Area Altitude Balance Ratio Research Articles

Equilibrium line altitudes of alpine glaciers in Alaska suggest Last Glacial Maximum summer temperature was 2–5 °C lower than during the pre-industrial

Abstract. The lack of continental ice sheets in Alaska during the Last Glacial Maximum (LGM; 26–19 ka) has long been attributed to extensive aridity in the western Arctic. More recently, climate model outputs, a few isolated paleoclimate studies, and global paleoclimate synthesis products show mild summer temperature depressions in Alaska compared to much of the high northern latitudes. This suggests the importance of limited summer temperature depressions in controlling the relatively limited glacier growth during the LGM. To explore this further, we present a new statewide map of LGM alpine glacier equilibrium line altitudes (ELAs), LGM ΔELAs (LGM ELA anomalies relative to the Little Ice Age, LIA), and ΔELA-based estimates of temperature depressions across Alaska to assess paleoclimate conditions. We reconstructed paleoglacier surfaces in ArcGIS to calculate ELAs using an accumulation area ratio (AAR) of 0.58 and an area–altitude balance ratio (AABR) of 1.56. We calculated LGM ELAs (n= 480) in glaciated massifs in the state, excluding areas in southern Alaska that were covered by the Cordilleran Ice Sheet. The data show a trend of increasing ELAs from the southwest to the northeast during both the LGM and the LIA, indicating a consistent southern Bering Sea and northernmost Pacific Ocean precipitation source. Our LGM–LIA ΔELAs from the Alaska Range, supported with limited LGM–LIA ΔELAs from the Brooks Range and the Kigluaik Mountains, average to −355 ± 176 m. This value is much greater than the global LGM average of ca. −1000 m. Using a range of atmospheric lapse rates, LGM–LIA ΔELAs in Alaska translate to summer cooling of < 2–5 ∘C. Our results are consistent with a growing number of local climate proxy reconstructions and global data assimilation syntheses that indicate mild summer temperature across Beringia during the LGM. Limited LGM summer temperature depressions could be explained by the influence of Northern Hemisphere ice sheets on atmospheric circulation.

Glacial geomorphology and Pleistocene glacier reconstruction in the Demänovská Valley, Low Tatra Mountains, Slovakia

In the Western Carpathians, clear evidence of the Pleistocene glaciations only occurs in two mountain massifs – the Tatra and Low Tatra Mountains. The Low Tatra Mountains (2043 m a.s.l.), contrary to the higher and more strongly glaciated Tatra Mountains (2654 m a.s.l.), have previously been much less attractive for scientific research. Hence, in these mountains, both glacial landforms and chronology, together with a detailed reconstruction of glacier geometry and resulted equilibrium line altitude (ELA), are poorly documented. The aim of this paper is to characterize the glacial relief and reconstruction of geometry and ELA of the Zadná voda glacier in the Demänovská Valley system which belongs to the category of the largest Pleistocene glaciers on the northern slope of the Low Tatra Mountains. The mapping results show that a freshly shaped, massive terminal moraine of maximal ice extent (MIE, likely formed during the global Last Glacial Maximum – LGM) occurs 4.3 km distance down-valley from the glacial cirque backwalls. There is no evidence of deposits from older glaciations beyond the terminal moraine down the valley. The terminal zone of the MIE features a fresh morainic landscape with hummocky topography with kettle hollows and the only known morainic lake in the Low Tatra Mountains – Vrbické pleso. During the MIE, the Zadná voda glacier covered 7 km2 of the area and featured a mean thickness of 48 m. The ELA of this glacier was 1433 m, determined by the area-altitude balance ratio (AABR) 1.6 method, which is a similar value to the LGM ELA calculated in the Western Tatra Mountains. The recessional stages were only recognized in the cirques area, where one or two generations of debris-covered glaciers and rock glaciers mark the final deglaciation of the study area.

