Larch trees in the North Cascades
credit:
Jeremy Littell
Growth rings in some trees that grow at high altitudes correlate with snowpack.
credit:
Jeremy Littell
Trees, it turns out, hold the key to understanding the climate history of the West. In a recent study published in the journal Science, researchers took core samples from trees in 66 sites in the Columbia, Missouri, and Colorado river basins to gain a better understanding of weather patterns going back hundreds of years, and compared those patterns with recent changes in climate.
Jeremy Littell is a research scientist with the Climate Impacts Group at the University of Washington and a co-author of the study.
“In some cases the living trees are approaching a thousand years or even more. Some of these trees were witnessing climate at the time of the Norman invasion. And that’s exciting to scientists because being able to look that far back in history gives some context to the recent changes in climate,” he said.
The tree samples show that for the past thousand years, anyway, climate has cycled through wetter and drier periods — but the current warming period is out of whack with the historical pattern. Littell says consider the sub alpine larch tree. When there’s less snowpack, these trees grow better so their growth rings are wider and this century scientists are consistently seeing more wide growth rings.
“If you’re a larch that’s been standing in the North Cascades for 1,000 years then the last 50-100 years look quite different than what had been going on before that,” Littell said.
The tree record shows that the recent decreases in snowpack across the West are inconsistent with tree records that go back hundreds of years. And that, Littell explains, could be our fault.
“There are a couple of analyses that suggest that somewhere between half and two thirds of the trend in snow pack and water resources are attributable to human-caused climate warming.”
Think of snowpack as a sort of watery bank account — where water is stored in solid form and then slowly dispensed as the snow melts throughout the drier summer months. Those later water installments are critical for crop irrigation and the health of river ecosystems.
But climate models for the Northwest suggest that as average annual temperatures rise, we’ll get less snow and more rain that will be immediately absorbed. Sort of like cashing that water check upfront, instead of saving it for a sunny day.
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