Long-term mass balance records from Lemon Creek Glacier in Alaska show slightly declining mass balance with time. The mean annual balance for this glacier was − each year during the period of 1957 to 1976. Mean annual balance has been increasingly negatively averaging − per year from 1990 to 2005. Repeat glacier altimetry, or altitude measuring, for 67 Alaska glaciers find rates of thinning have increased by more than a factor of two when comparing the periods from 1950 to 1995 ( per year) and 1995 to 2001 ( per year). This is a systemic trend with loss in mass equating to loss in thickness, which leads to increasing retreat—the glaciers are not only retreating, but they are also becoming much thinner. In Denali National Park, all glaciers monitored are retreating, with an average retreat of per year. The terminus of the Toklat Glacier has been retreating per year and the Muldrow Glacier has thinned since 1979. Well documented in Alaska are surging glaciers that have been known to rapidly advance, even as much as per day. Variegated, Black Rapids, Muldrow, Susitna and Yanert are examples of surging glaciers in Alaska that have made rapid advances in the past. These glaciers are all retreating overall, punctuated by short periods of advance.