Spatio-temporal variability of glacier surface area, ELA, and AAR in the Mago river basin of Arunachal Pradesh

Abstract The spatial and temporal variations of glacier surface area, annual equilibrium line altitude (ELA), and accumulation area ratio (AAR) in Mago basin, Eastern Himalaya, are studied over 1988–2019 using Landsat data. The glacier surface area was mapped using automatic glacier extraction index (AGEI) and 49 glaciers named as G1–G49 were found in 2019. Fifteen glaciers with area greater than 1 km2 in 2019 were selected for analysis. An automatic GIS tool comprising area altitude balance ratio (AABR), AAR, and mean glacier elevation (MGE), and combination of NIR band and Otsu threshold, were used for determining ELA. All glaciers experienced area shrinkage with a total loss of 15.22 km2 (28.48%) at 0.49 km2 (0.92%) per annum. G1, G9, G15, G35, G41, and G42 showed significant decreasing trend in area. Glaciers facing South-West showed highest deglaciation. For the annual ELA, all glaciers showed increasing trend and significant increasing trend was observed in G2, G15, and G40. Overall, there was an increase in ELA by 137.3 m at 4.43 m/year. The average AAR varied from 0.44 to 0.97. Average AAR of about 0.8 was observed in South, South-East, and North facing, about 0.6 in South-West and West facing, and about 0.7 in East facing glaciers.

Testing the area–altitude balance ratio (AABR) and accumulation–area ratio (AAR) methods of calculating glacier equilibrium-line altitudes

AbstractIn this study, we compare equilibrium-line altitudes (ELAs) calculated using the area–altitude balance ratio (AABR) and the accumulation–area ratio (AAR) methods, with measured ELAs derived from direct field observations. We utilise a GIS toolbox to calculate the ELA for 64 extant glaciers by applying the AABR and AAR methods to DEMs and polygons of their geometry. The calculated ELAs (c-ELAs) are then compared to measured zero-net balance ELAs (znb-ELAs) obtained from mass-balance time series held by the WGMS for the same glaciers. The correlation between znb-ELAs and AABR (1.56)/AAR (0.58) c-ELAs is very strong, with an r2 = 0.99. The smallest median difference between znb-ELAs and c-ELAs (i.e. 65.5 m) is obtained when a globally representative AABR of 1.56 is used. When applied to palaeoglacier-climate applications, this difference translates to ~0.42°C, well within the uncertainty of palaeotemperature proxies used to determine mean summer temperature at the ELA. The more widely used mean AABR of 1.75 is shown to be statistically invalid due to the skewness of the dataset. On this basis, when calculating glacier ELAs, we recommend the use of a global AABR value of 1.56.

Tropical glacier reconstructions during the Last Glacial Maximum in Costa Rica

Numerous high elevation tropical mountains around the world show evidence of past glacial activity during the Last Glacial Maximum (LGM). Cerro Chirripó in Costa Rica exhibits paleoglacial landforms such as glacial cirques, moraine deposits and polished and striated bedrock surfaces. We used aerial imagery (1:25000) and contour lines to develop a Digital Elevation Model (DEM) for the LGM. We determined paleo-equilibrium line altitudes (paleo-ELAs) using Area-Altitude Balance Ratio (AABR) during the LGM for Cerro Chirripó in Costa Rica. Additionally, a Generalized Linear Model (GLM) was performed to statistically analyze the paleoglacier volumes and ice thickness combined with ten land surface parameters (LSP). Our results identified thirty-one paleoglaciers covering an area of 28.26 km2 during the global LGM, with a maximum ice thickness of 178 meters in Cerro Chirripó, a total volume of 13863 × 105 m3 and a mean paleo-ELA of 3490 meters. In addition, Area and Slope were the LSP with the highest statistical correlation to explain the paleoglacier volumes, while Area and Diurnal Anisotropic Heating were best for the paleoglacier ice thickness. As one of the first studies in the tropical high mountain environments, this work expands the geographic scope of glacier volume and thickness reconstructions during the maximum expansion of the LGM.

