Climate Denial Crock of the Week

After an unsettlingly warm winter across North America, budding trees are coming to life earlier than usual, setting them up for the next arctic outbreak, which is spreading across the country now. (see first few minutes of video above)

The Washpost piece below is specific to DC’s cherry blossoms, but the problem applies to apple, cherry and other growers across the states.

Washington Post:

As fast-warming winters encourage early blooms, they also risk exposing the trees to damage from cold snaps that remain common into March at the mid-latitudes where the ornamental cherry trees have long thrived. A close call is in the forecast this weekend, with temperatures possibly dropping into the upper 20s in Washington.

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Winter is warming faster than any other season in the northern hemisphere, and that is sending confusing signals to cherry trees and other plant life.

While hibernating for winter, the trees keep track of how long temperatures spend below about 45 degrees. Once the weather starts to warm, though, they start another count, Walsh said, gauging whether it’s safe to begin growing again based on how much temperatures rise.

Increasingly mild winters mean the cherry trees risk blooming so early they could still face damaging hard freezes, as occurred in Washington in 2017.

That year, the Tidal Basin blossoms were on the verge of a mid-March peak bloom, almost two weeks earlier than average, when temperatures dropped into the lower to mid-20s on back-to-back mornings. That wiped out nearly half of Washington’s cherry blossoms.

When temperatures remain that cold for several hours, it harms the blossoms if the buds have begun to flower. Even colder temperatures, as well as sunlight reflecting off snow cover, can also damage cherry trees’ bark and internal tissue, Walsh said. Temperatures are forecast to skirt the danger zone for blossom damage this weekend, with lows expected in the upper 20s to near 30 early Sunday and early Monday mornings.

Both Washington and Japan, where the Yoshino variety of ornamental cherry trees originate, have seen marked trends of earlier blooms. Over the past century, the average peak bloom date in Washington has advanced from around April 5 to March 31 as spring temperatures have warmed.

National Park Service data indicates peak bloom has occurred before April 5 in 15 of the last 20 years; this year’s peak is also expected to be early, probably within the next week.

In Kyoto, Japan, where records go back 1,200 years, the date of peak bloom has rapidly shifted earlier over the past century and a half. Peak bloom arrived earlier than ever in 2021, on March 26.

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Months after the risk of freezes has passed, cherry trees face risks of similar damage from heat stress. When temperatures surge toward the triple digits, they sometimes must shut down their growing and energy-gathering processes to survive, said Richard Olsen, director of the National Arboretum.

And Olsen said the stress of such high temperatures can compound when heat is slow to dissipate on summer nights, an increasingly common but often overlooked consequence of global warming and climate change.

When that happens, the trees produce so little energy during the day, they don’t have enough to devote to repairing themselves and their regular metabolism to stay alive, he said.

Both heat and freezes can cause damage to the trees’ bark and trunks that produces vertical cracks and gashes that affect their growth and make them vulnerable to further problems, Walsh said.

The issues create a “perennial question” about the survivability of the cherry blossoms in a changing climate, Olsen said. The situation isn’t dire just yet, though.

“It’s more going to be a chronic issue that we can solve with diligent planting and replacement of the species mix we use,” he said.