 90% of its oil imports, most of which pass through the strait of Hormuz. About 20% of the natural gas used in South Korea comes via the same route.

Vietnam, South Korea and other countries have taken steps to ration energy use, while other Asian nations have encouraged government officials to work from home or reduced the length of the working week. In Seoul, authorities have urged residents to make short trips on foot or by bicycle.

 

Japan has already tapped into its large strategic oil reserves, with local media reporting on Friday that it plans to release a further 20 days’ worth from 1 May. It is also sourcing oil imports from suppliers that do not use the strait of Hormuz.

Experts have warned that if shipping in the vital waterway doesn’t return to normal soon, the world’s fourth-biggest economy will eventually face a crude oil shortage. That could force businesses and households to make more drastic cuts in petrol and electricity usage reminiscent of those introduced during the oil shocks of the 1970s.

The threat of an energy crunch had been “one of the factors” in allowing Tokyo government staff to wear casual clothes to work, Agence France-Presse said, adding that some employees had been spotted in shorts, T-shirts and short-sleeved blouses since the initiative’s launch this week to coincide with the start of Japan’s heatstroke warning system.

“I was a bit nervous, but it’s very comfortable, and I feel like it’ll improve my work efficiency,” a metropolitan government official who was wearing shorts to the office for the first time told the Yomiuri Shimbun newspaper. “As it gets even hotter, I’d like to come in earlier and work from home as well.”

Citing “a severe outlook for electricity supply and demand,” Tokyo’s governor, Yuriko Koike, who as environment minister was behind the Cool Biz campaign two decades ago, told reporters: “We encourage cool attire that prioritises comfort, including polo shirts, T-shirts and sneakers and – depending on job responsibilities – shorts”.

War notwithstanding, Japanese employers have been forced to rethink old rules on workplace attire as a result of the climate crisis. Last year, the country endured its hottest summer since records began in 1898, according to the meteorological agency.

Now that it is no longer unheard of for temperatures to rise to 40C or above, the agency last week announced a new extreme weather event: kokusho, or “cruelly hot”.

After 1,200 years, cherry blossom record to live on despite Japanese scientist’s death. Prof Yasuyuki Aono’s meticulous work charted shifting bloom dates as a marker of climate change

 

People viewing cherry blossoms in Osaka last week. Photograph: Jiji Press/EPA

by  Chris Baraniuk

 

Even in his final months, he counted the days until the cherry blossoms. Prof Yasuyuki Aono of Osaka Metropolitan University spent his career gathering data on the spring flowering dates of cherry trees in Japan in what is one of the world’s longest climate records tracking a seasonal occurrence.

Using sources dating as far back as the 9th century, he revealed that cherry tree flowerings have occurred progressively earlier in recent decades – a now famous marker of climate change.

 

Last April, Aono posted a photo to social media of his spreadsheet. He had just completed the 2025 entry, recording “4 [April]” as the peak flowering date for the particular cherry tree species he tracked, the mountain cherry, or Prunus jamasakura.

Below this, the next row was already marked “2026” but Aono never got to fill it in. He died on 5 August last year, according to former colleagues contacted by the Guardian.

Yasuyuki Aono learned old forms of Japanese so he could read 9th-century records. Photograph: Richard Primack

“You can very much see that he planned to continue,” said Tuna Acisu, a data scientist at Our World in Data, an online platform that publishes a chart based on Aono’s cherry tree data. “That made me a little bit emotional.”

Now, following a search launched by Acisu last week – sparked by fears that no one would be able to continue the 1,200-year cherry blossom record – a researcher in Japan has stepped forward and offered to make formal observations of the mountain cherry’s spring flowerings.

“He is consulting the same sources as Prof Aono to get us this year’s cherry blossom peak bloom and said he will confirm the date in the coming days,” Acisu said. The researcher in question asked to remain anonymous until the arrangement is finalised.

Acisu and colleagues first realised that something may have happened to Aono when they noticed in January that his university web page was no longer active. They then learned that he had died and that no other researcher or institution had emerged to carry on his observations. Spring arrived with no new mountain cherry data.

After Acisu launched her campaign to find a new cherry blossom observer, she received dozens of messages. “It’s really great to know that the dataset is being continued,” she said, expressing her gratitude to the new researcher. “I feel very relieved.”

 

Crucially, Acisu had sought a contact in Japan who could continue tracking not only the same species of cherry, but also in the same location: Arashiyama, Kyoto.

There are other projects that monitor cherry tree flowerings around Japan, since cherry bloom festivals are an important part of culture and tourism in the country, but not this specific species. For example, the Japan Weather Association monitors a different species: the Somei-yoshino cherry (Prunus x yedoensis), which was cultivated in the 19th century.