Equilibrium-line altitude and temperature reconstructions during the Last Glacial Maximum in Chirripó National Park, Costa Rica

Several high areas of Central America show evidence of past glacial activity, including glacial cirques, polished and striated bedrock surfaces and moraine deposits. As glacial remnants, these morphologies can guide the understanding of past climate conditions, such as during the Last Glacial Maximum (LGM). Based on aerial imagery (1:25,000), detailed Digital Elevation Model (DEM) and field geomorphic assessments, we establish paleo-equilibrium line altitudes using four methods: Median Glacier Elevation (MGE), Accumulation Area Ratio (AAR), Area x Altitude (AA), and Area-Altitude Balance Ratio (AABR) during the LGM for the Chirripó National Park in Costa Rica. In addition, we calculate the LGM paleo-temperature decrease multiplying several modern atmospheric lapse rates by the equilibrium line altitude depression. In addition, a Generalized Linear Model (GLM) was applied to describe linkages between obtained paleoglacier areas and multiple land surface parameters such as Basin location, Analytical Hillshading, Aspect, Area Surface Radiation, Diurnal Anisotropic Heating, Slope, Terrain Ruggedness Index, and Wind Exposition. Our results determined thirty-one paleo equilibrium line altitudes (paleo-ELAs) and a mean altitude of ~3490 m a.s.l. Some of the moraines used in this study have been recently dated, confirming the extension of the glaciers during LGM used to calculate the ELAs. We also obtained an annual average temperature decrease of ~10.5 °C applying a lapse rate of 0.65 °C/100. Moreover, Wind Exposition and Terrain Ruggedness Index were the land surface parameters with greater statistical correlation for paleo-glacier areas. Therefore, our results provide new knowledge into the reconstruction of the maximum expansion of the LGM on tropical landscapes.

A GIS tool for automatic calculation of glacier equilibrium-line altitudes

A toolbox for the automated calculation of glacier equilibrium-line altitudes (ELAs) using the Accumulation Area Ratio, Area-Altitude Balance Ratio, Area-Altitude and Kurowski methods is presented. These are the most commonly-used methods of ELA calculation in palaeo-glacier reconstructions. The toolbox has been coded in Python and runs in ArcGIS requiring only the reconstructed surface of the palaeo-glacier (a DEM) as input. Through fast and automatic calculation this toolbox simplifies the process of ELA determination and can successfully work both for a single glacier and for large datasets of multiple glaciers.

Palaeoclimatic reconstruction from Lateglacial (Younger Dryas Chronozone) cirque glaciers in Snowdonia, North Wales

The cirques of Snowdonia, North Wales were last occupied by glacier ice during the Younger Dryas Chronozone (YDC), c. 12.9–11.7ka. New mapping presented here indicates 38 small YDC cirque glaciers formed in Snowdonia, covering a total area of 20.74km2. Equilibrium line altitudes (ELAs) for these glaciers, calculated using an area–altitude balance ratio (AABR) approach, ranged from 380 to 837m asl. A northeastwards rise in YDC ELAs across Snowdonia is consistent with southwesterly snow-bearing winds. Regional palaeoclimate reconstructions indicate that the YDC in North Wales was colder and drier than at present. Palaeotemperature and annual temperature range estimates, derived from published palaeoecological datasets, were used to reconstruct values of annual accumulation and ‘winter balance plus summer precipitation’ using a degree-day model (DDM) and non-linear regression function, respectively. The DDM acted as the best-estimate for stadial precipitation and yielded values between 2073 and 2687mma−1 (lapse rate: 0.006°Cm−1) and 1782–2470mma−1 (lapse rate: 0.007°Cm−1). Accounting for the potential input of windblown and avalanched snow onto former glacier surfaces, accumulation values dropped to between 1791 and 2616mma−1 (lapse rate: 0.006°Cm−1) and 1473–2390mma−1 (lapse rate: 0.007°Cm−1). The spatial pattern of stadial accumulation suggests a steep precipitation gradient and provides verification of the northeastwards rise in ELAs. Glaciers nearer the coast of North Wales were most responsive to fluctuations in climate during the YDC, responding to sea-ice enforced continentality during the coldest phases of the stadial and to abrupt warming at the end of the stadial.

