Summer Rupper

Glacier Changes and Regional Climate: A Mass and Energy Balance Approach*

The mass balance of a glacier is a complex consequence of the combination of atmospheric variables that control it. However, the understanding of past, present, and future glacier states is often predicated on very simplified representations of the mass balance–climate relationship. Here, a full surface energy and mass balance (SEMB) model is developed to explore the relationship between glacier equilibrium-line altitudes (ELAs) and climate at a regional scale. This model is applied to central Asia because of the diverse climate regimes and glacier history. The model captures the pattern in ELAs well; the seasonal cycle in energy balance terms are comparable to studies on individual glaciers in central Asia, and the proportionality factor relating melt to temperature is within the range of those reported for individual glaciers within the area. In regions where precipitation is low, ablation at the ELA is dominated by sublimation. Conversely, where precipitation is high, ablation at the ELA is dominated by melt and surface runoff. In turn, the sensitivity of the ELA to changes in climate is strongly tied to the dominant ablation process. In particular, ELAs in melt-dominated regions are most sensitive to interannual variability in air temperature, while ELAs in sublimation-dominated regions are most sensitive to interannual variability in precipitation. Glaciers in sublimation-dominated regions are acutely sensitive to even small changes in atmospheric variables. Finally, changes in clouds are shown to be important in all regions through their influence on the shortwave and longwave radiative fluxes, which dominate the surface energy balance at the ELA.

Sensitivity and response of Bhutanese glaciers to atmospheric warming

] Glacierized change in the Himalayas affects river-discharge, hydro-energy and agricultural production, andGlacial Lake Outburst Flood potential, but its quantificationand extent of impacts remains highly uncertain. Here wepresent conservative, comprehensive and quantitative pre-dictions for glacier area and meltwater flux changes inBhutan, monsoonal Himalayas. In particular, we quantifythe uncertainties associated with the glacier area and melt-water flux changes due to uncertainty in climate data, acritical problem for much of High Asia. Based on a suiteof gridded climate data and a robust glacier melt model,our results show that glacier area and meltwater changeprojections can vary by an order of magnitude for differentclimate datasets. However, the most conservative resultsindicate that, even if climate were to remain at the present-day mean values, almost 10% of Bhutan’s glacierized areawould vanish and the meltwater flux would drop by as muchas 30%. Under the conservative scenario of an additional1 C regional warming, glacier retreat is going to continueuntil about 25% of Bhutan’s glacierized area will have dis-appeared and the annual meltwater flux, after an initialspike, would drop by as much as 65%.

The Relationship between Snow Accumulation at Mt. Logan, Yukon, Canada, and Climate Variability in the North Pacific

Abstract An ice core from Mt. Logan, Yukon, Canada, presents an opportunity to evaluate the degree to which ice core accumulation records can be interpreted as meaningful measures of interannual climate variability. Statistical analyses and comparisons with synoptic station data are used to identify the physical relationships between Mt. Logan ice core accumulation data and large-scale atmospheric circulation. These analyses demonstrate that only the winters of high accumulation years have a robust connection with atmospheric circulation. There are no consistent relationships during anomalously low and average accumulation years. The wintertime of high accumulation years is associated with an enhanced trough–ridge structure at 500 hPa and in sea level pressure over the northeast Pacific and western Canada, consistent with increased southerly flow bringing in warmer, moister air to the region. While both storm (i.e., 2–6 days) and blocking (i.e., 15–20 days) events project onto the same climate pattern, on...

Six hundred thirty‐eight years of summer temperature variability over the Bhutanese Himalaya

High-resolution tree ring reconstructions from the Himalaya provide essential context for assessing impacts of future climate change on regional water reserves and downstream agriculture. Here we e ...

