Glacier projections sensitivity to temperature-index model choices and calibration strategies

The uncertainty of glacier change projections is largely influenced by glacier models. In this study, we focus on temperature-index mass-balance (MB) models and their calibration. Using the Open Global Glacier Model (OGGM), we examine the influence of different surface-type dependent degree-day factors, temporal climate resolutions (daily, monthly) and downscaling options (temperature lapse rates, temperature and precipitation corrections) for 88 glaciers with in-situ observations. Our findings indicate that higher spatial and temporal resolution observations enhance MB gradient representation due to an improved calibration. The addition of surface-type distinction in the model also improves MB gradients, but the lack of independent observations limits our ability to demonstrate the added value of increased model complexity. Some model choices have systematic effects, for example weaker temperature lapse rates result in smaller projected glaciers. However, we often find counter balancing effects, such as the sensitivity to different degree-day factors for snow, firn and ice, which depends on how the glacier accumulation area ratio changes in the future. Similarly, using daily versus monthly climate data can affect glaciers differently depending on the shifting balance between melt and solid precipitation thresholds. Our study highlights the importance of considering minor model design differences to predict future glacier volumes and runoff accurately

Global glacier change in the 21st century: Every increase in temperature matters

Glacier mass loss affects sea level rise, water resources, and natural hazards. We present global glacier projections, excluding the ice sheets, for shared socioeconomic pathways calibrated with data for each glacier. Glaciers are projected to lose 26 ± 6% (+1.5°C) to 41 ± 11% (+4°C) of their mass by 2100, relative to 2015, for global temperature change scenarios. This corresponds to 90 ± 26 to 154 ± 44 millimeters sea level equivalent and will cause 49 ± 9 to 83 ± 7% of glaciers to disappear. Mass loss is linearly related to temperature increase and thus reductions in temperature increase reduce mass loss. Based on climate pledges from the Conference of the Parties (COP26), global mean temperature is projected to increase by +2.7°C, which would lead to a sea level contribution of 115 ± 40 millimeters and cause widespread deglaciation in most mid-latitude regions by 2100

A rockfall-induced glacial lake outburst flood, Upper Barun Valley, Nepal

On April 20, 2017, a flood from the Barun River, Makalu-Barun National Park, eastern Nepal formed a 2&ndash;3-km-long lake at its confluence with the Arun River as a result of blockage by debris. Although the lake drained spontaneously the next day, it caused nationwide concern and triggered emergency responses. We identified the primary flood trigger as a massive rockfall from the northwest face of Saldim Peak (6388&nbsp;m) which fell approximately 570&nbsp;m down to the unnamed glacier above Langmale glacial lake, causing a massive dust cloud and hurricane-force winds. The impact also precipitated an avalanche, carrying blocks of rock and ice up to 5&nbsp;m in diameter that plummeted a further 630&nbsp;m down into Langmale glacial lake, triggering a glacial lake outburst flood (GLOF). The flood carved steep canyons, scoured the river&rsquo;s riparian zone free of vegetation, and deposited sediment, debris, and boulders throughout much of the river channel from the settlement of Langmale to the settlement of Yangle Kharka about 6.5&nbsp;km downstream. Peak discharge was estimated at 4400&thinsp;&plusmn;&thinsp;1800&nbsp;m3&nbsp;s&minus;1, and total flood volume was estimated at 1.3&thinsp;&times;&thinsp;106&nbsp;m3 of water. This study highlights the importance of conducting integrated field studies of recent catastrophic events as soon as possible after they occur, in order to best understand the complexity of their triggering mechanisms, resultant impacts, and risk reduction management options. </div

Seasonally stable temperature gradients through supraglacial debris in the Everest region of Nepal, Central Himalaya

Rock debris covers about 30% of glacier ablation areas in the Central Himalaya and modifies the impact of atmospheric conditions on mass balance. The thermal properties of supraglacial debris are diurnally variable but remain poorly constrained for monsoon-influenced glaciers over the timescale of the ablation season. We measured vertical debris profile temperatures at 12 sites on four glaciers in the Everest region with debris thickness ranging from 0.08–2.8 m. Typically, the length of the ice ablation season beneath supraglacial debris was 160 days (15 May to 22 October)—a month longer than the monsoon season. Debris temperature gradients were approximately linear (r2 > 0.83), measured as –40°C m–1 where debris was up to 0.1 m thick, –20°C m–1 for debris 0.1–0.5 m thick, and –4°C m–1 for debris greater than 0.5 m thick. Our results demonstrate that the influence of supraglacial debris on the temperature of the underlying ice surface, and therefore melt, is stable at a seasonal timescale and can be estimated from near-surface temperature. These results have the potential to greatly improve the representation of ablation in calculations of debris-covered glacier mass balance and projections of their response to climate change.Peer reviewe

