While record-sized snowpack and attendant flooding in the Rocky Mountains have made headlines this spring, the long-term trend for snowpack levels in the Rockies tells quite a different story, according to climate scientists.
Snowpack has in fact been declining in recent decades, and a new U.S. Geological Survey-led study shows the decrease since the 1980s is more significant than at any other time in the past 1,000 years.
Numerous studies have documented snowpack decline over the past 50 to 100 years, but Gregory Pederson, a USGS research scientist, wondered how recent losses stacked up against those of the past millennium.
To put the recent observations in a historical context, Pederson and his colleagues examined tree rings to reconstruct winter snow accumulation in the mountains that drain to the Colorado, Columbia and Missouri river basins -- which collectively provide 60 percent to 80 percent of the water needs for more than 70 million people.
"It is amazing that tree growth across the West tells the snowpack story like our snow courses do, and it tells it quite skillfully over the past 1,000 years," Pederson said. The study appears this week in the journal Science.
Reading tree rings
Tree rings indicate snowpack levels in two ways: Lower-elevation trees like Douglas fir and ponderosa pine produce thicker growth rings during large snowpack years because more water is available. Higher-elevation trees like hemlock and subalpine larch show the opposite -- heavier snowfall that takes longer to melt means the growing season is shorter, so the growth rings are thinner.
The team analyzed 66 tree ring chronologies, which were gathered over the past 20 years by numerous researchers. Pederson and his colleagues spent much of the last 10 years collecting new data to ensure the record was as current as possible.
To collect the data, scientists bored small holes into trees and extracted wood cores about the width of a pencil. They did this to at least 20 to 30 trees to build chronologies for each location.
"It doesn't really hurt the tree at all; they seal that up pretty quick," Pederson said.
Most of the living trees sampled are only 300 to 800 years old, so the team also took samples from dead, fallen trees to extend the record to more than 1,000 years.
After analyzing hundreds of thousands of tree rings, the researchers found only two instances of sustained low snowpack in the northern Rockies comparable to the 20th century from about 1300 to 1330 and about 1511 to 1530. However, those dips were not as severe as current declines.
"You have a pretty severe early 20th century and especially a post-1980s decline that is really isn't matched most anywhere in the paleo-climate record," Pederson said.
Also, typically when there is more snow in the northern Rockies, there is less snow further south, and vice versa. This "North-South dipole" is caused by natural fluctuations in sea surface temperatures that change on an annual and decadal basis, including El Niño and La Niña, which determine storm tracks.
However, after the 1980s, the "North-South dipole" disappeared and snowpack declined across the entire range, indicating temperature is playing an increasingly important role in snowpack levels. This West-wide decline appeared to occur only one other time in the tree-ring record during the 1350s and early 1400s when temperatures were nearly as warm as today.
"We've seen this in the instrumental record, but this puts it in context of multiple centuries of data," said Greg McCabe, a USGS climatologist in Denver. "This really gives us a better idea of the full range of climate variability."
Confirming earlier studies
The latest findings support conclusions from other studies suggesting that 30 to 60 percent of recent snowpack decline is due to human-induced warming. Pederson did not state exact percentages for declining snowpack because snow varies greatly across the landscape. Instead, he averaged the change in snowpack for each watershed in the study area.
"It's a really well-done study," McCabe said. "It is both spatially and temporally extensive."
Having a long-range view of snowpack levels is beneficial to more than climate scientists -- water managers will find such information useful when studying the cycles of droughts and wet periods, McCabe said.
The contrast between the study's findings and this year's record snowpack is a prime example of the difference between a single weather event and climate, Pederson said. Weather is the state of the atmosphere at a given time and place, whereas climate is the average condition over multiple decades.
"The actual definition of a 'climate normal' is a 30-year moving average," Pederson said. "So when you hear people say, 'Well, snowpack this year is 300 percent of normal,' it's really anomalous to the last 30 years. ... But we're extremely anomalously low compared to the last century and millennium."
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