Late Quaternary glaciation and equilibrium-line altitudes of the Mayan Ice Cap, Guatemala, Central America

The Sierra los Cuchumatanes (3837 m), Guatemala, supported a plateau ice cap and valley glaciers around Montaña San Juan (3784 m) that totaled ∼ 43 km 2 in area during the last local glacial maximum. Former ice limits are defined by sharp-crested lateral and terminal moraines that extend to elevations of ∼ 3450 m along the ice cap margin, and to ca. 3000–3300 m for the valley glaciers. Equilibrium-line altitudes (ELAs) estimated using the area–altitude balance ratio method for the maximum late Quaternary glaciation reached as low as 3470 m for the valley glaciers and 3670 m for the Mayan Ice Cap. Relative to the modern altitude of the 0°C isotherm of ∼ 4840 m, we determined ELA depressions of 1110–1436 m. If interpreted in terms of a depression of the freezing level during maximal glaciation along the modern lapse rate of − 5.3°C km − 1 , this ΔELA indicates tropical highland cooling of ∼ 5.9 to 7.6 ± 1.2°C. Our data support greater glacial highland cooling than at sea level, implying a high tropical sensitivity to global climate changes. The large magnitude of ELA depression in Guatemala may have been partially forced by enhanced wetness associated with southward excursions of the boreal winter polar air mass.

Quaternary glaciation in the Nubra and Shyok valley confluence, northernmost Ladakh, India

Three glacial stages (Deshkit 1, Deshkit 2 and Dishkit 3 glacial stages) are identified in the Nubra and Shyok valleys in northernmost Ladakh, northwest India, on the basis of geomorphic field mapping, remote sensing, and 10 Be terrestrial cosmogenic nuclide surface exposure dating. The glacial stages date to ∼ 45 ka (Deshkit 1 glacial stage), ∼ 81 ka (Deshkit 2 glacial stage) and ∼ 144 ka (Deshkit 3 glacial stage). A mean equilibrium line altitude depression of ∼ 290 m for the Deshkit 1 glacial stage was calculated using the area accumulation ratio, toe-to-headwall ratio, area–altitude, and area–altitude balance ratio methods. Comparison of glaciation in the Nubra and Shyok valleys with glaciations in the adjacent Central Karakoram of northern Pakistan and northern side of the Ladakh Range of northern India indicates that glaciation was synchronous on Milankovitch timescales across the region during MIS-6, but differed greatly in extent, with more extensive glaciation in the Karakoram than the morphostratigraphically equivalent glaciation on the northern slopes of the Ladakh Range. This highlights the strong contrast in the extent of glaciation across ranges in the Himalaya–Tibetan orogen, necessitating caution when correlating glacial successions within and between mountain ranges.

Climatic implications of reconstructed early–mid Pliocene equilibrium-line altitudes in the McMurdo Dry Valleys, Antarctica

AbstractEarly–mid Pliocene moraines in the McMurdo Dry Valleys, Antarctica, are more extensive than the present alpine glaciers in this region, indicating substantial climatic differences between the early–mid Pliocene and the present. To quantify this difference in the glacier–climate regime, we estimated the equilibrium-line altitude (ELA) change since the early–mid Pliocene by calculating the modern ELA and reconstructing the ELAs of four alpine glaciers in Wright and Taylor Valleys at their early–mid Pliocene maxima. The area–altitude balance ratio method was used on modern and reconstructed early–mid Pliocene hypsometry. In Wright and Victoria Valleys, mass-balance data identify present-day ELAs of 800–1600ma.s.l. and an average balance ratio of 1.1. The estimated ELAs of the much larger early–mid Pliocene glaciers in Wright and Taylor Valleys range from 600 to 950±170ma.s.l., and thus are 250–600±170m lower than modern ELAs in these valleys. The depressed ELAs during the early–mid-Pliocene most likely indicate a wetter and therefore warmer climate in the Dry Valleys during this period than previous studies have recognized.