Bathymetric survey of Imja Lake, Nepal in 2012

Imja Lake is one of the most studied lakes in the Himalaya as well as one of the most rapidly evolving glacial lakes in Nepal. Many researchers have studied the lake and the potential of a glacier lake outburst flood from the lake. One of the important factors in assessing the outburst flood risk is the volume that could be released in the flood and good bathymetric data is necessary to estimate that value. This work reports on the 2012 bathymetric survey of Imja Lake and the rate of expansion that has been observed in the lake over the last two decades, since 1992. The survey was somewhat hampered by the extensive iceberg coverage of the lake in September 2012, but a good estimate of the bottom bathymetry and the current volume was obtained. When compared to previous surveys, it is very clear that the lake bottom has continued to deepen as the ice beneath the lake has melted. The average depth has increased by 62% since 2002 and continues to increase at a rate of 1.8 m/yr. The maximum depth has increased 28% since 2002 and is increasing currently at a rate of 5.8 m/yr. Perhaps more important in terms of glacier lake outburst flood risk is the continued rapid areal expansion of the lake which has expanded 41% since 2002 and is growing at a rate of 0.02 km2/yr. This expansion has resulted in an additional 6 million m3 of water for an outburst flood event, and increasing the maximum possible flood volume to 36.3 million m3 a 73% increase from what was calculated using 2002 data.Center for Research in Water Resource

Glacier projections sensitivity to temperature-index model choices and calibration strategies

The uncertainty of glacier change projections is largely influenced by glacier models. In this study, we focus on temperature-index mass-balance (MB) models and their calibration. Using the Open Global Glacier Model (OGGM), we examine the influence of different surface-type dependent degree-day factors, temporal climate resolutions (daily, monthly) and downscaling options (temperature lapse rates, temperature and precipitation corrections) for 88 glaciers with in-situ observations. Our findings indicate that higher spatial and temporal resolution observations enhance MB gradient representation due to an improved calibration. The addition of surface-type distinction in the model also improves MB gradients, but the lack of independent observations limits our ability to demonstrate the added value of increased model complexity. Some model choices have systematic effects, for example weaker temperature lapse rates result in smaller projected glaciers. However, we often find counter balancing effects, such as the sensitivity to different degree-day factors for snow, firn and ice, which depends on how the glacier accumulation area ratio changes in the future. Similarly, using daily versus monthly climate data can affect glaciers differently depending on the shifting balance between melt and solid precipitation thresholds. Our study highlights the importance of considering minor model design differences to predict future glacier volumes and runoff accurately.</p

A Field-based Study of Impacts of the 2015 Earthquake on Potentially Dangerous Glacial Lakes in Nepal

Following the earthquake that occurred on April 25, 2015 in Nepal, and the second major earthquake that occurred on May 12, 2015, we conducted field-based studies of five potentially dangerous glacial lakes in Nepal— Imja Tsho, Tsho Rolpa, Dig Tsho, Panga Dinga, and Thulagi. This research was undertaken in an effort to better understand what impacts the earthquake may have had on lake stability, flood potential as well as local perceptions of the dangers that post-earthquake outburst floods pose. Although only one relatively small, earthquakerelated glacial lake outburst flood (GLOF) was found to have occurred, the presence of new cracks, slumps, shifted or displaced boulders, and landslide activity within the already deteriorating terminal and lateral moraines of the lakes suggests that they may have been further de-stabilized by the earthquake and its aftershock. Downstream communities feared the increased likelihood of aftershocks and GLOFs, and at least two downstream regions experienced panic when rumors of imminent floods started spreading. In all our visits to these downstream villages, we encountered inadequate awareness of: early flood warning systems, lake risk reduction methods, and disaster management planning. In order to eliminate the potential impacts of future glacial lake outburst floods to downstream communities, agricultural land, and infrastructure, the authors recommend the development: of standardized glacial lake risk assessment methods, Himalayan-specific lake lowering and risk reduction approaches, user friendly early warning systems, and disaster preparedness training

Modeling Mitigation Strategies for Risk Reduction at Imja Lake, Nepal

A model was developed to assess the impact of a potential glacial lake outburst flood (GLOF) from Imja Lake in Nepal and its impact on downstream communities. Implications of proposed GLOF risk reduction alternatives, including one suggested by local community members, were assessed. Results provided three alternatives that offer significant risk reduction for the communities, including (1) no lowering of the lake and constructing a 60 m flood detention dam, resulting in a 43.2 percent reduction of risk, (2) lowering the lake 10 m with a 40 m dam, resulting in a 57.8 percent reduction of risk, and (3) lowering the lake 20 m with no dam, resulting in a risk reduction of 66.7 percent. An alternative to lower the lake by 3 m with no check-dam, currently under consideration by the Government of Nepal, would result in a 5.2 percent reduction of risk. This alternative does not appear to offer significant risk reduction benefits to downstream communities compared to lowering the lake by 20 m. Results suggest that either the lake must be lowered by significantly more than 3 m (20 m is recommended) or that a downstream flood detention dam be included in the project. One possible method of lowering Imja Lake is to use siphons to drain lake water by 3 m, excavate to the new water level, repeating the process until a total lowering of 20 m is achieved. This method would require the use of 13 pipes of 0.350 m diameter to lower the lake.Center for Research in Water Resource