Modelling Glacier Evolution in Bhutanese Himalaya during the Little Ice Age

Mountain glaciers provide us a window into past climate change and landscape evolution, but the pattern of glacier evolution at centennial or suborbital timescale remains elusive, especially in monsoonal Himalayas. We simulated the glacier evolution in Bhutanese Himalaya, a typical monsoon influenced region, during the Little Ice Age (LIA), using the Open Global Glacier Model and six paleo-climate datasets. Compared with the mapped glacial landforms, the model can well capture the glacier length changes, especially for the experiment driving by the GISS climate dataset, but overestimates the changes in glacier area. Simulation results reveal four glacial substages at 1270s–1400s, 1470s–1520s, 1700s–1710s, and 1820s–1900s in the study area. From further analysis, a negative correlation between the number of the substages and glacier length was found, which suggests that the number and occurrence of glacial substages are regulated by the heterogeneous responses of glaciers to climate change. In addition, the changes in summer temperature dominated the glacier evolution in this region during the LIA.

The Holocene Glacial History of Dart Glacier, Southern Alps, New Zealand

<p>Mountain glaciers are sensitive climate indicators, as climate variability drives mass changes that are expressed in glacier length fluctuations. These length changes are preserved in the geological record, thus offering the potential to generate new palaeoclimate proxy data that can be used to extend instrumental climate records. This study presents geomorphological mapping and cosmogenic ¹⁰Be surface exposure dating of the Holocene moraines at Dart Glacier, New Zealand. These findings show that an early Holocene advance (~6 km longer than present-day) took place ~7817 ± 336 years ago. Moraine ages also show that a more restricted glacier readvance (~4 km longer than present-day) occurred ~321 ± 44 years ago. Through better constraining the timing and magnitude of Holocene glacier length changes, we extend the ~100-year history of observational records in the upper Dart valley. Net retreat of Dart Glacier during the Holocene is consistent with other moraine chronologies from New Zealand, which supports existing hypotheses that suggest summer insolation was the dominant driver of multi-millennial climate change at southern mid-latitudes during the current interglacial. Individual moraine forming events at Dart Glacier also coincide with moraine ages from several other catchments in the Southern Alps and likely reflect shorter-term (decadal-centennial-scale) climatic changes. The new geological record constraints of length changes at Dart Glacier offer the opportunity to test such hypotheses more formally using physics-based modelling. Connecting Holocene moraine records to historical glacier observations using ¹⁰Be surface exposure dating requires consistently low background levels of this rare isotope. Systematic blank experiments show that concentrated analytical grade hydrofluoric acid and reused beakers are likely the largest contributors of ¹⁰Be to the average process blank in the VUW Cosmogenic Laboratory. Based on these findings I recommend small methodological improvements that could be implemented to lower process blank ratios for routine application of ¹⁰Be surface exposure dating to near-historic glacial landforms.</p>

The Holocene Glacial History of Dart Glacier, Southern Alps, New Zealand

<p>Mountain glaciers are sensitive climate indicators, as climate variability drives mass changes that are expressed in glacier length fluctuations. These length changes are preserved in the geological record, thus offering the potential to generate new palaeoclimate proxy data that can be used to extend instrumental climate records. This study presents geomorphological mapping and cosmogenic ¹⁰Be surface exposure dating of the Holocene moraines at Dart Glacier, New Zealand. These findings show that an early Holocene advance (~6 km longer than present-day) took place ~7817 ± 336 years ago. Moraine ages also show that a more restricted glacier readvance (~4 km longer than present-day) occurred ~321 ± 44 years ago. Through better constraining the timing and magnitude of Holocene glacier length changes, we extend the ~100-year history of observational records in the upper Dart valley. Net retreat of Dart Glacier during the Holocene is consistent with other moraine chronologies from New Zealand, which supports existing hypotheses that suggest summer insolation was the dominant driver of multi-millennial climate change at southern mid-latitudes during the current interglacial. Individual moraine forming events at Dart Glacier also coincide with moraine ages from several other catchments in the Southern Alps and likely reflect shorter-term (decadal-centennial-scale) climatic changes. The new geological record constraints of length changes at Dart Glacier offer the opportunity to test such hypotheses more formally using physics-based modelling. Connecting Holocene moraine records to historical glacier observations using ¹⁰Be surface exposure dating requires consistently low background levels of this rare isotope. Systematic blank experiments show that concentrated analytical grade hydrofluoric acid and reused beakers are likely the largest contributors of ¹⁰Be to the average process blank in the VUW Cosmogenic Laboratory. Based on these findings I recommend small methodological improvements that could be implemented to lower process blank ratios for routine application of ¹⁰Be surface exposure dating to near-historic glacial landforms.</p>

