Can climate warming induce glacier advance in Taylor Valley, Antarctica? Andrew G. FOUNTAIN, 1 Thomas A. NEUMANN, 2 Paul L. GLENN, 1* Trevor CHINN 3 1 Departments of Geology and Geography, Portland State University, PO Box 751, Portland, Oregon 97207, USA E-mail: andrew@pdx.edu 2 Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98195-1310, USA 3 R/20 Muir RD. Lake Hawea RD 2, Wanaka, New Zealand ABSTRACT. Changes in the extent of the polar alpine glaciers within Taylor Valley, Antarctica, are important for understanding past climates and past changes in ice-dammed lakes. Comparison of ground-based photographs, taken over a 20 year period, shows glacier advances of 2–100 m. Over the past 10 3 years the climate has warmed. We hypothesize that an increase in average air temperature alone can explain the observed glacier advance through ice softening. We test this hypothesis by using a flowband model that includes a temperature-dependent softness term. Results show that, for a 28C warming, a small glacier (50 km 2 ) advances 25 m and the ablation zone thins, consistent with observations. A doubling of snow accumulation would also explain the glacial advance, but predicts ablation-zone thickening, rather than thinning as observed. Problems encountered in modeling glacier flow lead to two intriguing but unresolved issues. First, the current form of the shape factor, which distributes the stress in simple flow models, may need to be revised for polar glaciers. Second, the measured mass-balance gradient in Taylor Valley may be anomalously low, compared to past times, and a larger gradient is required to develop the glacier profiles observed today. 1. INTRODUCTION The McMurdo Dry Valleys is one of the few ice-free regions in Antarctica. Consequently, it is one of the few places the glacially modified landscape can be used to infer past changes of the Antarctic ice sheet (Denton and Hall, 2000). Large changes in ice-sheet extent, relative to the scale of the dry valleys, have been documented (Hall and others, 2000; Higgins and others, 2000). In contrast, the extent of the local alpine glaciers has changed little since the Pliocene (Hall and others, 1993; Wilch and others, 1993). Yet the small changes in the alpine glaciers are important to the maintenance of the enclosed ice-covered lakes in the dry valleys and, in turn, to the ecosystems that inhabit the valleys (Fountain and others, 1999). We use repeat photog- raphy to quantify the change in extent of several glaciers over the past 20 years. To interpret these changes, we developed a numerical model to examine possible processes controlling the observed changes. Results from this study are important to several glaciological issues. First, the results bear on the possibility of using glacially modified terrain around these glaciers for paleoclimatic reconstruction. This is particularly apt in view of the climatic results obtained from the Taylor Dome ice core located about 100km away (Steig and others, 2000). Second, a history of glacier advance and retreat will help define the history of ice- dammed lake formation and drainage, a subject important to ecological evolution within the valleys (Fountain and others, 1999). Third, we use our model to explore the response of polar alpine glaciers to climatic variations, a subject that has not been as thoroughly explored as for their temperate counterparts. 2. SITE DESCRIPTION The McMurdo Dry Valleys are located in southern Victoria Land on the edge of the Antarctic continent (Fig. 1). They are named the ‘dry valleys’ (Scott, 1905) because the Trans- antarctic mountain range blocks the seaward expansion of the East Antarctic ice sheet (EAIS) and the valleys are largely empty of ice. A few outlet glaciers from the ice sheet terminate in the western ends of the valleys. Alpine glaciers populate the mountains, the largest of which reach the valley floor. Average annual air temperatures in the valleys vary between –30 and –178C, and summer air temperatures typically reach only a few degrees above freezing (Doran and others, 2002b). Consequently, the glaciers in the valleys are polar and are frozen to the substrate (Cuffey and others, 2000). Melting is restricted to the glacier surface and to the 20 m high vertical cliffs which define the glacier margin in the ablation zone (Chinn, 1985; Fountain and others, 1998). Streams originate at the glacier termini and flow for perhaps 10 weeks during the austral summer (McKnight and others, 1999). Some of the streams discharge into perennially ice- covered lakes, which are present in every valley. The annual mass exchange of these polar glaciers is relatively small compared to temperate glaciers. About 0.1 m of snow accumulates in the upper zones and about 0.1–0.2 m of ice is lost from the ablation zone (Bull and Carnein, 1970; Chinn, 1980; Fountain, unpublished data). Ablation in the accumulation zones is limited to sublimation and wind erosion. In the ablation zone, about 40–80% of the mass loss occurs through sublimation and the remainder is lost by melting (Lewis and others, 1998). The glacial history of the valleys is dominated by the activity of the EAIS. Taylor Glacier advanced almost to the eastern end of Taylor Valley about 70–100kyr BP (Hendy and others, 1979; Higgins and others, 2000). During the Last Glacial Maximum, lowered sea levels caused the Ross Ice Journal of Glaciology, Vol. 50, No. 171, 2004 *Present address: Landau Associates, Inc., 130 Second Avenue South, Edmonds, Washington 98020, USA. 556