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.

“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first realised that something may have happened to Aono when they noticed in January that his university web page was no longer active.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.

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.”