Mass balance loss of Mount Baker, Washington glaciers 1990–2010 Mauri Pelto 1 * and Courtenay Brown 2 1 Dept. of Environmental Science, Nichols College, Dudley MA 0157, USA 2 Department of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada, V5A 1S6 Abstract: Mount Baker, North Cascades, WA, has a current glacierized area of 38.6 km 2 . From 1984 to 2010, the North Cascade Glacier Climate Project has monitored the annual mass balance (Ba), accumulation area ratio (AAR), terminus behaviour and longitudinal profiles of Mount Baker glaciers. The Ba on Rainbow, Easton and Sholes Glaciers from 1990 to 2010 averaged À0.52 m w.e. a À1 (m a À1 ). Terminus observations on nine principal Mount Baker glaciers, 1984–2009, indicate retreat ranging from 240 to 520 m, with a mean of 370 m or 14 m a À1 . AAR observations on Rainbow, Sholes and Easton Glaciers for 1990–2010 indicate a mean AAR of 0.55 and a steady state AAR of 0.65. A comparison of Ba and AAR on these three glaciers yields a relationship that is used in combination with AAR observations made on all Mount Baker glaciers during 7 years to assess Mount Baker glacier mass balance. Utilizing the AAR–Ba relationship for the three glaciers yields a mean Ba of À0.55 m a À1 for the 1990–2010 period, 0.03 m a À1 higher than the measured mean Ba. The mean Ba based on the AAR–Ba relationship for the entire mountain from 1990 to 2010 is À0.57 m a À1 . The product of the mean observed mass balance gradient determined from 11 000 surface mass balance measurements and glacier area in each 100-m elevation band on Mount Baker yields a Ba of À0.50 m a À1 from 1990–2010 for the entire mountain. The median altitude of the three index glaciers is lower than that of all Mount Baker glaciers. Adjusting the balance gradient for this difference yields a mean Ba of À0.77 m a À1 from 1990 to 2010. All but one estimate converge on a loss of À0.5 m a À1 for Mount Baker from 1990 to 2010. This equates to an 11-m loss in glacier thickness, 12–20% of the entire 1990 volume of glaciers on Mount Baker. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS glacier mass balance; North Cascades; accumulation area ratio; balance gradient Received 24 August 2011; Accepted 17 April 2012 INTRODUCTION Glaciers have been studied as sensitive indicators of climate for more than a century (Forel, 1895). Annual mass balance measurements are the most accurate indicator of short-term glacier response to climate change (Haeberli, 1995). The importance of monitoring glacier mass balance was recognized during the International Geophysical Year (IGY) in 1957. For the IGY, a number of benchmark glaciers around the world were chosen where mass balance would be monitored. This network has proven valuable, but in many areas, the number of glacier is limited; for example, there is just one benchmark glacier in the conterminous United States South Cascade Glacier (Figure 1) (Fountain et al., 1991). Glacier mass balance varies because of geographic characteristics such as aspect, elevation and location with respect to prevailing winds. Because no single glacier is representative of all others to understand the causes and nature of changes in glacier surface mass balance throughout a mountain range, it is necessary to monitor a significant number of glaciers (Fountain et al., 1991). The North Cascade region contains more than 700 glaciers, which covered 250 km 2 (Post et al., 1971; Granshaw and Fountain, 2006). The North Cascade Glacier Climate Project (NCGCP) was founded in 1983 to monitor 10 glaciers throughout the range and identify the response of North Cascade Range, WA, glaciers to regional climate change (Pelto, 1988). The annual observations include mass balance, terminus behaviour, glacier surface area and accumulation area ratio (AAR). Annual mass balance (Ba) measurements have been continued on the eight original glaciers that still exist. Two glaciers have disappeared: the Lewis Glacier and Spider Glacier (Pelto, 2008). In 1990, Easton and Sholes Glaciers were added to the annual balance programme to offset the loss. The AAR is the fraction of a glacier that is in the accumulation area at the end of the melt season; an AAR of 0.90 indicates that 90% of the glacier has retained firn at summer’s end. Three of the glaciers currently monitored by the NCGCP, Easton, Rainbow and Sholes Glaciers, are on Mount Baker. A stratovolcano, Mount Baker is the highest peak in the North Cascades at 3286 m. Mount Baker has the largest contiguous network of glaciers in the mountain range with 12 significant glaciers covering 38.6 km 2 and ranging in elevation from 3250 m to 1320 m a.s.l. (Figure 1). That there are three glaciers on this single mountain with long-term mass balance records of at least 20 years is unique globally. A combination of AAR observations and annual balance measurements on Mount Baker glaciers *Correspondence to: Mauri Pelto, Dept. of Environmental Science, Nichols College, Dudley, MA 0157, USA. E-mail: mspelto@nichols.edu HYDROLOGICAL PROCESSES Hydrol. Process. 26, 2601–2607 (2012) Published online 17 July 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9453 Copyright © 2012 John Wiley & Sons, Ltd.