Inferring Past Climate from Moraine Evidence Using Glacier Modelling

<p>Glacier length fluctuations reflect changes in climate, most notably temperature and precipitation. By this reasoning, moraines, which represent former glacier extent, can be used to estimate past climate. However, estimating palaeoclimate from moraines is not a straight-forward process and involves several assumptions. For example, recent studies have suggested that interannual stochastic variability in temperature in a steady-state climate can cause a glacier to experience kilometre-scale fluctuations. Such studies cast doubt on the usefulness of moraines as climate proxy indicators. Detailed glacial geomorphological maps and moraine chronologies have improved our understanding of the spatial and temporal extent of past glacial events in New Zealand. Palaeoclimate estimates associated with these moraines have thus-far come from simple methods, such as the accumulation area ratio, with unquantifiable uncertainties. I used a numerical modelling approach to approximate the present-day glacier mass balance pattern, which includes the effects of snow avalanching on glacier mass balance. I then used the models to reconstruct palaeoclimate for Lateglacial and Holocene glacial events in New Zealand, and to better understand moraine-glacier-climate relationships. The climate reconstructions come from simulating past glacier expansions to specific terminal moraines, but I also simulated glacier fluctuations in response to a previously derived temperature reconstruction, and to interannual stochastic variability in temperature. The purpose behind each simulation was to identify the drivers of significant glacier fluctuations. The modelling results support the hypothesis that New Zealand moraine records reflect past climate, especially changes in temperature. Lateglacial climate was reconstructed to be 2-3 C lower than the present day. This temperature range agrees well with previous estimates from moraines and other climate proxy records in New Zealand. Modelled temperature estimates for the Holocene moraines are slightly colder than those derived from simpler methods, due to a non-linear relationship found between snowline lowering and glacier length. This relationship results from the specific valley shape and glacier geometry, and is likely to occur in other, similarly-shaped glacier valleys. The simulations forced by interannual stochastic variability in temperature do not show significant (>300 m) fluctuations in the glacier terminus. Such fluctuations can not explain the Holocene moraine sequence that I examined, which extends >2 km beyond the present-day glacier terminus. Stochastic temperature change could, however, in part, cause fluctuations in glacier extent during an overall glacier recession. Modelling shows that it is also unlikely that glaciers advanced to Holocene and Lateglacial moraine positions as a result of precipitation changes alone. For these reasons, temperature changes are a necessary part of explaining past glacier extents, especially during the Lateglacial, and the moraines examined here likely reflect changes in mean climate in New Zealand. The glacier modelling studies indicate that simpler methods, such as the accumulation area ratio, can be used to appropriately reconstruct past climate from glacial evidence, as long as the glacier catchment has a straight forward geometry, shallow bed slope and no tributary glaciers. Non-linear relationships between climate change and glacier length develop when valley shape is more complex, and glaciers within these systems are probably better simulated using a modelling approach. Using a numerical modelling approach, it is also possible to gain a greater understanding of glacier response time, length sensitivities, and estimates of ice extent in valleys within the model domain where geomorphic evidence is not available. In this manner, numerical models can be used as a tool for understanding past climate and glacier sensitivity, thus improving the confidence in the palaeoclimate interpretations.</p>

Inferring Past Climate from Moraine Evidence Using Glacier Modelling

<p>Glacier length fluctuations reflect changes in climate, most notably temperature and precipitation. By this reasoning, moraines, which represent former glacier extent, can be used to estimate past climate. However, estimating palaeoclimate from moraines is not a straight-forward process and involves several assumptions. For example, recent studies have suggested that interannual stochastic variability in temperature in a steady-state climate can cause a glacier to experience kilometre-scale fluctuations. Such studies cast doubt on the usefulness of moraines as climate proxy indicators. Detailed glacial geomorphological maps and moraine chronologies have improved our understanding of the spatial and temporal extent of past glacial events in New Zealand. Palaeoclimate estimates associated with these moraines have thus-far come from simple methods, such as the accumulation area ratio, with unquantifiable uncertainties. I used a numerical modelling approach to approximate the present-day glacier mass balance pattern, which includes the effects of snow avalanching on glacier mass balance. I then used the models to reconstruct palaeoclimate for Lateglacial and Holocene glacial events in New Zealand, and to better understand moraine-glacier-climate relationships. The climate reconstructions come from simulating past glacier expansions to specific terminal moraines, but I also simulated glacier fluctuations in response to a previously derived temperature reconstruction, and to interannual stochastic variability in temperature. The purpose behind each simulation was to identify the drivers of significant glacier fluctuations. The modelling results support the hypothesis that New Zealand moraine records reflect past climate, especially changes in temperature. Lateglacial climate was reconstructed to be 2-3 C lower than the present day. This temperature range agrees well with previous estimates from moraines and other climate proxy records in New Zealand. Modelled temperature estimates for the Holocene moraines are slightly colder than those derived from simpler methods, due to a non-linear relationship found between snowline lowering and glacier length. This relationship results from the specific valley shape and glacier geometry, and is likely to occur in other, similarly-shaped glacier valleys. The simulations forced by interannual stochastic variability in temperature do not show significant (>300 m) fluctuations in the glacier terminus. Such fluctuations can not explain the Holocene moraine sequence that I examined, which extends >2 km beyond the present-day glacier terminus. Stochastic temperature change could, however, in part, cause fluctuations in glacier extent during an overall glacier recession. Modelling shows that it is also unlikely that glaciers advanced to Holocene and Lateglacial moraine positions as a result of precipitation changes alone. For these reasons, temperature changes are a necessary part of explaining past glacier extents, especially during the Lateglacial, and the moraines examined here likely reflect changes in mean climate in New Zealand. The glacier modelling studies indicate that simpler methods, such as the accumulation area ratio, can be used to appropriately reconstruct past climate from glacial evidence, as long as the glacier catchment has a straight forward geometry, shallow bed slope and no tributary glaciers. Non-linear relationships between climate change and glacier length develop when valley shape is more complex, and glaciers within these systems are probably better simulated using a modelling approach. Using a numerical modelling approach, it is also possible to gain a greater understanding of glacier response time, length sensitivities, and estimates of ice extent in valleys within the model domain where geomorphic evidence is not available. In this manner, numerical models can be used as a tool for understanding past climate and glacier sensitivity, thus improving the confidence in the palaeoclimate interpretations.</p>

