(CNN) -- A shrinking snowpack in the Rocky Mountains may make it harder to slake the thirst of a growing population in the Western United States, according to new research from the U.S. government and several universities.
The frosted peaks of the Rockies store much of the water that allows modern cities to thrive in the largely arid Western United States. But despite record snows over the past winter, frozen precipitation in the mountains where the Colorado, Missouri and Columbia rivers begin has been receding over the past several decades at a pace not seen since the medieval era, the U.S. Geological Survey has found.
The shrinking snowpack serves as a "bank account" for those river systems, which supply drinking water and electric power to more than 70 million people from the Pacific Coast to the upper Great Plains, said Greg Pederson, the study's lead researcher.
"What water managers are more or less facing is not that the precipitation is going away," said Pederson, a USGS ecologist at the Northern Rocky Mountain Science Center in Bozeman, Montana. "More of the precipitation will be coming as rain rather than snow. They'll be dealing with what is harder to capture in the water management world and less reliable.
"It's free storage for us. It's dams we don't have to build," he added.
The findings were released online last week by the journal Science. Researchers from the USGS and the universities of Colorado, Wyoming, Arizona and Western Ontario used the patterns of tree rings to track how much snow was retained on the peaks of the Rockies over about 800 years.
A thicker ring indicates more snow for some species, like Douglas firs or ponderosa pines; for others, like subalpine larch, more snow leads to thinner rings. Those rings can be anchored back to a calendar year, "kind of like fingerprints," Pederson said.
The study dates to 2004. Although Rocky Mountain states recorded record snowpacks in the winter of 2011, "It's really just a blip on this long-term decline in the northern Rockies," he said.
Climate researchers expect global average temperatures to increase by 1 to 3 degrees Celsius (1.8 to 5.4 degrees Fahrenheit) over the next half-century, so there's "no reason to expect" the trend to stop, he said.
Pederson said the findings add further context to studies that indicate a 30% to 60% decrease in the snows that gather at the upper elevations of the mountains. They also suggest that in the past three decades, a longstanding pattern that typically leads to lower snowpacks in the southern Rockies when the northern end of the range gets more has "uncoupled," he said.
"It's a two-part story," he said. "In a nutshell, what you're seeing is synchronous, declining snowpacks across the West since the 1980s." Based on the new study, the last time that pattern was seen as the mid-1300s to early 1400s, he said.