Just five days ago I posted a paper that addressed the loss of glacial mass in South America. I learned of those losses in a Scientific American article which also described glacial losses in western North American glaciers published in Geophysial Research Letters. The latter paper was not available at that time but it is now: Heterogeneous Changes in Western North American Glaciers Linked to Decadal Variability in Zonal Wind Strength by B. Menounos, R. Hugonnet, D. Shean, A. Gardner, I. Howat, E. Berthier, B. Pelto, C. Tennant, J. Shea, Myoung‐Jong Noh, F. Brun, and A. Dehecq
Here are the summaries, introduction, and the last section on discussion and implications. If the links do not work, go to the online version of the paper here.
Summary
Western North American (WNA) glaciers outside of Alaska cover 14,384 km2 of mountainous terrain. No comprehensive analysis of recent mass change exists for this region. We generated over 15,000 multisensor digital elevation models from spaceborne optical imagery to provide an assessment of mass change for WNA over the period 2000–2018. These glaciers lost 117 ± 42 gigatons (Gt) of mass, which accounts for up to 0.32 ± 0.11 mm of sea level rise over the full period of study. We observe a fourfold increase in mass loss rates between 2000–2009 [−2.9 ± 3.1 Gt yr−1] and 2009–2018 [−12.3 ± 4.6 Gt yr−1], and we attribute this change to a shift in regional meteorological conditions driven by the location and strength of upper level zonal wind. Our results document decadal‐scale climate variability over WNA that will likely modulate glacier mass change in the future.Plain Language Summary
Glaciers in western North America provide important thermal and flow buffering to streams when seasonal snowpack is depleted. We used spaceborne optical satellite imagery to produce thousands of digital elevation models to assess recent mass loss for glaciers in western North America outside of Alaska. Our analysis shows that glacier loss over the period 2009–2018 increased fourfold relative to the period 2000–2009. This mass change over the last 18 years is partly explained by changes in atmospheric circulation. Our results can be used for future modeling studies to understand the fate of glaciers under future climate change scenarios.
Introduction
Anthropogenic warming is expected to drive continued mass loss from alpine glaciers throughout the remainder of this century (Marzeion et al., 2017). Relative to other alpine environments, glaciers in western North America (WNA) are expected to play a minor role in future sea level rise (Levermann et al., 2013; Marzeion et al., 2018; Radić et al., 2014) given their small, cumulative volume (Huss & Farinotti, 2012). These ice masses, however, represent important freshwater reservoirs that provide late‐summer meltwater runoff when seasonal snowpacks have been depleted (Frans et al., 2018; Moore et al., 2009) or during years characterized by drought (Jost et al., 2012). Water managers require up‐to‐date assessments of how these ice masses have and will likely change in the coming decades.
A global assessment of glacier mass change by Gardner et al. (2013) included an estimate of WNA glacier mass loss of 14 ± 3 Gt yr−1 for the period 2003–2009 based on the extrapolation of sparse in situ surface mass balance (SMB) measurements (Cogley, 2009). Other approaches to estimate glacier mass change, such as satellite laser altimetry and satellite gravimetry employed by Gardner et al. (2013) for other glacierized regions, have so far proven unsuccessful for WNA due to sparse repeat‐track spacing at lower latitudes and challenges associated with deconvolving competing mass change signals (groundwater, seasonal snow, reservoir volumes and glacio‐isostatic adjustment). Such methods perform particularly poorly for lower‐latitude mountain ranges with disperse glacier coverage, such as those that characterize WNA (Gardner et al., 2013; Jacob et al., 2012).
Repeat mapping of surface elevation through stereophotogrammetry provides an additional approach to measure glacier thickness change on a regional scale that can circumvent spatial and temporal biases imposed by using SMB observations to estimate regional mass change. Geodetic surveys exist for many glacierized regions of WNA (e.g., Basagic & Fountain, 2011; Schiefer et al., 2007), but no study samples all of these regions in a systematic fashion. Novel methods to infer elevation change from medium resolution satellite imagery (Brun et al., 2017) coupled with automated processing of both medium and very high resolution optical satellite imagery (Noh & Howat, 2017; Shean et al., 2016) provide new opportunities to improve global estimates of glacier mass change.
The primary motivation of our paper is to provide the first, regionally complete estimate of glacier mass change for WNA for the period 2000–2018. We then use these data to (i) quantify the contribution of WNA glaciers to sea level rise over the last 18 years, (ii) determine the reliability and representativeness of existing WNA in situ SMB records, and (iii) assess the climatic drivers that affect mass change at the subregional scale.
Cutting to the chase....
Discussion and Implications
Our study provides a comprehensive assessment of glacier mass change (−6.5 ± 2.3 Gt yr−1) for nearly all glacierized terrain in WNA over the period 2000–2018. Our estimated rate of WNA mass change for the early period (−2.9 ± 3.1 Gt yr−1) is considerably less negative than the rate (−14 ± 3 Gt yr−1) previously reported for the period 2003–2009 (Gardner et al., 2013).In situ observations of SMB are invaluable given their temporal continuity and value in understanding climatic drivers of mass change (Hodge et al., 1998), but their use may bias regional estimates of mass change (Gardner et al., 2013). Our analysis (supporting information S1) shows that glaciological measurements of mass change are in broad agreement with geodetic estimates at the local scale, but they do not effectively sample large ice masses that dominate the regional signal of mass change from WNA, namely, large icefields in the southern Coast Mountains. Using available SMB measurements for 14 glaciers in WNA (supporting information S1), we calculate an average mass change of −874 ± 100 kg m−2 yr−1over the period 2000–2017. When multiplied by the total glacierized area of WNA, this value yields an annual mass loss of 13.6 ± 4.3 Gt yr−1, close to the value (14 ± 3 Gt yr−1) calculated by Gardner et al. (2013) using a similar approach. These values are twice as large as those based on our trend analysis. This discrepancy suggests that glaciers chosen for long‐term monitoring programs are losing mass more rapidly than the region as a whole. Our findings accord with a study by Fountain et al. (2009), who concluded that South Cascade Glacier, chosen for long‐term in situ SMB observations, lost three times more mass than glaciers in the north Cascade Mountains (subregion 11 of Table 1).