Spatial patterns in Central Asian climate and equilibrium line altitudes

A suite of general circulation model (GCM) simulations and a glacier equilibrium line altitude (ELA) model are compared to reconstructed glacier advances from geomorphic data and used to test the sensitivity of Central Asian glaciers to simulated climate changes at the Last Glacial Maximum (LGM). Results highlight temperature changes as being the most important influence on glacier ELA changes during the LGM. With the exception of the southern Himalaya, for much of Central Asia there is consistency between GCMs for simulated LGM temperature changes, with a mean cooling of 4° C. Further research will necessarily need to focus on detailed analysis of the inter-model differences in temperature in the southern Himalaya, and acquiring additional paleoclimate proxies in the region in order to further constrain the GCMs.

Response of closed basin lakes to interannual climate variability

AbstractLakes are key indicators of a region’s hydrological cycle, directly reflecting the basin-wide balance between precipitation and evaporation. Lake-level records are therefore valuable repositories of climate history. However, the interpretation of such records is not necessarily straightforward. Lakes act as integrators of the year-to-year fluctuations in precipitation and evaporation that occur even in a constant climate. Therefore lake levels can exhibit natural, unforced fluctuations that persist on timescales of decades or more. This behavior is important to account for when distinguishing between true climate change and interannual variability as the cause of past lake-level fluctuations. We demonstrate the operation of this general principle for the particular case-study of the Great Salt Lake, which has long historical lake-level and climatological records. We employ both full water-balance and linear models. Both models capture the timing and size of the lake’s historical variations. We then model the lake’s response to much longer synthetic time series of precipitation and evaporation calibrated to the observations, and compare the magnitude and frequency of the modeled response to the Great Salt Lake’s historical record. We find that interannual climate variability alone can explain much of the decadal-to-centennial variations in the lake-level record. Further, analytic solutions to the linear model capture much of the full model’s behavior, but fail to predict the most extreme lake-level variations. We then apply the models to other lake geometries, and evaluate how the timing and amplitude of a lake-level response differs with climatic and geometric setting. A lake’s response to a true climatic shift can only be understood in the context of these expected persistent lake-level variations. On the basis of these results, we speculate that lake response to interannual climate variability may play an important part in explaining much of Holocene lake-level fluctuations.

Equilibrium line altitudes along the Andes during the Last millennium: Paleoclimatic implications

Deciphering the climate changes that influenced the glacial fluctuations of the last millennium requires documenting the spatial and temporal patterns of these glacial events. Here, we estimate the change in equilibrium line altitudes (ELAs) between the most prominent glacial advance of the last millennium and the present for four alpine glaciers located in different climatic regimes along the Andes. For each glacier, we reconstruct scenarios of climatic conditions (temperature and precipitation anomalies) that accommodate the observed ELA changes. We focus on the following glaciers: an alpine glacier in the Cordillera Vilcanota (13°S), Tapado glacier (30°S), Cipreses glacier (34°S), and Tranquilo glacier (47°S). Our results show that the range of possible temperature and precipitation anomalies that accommodate the observed ELA changes overlap significantly at three of the four sites (i.e. Vilcanota, Cipreses, and Tranquilo). Only Tapado glacier exhibits a set of climate anomalies that differs from the other three sites. Assuming no change in precipitation, the estimated ELA changes require a cooling of at least 0.7°C in the Cordillera Vilcanota, 1.0°C at Tapado glacier, 0.6°C at Cipreses glacier, and 0.7°C at Tranquilo glacier. Conversely, assuming no change in temperature, the estimated ELA changes are explained by increases in precipitation exceeding 0.52 m yr−1 (64% of the annual precipitation) in the Cordillera Vilcanota, 0.31 m yr−1 (89%) at Tapado glacier, 0.22 m yr−1 (27%) at Cipreses glacier, and 0.3 m yr−1 (27%) at Tranquilo glacier. By mapping the ELA changes and modeling the potential climate forcing across diverse climate settings, we aim to contribute toward documenting the spatial variability of climate conditions during the last millennium, a key step to decipher the mechanisms underlying the glacial fluctuation that occurred during this period.

