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UNH Study: Slower Warming Trend Stabilizes Seasonal Change

Allegra Boverman for NHPR
The 2018 foliage, "leaf-out" season in Ashland.

A first-of-its-kind study from UNH shows that slower rates of global warming can make seasonal change more predictable.

The study looked at the period between 1998 and 2012, when global temperatures increased more slowly than in years prior. Scientists still aren't sure exactly why.

But UNH researcher Jingfeng Xiao says it made a difference for what’s known as phenology – the timing of seasonal change. His study is the first to look at this period of slowed warming in the context of phenology in North America.

“Phenology regulates the exchange of carbon dioxide, water and energy between the land’s surface and atmosphere,” he says. “So if we have an earlier leaf-out or late fall phenology, we could expect to have a longer growing season.”

That’s what he says happened when warming was occurring faster: trees were losing their leaves later and later in autumn, and growing them back earlier and earlier in spring, lengthening the summer season when photosynthesis is at its peak.

“With a longer growing season, plants could absorb more carbon from the atmosphere,” he says. “We could expect to see croplands would have higher yield and we could also expect to have more food production.”

But he says this shift also has downsides – particularly around the water cycle. 

“If plants have a longer growing season, the plants will need to use more water, and water will be lost and released into the atmosphere,” Xiao says. “So … it could be an issue for some regions, particularly those regions with water shortage issue.”

Xiao’s study found that when warming slowed, these seasonal changes stabilized.

He says there was still some variability from year to year, but no continuation of the long-term trend of a longer summer observed when warming was more rapid.

“If we could potentially reduce greenhouse gas emissions … the average surface temperature may not consistently increase anymore,” Xiao says. “Then … the phenology will stabilize as well, so we will not expect to see a longer growing season anymore.”

He says it means human action to continually slow the rise in global temperatures could help preserve the seasonal timing on which many human systems rely.

Annie has covered the environment, energy, climate change and the Seacoast region for NHPR since 2017. She leads the newsroom's climate reporting project, By Degrees.
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