From roughly 800 to 1850, the blossom flowering time was fairly stable. While the bloom dates bounced around quite a bit from year to year during April, the long-term average hovered between April 10 and April 17 (the 100th to 107th day of the year).
But from 1850 to present day, the flowering has surged toward earlier dates at the rate of about one week per century. Consider that in 1850, the average flowering date was around April 17; now, it’s closer to April 6.
The bloom dates have advanced as March temperatures in Kyoto have rapidly risen. While there are different environmental factors that influence the flowering dates, usually the warmer it is in March, the earlier the cherry blossoms bloom.
Aono said the estimated temperature had warmed 6.1 degrees (3.4 Celsius) in Kyoto since 1820. In his temperature reconstructions dating back as far as 800 A.D. posted online, no other period was as consistently warm as the present.
The shape of the trend line through the blossom flowering dates resembles a hockey stick — with a flat handle but sharply sloping blade. Indeed, multiple 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.
Perhaps no long-term temperature reconstruction is more famous than the original “hockey stick” chart published by Penn State climate scientist Michael Mann nearly two decades ago. It showed that temperatures in the 20th century were warmer than any time in at least the past 1,000 years.
Mann was not the least surprised when presented with the Kyoto data. “Kyoto is just one location on the planet,” he said in an email. “But the large-scale warming of the past century is so distinct and widespread that it is increasingly evident from diverse records all around the globe.”
Both Kyoto’s cherry tree flowering and temperature data suggest that its climate is warmer than it has been in at least a millennium.
Interestingly, recent trends in Kyoto’s cherry blossom flowering dates are very similar to those in D.C.
In 1912, Japan gifted Washington 3,000 cherry trees and, since 1921, the National Park Service has kept a record of their peak flowering dates.
Washington’s peak bloom date has advanced about five days, from April 6 to April 1 between 1921 and 2017. At the same time, Kyoto’s peak bloom date has moved forward from around April 12 to April 5.
Just as warming March temperatures are likely to blame for the earlier bloom dates in Kyoto, the same holds true in Washington. Since 1921, Washington’s March temperatures have warmed at the rate of about 2.8 degrees per 100 years.
In both Kyoto and Washington, the warming trends and earlier blooms are most likely due to a growing urban heat island effect and increasing concentrations of greenhouse gases in the atmosphere.