Sierra Nevada Glacier Change
Glaciers of the Sierra NevadaThis page highlights my research on glacier change in the Sierra Nevada. A publication and my thesis entitled "Quantifying Twentieth Century Glacier Change in the Sierra Nevada, California" are available: Thesis (2008) Publication (2011)
Numerous small alpine glaciers occupy the high elevation regions of the central and southern Sierra Nevada, California. These glaciers change size in response to variations in climate and are therefore important indicators of climate change. While knowledge of glacier shrinkage in the Sierra Nevada is common, there was little quantitative information on the magnitude or rate of reduction. The purpose of my thesis was to define the number and spatial extent of the Sierra Nevada glacier population, quantify the magnitude and rate of change in glacier extent for a small subset of glacier, and compare the rates of change against climate variations.
Where do glaciers occur in the Sierra Nevada?
Glaciers are found high in the Sierra Nevada, generally above 10,000 feet in elevation, on the north sides of steep mountains from Yosemite National Park in the north to Sequoia National Park in the south. Sierra Nevada glaciers are typically located in mountain cirques, which are small bowl-like depressions on the sides of mountains which were created by previous glaciers.The number of glaciers in the Sierra Nevada depends on the definition. The definition of a glacier is a body of perennial ice or snow that moves. Movement of ice can be observed as cracks in the ice known as crevasses. Because Sierra Nevada glaciers are small and remote previous glacier inventories have placed the number between 50 and 500 glaciers. My survey, based on USGS topographic maps (1:24,000 scale), counted over 1700 glaciers and snow and ice patches. Many snow and ice bodies which do meet the definition of a glacier. I estimated the number of "true glaciers" at 118 glaciers in the Sierra Nevada based on theoretical considerations.
How have glaciers changed over the past century?Sierra Nevada glaciers have become smaller over the past century. This is evident from early photographs which reveal these glaciers occupying their moraines around 1900. Today, the glaciers have receded up the mountains and appear deflated. I quantified the surface area change by selecting fourteen glaciers throughout the Sierra Nevada and measured the change using historical photographs, geologic evidence, and field mapping. I determined the glaciers lost and average of 55 percent of their surface area by 2004. The decrease in surface area of individual glaciers ranged between 31 and 78 percent.I selected a subset of seven glaciers to determine the rate of change over the past century. The results indicated that glaciers rapid retreated occurred over the first half of the twentieth century beginning in the 1920s in response to warm/dry conditions and continued through the mid-1970s. Recession ceased during the early 1980s, when some glaciers advanced. Since the 1980s each of the seven study glaciers resumed retreat.
Glacier Repeat PhotographyComparisons of the repeat photographs reveal that all ten glaciers surveyed in 2003 and 2004 have experienced a reduction in ice volume and surface extent over the past century. Repeat photography is a valuable tool in determining change through time, especially when combined with aerial photos and field measurements. Please contact me if you are interested in using these photographs.