Modelling the Vadret da Tschierva, Switzerland: calibration with the historical length record and future response to climate change

The Vadret da Tschierva (Vd Tschierva) is a 4 km long glacier in the Swiss Alps spanning an altitude range of 2400–4049 m a.s.l. Length observations since 1855 show steady retreat interrupted by a period of advance from 1965 until 1985. The total retreat is ~2200 m (period 1855–2018). We have studied the Vd Tschierva with a flowline model, combined with ‘buckets’ that represent steep hanging glaciers and ice-free rock faces delivering mass to the main stream. The model is calibrated by a control method, in which an ELA history is objectively determined by finding the best match between observed and simulated glacier length. There is a modest correlation between the reconstructed ELA and an ELA record based on meteorological observations at Segl-Maria (only 8 km away from the glacier). It is difficult to reproduce the observed length record when the glacier model is driven by climate model output (Coupled Model Intercomparison Project 5). We have calculated the future evolution of the Vd Tschierva for different rates of ELA rise. For a constant rise of 4 ${\rm m\;}{\rm a}^{ \hbox{-} 1}$ , we predict that the glacier length will change from the current 3.2 km to ~1.7 km in the year 2100.

Modelling the mass budget and future evolution of Tunabreen, central Spitsbergen

Tunabreen is a 26-km long tidewater glacier. It is the most frequently surging glacier in Svalbard, with four documented surges in the past hundred years. We have modelled the evolution of this glacier with a Minimal Glacier Model (MGM), in which ice mechanics, calving and surging are parameterized. The model geometry consists of a flow band to which three tributaries supply mass. The calving rate is set to the mean observed value for the period 2012–2019, and kept constant. For the past 120 years, a smooth Equilibrium Line Altitude (ELA) history is reconstructed by finding the best possible match between observed and simulated glacier length. There is a modest correlation between this ELA history and meteorological observations from Longyearbyen. The simulated glacier retreat is in good agreement with observations. Runs with and without surging show that the effect of surging on the long term glacier evolution is limited. Due to the low surface slope and associated strong height -mass balance feedback, Tunabreen is very sensitive to changes in ELA. For a constant future ELA equal to the reconstructed value for 2020, the glacier front will retreat by 8 km during the coming hundred years. For an increase of the ELA of 2 m per year, the retreat is projected to be 13 km and Tunabreen becomes a land-based glacier around 2100. The calving rate is an important parameter: increasing its value by 50 % has about the same effect as a 50 m increase in the ELA, the corresponding equilibrium glacier length being 18 km (as compared to 25.8 km in the reference state). Response times vary from 150 to 400 years, depending on the forcing and on the state of the glacier (tidewater or land-based).

