Glacier Response to Intrinsic Climate Variability in the Pacific Northwest

In documenting and interpreting a glacier's history, substantial advances or retreats are often interpreted as reflections of a shift in the climate. However, essentially random interannual variability is an intrinsic feature of the climate system. A steady climate is really a climate whose statistics are steady (i.e. same mean and variance). Glaciers will respond to interannual variations in temperature and precipitation, even though a mean shift in the climate has not necessarily taken place. Because glaciers typically have a dynamic response time that is much longer than a year, they will integrate this short-term forcing and respond with fluctuations at decadal and centennial timescales. These natural variations must be accounted for before confidently interpreting a geologic field observation as a reflection of a true shift in climate. We use two glacier models, one linear and the other dynamic, to investigate what magnitude of glacier length variabilty ought to be expected in response to typical interannual variations in climate. We focus on the Pacific Northwest of the United States, and in particular the question of whether advances during the late 19th Century, commonly attributed to the global Little Ice Age, might have arisen from natural variations of a steady climate state.