“The Kyoto Cherry Blossom record is incredibly valuable for climate change research because of its length and the strong sensitivity of flowering to springtime temperatures (warmer springs = earlier flowering, typically),” Benjamin Cook, a research scientist at Columbia University who specializes in reconstructing climate data from the past, said in an email.
Unique for its longevity, the cherry blossom time series shows the average peak bloom date was relatively stable for about 1,000 years, from about 812 to 1800. But then, the peak bloom dates slope abruptly downward, revealing a shift earlier and earlier in the spring.
“Since the 1800s, warming has led to a steady trend toward earlier flowering that continues to the present day,” Cook said. “Some of this warming is due to climate change, but some is also likely from an enhanced heat island effect due to increased urbanization of the environment over the last couple of centuries.”
Due to its length, the data record is a treasure, having been maintained by emperors, aristocrats, governors and monks over the centuries. Most recently, Yasuyuki Aono, a scientist at Osaka Prefecture University, has tracked the blossoms and posted the data online.
Cherry blossoms have burst unusually early all over Japan this spring. In Tokyo, they reached full bloom March 22, their second-earliest date on record and earliest since 1953, according to Japan Forward. It marked the ninth earlier than normal bloom in a row, the Forward reported, following nearly a week during which high temperatures climbed to at least 68 degrees (20 Celsius). The average full bloom date is March 25.
In Kyoto, the shift toward earlier blooms has been most rapid in the past 100 to 150 years. In 1850, the average flowering date was about April 17; now, it’s closer to April 5. During this time, the average temperature in Kyoto has risen by about 6 degrees (3.4 Celsius).
The shape of the trend line through the blossom flowering dates resembles a hockey stick — with a flat handle but sharply sloping blade. Indeed, many studies documenting temperature changes over the past 1,000 to 2,000 years have taken on a similar shape, reflecting a relatively stable climate through the mid-1800s and a quickly warming one ever since.
“Evidence, like the timing of cherry blossoms, is one of the historical ‘proxy’ measurements that scientists look at to reconstruct past climate,” said Michael Mann, a professor and climate scientist at Pennsylvania State University, who has published numerous studies on temperature changes in recent millennia. “In this case, that ‘proxy’ is telling us something that quantitative, rigorous long-term climate reconstructions have already told us — that the human-caused warming of the planet we’re witnessing today is unprecedented going back millennia.”
The trends toward earlier blooms seen in Japan have also been observed in the cherry trees at the Tidal Basin in Washington, D.C., gifted by Japan in 1912. Since 1921, the National Park Service has kept a record of their peak flowering dates.
In the century of records, the average peak bloom date of the Yoshino cherry trees in Washington has advanced six to seven days, from about April 5 to March 31.
In 2021, the blossoms in Washington peaked on Sunday, several days ahead of the recent 30-year average. In 2020, they peaked March 20, tied for the third earliest on record.
This trend toward earlier bloom dates potentially makes the cherry blossoms more vulnerable to spring freezes, explained Theresa Crimmins, director of the USA National Phenology Network, which tracks the timing of the emergence of plant species.
“For trees that flower before they leaf out, like cherry trees, [a spring freeze] is especially damaging because they don’t generate another set of flower buds," Crimmins wrote in an email. “If the flower buds are frost-damaged, that is it for their flowering - and fruiting - for the year. There have been recent massive impacts to fruit crops because of early blooming followed by subsequent freezing events.”
“Another key concern,” Crimmins wrote, “is that very early flowering can lead to mismatches in the presence of the open flowers and the presence of the pollinators dependent upon the flowers as a food source. Bees are a key pollinator for cherry trees, and if bees were not active at the time of the very early cherry flowering this past spring in Japan, the trees may have had poor pollination. In addition, those bees may be going hungry without the cherry flowers as a food resource when they do become active.”