Three analyses link Typhoon Dujuan’s heavier rain in Japan to climate change

Human-caused warming made the rain from Typhoon Dujuan heavier when the storm battered the coast east of Tokyo in September, according to a rapid analysis published Oct. 28 by World Weather Attribution (WWA). Two other analyses, one from the ClimaMeter project and one from Japanese government scientists, also found more rain, though their estimates of the size of the effect differ.

Dujuan, the 25th typhoon of Japan’s 2026 season, intensified over the Western Pacific into a typhoon with sustained winds exceeding 120 km/h, WWA said. Rather than making direct landfall, it tracked along Japan’s eastern coast, interacting with an autumn rain front, and particularly affected the Boso Peninsula in Chiba Prefecture on Sept. 21. WWA, citing Japanese government figures, said the storm was associated with 17 deaths, including 10 in Chiba. A road collapsed in Chiba City, embankments around Lake Inbanuma were breached at three locations, and nearly 45,000 Tokyo Electric Power Company customer accounts lost electricity.

What WWA found

The WWA team, with scientists from Japan, the United States, the United Kingdom, Ireland and the Netherlands, defined the event as the two days of most extreme rainfall over the Kantō region. Across that region, similar totals are expected roughly every three years in today’s climate. Locally the rain was far rarer: about a 1-in-20-year event at Oshima, about 1-in-60 at Funabashi and Abiko in Chiba, and about 1-in-150 at Miura in Kanagawa.

The standard approach of comparing observed trends with climate models did not give a clear answer. Observations suggested such downpours have become more likely and intense, but uncertainties were very high, models disagreed substantially, and the coarse resolution of models and gridded data prevented a reliable probabilistic attribution for such a small, topographically complex area, the authors said.

Other lines of evidence were clearer. The study found “an increase in the amount of water vapour of 9% attributable to human-induced climate change,” which the authors said is consistent with the Clausius–Clapeyron relation. Sea surface temperatures along the storm’s path before peak intensity were around 2–3°C warmer than in a preindustrial climate. Using Dujuan-like weather patterns as analogues, the team found “a 20-40% increase in rainfall in the locations that received the highest rainfall.” Wind intensity rose slightly, with no statistically significant change in frequency. El Niño, the team said, “did not intensify the rainfall or increased sea surface temperatures” under any of the definitions tested.

Two other estimates

An analysis by ClimaMeter, a project hosted by France’s Institut Pierre Simon Laplace and CNRS, compared the storm’s sea-level pressure pattern with similar past systems in 1950–1987 and 1988–2025. It found “about 5-15% heavier rainfall over the storm core” in the recent period, no statistically significant change in wind speed, and that about 6% of rain-linked damage (10th–90th percentile: 3 to 9%) was attributable to human-driven climate change. ClimaMeter described the storm as making landfall on the Boso Peninsula and reported 832 mm of rain over 48 hours on Oshima island.

On Oct. 23, Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT) released event attribution results from its climate projection research program and the Japan Meteorological Agency’s Meteorological Research Institute. Covering record heavy rain in August and September 2026, including rain associated with Typhoon No. 25, it said global warming may have increased rainfall by around 20% in each case. WWA summarized that result as an increase of over 20%.

The three used different methods and event definitions, so their numbers are not directly comparable.

Why it matters

Japan has extensive forecasting, early warning systems, flood defenses and disaster response, WWA noted, yet Dujuan still proved deadly. “Dujuan highlights the limits of preparedness, such as when rainfall exceeds design values for infrastructure,” the authors wrote. They called for stronger structural and physical adaptation alongside social measures, and said older adults and people reliant on care faced heightened risk.

WWA and ClimaMeter both located the warming signal in the rain rather than the wind. WWA concluded that warming had a relatively limited effect on the typhoon’s frequency and intensity, but a more pronounced effect on the rainfall from the combined typhoon and autumn rain front.

The caveats

None of the three has been presented as a peer-reviewed paper. WWA says its rapid studies are published before peer review, using established methods that have been peer reviewed, and that about a quarter of its studies later appear in peer-reviewed journals. The ClimaMeter page does not say its Dujuan analysis was peer reviewed, though its methods draw on published work, and the MEXT announcement summarizes results whose method details sit in accompanying reports. Casualty and outage figures also vary by source and date: ClimaMeter reported seven dead and five missing and power cut to some 58,000 households in Chiba, while WWA reported the figures cited above.

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