Parsing complex terrain controls on mountain glacier response to climate forcing
Introduction
Glacier hypsometry and existing ice volume exert a first order control on how a glacier's mass and spatial footprint change in response to a climate perturbation (Cuffey and Paterson, 2010; Oerlemans, 2001). However, local topography sometimes strongly affects accumulation and ablation processes on small (on the order of 10−1 to 100 km2) mountain glaciers (Benn and Lehmkuhl, 2000; Carrivick et al., 2015; Garg et al., 2017; Hock, 1999; Hoffman et al., 2007; Hughes, 2010; Kuhn, 1995), such that surface mass balance can be highly variable between proximal glaciers. Thus, the volume and area changes that different glaciers across a mountain range undergo are not necessarily expected to be uniform or to coincide perfectly with the primary factors that control climatic conditions such as elevation, aspect, or latitude (Benn and Evans, 2010). For example, analysis of 1848 mountain glaciers located from 70° to 74° N in northeastern Greenland revealed no spatial pattern in the last century of glacier volume change (Carrivick et al., 2019). Latitudinal air temperature gradients do not necessarily drive complex variations in mountain ice volume.
For an example of complexity in the glacier-climate relationship outside the Arctic, one study in the Canadian Rockies showed that amidst the wider pattern of glacier retreat from the mid-20th century to the early 2000s, the area of the smallest glaciers essentially did not change (DeBeer and Sharp, 2007). In this Canadian Rocky Mountain setting (DeBeer and Sharp, 2009) and other settings such as the Austrian Alps (Carrivick et al., 2015) such unexpected persistence of small glaciers has been explained as a decoupling from regional climate: Mountain glaciers retreat into topographically favorable settings (“refugia”) where the glacier is shaded from radiation and fed by snow avalanching or preferential deposition of wind-drifted snow, enabling glacier persistence in a climate otherwise unsuitable for maintaining glacier ice. Similarly, the sparsely glaciated terrain within Glacier National Park (GNP), U.S.A., exhibited overall glacier thinning rates that were near zero during the period 2000–2018 (Menounos et al., 2018). Previous analyses of regional snowlines (Matthes, 1942) and the topographic setting of GNP glaciers (Allen, 1998) concluded that GNP glaciers generally reside in locally favorable, refugial settings, potentially explaining these recent minimal glacier thinning rates.
In contrast to the refugia effect, there are circumstances where glaciers are far out of balance with climate, with remaining small glaciers most prone to wastage and most vulnerable to disappearance. For instance, an assessment of glacier change since the Little Ice Age (LIA) in Patagonia showed that small glaciers had the highest rates of area loss (Meier et al., 2018). Likewise, recent glacier change measured across western North America (Menounos et al., 2018) show that small glaciers in the Northern Interior region of British Columbia experienced greatest thinning from 2000 to 2018. Collectively, these studies emphasize the complexities in interpreting the underlying controls on geometric responses of small mountain glaciers to overall changing climate.
In this paper we unravel the controls of initial glacier volume, ice thickness, elevation, and local refugia effects in governing the fate of glaciers in GNP over the last approximately 150 years, since the LIA glacial maximum. We start by simply analyzing glacier area changes from LIA to present and identify glaciers that persisted versus diminished to <0.1 km2. Next, we define terrain parameters that approximate topographically driven mass balance processes and perform principal component analyses to constrain multivariance. Finally, we evaluate the expected stability of LIA glaciers with respect to various controls and make comparisons to observations of glacier change.
Section snippets
Study area and climate forcing
The mountainous terrain of GNP in the U.S. Northern Rocky Mountains (Fig. 1) is characterized by glacially carved valleys, cirques, horns, and arêtes (Fig. 2a). During the period of cooling associated with the LIA (14th to 19th centuries) GNP glaciers advanced (Carrara, 1989) and deposited distinct moraines, distinguishable as centuries (and not millennia) young because of the sharply crested morphology and lack of vegetative cover. Tree ring dates of 1860 and 1859 from just inside the Jackson
Data and methods
To ensure a complete and consistent inventory of glaciers present on the modern, 21st century landscape, we complemented the glacier margin dataset available for 28 named glaciers in and near GNP derived from 2015 WorldView imagery (Fagre et al., 2017) with five additional unnamed glaciers identified during an inventory of 2005 National Agriculture Imagery Program (NAIP) imagery (Fagre et al., 2019). We distinguished glaciers from perennial snow fields using the criteria that glaciers had to be
Results
LIA glaciers in GNP were not long valley glaciers (Fig. 1). At the LIA glacial maxima there were 82 cirque glaciers >0.1 km2, individually <5 km2 in area, but collectively comprising 56 km2 of ice coverage in and near GNP. By the beginning of the 21st century, according to our inventory, only 33 modern glaciers >0.1 km2 remained, meaning that 49 glaciers diminished. Modern glaciers cover 75% less area, at 14.4 km2 of ice coverage. This is greater than the 50% reduction in percentage glacier
Discussion
The glaciers in GNP at the LIA glacial maxima were cirque type glaciers (Figs. 1, 2) and were therefore less subject to rapid terminus retreat than would be expected for valley-type glacier area altitude distributions (Oerlemans, 2001). In general, the small and/or low elevation LIA glaciers in GNP have diminished to <0.1 km2, while the large and/or high elevation glaciers have persisted. This is the expected pattern of glacier loss due to regional climate change since the LIA when considering
Conclusions
When glaciated mountain ranges are subject to climate warming, glaciers with large initial ice volume, high median elevation, aspects that are shaded and preferable for wind loading (leeward), and that are situated at the base of cliffs delivering snow avalanches, will likely persist longer than glaciers with small initial ice volume, low median elevation, and less favorable terrain settings. Yet beyond this generality, how do complex terrain factors relate to glacier area change? We addressed
Data availability
Little Ice Age and modern glacier extents analyzed in this paper are publicly available: https://www.sciencebase.gov/catalog/item/5b1564ebe4b092d9651e1cee
https://www.sciencebase.gov/catalog/item/5c8152d5e4b09388244762be
https://www.sciencebase.gov/catalog/item/58af7022e4b01ccd54f9f542
Author contributions
All authors contributed to the conceptualization of this study. CF developed the methodology, executed analyses, created figures, and wrote the original manuscript draft. DF was responsible for project administration. JH provided supervision. All authors contributed to writing via review and editing.
Declaration of Competing Interest
The authors declare that they have no conflict of interest.
Acknowledgements
This work was funded by the U.S. Geological Survey Land Resources Mission Area, Research and Development Program. Adam Clark, Chelsea Martin-Mikle, and Lisa McKeon contributed expert local knowledge. Erich Peitzsch advised classification and regression tree methods, helped with principal component analysis, and provided invaluable insight on data visualization. Christopher Nuth wrote the original script for calculating aspect and slope that was adapted for use in this study. Ben Hills provided
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