Late little ice age glacier fluctuations in the Cascade Range of Washington and northern Oregon

During the late Holocene, the large valley glaciers in the Cascade Range of Washington and northern Oregon advanced and retreated several times, depositing a series of nested moraines that lie within a few kilometers downvalley of their current terminal positions. Because glaciers are sensitive to changes in precipitation and temperature, glacier-length chronologies derived from these moraines are often used as a proxy record of past climate variation.; Direct observations of glacier extents prior to the widespread collection of air photos in the 1940s are limited to indirect field data. A regionally applicable lichen growth curve with an accuracy of +/-10 years was developed to characterize the timing of glacier fluctuations using moraine ages between about 1850 and 1950. The resulting data show decadal variations in glacier extent and suggest that, since the late 19th century, glaciers in the Cascade Range of Washington and northern Oregon have responded synchronously to regional climate patterns.; The poorly constrained ages of pre-19th-century moraines, however, means that these older landforms may be a result of substantially earlier ice advances of indeterminate age(s), and thus they may not reflect regional or global patterns of climate fluctuations. Using historical climate data characteristic of the 20th-century record, a linearized numerical model that produces glacier-length variations from temperature and precipitation data was used to simulate possible length changes that would be anticipated under naturally varying climate conditions. These variations can account for virtually all of the glacier-length fluctuations identified in the moraine record. Attributing the 19th century (or other late Holocene) moraines to distinct climate changes thus requires that any such climatic shift be larger and (or) longer in duration than the observed 20th-century climatic variability. If any systematic climate shifts have occurred, the glacial record suggests that they have been of a magnitude no greater than the modeled natural variability and the moraine record may not provide substantial evidence that they ever existed.