Tuesday, May 11, 2010
Glacial History of Climate Change in Skagit Valley, Washington
Glaciers are invaluable recorders of climate change because they are sensitive to both winter and summer weather, dramatically reveal climate trends, and leave clear evidence on the landscape of past climate changes. There is a rich glacial record of climate change in Skagit Valley, which holds more than 390 active glaciers. This record spans the last 30,000 years, and includes evidence of multiple glaciations from the last ice age, glacial advance and retreat cycles of the last 7,000 years, and detailed photographic and mass balance records from the last half-century.
During the alpine phase of the last ice age, large tributary glaciers dammed Skagit Valley at two locations and created large lakes. Macrofossils recovered from these lakes, as well as reconstructed glacial equilibrium line altitudes provide an important measure of the magnitude of climate change and the ensuing environmental response. The last ice age reached a climax when the massive Cordilleran Ice Sheet filled the valley to elevation 6500 m a.s.l. Ice sheet glaciation had dramatic effects on the drainage pattern of the Skagit valley, which once flowed north to the Fraser River. Moraines mark the last advance of relatively small alpine glaciers at the end of the last ice age about 13,000 years ago. This advance was recorded in terrestrial systems globally, and is related to rapid climate fluctuations caused by rapid drainage of glacial Lake Aggasiz. Evidence from the Middle Fork Nooksack Valley indicates that Deming Glacier advanced and retreated four times within a 3,000 year period.
Buried forest beds below glaciers on Mount Baker and near Garibaldi Peak provide important records of climate fluctuations spanning the last 7,000 years. On Mount Baker alone, eight buried forest layers have been identified and radiocarbon dated. When combined with similar regional records, these data show that alpine glaciers advanced and retreated as many as seven times in the Holocene. Advances were growing larger through this period, with the most recent advance being the largest at most sites in western North America. This trend indicates that solar forcing associated with changes in axial tilt controlled a long term climate cooling trend, with cyclic climate changes at the millennial scale. Further, it is clear that the recent loss of glaciers in this region canít be explained by natural cycles of climate change alone.
Rapid retreat of alpine glaciers in Skagit Valley since the end of the Little Ice Age has been documented by aerial photography and geologic mapping, and in the past 50 years, by glacial mass balance measurements. Results from an NPS mapping program indicate that North Cascades National Park has lost about 50% of its glacial area since 1900 A.D., comparable to the loss of alpine glaciers at regional and global scales. Oblique aerial photographs of park glaciers taken by Austin Post in the late 1950ís, when compared to photographs taken by John Scurlock in the past decade, provide dramatic evidence of climate warming. Glacial mass balance monitoring in the watershed provides and important index of climate at high elevations in the North Cascades. This program has also quantified the glacial contribution to summer base flow on Skagit River at 120-180B gallons. Since 1993, the net loss of ice is 400B gallons, or one monthís continuous flow on Puget Soundís largest river.