 

That Aono was able to compile flowering data for the mountain cherry over a period spanning more than 1,200 years is what gave his data series such significance, said Acisu.

Scientists have found the signature of climate change in a wide variety of other sources, including tree rings, plant pigments deposited in seabed sediments, and even temperature and humidity records jotted down by organ tuners in British churches.

Among the revelations made by Aono were the 2021 and 2023 peak flowering dates – they are the earliest in the entire mountain cherry record, occurring on the 85th and 84th day of those years, respectively.

Aono’s work on the mountain cherry was “extremely important”, said Toshio Katsuki, a dendrologist at the Forestry and Forest Products Research Institute in Ibaraki prefecture, who added that efforts to continue recording the same species’ spring flowering dates would be academically valuable.

Tourists wearing kimonos pose with cherry blossoms in Kyoto at the end of March this year. Photograph: Manami Yamada/Reuters

Richard Primack, a professor of biology at Boston University, met Aono on a trip to Japan in 2006. Aono told him that he had learned to read old forms of Japanese in order to build up his dataset of mountain cherry flowering dates. In dusty historical archives, Aono would find references to cherry blossom festivals in Kyoto and, from that, was able to calculate the flowering dates for specific years.

While some years are missing, the earliest record he found dated to 812. “It was really quite an amazing experience,” said Primack, remembering the meeting. “You just realise how dedicated an individual he was.”

In a paper published earlier this month, Primack and Katsuki described how flowering of the Somei-yoshino cherry also appears to be affected by climate change in southern areas of Japan. Data from 1965 to 2024 shows that milder winters were increasingly causing the spring-flowering cherries to have “a kind of bedraggled look, rather than a full, dazzling display,” said Primack. “Many of the flower buds were falling off without opening.”

EUA e Japão anunciam projeto de energia nuclear de US$ 40 bilhões

Usina nuclear na Pensilvânia, nos EUA              

 

Washington | AFP

Os Estados Unidos e o Japão anunciaram nesta quinta-feira (19) um projeto de US$ 40 bilhões (R$ 210 bilhões) para construir reatores nucleares nos estados de Tennessee e Alabama, após uma reunião dos líderes dos dois países na Casa Branca.

Os diálogos entre o presidente americano, Donald Trump, e a primeira-ministra japonesa, Sanae Takaichi, ocorrem depois que Tóquio aceitou, no ano passado, investir US$ 550 bilhões como parte de um novo acordo comercial com Washington.

Na declaração conjunta desta quinta-feira (19) sobre os chamados pequenos reatores modulares (SMR, sigla em inglês) também foi anunciado um investimento de US$ 33 bilhões (R$ 173,5 bilhões) em instalações de geração de energia e gás natural na Pensilvânia e no Texas.

Em fevereiro, os dois países anunciaram a primeira parte de projetos por conta do novo fundo de investimentos, com compromissos de US$ 36 bilhões em três projetos de infraestrutura.

A declaração desta quinta-feira assinala que os projetos garantiriam a segurança, ao "acelerarem o crescimento econômico de ambos os países, abrindo, assim, caminho para uma Nova Idade do Ouro da sempre crescente Aliança Japão-Estados Unidos". Os SMR serão construídos pela empresa nipo-americana GE Vernova Hitachi.

 Estados Unidos e Japão também divulgaram hoje um plano de ação sobre o desenvolvimento de cadeias de suprimento de minerais críticos, em meio à preocupação com o papel dominante da China nesse setor.

Também foi revelado investimento de US$ 33 bilhões em geração de energia e gás natural nos EUA

Em 2025, o país asiático se comprometeu a investir US$ 550 bilhões nos Estados Unidos em acordo comercial

Why the weirdest sea level changes on Earth are happening off the coast of Japan

Fishing boats sail toward a kelp harvest area in the Northern Territories, 3.7 kilometers off Nemuro, Hokkaido, Japan on June 2, 2018. Fish and kombu seaweed harvested in this region are crucial to Japan's food culture, but recent changes in a warm ocean current called the Kuroshio have damaged these fisheries.

The Asahi Shimbun/Getty Images

 

 By Chris Mooney, Yumi Asada

 

 

Bathtubs and pools mislead us about the ocean: Its surface is anything but flat.

Seas pile up in some spots, pushed by trade winds or pulled by gravity toward big things like ice sheets. Amid it all, at the western end of large ocean basins, the fastest surface currents — veins of warm water — race toward the poles, causing additional slopes at the surface.

The ocean is uneven to begin with, and its unevenness is also changing. Maps of recent changes show intricate patterns of watery hills and valleys, but also call attention to one extraordinary location. Off the coast of Japan, one region of the ocean has been rising by nearly an inch every year, right next to another where it has been falling even faster.

