Coaxed by warming air and stronger sunlight, flowers unfurl on cherry trees and eager green buds burst forth from horse chestnuts.
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.
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.
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.
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.
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.
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.
How will global warming disturb these delicate relationships, accelerate the extinction of species, and drive up costs for human agriculture? Scientists don’t know.
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