The latest study compares records of snow and ice thickness and densities over the years, the factors used to calculate mass. The glaciers have lost mass as melting outpaced new snow and ice accumulation, and for all three, the losses were especially dramatic over the past 15 years, according to the USGS study.The three glaciers — Gulkana and Wolverine in Alaska and South Cascade in Washington — are considered benchmarks for those in alpine and maritime climates because they closely parallel other glaciers in their regions. They have also been the subject of close scientific scrutiny since 1957.