GREEN LAKE 4 - University of Colorado hydrologist Nel Caine cranks the hand augur's handle, and the razor-sharp steel blades bite the blue-black lake ice.

The blades grind and scrape as they bore a 6-inch hole through the ice, churning up shavings.

Finally, the rasping stops, and burbling water whooshes into the drill hole, filling it with slush.

"All day for half a pint of water," Caine muttered last February as he lowered a plastic bottle, strapped to the end of a ski pole, down through the hole.

Every winter month for the past 23 years, Caine has clicked into cross-country skis and trudged uphill six miles, from CU's Mountain Research Station west of Boulder to a remote lake along the Continental Divide.

Caine, 64, makes the monthly winter trips to Green Lake 4 to collect water for a long-term lake-chemistry study. But the treks have also yielded a scientific windfall - a record of thinning lake ice that tells an unexpected story about climate change.

When Caine began the study in 1982, he had to drill through 4.5 feet of late-March ice to reach water. He screwed extra sections onto the auger's end to get through all the ice, which thickens throughout the winter and peaks in late March.

But a few years ago, he noticed that the augur extensions weren't needed.

The ice had thinned significantly. In 2000 and 2002, the ice was about 18 inches thinner than in 1982.

"One of the things I never paid any attention to, but used to record in my field book every time, was how much ice we had to drill through to get to the lake water," said Caine, a researcher at CU's Institute of Arctic and Alpine Research, known as INSTAAR.

Caine pulled out his old field notebooks and plotted late-March ice thickness on a piece of graph paper. The dots formed a clear pattern, revealing a steady decline at Green Lake 4 since 1982.

"My first thought was, 'Hey, things must be getting warmer. Is this global warming?' " Caine recalled. "That would be the first thing that would jump into anybody's mind."

Along with shrinking mountain glaciers, the thinning of lake and river ice is viewed by many climate researchers as a response to a warming world. Lake and river ice now melts about two weeks earlier than it did a century ago at many mid- and high-latitude locations in the Northern Hemisphere, according to the Intergovernmental Panel on Climate Change.

Caine maintains a small weather station at Green Lake 4. Less than a mile north of the lake, CU's Mountain Research Station operates the D-1 weather station on Niwot Ridge, with unbroken climate records stretching back to 1953.

The ice on Green Lake 4 has been thinning by two-thirds of an inch per year since 1982.

But weather records indicated that the temperatures at D-1 have been holding steady during that period. If it's not getting warmer, why is the ice on Green Lake 4 thinning?

Caine believes two mechanisms account for it. Since 1982, more groundwater has flowed into the lake in the fall and more snow has fallen on its surface in the winter.

October-through-March precipitation (the amount of water the melted snow would yield) has increased by a quarter inch per year.

Caine suspects the extra snow acts as an insulating blanket to warm the lake's surface. Groundwater flowing into the lake keeps it stirred up and inhibits the formation of thick ice, he said.

So thinning ice at Green Lake 4 may be unrelated to global climate change - unless worldwide warming is responsible for the increased snowfall and runoff.

"The classic story is that it's going to get warmer, and the warming is going to increase the same amount or more as you go to higher elevations," said INSTAAR ecologist Tim Kittel.

"But that may not be the case at places like Niwot Ridge," Kittel said. "And that makes it really difficult to suggest what's going to happen in the future for our alpine and subalpine sites."

Green Lake 4 is a 13-acre lake in a glacier-scoured bedrock basin just east of the Continental Divide and Arikaree Glacier, at an elevation of 11,550 feet. Runoff from the Green Lakes Valley and the adjacent Silver Lake Valley, home to Arapaho Glacier, empties into Silver Lake. North Boulder Creek, a tributary of the South Platte River, flows out of Silver Lake.

The Silver Lake watershed is owned by the city of Boulder and supplies about 40 percent of the city's water, according to Craig Skeie, water source facilities manager for the city.

The watershed, including the glaciers, is closed to the public.

Caine estimates that melted ice from the Arapaho and Arikaree glaciers provides 150 to 200 acre-feet of water per year - less than 1 percent of the city's annual consumption. An acre-foot of water is the amount that two typical four-person families use in a year.