It’s the fingerprint of one of those surface currents changing its location, an event that has had dramatic repercussions. The Kuroshio, or “Black Current,” is one of the largest streams of water anywhere in the world, and its recent movement has triggered record-warm ocean temperatures and upended fisheries, an indelible staple of Japanese culture. Scientists say the warm waters have even amplified heatwaves on land and driven extreme rainfall. 

 

And while there are signs some of the changes are now waning, fishing communities say they aren’t yet back to normal. Meanwhile, scientists worry it could be a sign of more volatility to come.

The position of the current could keep fluctuating, said Bo Qiu, a leading Kuroshio expert at the University of Hawaiʻi at Mānoa. “It’s hard to predict the future, but given the data we have so far, I can only see the intensity becoming larger and larger,” he said. 

The Boso Peninsula forms the eastern edge of Tokyo Bay. Normally, the Kuroshio Extension banks away from Japan and heads into the Pacific near this spot. But in recent years, it's continued northward, bringing unprecedented warm water with it.

John S Lander/LightRocket/Getty Images

 

 

A river in the ocean

The deep, warm Kuroshio transports more than 200 times as much water as the Amazon River, traveling north from the equator and normally banking east around Japan’s Boso peninsula, near Tokyo. Here, it becomes known as the Kuroshio Extension as it heads into the open Pacific. 

 

But in recent years, the current has been behaving in anything but the usual way, and the Extension, in particular, made a major divergence along Japan’s coast. Its northern edge shifted as much as 300 miles farther poleward, leading to unprecedented warm waters in the surrounding region.

“I was so surprised I don’t even know if ‘surprised’ is the right word,” said Shusaku Sugimoto, an associate professor at Tohoku University in Sendai, Japan, a northern coastal city. 

 

When a major ocean current changed course, sea levels changed with it

Recent changes in a warm current called the Kuroshio have created some of the fastest rising seas on the planet — right next to seas that are falling even faster. 

Sugimoto led a study analyzing ocean temperatures off the coast in locations the Extension didn’t historically reach, but has in recent years. “The fact that the temperature rose 6 degrees (Celsius) off the Sanriku coast, and the elevated temperature persisted for two years, represents a level of water temperature rise we’ve never seen before,” he said. 

It’s not the only change.

In August of 2017, the Kuroshio current south of Japan settled into a “large meander” pattern, leaving the coastline and looping southward, taking its warm waters with it. This big shift in water temperatures south of Japan changes the distributions of fish species offshore.

Large meanders themselves are a well-known recurring feature of the current, explained Shinichiro Kida, an oceanographer at Kyushu University. Records of these events date back to the 1960s. During a lengthy meander event from 1975 to 1980, scientists saw a severe decline in anchovy in the Enshunada Sea, a major fishing region to the south of Japan’s main island of Honshu. The anchovy were replaced by sardines, which favor the warmer water the current brought to the region.

But no Kuroshio large meander that we know of has lasted as long as this one. In August, the Japan Meteorological Agency finally declared that it had ended after nearly eight years. But overall both changes have had a big effect on this country that is home to more than 100 million. 

 

The meander and the Extension shift are connected, Qiu said. He co-authored a new paper in the Journal of Climate arguing as much, and calling the configuration a “new dynamic regime.”

“I’ve been working on the Kuroshio Extension for more than three decades,” Qiu said. “I never expected this.”

As the Kuroshio powers along, it not only brings warmer water but also, depending on its location, higher sea level. There can be a several-foot difference between ocean heights on different sides of the current, thanks to its warmth and speed .

But because of these factors, any new movement of the current can have dramatic effects. 

 

The large meander, for example, triggered a sea level fall in one region and pronounced sea level rise — by as much as half a foot — along the coast of the island of Honshu south of Tokyo. In October of 2017, when Typhoon Lan struck the Shizuoka Prefecture along this coastline, the higher sea level amplified the damage, according to the Japan Meteorological Agency.

It is not only the height of the sea: As the Kuroshio is a warm current, and one traveling northward into cooler waters, its arrival in a new location has enormous effects on ocean temperatures.

The big question is how much the recent phenomena are part of a natural cycle, and how much it’s influenced by something else, like climate change. (And how much it’s both, combined.)

The impact of climate change on the large meander is unclear, because of its long history. But as far as the Extension shift goes, there are mounting reasons to implicate greenhouse gases and a changing climate.

When the tropics travel to you

To understand the recent changes in the Kuroshio is to understand how water moves through the world’s oceans, especially the large rotating “gyres” found in the world’s major ocean basins. 

 Five of these basins — North and South Pacific, North and South Atlantic, and Indian — feature a similar pattern. Warm water travels west along the equator, then turns toward the poles. The currents carrying the warm water north or south on poleward transits are called “western boundary currents,” and they include the Kuroshio and its four famed cousins: the Gulf Stream, the Brazil, the East Australian, and the Agulhas.

