Terminus behavior and response time of North Cascade glaciers, Washington, U.S.A.

Abstract Observation of the terminus behavior of 38 North Cascade glaciers, Washington, U.S.A., since 1890 shows three different types of glacier response: (1) Continuous retreat from the Little Ice Age (LIA) advanced positions from 1890 to approximately 1950, followed by a period of advance from 1950 to 1976, and then retreat since 1976. (2) Rapid retreat from 1890 to approximately 1950, slow retreat or equilibrium from 1950 to 1976, and moderate to rapid retreat since 1976. (3) Continuous retreat from 1890 to the present. Type 1 glaciers are notable for steeper slopes, extensive crevassing and higher terminusregion velocities. Type 2 glaciers have intermediate velocities, moderate crevassing and intermediate slopes. Type 3 glaciers have low slopes, modest crevassing and low terminusregion velocities. This indicates that the observed differences in the response time and terminus behavior of North Cascade glaciers in reaction to climate change are related to variations in specific characteristics of the glaciers. The response time is approximately 20–30 years on type 1 glaciers, 40–60 years on type 2 glaciers and a minimum of 60–100 years on type 3 glaciers. The high correlation in annual balance between North Cascade glaciers indicates that microclimates are not the key to differences in behavior. Instead it is the physical characteristics — slope, terminus velocity, thickness and accumulation rate — of the glacier that determine recent terminus behavior and response time. The delay between the onset of a mass-balance change and initiation of a noticeable change in terminus behavior has been observed on 21 glaciers to be 4–16 years. This initial response time applies to both positive and negative changes in mass balance.