Cirque glacier sensitivity to 21st century warming: Sperry Glacier, Rocky Mountains, USA Joel Brown a,b, ⁎, Joel Harper a , Neil Humphrey c a Department of Geosciences, University of Montana, 32 Campus Dr., Missoula, MT 59812, USA b Center for Geophysical Investigation of the Shallow Subsurface, Boise State University, 1910 University Dr., Boise, ID 83725, USA c Geology and Geophysics Department 3006, University of Wyoming, Laramie, WY 82071, USA abstract article info Article history: Received 25 February 2010 Accepted 3 September 2010 Available online xxxx Keywords: glacier cirque glacier glacier modeling The interpretation of climate change based on the behavior of small cirque glaciers is not always straightforward or unique. In this study of Sperry Glacier, Glacier National Park, Montana, we model future change of the glacier under 11 different warming scenarios. The scenarios vary from no warming from present conditions to warming at a linear rate of 10 °C/century. We assume constant precipitation and only consider change invoked by warming. Our cellular automata model is based on simple rules that account for mass balance gradient, aspect, avalanching, and the flow of ice to redistribute mass. We constrain the model with glaciological data including georadar-measured ice depth, field-measured surface mass balance, and field-mapped ice surface topography. Under the most probable temperature increase based on downscaled OA-GCM output for the IPCC A1B scenario, we conservatively estimate the glacier persisting through at least 2080. By comparing glacier volume responses to different warming scenarios we elucidate a relationship between the magnitude of temperature change and the sensitivity of the glacier to small variations in the temperature increase. We find that the greater the magnitude of the temperature increase, the less sensitive the glacier area and volume become to slight differences in the warming rate. If we generalize this relationship to the region, we expect that a small change in climate will produce varying responses for glaciers throughout the region, whereas the glacier response to a large change in climate will likely be very similar over the entire region. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Area and volume adjustments of mountain glaciers have important impacts on society and natural systems. Most notable are the contributions of mountain glaciers to sea level rise (e.g., Meier and Dyurgerov, 2002; Bahr et al., 2009), and the influence of mountain glaciers on water resources and geomorphic hazards (e.g., Moore et al., 2009; Leiva et al., 2007). Since mountain glaciers are considered sensitive indicators of climate, they are used to detect and monitor local climate change in regions not typically monitored by instru- mentation (e.g., Haeberli et al., 2007). Further, observations of glacier change are independent from potential issues related to the location, instrumentation, and processing of weather station data. Consequently, the general global retreat of mountain glaciers (Dyurgerov and Meier, 2000) is commonly cited as corroborating evidence for 20th century climate warming of the instrumental temperature record. For example, Oerlemans (2005) used the length records of 169 glaciers located around the world to construct a quantitative record of 20th century warming, and found that the glacier record agreed remarkably well with the instrumental record. With projected increases to the rate of warming in the 21st century (IPCC, 2007), a general acceleration of rates of glacier retreat appears likely. For many small mountain glaciers, projecting their recent rate of retreat forward implies they will disappear within the 21st century (e.g., Nesje et al., 2008). However, the small glaciers within a region do not always advance or retreat at the same rate as large ones (Granshaw and Fountain, 2006; Fountain et al., 2009) and past advances or retreats of a glacier may not indicate how that glacier will change in the future. As mountain glaciers become small, many begin to occupy little more than the area below their cirque headwall. Near the cirque, winter snow accumulation is often enhanced from wind drifting and avalanching from the steep cliffs above, while radiation shading reduces summer ablation (Kuhn, 1995). Consequently, cirque glaciers are sometimes considered products of topography and therefore inappropriate indicators of climate variability and change (Kuhn, 1995). In addition, climate change within a region is not typically spatially uniform (e.g., Shindell and Faluvegi, 2009). Therefore, similar glaciers in different basins within the same region may not experience identical changes in climate and thus may have slightly different volume and area changes. Global and Planetary Change xxx (2010) xxx–xxx ⁎ Corresponding author. Geosciences Department, The University of Montana, 32 Campus Drive #1296, Missoula, MT 59812-1296, USA. Tel.: +1 406 370 1775; fax: +1 406 243 4028. E-mail address: jbrown@cgiss.boisestate.edu (J. Brown). GLOBAL-01613; No of Pages 8 0921-8181/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2010.09.001 Contents lists available at ScienceDirect Global and Planetary Change journal homepage: www.elsevier.com/locate/gloplacha Please cite this article as: Brown, J., et al., Cirque glacier sensitivity to 21st century warming: Sperry Glacier, Rocky Mountains, USA, Glob. Planet. Change (2010), doi:10.1016/j.gloplacha.2010.09.001