Brown, C.E., Clague, J. and Reidel, J., 2010, Modern area-altitude balance ratios of glaciers on Mount Baker, Washington: Geological Society of America Abstracts with Programs, Vol. 42, No. 5, p. 229

Modern area-altitude balance ratios of glaciers on Mount Baker, Washington

Glaciers in the North Cascade Range and Coast Mountains in the Pacific Northwest have thinned and retreated during the twentieth century. Researchers have examined the impact of recent climate change on these glaciers using tools such as the Accumulation Area Ratio (AAR) and Area-Altitude Balance Ratio (AABR), which provide estimates of modern and former glacier mass balance and Equilibrium Line Altitudes (ELA). AABRs have not been reliably established for glaciers in the Pacific Northwest, including Mount Baker, the focus of our study. We used a set of National Agriculture Imagery Program (NAIP) color aerial photographs (scale 1:12,000) taken at the end of the 2009 ablation season and a 10 m DEM to map at high resolution the areal distribution of snow and ice/firn on Mount Baker and to precisely determine the 2009 ELA of its glaciers. Glaciers <1 km2 and those with significant debris cover were not included in the analysis. We compared our data to the long-term mass balance data of the North Cascade Glacier Climate Project (NCGCP), and determined both the 2009 and zero net balance AABRs. We then calculated ELA depressions for Mount Baker glaciers at the Little Ice Age maximum, based on the mapped extent of eighteenth and nineteenth-century lateral and end moraines. The 2009 AAR values range from 0.18 to 0.49; the highest values are associated with the largest glaciers. Glacier size and hypsometry are likely responsible for most of the differences in the AAR and AABR values. 2009 and steady-state ELA values based on NCGCP data were found to correlate well for a zero net balance AAR of 0.66. Modern AABRs are approximately 3-3.3 for the larger Mount Baker glaciers. These values are much higher than 2-2.2, the range that is assumed to be representative of mid-latitude glaciers. The ELA depression on Mount Baker at the peak of the Little Ice Age is comparable to that determined in the North Cascades by previous researchers. Given the widespread availability of public-domain high-resolution ortho-imagery for glacierized areas in the United States, we recommend that it be used with field-based measurements for ELA reconstruction and alpine climate studies. These images also can help identify ancient glacier margins and refine methods of glacier reconstruction.