Abstract
Abstract. We describe a method to calculate regional snow line elevations and annual equilibrium line altitudes (ELAs) from daily MODIS imagery (MOD02QKM) on large glaciers and icefields in western North America. An automated cluster analysis of the cloud-masked visible and near-infrared bands at 250 m resolution is used to delineate glacier facies (snow and ice) for ten glacierized regions between 2000–2011. For each region and season, the maximum observed value of the 20th percentile of snow-covered pixels (ZS(20)) is used to define a regional ELA proxy (ELAest). Our results indicate significant increases in the regional ELA proxy at two continental sites (Peyto Glacier and Gulkana Glacier) over the period of observation, though no statistically significant trends are identified at other sites. To evaluate the utility of regional ELA proxies derived from MOD02QKM imagery, we compare standard geodetic estimates of glacier mass change with estimates derived from historical mass balance gradients and observations of ZS(20) at three large icefields. Our approach yields estimates of mass change that more negative than traditional geodetic approaches, though MODIS-derived estimates are within the margins of error at all three sites. Both estimates of glacier mass change corroborate the continued mass loss of glaciers in western North America. Between 2000 and 2009, the geodetic change approach yields mean annual rates of surface elevation change for the Columbia, Lillooet, and Sittakanay icefields of −0.29 ± 0.05, −0.26 ± 0.05, and −0.63 ± 0.17 m a−1, respectively. This study provides a new technique for glacier facies detection at daily timescales, and contributes to the development of regional estimates of glacier mass change, both of which are critical for studies of glacier contributions to streamflow and global sea level rise.
Highlights
In the century, wastage of mountain glaciers and icecaps in response to anthropogenic climate change is expected to increase mean global sea levels by 0.051–0.124 m (Cazenave and Nerem, 2004; Raper and Braithwaite, 2006; Radicand Hock, 2011)
ELAest anomalies derived for Peyto Glacier region, for example, are significantly correlated (r = 0.75) with those obtained for the Columbia Icefield, but regional equilibrium line altitudes (ELAs) at Place Glacier are not significantly correlated with those observed at the Lillooet Icefield, which is located only 75 km to the northwest (Fig. 1)
Values of MODIS-derived ELA proxies will be sensitive to the availability of cloud-free imagery, though the use of lowess smoothers provides a conservative estimate of the maximum value of ZS(20)
Summary
Wastage of mountain glaciers and icecaps in response to anthropogenic climate change is expected to increase mean global sea levels by 0.051–0.124 m (Cazenave and Nerem, 2004; Raper and Braithwaite, 2006; Radicand Hock, 2011). Regional glacier mass change can be estimated from (a) groundbased measurements (Radicand Hock, 2010), (b) anomalies in regional gravity fields (Arendt et al, 2009; Jacob et al, 2012), (c) empirical models (Radicand Hock, 2011), (d) distributed mass balance models (Machguth et al, 2009), and (e) geodetic measurements (Schiefer et al, 2007; Tennant et al, 2012) These approaches may be limited in terms of either spatial or temporal resolution, or both. The MOD02QKM product contains calibrated and geolocated radiances in the red (0.620–0.670 μm) and nearinfrared (0.841–0.876 μm) bands with spatial resolutions of 250 m at nadir Others have used these bands to calculate subpixel fractional snow cover and monitor snow line evolution (Lopez et al, 2008; Sirguey et al, 2009), and a recent study developed estimates of mass change from MODIS-derived surface albedos (Dumont et al, 2012). Basin-scale estimates of glacier mass change are useful for diagnosing glacier contributions to streamflow and sea level rise, and the approach developed here complements other estimates of glacier mass change
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