Scary Spring: Earlier Blooms Are a Sign of Climate Change

The stirrings of springtime show nature awakening. Coaxed by warming air and stronger sunlight, flowers unfurl on cherry trees and eager green buds burst forth from horse chestnuts. A little hope returns, as bees buzz and birds build nests. This year, it’s been happening a little earlier — and the reason isn’t hard to find.

In Washington DC, the city’s famous cherry trees — the originals a gift from Japan in 1912 — reached peak blossom on March 21, rather earlier than a century ago. In Kyoto, where these trees’ cousins live, records show the first blooms advancing by a week over the past century, alongside a temperature increase of more than 5 degrees Fahrenheit. Planetary warming is driving a similar trend globally, shifting the timings of not only the first leaves and flowers, but bird migrations and egg-hatchings. These changes have accelerated in the past 20 years.

We often think of global warming as something made evident only through difficult scientific measurements of atmospheric CO2 levels or average sea-surface temperatures. But signs of warming are all around us in distortions of the historical rhythms of the natural world, adding ominous overtones to the joyous springtime greenery. All these shifts reflect nature under increasing pressure — and hold unpredictable consequences for our well-being and the resilience of global ecosystems in coming decades.

The study of the timing of important biological events is known as “phenology,” and much of what we know about it comes from painstaking observations made over centuries. The longest time series recorded anywhere is for the cherry trees in Kyoto; remarkably, it goes all the way back to the year 812. This data, and a variety of other records — including data in the U.K. extending over 250 years — show that plants’ flowering and first-leaf dates remained fairly stable through the 19th century, then, along with rising planetary temperatures, started to creep earlier in the first half of the 20th century.

Since then, the changes have accelerated, while showing significant regional variation: Spring phenology has advanced by six days in China over the past 35 years and by 30 days in Switzerland. In Kyoto in 2021, the cherry trees bloomed on the earliest date in 1,200 years of meticulous record-keeping.

Adapting to the pace and unpredictability of planetary warming is difficult — not only for animals and humans, but for plants. One recent study examined shifts in the beginning and end of the growing season in the northern hemisphere over the past 30 years, comparing changes in temperature to plants’ responses. The researchers found that most plants lagged behind the pace of recent warming. Some even shifted their timings in the wrong direction — blooming later, rather than earlier. And some went dormant earlier in the fall, even though you’d think warmer autumns might extend their growing season.

These mismatches were more pronounced in landscapes dominated by human activities such as intensive farming. It’s not clear why, but here’s one possible reason: Species change their behavior not only in direct response to climate variables — temperatures, rainfall patterns and so on — but also in response to the shifting activities of other species with which they interact. The faster responses of species in undisturbed regions could reflect these species’ exposure to a broader spectrum of signals about the changing environment coming from many other plant species. In contrast, the sluggish plants in zones degraded by humans may be adapting slowly because they’re not getting signals from other plants.

Whatever the reason, plants’ lagging response is worrying. The consequences could be unpleasant for nature and humanity alike. It’s not so important, perhaps, precisely when the flowers bloom or birds hatch, but the integrity of the natural world depends on millions of such events taking place in delicate coordination and synchrony. When flowers bloom affects when bees can pollinate them, which later determines when birds and other animals find fruit to raise their young, which potentially provide food to other animals — including us. In nature, nothing happens in isolation.

How will global warming disturb these delicate relationships, accelerate the extinction of species, and drive up costs for human agriculture? Scientists don’t know. Nobody knows. We’re running a sweeping and dangerous experiment, and it would be far, far better if we never have to find out.