Glaciers gain mass through snowfall and lose mass through melting and sublimation (when water evaporates directly from solid ice). Glaciers that terminate in a lake or the ocean also lose mass through iceberg calving. Those that end in the ocean are called tidewater glaciers, and they have more complex cycles of advance and retreat than glaciers that terminate on land, at least on annual and decades-long time scales. Even in a stable climate, such glaciers can experience periods of rapid retreat that are more influenced by seafloor topography and ocean circulation at their terminus than in recent climate conditions. (climate.gov ) Global climate has changed rapidly with an average increase of 1.5°F over the past 100 years. While this number may seem insignificant, noticeable changes have occurred in the glaciers in the park. In 1850, glaciers numbered around 150. By 1966, the number dropped to 50 named and unnamed glaciers. As of 2009, 26 glaciers remained. (USGS 2010). Melt runoff from glaciers in mountain environments provides more than 50% of the world’s fresh water supply. Populations downstream depend on this dwindling source not just for drinking water, but also as a means of dilution of pollutants generated at lower elevations. As climate warms, less snow and more rain falls in winter. Spring runoff is happening earlier, leaving less water for the drier months, possibly causing some streams to become intermittent, impacting stream ecology. Data were not available for Grinnell Glacier for 1900. However, interpolation, a means of estimating a value within a given set of data, can be used to get an estimate of the area for that year. Extrapolated data finds zero ice at Grinnell Glacier in 2039. Glaciers are no longer considered glaciers when their area diminishes to 25 acres. -Judy McLirath, USF Tampa