Defining modern day Area-Altitude Balance Ratios (AABRs) and their use in glacier-climate reconstructions

With the increasing use of digital elevation models in palaeo-glacier reconstructions and the availability of freeware spreadsheets the Area-Altitude Balance Ratio (AABR) and Balance Ratio (BR) methods are becoming increasingly used in palaeo-glacier reconstruction for estimating Equilibrium Line Altitudes (ELA) and subsequently deriving quantitative estimates of palaeo-climate. While there are many data detailing contemporary Accumulation Area Ratios, there are still only a few studies that have established, from contemporary environments, AABR/BR ratios. Publicly available glacier mass balance (World Glacier Monitoring Service, US Geological Survey, and Norwegian Water Directorate) and spatial extent datasets provided the basis for this research. From a time series of mass balance, regressing specific net balance against ELA allows the zero net balance ELA to be identified. Once the zero balance ELA is established, the glacier hypsometry above and below the ELA is defined. The AABR/BR is calculated by using (the right hand side) the following: AABR = b n a b / b n a c = ( z ¯ a c A a c ) / ( z ¯ a b A a b ) , where, b nab and b nac are the net mass balance gradients in the ablation and accumulation zones respectively, z ¯ a c and z ¯ a b are the area-weighted mean altitudes of the accumulation and ablation areas respectively and A ac and A ab are the areas of the accumulation and ablation areas respectively. AABRs are calculated for a suite of glaciers located across a range of climatic zones and glacier types, with Antarctica being excluded. The following “representative” AABRs are found: a global AABR = 1.75 ± 0.71; Mid-latitude maritime = 1.9 ± 0.81; High-latitude = 2.24 ± 0.85; North America – West Coast = 2.09 ± 0.93; North America – Eastern Rockies = 1.11 ± 0.1; Canadian Arctic = 2.91 ± 0.35; Svalbard = 2.13 ± 0.52; Western Norway = 1.5 ± 0.4; European Alps = 1.59 ± 0.6; Central Asia = 1.75 ± 0.56; Kamchatka = 3.18 ± 0.16. This study provides an empirically derived dataset characterising AABR ratios which may be used for ELA estimation in palaeo-glacier reconstructions and for palaeo-climate quantification.

Glacial geomorphology of the Creag Meagaidh Massif, Western Grampian Highlands: Implications for local glaciation and palaeoclimate during the Loch Lomond Stadial

Detailed mapping of the Creag Meagaidh massif reveals two geomorphological signatures. One suggests widespread ice retreat to a western source, while a second indicates the existence of a small, locally sourced, icefield and three possible independent glaciers. Based on morphostratigraphic criteria, a Loch Lomond Stadial (Younger Dry as) age is proposed for the local ice masses, indicating that ice extent was greater than previously thought. When constrained by the inferred limits of erosive and depositional ice, the Creag Meagaidh Icefield (18.2 km ) yields an area altitude balance ratio (AABR) equilibrium line altitude (ELA) of between 621 and 628 m (AABR = 1.67–2.0). However, uncertainties concerning the potential input of cold‐based plateau ice mean that these ELAs are regarded as minimal values. Mean annual precipitation at the icefield ELAs is calculated to have been 2090±430 mm. Modern precipitation at these altitudes lies within the lower end of this range, suggesting that precipitation over the Creag Meagaidh area was similar to, or slightly higher than, today during the Loch Lomond Stadial.