The glacial history of Rocky Top cirque, southeast Fiordland, New Zealand

<p><b>Understanding natural climate variability is a fundamental goal of paleoclimate science. Temperate mountain glaciers are sensitive to climate variability, changing volume, and thus thickness and length, in response to changes in temperature and precipitation. Glaciers deposit moraines at their margins, which if well-preserved may provide evidence of glacier length fluctuations following glacial retreat. Therefore mountain glaciers can be used as proxies to investigate past climatic changes, offering the potential to reconstruct the timing and magnitude of natural climate variability and paleoclimate for the former glacier extent(s). </b></p><p>This study applies methods of detailed geomorphological mapping and cosmogenic 10Be surface exposure dating to establish a high-precision moraine chronology and examine the timing and magnitude of glacier length changes at Rocky Top cirque. A quantitative reconstruction of paleoclimate for the identified former glacier extents was produced using an equilibrium-line altitude (ELA) reconstruction method and application of a temperature lapse rate. Findings show a clear pattern of glacial retreat at the end of the Last Glacial Maximum, with exposure ages from moraine boulders successfully constraining the timing of five distinct periods of glacier readvance or standstills. The most recent glacial event at Rocky Top cirque occurred between 17342 ± 172 yrs BP and during this period the ELA was depressed by 611 m. The second innermost moraine produced an indistinguishable age of 17196 ± 220 yrs BP and had an ELA depression of 616 m, indicating rapid glacial retreat. Progressively older moraines produced surface exposure ages of 18709 ± 237 and 19629 ± 308 yrs BP, with ELA depressions of 618 and 626 m respectively. The oldest moraine of 34608 ± 8437 yrs BP had insufficient geomorphic constraint to produce an ELA. Paleoclimate reconstruction results suggest that a best estimate of paleotemperature at the time of moraine formation (~19-17 ka) was between 3.2 ± 0.8 to 3.3 ± 0.8°C cooler than present-day. </p><p>Net retreat of the former glacier is consistent with other similar moraine chronologies from the Southern Alps, which supports the regional trend and suggests that glaciers in the Southern Alps responded to common climatic forcings between ~19-17 ka. </p>

The glacial history of Rocky Top cirque, southeast Fiordland, New Zealand

<p><b>Understanding natural climate variability is a fundamental goal of paleoclimate science. Temperate mountain glaciers are sensitive to climate variability, changing volume, and thus thickness and length, in response to changes in temperature and precipitation. Glaciers deposit moraines at their margins, which if well-preserved may provide evidence of glacier length fluctuations following glacial retreat. Therefore mountain glaciers can be used as proxies to investigate past climatic changes, offering the potential to reconstruct the timing and magnitude of natural climate variability and paleoclimate for the former glacier extent(s). </b></p><p>This study applies methods of detailed geomorphological mapping and cosmogenic 10Be surface exposure dating to establish a high-precision moraine chronology and examine the timing and magnitude of glacier length changes at Rocky Top cirque. A quantitative reconstruction of paleoclimate for the identified former glacier extents was produced using an equilibrium-line altitude (ELA) reconstruction method and application of a temperature lapse rate. Findings show a clear pattern of glacial retreat at the end of the Last Glacial Maximum, with exposure ages from moraine boulders successfully constraining the timing of five distinct periods of glacier readvance or standstills. The most recent glacial event at Rocky Top cirque occurred between 17342 ± 172 yrs BP and during this period the ELA was depressed by 611 m. The second innermost moraine produced an indistinguishable age of 17196 ± 220 yrs BP and had an ELA depression of 616 m, indicating rapid glacial retreat. Progressively older moraines produced surface exposure ages of 18709 ± 237 and 19629 ± 308 yrs BP, with ELA depressions of 618 and 626 m respectively. The oldest moraine of 34608 ± 8437 yrs BP had insufficient geomorphic constraint to produce an ELA. Paleoclimate reconstruction results suggest that a best estimate of paleotemperature at the time of moraine formation (~19-17 ka) was between 3.2 ± 0.8 to 3.3 ± 0.8°C cooler than present-day. </p><p>Net retreat of the former glacier is consistent with other similar moraine chronologies from the Southern Alps, which supports the regional trend and suggests that glaciers in the Southern Alps responded to common climatic forcings between ~19-17 ka. </p>

A new automatic approach for extracting glacier centerlines based on Euclidean allocation

Glacier centerlines are crucial input for many glaciological applications. From the morphological perspective, we proposed a new automatic method to derive glacier centerlines, which is based on the Euclidean allocation and the terrain characteristics of glacier surface. In the algorithm, all glaciers are logically classified as three types including simple glacier, simple compound glacier, and complex glacier, with corresponding process ranges from simple to complex. The process for extracting centerlines of glaciers introduces auxiliary reference lines and follows the setting of not passing through bare rock. The program of automatic extraction of glacier centerlines was implemented in Python and only required the glacier boundary and digital elevation model (DEM) as input. Application of this method to 48 571 glaciers in the second Chinese glacier inventory automatically yielded the corresponding glacier centerlines with an average computing time of 20.96 s, a success rate of 100 % and a comprehensive accuracy of 94.34 %. A comparison of the longest length of glaciers to the corresponding glaciers in the Randolph Glacier Inventory v6.0 revealed that our results were superior. Meanwhile, our final product provides more information about glacier length, such as the average length, and the longest length, the lengths in the accumulation and ablation regions of each glacier.