Sensitivity of glacier runoff projections to baseline climate data in the Indus River basin

Quantifying the contribution of glacier runoff to water resources is particularly important in regions such High Mountain Asia, where glaciers provide a large percentage of seasonal river discharge and support large populations downstream. In remote areas, direct field measurements of glacier melt rates are difficult to acquire and rarely observed, so hydro-glaciological modeling and remote sensing approaches are needed. Here we present estimates of glacier melt contribution to the Upper Indus watershed over the last 40 years using a suite of seven reanalysis climate datasets that have previously been used in hydrological models for this region, a temperature-index melt model and > 29,000 km2 of ice cover. In particular, we address the uncertainty in estimates of meltwater flux that is introduced by the baseline climate dataset chosen, by comparing the results derived from each. Mean annual glacier melt contribution varies from 8 km3 yr-1 and 169 km3 yr-1, or between 4-78% of the total annual runoff in the Indus, depending on temperature dataset applied. Under projected scenarios of an additional 2-4°C of regional warming by 2100 AD, we find annual meltwater fluxes vary by >200% depending on the baseline climate dataset used and, importantly, span a range of positive and negative trends. Despite significant differences between climate datasets and the resulting spread in meltwater fluxes, the spatial pattern of melt is highly correlated and statistically robust across all datasets. This allows us to conclude with confidence that fewer than 10% of the >20,000 glaciers in the watershed contribute more than 80% of the total glacier runoff to the Indus. These are primarily large, low elevation glaciers in the Karakoram and Hindu Kush. Additional field observations to ground-truth modeled climate data will go far to reduce the uncertainty highlighted here and we suggest that efforts be focused on those glaciers identified to be most significant to water resources.

Effect of Atmospheric Deposition and Weathering on Trace Element Concentrations in Glacial Meltwater at Grand Teton National Park, Wyoming, U.S.A.

ABSTRACT Glaciers are reservoirs of atmospherically deposited trace elements that are released during melt. Weathering in glacial environments also contributes solutes to proglacial streams. To investigate the relative importance of atmospheric deposition and weathering on trace element chemistry of glacial streams, we sampled supraglacial and proglacial meltwater at two glacierized catchments in Grand Teton National Park, Wyoming, which is located downwind of agricultural/industrial emissions and dust sources. Concentrations of major ions (Mg2+, K+, Na+, Ca2+, SO42-), alkalinity, conductivity, and a subset of trace elements (U, Mo, Sr, Rb, Li, Ba) were low in supraglacial meltwater but increased in proglacial streams because of water-rock interactions. In contrast, concentrations of the trace metals Mn, Co, Zn, Pb, Cd, and Hg had relatively high concentrations in supraglacial meltwater and decreased downstream. These metals are not abundant in the local bedrock and thus are likely sourced from atmospheric deposition. Stable isotopes indicated different water sources in July (snowmelt-dominated) and August (ice melt-dominated), but water chemistry was similar during both months, indicating similar composition of recent snowfall and older ice. These findings have implications for evaluating the relative impacts of atmospheric deposition and weathering in glacier- and snow-dominated catchments.

Impacts of topographic shading on direct solar radiation for valley glaciers in complex topography

Topographic shading, including both shaded relief and cast shadowing, plays a fundamental role in determining direct solar radiation on glacier ice. However, this parameter has been oversimplified or incorrectly incorporated in surface energy balance models in some past studies. Here we develop a topographic solar radiation model to examine the variability in irradiance throughout the glacier melt season due to topographic shading and combined slope and aspect. We apply the model to multiple glaciers in High Mountain Asia (HMA), and test the sensitivity of shading to valley-aspect and latitude. Our 10 results show that topographic shading significantly alters the potential direct clear-sky solar radiation received at the surface for valley glaciers in HMA, particularly for northand south-facing glaciers. Additionally, we find that shading can be extremely impactful in the ablation zone. Cast shadowing is the dominant mechanism in determining total shading for valley glaciers in parts of HMA, especially at lower elevations. Although shading has some predictable characteristics, it is overall extremely variable between glacial valleys. Our results suggest that topographic shading is not only an important factor 15 contributing to surface energy balance, but could also influence glacier response and mass balance estimates throughout HMA.