Colorado scientists using ground-penetrating radar have found climate change is shrinking glaciers and other icy terrain in the Rocky Mountains — raising concerns about water supplies.

The Arikaree Glacier — likely more than 1,000 years old — has been thinning by about 1 meter a year over the past 15 years and will disappear completely in 25 years, a team of scientists concluded.

Their peer-reviewed research, unveiled Tuesday, found that rock glaciers and other ice that holds water west of Boulder also will vanish. The Arikaree Glacier, which feeds North Boulder Creek, had held steady from 1965 through 1997.

“What ice provides is insurance so that, when we have bad years, we can get more contribution from glaciers and permafrost,” said CU hydrologist Mark Williams of the Institute of Arctic and Alpine Research, lead author of the study and a principal investigator at CU’s Niwot Ridge station.

He compares melting ice to pulling from a bank account, saying more water may flow for now but that reserves needed to endure droughts won’t be there. Mountains that held snow and ice for hundreds of thousands of years, with fluctuations, rapidly are losing it, Williams said.

“We can’t get out of this. It is wired now, on the Colorado Front Range and in the Arctic,” he said. “There’s going to be less ability, on the water side, to address low flows in July and August … .

“Glacier National Park is not going to have glaciers in another couple decades. People are going to be upset about that. We’re in a loop we cannot get out of.”

A loss of sunlight reflectivity, as snow and ice disappear, will push temperatures higher, he added.

The Colorado scientists, supported by the National Science Foundation, determined that ice directly influenced by air temperatures has shown “clear signs of increased melt rates.” Their report said trends are “unlikely to change” under the current climate conditions.

The melting of glaciers and other ice in Colorado mountains jibes with a worldwide trend that scientists and governments have linked to climate change. The shrinking of Earth’s frozen “cryosphere” has been documented across Arctic sea ice, tundra and other areas that traditionally hold packed snow.

Alpine glaciers form when net snow accumulation leads to snow compressing and re-crystallizing into dense ice, which flows slowly to lower elevations, sculpting land. Over the past three decades, scientists measuring glaciers worldwide, using remote sensing and other methods, have documented shrinkage.

The work in Colorado focused on the size of the glacier, which had a maximum depth of 25 meters, shallower at the top and toes. Scientists also measured ice thickness in the Green Lakes Valley area below Niwot Ridge, which includes mounds of ice, dirt and rock that store water as well as subsurface ice and seasonally frozen lakes. They used radar, electrical devices and seismometers that send signals through subsurface ice.

Colorado water supply managers are tracking climate change effects.

“They are concerned, and the answer may be more storage,” Williams said. “That’s something people don’t like to hear … but there needs to be more local storage.”

Rather than build dams to create more reservoirs, some utilities are looking at underground storage of water in aquifers so that evaporation losses could be minimized.

Boulder draws water supplies from areas off Niwot Ridge. Denver Water relies on flows down from high-mountain snowfields, and rain, diverting water from creeks and rivers to supply 1.3 million metro Denver residents.

The latest science on changes in frozen terrain “helps with operational and long-term planning,” utility climate adaptation manager Laurna Kaatz said, adding that Denver Water keeps weather and streamflow records covering nearly a century.

“This helps ensure that shorter-term changes to streamflow, like that from ‘fossil ice,’ do not overly influence our decisions,” Kaatz said.

“Changes in glaciers and ice are indicators of change, and can help inform an understanding of the changes in magnitude and lifespan of our mountain snowpack. … Climate change is just one of many changes we’re planning for in the future to ensure we have a secure water supply.”

Bruce Finley: 303-954-1700, bfinley@denverpost.com or @finleybruce