Cumulative mass loss from WNA glaciers over the period 2000–2018 could potentially account for 0.32 ± 11‐mm global sea level rise equivalent, about 0.6% of observed SLR over the period 1993–2017 (Nerem et al., 2018). Our estimate is an upper limit as it assumes that meltwater from glacier mass loss was directly conveyed to the ocean and not stored in intermediate locations (e.g., proglacial lakes formed over the last 18 years). While surface storage of this water might be small, it could contribute to aquifer recharge (Liljedahl et al., 2017).
One of the most surprising findings of our study is the dipole pattern of mass change between glaciers in the British Columbia central Coast [50–55°N] and the Cascade mountains [42–47°N] of the United States (Figure 1). Positive anomalies in zonal winds (250 hPa) over the Cascade Mountains imply a strengthening and southward shift in the jet stream between the early and late periods that would increase the frequency of mid‐latitude cyclones with attendant increases in precipitation. A change in meteorological conditions that favor changes in glacier mass are clearly revealed in the composite anomaly maps, especially for the central, southern Coast and Cascade glaciers (Figure 3). Glaciers of the southern Coast Mountains descend to lower elevations than those that flank high Cascade volcanoes. The partitioning of precipitation into either rain or snow may also explain why some of the Cascade glaciers gained mass during the recent period while Coast Mountain glaciers continued to experience strong thinning and mass loss. Future work employing surface mass and energy balance modeling at the glacier scale can be used to test this hypothesis.
The spatial distribution of subregional mass change is partly linked to regional changes in atmospheric circulation that affect accumulation and ablation. Others have noted the importance of zonal wind on controlling glacier mass balance (Marshall et al., 2011; Shea & Marshall, 2007), and some studies attribute a decline in zonal wind strength to explain the long‐term (1950–2005) decline in winter snow water equivalent (Luce et al., 2013). In our study, anomalies in zonal wind covary with many of the meteorological fields known to control glacier mass balance, namely, temperature and precipitation. Orographically enhanced precipitation in WNA is also favored when strong zonal flow delivers moist air masses that originate over the Pacific Ocean (Jarosch et al., 2012; Neiman et al., 2008). Regions of weak zonal wind also coincide with lower‐than‐average geopotential height over last 18 years.
It remains uncertain whether mass change observed over the last 18 years is related to natural climate variability known to affect glacier mass balance in WNA (Bitz & Battisti, 1999; Hodge et al., 1998; Moore & Demuth, 2001), stochastic variability, or whether these recent changes are related to anthropogenic climate change. We note no obvious relation between major climate indices and decadal changes in zonal wind strength or pressure at the subregional scale (supporting information S1). Using an ensemble of 25 global climate models, however, Luce et al. (2013) show that under a high emission scenario (RCP8.5), zonal wind (700 hPa) strength significantly decreases over the Cascade and Coast Mountains by the end of this century. Zonal winds weaken most over the central and southern Coast Mountains (50–55°N) but strengthen over southern latitudes of California (Luce et al., 2013). Based on the results of our study, weaker zonal winds would tend to favor stronger mass loss for glaciers in the Cascade and Coast Mountains.
Glaciers in both the CONUS and western Canada are expected to undergo continued mass loss throughout this century, even under moderate emission scenarios (Clarke et al., 2015; Frans et al., 2018; Huss & Hock, 2018). These changes will reduce or eliminate the thermal‐ and flow‐buffering capacity provided by glacier runoff for many watersheds, with implications for downstream ecosystems and water resources. If the last 18 years provide a suitable analogue for the next 30–50 years, future glacier change will be modulated by decadal‐scale climate variability. Like seasonal snow cover, improvement in understanding and forecasting climate variability at decadal time scales will be important to help guide estimates of glacier mass change for water management. Projections of future glacier mass change for both the CONUS (Frans et al., 2018) and western Canada (Clarke et al., 2015), like other regions, depend on well‐distributed observations of glacier area and mass change, and our results can be used to improve modeling efforts that seek to understand the fate of glaciers under future climate scenarios.
This glacial runoff can contribute to sea-level rise and it also represents water that can be stored underground for use meeting future water needs. The meltwater cannot, and doesn't have to be, stored everywhere there are melting glaciers but in some places it could be. Work to identify these areas should begin now.
Maria Gibson and I have looked at this in the Yakima Basin in Washington State, where the Yakima River is peaking earlier because the snowpack is melting. She identified places where the water could be stored in the subsurface. Here is the preliminary work. This does not include Maria's recent PhD dissertation.
Several years ago we proposed doing this in Peru but did not get funding to do so. Soon, perhaps.
Enjoy!
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