Alaskan Glacier Length and Area Responses to Natural and Anthropogenic Climate Changes and Non-climatic Forcings

Are glaciers responding to anthropogenic climate change (temperature and/or precipitation), natural climate variation, or other forcings? Commonly, it is all of the above. Little doubt the total Earth record of glacier changes points to recent anthropogenic climate changes as the major source of glacier area and length shrinkage. In Alaska, a host of variable phenomena is at work. Length and area changes of McCall Glacier and other simple, nearly debris-free glaciers in the Brooks Range, are probably among the truest indicators of climate change; however, in those examples, the response times are one to three centuries¹, and so they mix climate-change signals of the end of the Little Ice Age with more recent, dominantly anthropogenic climate change. Similarly sized glaciers in the Chugach Mountains and Saint Elias Range (not considering surging and tidewater calving glaciers) respond in length and area to climate changes on time scales of a few decades or less^2,3, and so they could better reflect anthropogenic climate changes; however, they are also influenced by landslides and glacier lakes, both of which are influenced by climate and tectonics. The difficulty of isolating discrete causes of glacier variations means that we must rely on glacier changes across entire regions of glaciers^4,5. We look toward a further advance, whereby we examine glacier sensitivity to climate change on various timescales. Differing glacier response times can be dealt with by considering many glaciers in a given region, and averaging the change signals of glaciers of similar sizes and then binning the response signals according to size increments. A remaining stochastic problem in deconvolution of multiple causes of glacier responses is seismicity, which can be dealt with by considering proximity to faults and isoseismic shaking radii. These approaches require a glacier-by-glacier measurement approach, such as by GLIMS (Global Land Ice Measurements from Space, www.glims.org). GLIMS is making progress with single time-snapshot mapping and more selective time series mapping, but we need systematic glacier boundary and mass balance time series.

¹ C. Delcourt et al. 2007, The Cryosphere Discuss. 1, 385-409.
² T. Johannesson et al. 1989, J Glaciol. 35, 355-369.
³ S.C.B. Raper & R.J. Braithwaite 2009, The Cryosphere 3, 183-194.
⁴ A.A. Arendt et al. 2002, Science 297, 382-386.
⁵ A.A. Arendt et al. 2009, Ann. Glaciol. 50, 1-7.