A picture posted on social media last April by Prof Yasuyuki Aono of a spreadsheet, with its blank row for 2026, carries a quiet poignancy. Prof Aono died before he got to fill in this year’s entry for when the cherry blossom fully bloomed in Kyoto. The academic had spent decades reconstructing dates of flowering that go back to the ninth century. His work illuminated how a botanical event long associated with the Japanese idea of mono no aware – a sadness at the passing of things – is shifting because of the climate crisis.

The “peak bloom” now occurs around two weeks earlier than in previous centuries. In the 1820s full bloom arrived in mid-April. In 2023 the full-flowering date was 25 March. An earlier blooming indicates warmer springs – and Prof Aono’s data provides a warning signal that Japan’s “sakura front” comes sooner each year.

But this change is more than just a biological response to rising temperatures. In Japan, it threatens to disrupt what the seasons mean. Springtime arrives with hanami – weeks of picnics and petals – as the blossom sweeps north from Okinawa to Hokkaido in a blaze of pink and white.

The timing matters beyond the aesthetic. Japan’s tourism industry relies on the $9bn a year generated by cherry blossom season. Such is the craze in the country that a town near Mount Fuji cancelled this year’s festivities because it was being overrun by visitors in search of “Instagrammable” spots.

Prof Aono’s work suggested that March temperatures in Kyoto have risen by several degrees since the early 19th century – enough to shift peak bloom by weeks rather than days. His records suggest that this century is much hotter than previous ones. The pattern is not unique to Japan. Since 1921, the US has recorded peak bloom dates for the cherry trees Japan had given as a gift to Washington a century ago. In both cases, it has advanced by about a week.

Another researcher will now maintain and update the records. Prof Aono learned classical Japanese script to read historical documents and reconstructed centuries of bloom dates. A millennium of book-keeping sounds permanent. But it depends on decades of effort by individuals whose lives are finite.

The information ultimately rests on a 1939 effort to compile a chronology: the seamless 1,200‑year dataset began as a painstaking act of archival recovery. By 1956 a Japanese meteorologist, Hidetoshi Arakawa, had made an intellectual leap. Writing about Kyoto’s cherry blossoms, he argued that the dates of their flowering were more than cultural markers of spring: they were climate records. By the late 1960s researchers had expanded the dataset and used it to analyse long‑term trends.

To the Japanese, the flowering cherry has always been more than a plant and its significance has been woven into the fabric of their history. In the 10th-century masterpiece The Tale of Genji, arguably the world’s first novel, there’s a whole chapter on the cherry-blossom festival staged in the imperial palace. Nine centuries later the Meiji restoration of rapid industrialisation promoted it as a symbol of both modernity and loyalty to the emperor. Disputes over the blossom’s origin periodically burst open in east Asia. The stakes could not be higher. Earlier blooms due to global heating risk breaking the natural rhythms that give meaning to sakura’s fleeting beauty.