 

Japan has seen some of the world's most extreme ocean changes

In recent years, waters off Japan’s Pacific Coast have experienced some of the world’s highest rates of ocean warming, as well as extreme changes in sea level.

 

Scientists are now seeing most of these currents changing in a similar way — getting warmer and pushing even farther poleward.

This change is rooted in a phenomenon known as the Hadley Cell, a global zone of warm, rising air across the tropics. It’s now expanding due to climate change. 

 

“This expansion shifts not just rainfall patterns but also the zones of sinking air that anchor high-pressure systems, such as the Pacific High,” explains Brown University climate scientist Emanuele Di Lorenzo.

In the middle latitudes, these massive high-pressure systems are the driving force behind currents like the Kuroshio. So, when the winds move, the currents do too.

Both models and data suggest the Kuroshio Extension, in particular, has been moving north partly because of the atmospheric shift, Di Lorenzo explained.

Between 1993 and 2021, driven by wind changes, the northern edge of the Kuroshio Extension shifted north by about 130 miles, a recent study found. This was before an even larger shift in 2023 and 2024. 

 

The Kuroshio Extension’s leap in 2023 and 2024 was an extreme event. The northern edge reached nearly to the northern tip of Honshu, Japan’s largest island. In a different study, Sugimoto and colleagues with the Japan Meteorological Agency and the Meteorological Research Institute took oceanographic measurements in the Extension’s new location. They found water as much as 18 degrees Fahrenheit warmer than usual, extending to a depth of around 400 meters, or 1,300 feet.

The authors note that for an entire year and a half between April 2023 and August 2024, the region saw “intense marine heatwave conditions almost every day.”

And it was not just in the water: the Japan Meteorological Agency also found that the extreme ocean conditions contributed to record summer heat over land in northern Japan in 2023. Another group of scientists, meanwhile, linked the warm offshore current to extreme rainfall over Japan’s Chiba Prefecture, near Tokyo, in September of 2023. 

The first saury catch of the season is landed at a port in Nemuro on Japan's northernmost main island of Hokkaido on Aug. 15, 2025. This species has been particularly affected by changes in the Kuroshio.

Kyodo News/Getty Images

 

An uncertain future for fisheries

These ocean changes have altered the distributions of fish populations along Japan’s Pacific Coast, creating extreme impacts on Japan’s iconic fisheries. 

 

In central Japan, for instance, a major mackerel fishery was upended, and fishers say that even though the large meander has now ended, that doesn’t mean things can instantly go back to the way they were.

“Immediate recovery is not a reality, and while conditions may gradually improve, at present, there has been no recovery in catches,” said Osamu Nagai, executive director of the Mie Gaiwan (Outer Bay) Fisheries Cooperative Association.

“The catch has fallen to less than half of what it was 10 years ago — we’re now only catching about 20 to 30 percent of the mackerel. This is a major blow,” Nagai said.

Northeastern Japan’s Sanriku coast, known for its rich fisheries, is a different story, but still a bad one. Here, the southward flowing Oyashio current traditionally brings down cool waters and supports rich fisheries. But when the Kuroshio Extension moved northward into this region, it displaced the Oyashio, bringing on a stark change in ocean temperature. 

 

And it is not just fish species, like Pacific salmon and saury, that became harder to catch.

“What matters most is the foundation of Japan’s most important food culture, which is the flavor made out of Kombu seaweed — it can only be harvested in Hokkaido, near Japan,” said Yoshihiro Tachibana, a professor who specializes in climate dynamics at Mie University. 

 

Kombu kelp is laid in the sun to dry Wakkanai, Hokkaido, on July 15, 2012. This seaweed is a vital part of Japan's food culture and economy. But it's becoming harder to harvest.

Hiroaki Murata/The Yomiuri Shimbun/AP

 

 

“Kombu stocks are declining dramatically. Dashi (the fundamental Japanese soup stock) culture might collapse. It’s declining. We can’t get any at all. So I believe this has a tremendous impact on our food culture as well,” he said.

Recently, the northern edge of the Kuroshio extension has retreated a bit. It has moved back to around 37 degrees North Latitude, according to Qiu. That’s still a high location historically, but hardly as extreme as before. 

 

But the questions remain: what do these extreme ocean events mean, how they are linked to climate change? For at least one researcher, they’re an early sign of things to come.

“It’s a great opportunity to learn what the oceans will be like 100 years from now,” said Sugimoto, of Tohoku University in the northeastern region of Honshu, Japan’s largest island.

“An unprecedented ocean phenomenon is now occurring by chance in Tohoku,” he continued. “Understanding how this has altered the seas of Tohoku offers a chance to understand how the world’s oceans will change in the future.”

Elisabeth Doty contributed to this report.