Human influence increases the likelihood of extremely early cherry tree flowering in Kyoto

The full flowering of Kyoto's cherry trees in 2021 was observed on the 26th of March, the earliest date recorded in over 1200 years. An early shift of the flowering season is consistent with Kyoto's warming climate and could have serious repercussions for the local economy. It is therefore crucial to assess how human activity impacts flowering dates and alters the likelihood of extremely early flowering. To make this assessment, our study combines a risk-based attribution methodology with a phenological model that estimates full flowering dates from daily temperature data. We employ 14 state-of-the-art climate models that provide ensembles of simulations with and without the effect of anthropogenic forcings, and, using the simulated temperatures at Kyoto, we obtain representations of the cherry flowering season under different climatic conditions. An observationally-based correction is also applied to the simulated temperatures to introduce the effect of urban warming. We find a significant anthropogenic shift in the mean flowering season of over a week, about half of which is due to urban warming. By the end of the century and under medium emissions, the early shift is estimated to further increase by almost a week. Extremely early flowering dates, as in 2021, would be rare without human influence, but are now estimated to be 15 times more likely, and are expected to occur at least once a century. Such events are projected to occur every few years by 2100 when they would no longer be considered extreme.

Automated summary

The full flowering of cherry trees in Kyoto occurred on March 26, 2021, the earliest date in over 1200 years. This early shift in flowering season is a consequence of Kyoto's warming climate, which may have serious implications for the local economy. A study combining risk-based attribution methodology and a phenological model reveals that human activity, particularly urban warming, significantly contributes to the shift in flowering dates. By the end of the century, the early shift is projected to increase further, making extremely early flowering more common occurrences.