Stability of Ice-Wedges in Kobuk Valley National Park and the Noatak National Preserve, 1951–2009

Swanson, David

Ice-wedge polygons are characteristic arctic landforms that are vulnerable to change in a warming climate. Degradation of ice wedges causes subsidence along the margins of ice-wedge polygons, which can produce small ponds that are visible on remotely sensed images. Ice-wedge polygons are common in the Ahnewetut Wetlands of Kobuk Valley National Park (AHN) and the Lower Noatak Lowlands of the Noatak National Preserve (LNL) in northwestern Alaska. I placed 300 random 1-ha plots in each of these regions on images from 1951-1952 and 2006-2009, and mapped surface water present within the portions of the plots occupied by ice-wedge polygons. The absolute area of small ponds was difficult to compare between image dates due to differences in image quality and potential short-term variations in wetness of the landscape. Thus I used the distribution of water relative to ice-wedge polygon centers as a measure of ice-wedge degradation. I mapped water by thresholding panchromatic images (pixels darker than the threshold were classified as water) and then computed the distance index 2a/(a+b) for each water pixel, where a and b are the distances to the nearest and second nearest polygon centers, respectively. This index ranges from near zero at polygon centers to near one at polygon margins, and thus can be used to analyze the location of water within the ice-wedge polygons. In the AHN study area, water was concentrated near polygon centers in 1951-52 and on polygon margins in 2006-2009, due to the appearance of numerous water-filled troughs over degraded ice wedges. The proportion of polygons with marginal ponds, indicating ice-wedge degradation, increased from 7% to 22% between the two time periods. Visual assessment of the plots confirmed that new polygon-marginal ponds formed between the image dates. In the LNL study area, water along polygon margins was more prevalent in 2006-2008 than 1951, but the change was not as marked as in the AHN study area: in 1951, 9% of the polygons had marginal ponds, while in 2006-08 this proportion had increased to 12%. Reasons for the greater amount of ice-wedge degradation in the AHN study area are unclear, but it may be due to AHN's position near the warm climatic limit for ice wedge development or its different topographic setting. A 1977 fire in the LNL study area had no effect on the prevalence of polygon-marginal ponds there in 2006-08. Continued ice-wedge degradation is expected in both study areas as a result of recent climate warming.

Stability of Ice-Wedges in Kobuk Valley National Park and the Noatak National Preserve, 1951–2009

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