The Loch Lomond Readvance on north Arran, Scotland: glacier reconstruction and palaeoclimatic implications

AbstractGeomorphological mapping of northern Arran provides evidence for two advances of locally nourished glaciers, the younger being attributable to the Loch Lomond Stade (LLS) of ca. 12.9–11.5 k yr BP, primarily through the mutually exclusive relationship between glacial limits and Lateglacial periglacial features. The age of the earlier advance is unknown. Inferred LLS glacier cover comprised two small icefields and eight small corrie or valley glaciers and totalled 11.1 km2. ELAs reconstructed using area–altitude balance ratio methods range from 268 m to 631 m for individual glaciers, with an area‐weighted mean ELA of 371 m. ELAs of individual glaciers are strongly related to snow‐contributing areas. The area‐weighted mean ELA is consistent with a north–south decline in LLS ELAs along the west coast of Great Britain. This decline has an average latitudinal gradient of 70 m 100 km−1, equivalent to a mean southwards ablation‐season temperature increase of ca. 0.42°C 100 km−1. Mean June–August temperatures at the regional climatic ELA, estimated from chironomid assemblages in SE Scotland, lay between 5.7 ± 0.1°C and 4.1 ± 0.2°C. Empirical relationships between temperature and precipitation at modern glacier ELAs indicate equivalent mean annual precipitation at the ELA lay between 2002 ± 490 mm and 2615 ± 449 mm. These figures suggest that stadial precipitation on Arran fell within a range between +8% and −33% of present mean annual precipitation. Copyright © 2006 John Wiley & Sons, Ltd.

Last glacial maximum equilibrium-line altitude and paleo-temperature reconstructions for the Cordillera de Mérida, Venezuelan Andes

The pattern and magnitude of glacier equilibrium-line altitude (ELA) lowerings in the tropics during the last glacial maximum (LGM) are topics of current debate. In the northern tropics, paleo-ELA data are particularly limited, inhibiting the ability to make regional and large-scale paleoclimatic inferences. To improve these records, nine paleo-glaciers in the Venezuelan Andes were reconstructed based on field observations, aerial photographs, satellite imagery and high-resolution digital topographic data. Paleo-glacier equilibrium-line altitudes (ELAs) were estimated using the accumulation-area ratio (AAR) and the area-altitude balance ratio (AABR) methods. During the local LGM in Venezuela (∼ 22,750 to 19,960 cal yr BP), ELAs were ∼ 850 to 1420 m lower than present. Local LGM temperatures were are at least 8.8 ± 2°C cooler than today based on a combined energy and mass-balance equation to account for an ELA lowering. This is greater than estimates using an atmospheric lapse rate calculation, which yields a value of 6.4 ± 1°C cooler. The paleo-glacial data from the Venezuelan Andes support other published records that indicate the northern tropics experienced a greater ELA lowering and possibly a greater cooling than the Southern Hemisphere tropics during the LGM.

Reconstruction of glacier equilibrium‐line altitudes for the Last Glacial Maximum on the High Plain of Bogotá, Eastern Cordillera, Colombia: climatic and topographic implications

AbstractThe High Plain of Bogotá in the Andes of Colombia provides an exceptionally detailed record of glaciation. A two‐stage Last Glacial Maximum (LGM) is noted in Bogotá; the older stage (max) presents an opportunity to reconstruct individual valley glaciers and explore spatial patterns. Well‐mapped geomorphic features on topographic base maps permit the reconstruction of 23 palaeoglacier surfaces. Glacier extent varies across the region, with lower altitudes reached farther to the east. Equilibrium line altitudes (ELAs) are reconstructed using the area–altitude balance ratio (AABR) method, with BRs in three groups reflecting the W–E gradient in glacier extent and selected by minimising variation from group means. Average LGM ELA for all palaeoglaciers is 3488 m with a standard deviation of 182 m. The average lowering in ELA from LGM to modern of ca. 1300 m is best explained by a considerable drop in temperature. Significant intra‐regional variance in LGM ELA can be ascribed to topography and its influence on precipitation and/or glacier form, with lower headwall elevations being correlated to larger accumulation areas. Copyright © 2005 John Wiley & Sons, Ltd.