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1. Reconstructing mountain glacier equilibrium line altitudes for the Last Glacial Maximum in the western United States

   https://doi.org/10.1130/abs/2021CD-363195  

   Halvorson, Victoria E.; Laabs, Benjamin J.C. (April 2021, Geological Society of America Abstracts with Programs)

   null (Ed.)

   Important information about past climates can be determined from reconstructed equilibrium line altitudes (ELA) of mountain paleoglaciers, specifically the temperature and precipitation accompanying a glacier in equilibrium. Previous reconstructions of Late Pleistocene ELAs of mountain glaciers across the western United States have been used to infer the pattern of temperature and precipitation change across the region, although most of the work was based on presumed ages and limited mapping of glacial deposits and landforms. Cosmogenic nuclide exposure dating of moraines combined with updated mapping and aerial imagery afford an opportunity to revisit the pattern of regional ELAs during multiple episodes of the last Pleistocene glaciation. The goal of this research is to reconstruct ELAs in the same region of previous reconstructions based on glacial sediments that have been dated using cosmogenic nuclide exposure ages. We focus on the large number of glacial valleys with moraines corresponding to the Last Glacial Maximum (LGM; 26.5-19.0 ka). Paleo-ELAs are estimated using the toe to headwall altitude ratio and the accumulation area ratio determined from published glacier reconstructions and existing glacial mapping. Cosmogenic-exposure ages of moraines are compiled from the informal cosmogenic nuclide exposure age database for alpine glacial features (ICE-D Alpine) and represented in a geographic information system along with ELAs for each glacial valley. A reconstructed ELA surface spanning the conterminous western United States is produced using existing algorithms in ArcGIS. Results show reconstructed ELAs generally lower than initially estimated and a larger range of ELAs across the region. In the Sierra Nevada, ELAs increase southeastward, which is consistent with previous estimates, spanning a range from 1800 to 2800 m asl. ELAs rise eastward across the Basin and Range toward the western shore of the area covered by Lake Bonneville, and then decrease eastward toward the Wasatch Mountains. This pattern is inconsistent with previous estimates and may reflect a west-to-east precipitation gradient that differs from modern climate. We discuss this pattern and broader features of the ELA surface of the LGM and later episodes of the last Pleistocene glaciation. 

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2. Last Glacial Maximum Reconstructions of Rwenzori Mountain Glaciers

   https://doi.org/10.1029/2022PA004527  

   Doughty, Alice M.; Kelly, Meredith A.; Russell, James M.; Jackson, Margaret S.; Anderson, Brian M.; Chipman, Jonathan; Nakileza, Bob R. (January 2023, Paleoceanography and Paleoclimatology)

   Abstract The magnitude of tropical cooling during the Last Glacial Maximum (LGM; ∼19–26.5 ka) remains controversial, with sea‐surface temperatures cooling by several degrees less than most temperatures reconstructed at high elevations. To explain this discrepancy, past studies proposed a steeper (increased) lapse rate—the temperature decrease with elevation—during the LGM relative to today. For instance, LGM temperatures in East Africa reconstructed from branched GDGTs from multiple elevations support an ∼0.9°C/km increase in the lapse rate during the LGM relative to present day. Lapse rates are a critical part of the Earth's climate sensitivity and atmospheric energy transfer, and it is vital to know whether and by how much the tropical lapse rate steepened during the LGM. Here, we simulate LGM glacier extents in the Rwenzori Mountains of Uganda with and without a change in lapse rate using a range of temperature and precipitation estimates. We find that the lapse rate must have been steeper than present for glaciers to reach their LGM positions using available sea‐level temperature and precipitation estimates for East Africa. 

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3. Late Holocene glacier and climate fluctuations in the Mackenzie and Selwyn mountain ranges, northwestern Canada

   https://doi.org/10.5194/tc-17-4381-2023  

   Hawkins, Adam C.; Menounos, Brian; Goehring, Brent M.; Osborn, Gerald; Pelto, Ben M.; Darvill, Christopher M.; Schaefer, Joerg M. (January 2023, The Cryosphere)

   Abstract. Over the last century, northwestern Canada experienced some of the highest rates of tropospheric warming globally, which caused glaciers in the region to rapidly retreat. Our study seeks to extend the record of glacier fluctuations and assess climate drivers prior to the instrumental record in the Mackenzie and Selwyn mountains of northwestern Canada. We collected 27 10Be surface exposure ages across nine cirque and valley glacier moraines to constrain the timing of their emplacement. Cirque and valley glaciers in this region reached their greatest Holocene extents in the latter half of the Little Ice Age (1600–1850 CE). Four erratic boulders, 10–250 m distal from late Holocene moraines, yielded 10Be exposure ages of 10.9–11.6 ka, demonstrating that by ca. 11 ka, alpine glaciers were no more extensive than during the last several hundred years. Estimated temperature change obtained through reconstruction of equilibrium line altitudes shows that since ca. 1850 CE, mean annual temperatures have risen 0.2–2.3 ∘C. We use our glacier chronology and the Open Global Glacier Model (OGGM) to estimate that from 1000 CE, glaciers in this region reached a maximum total volume of 34–38 km3 between 1765 and 1855 CE and had lost nearly half their ice volume by 2019 CE. OGGM was unable to produce modeled glacier lengths that match the timing or magnitude of the maximum glacier extent indicated by the 10Be chronology. However, when applied to the entire Mackenzie and Selwyn mountain region, past millennium OGGM simulations using the Max Planck Institute Earth System Model (MPI-ESM) and the Community Climate System Model 4 (CCSM4) yield late Holocene glacier volume change temporally consistent with our moraine and remote sensing record, while the Meteorological Research Institute Earth System Model 2 (MRI-ESM2) and the Model for Interdisciplinary Research on Climate (MIROC) fail to produce modeled glacier change consistent with our glacier chronology. Finally, OGGM forced by future climate projections under varying greenhouse gas emission scenarios predicts 85 % to over 97 % glacier volume loss by the end of the 21st century. The loss of glaciers from this region will have profound impacts on local ecosystems and communities that rely on meltwater from glacierized catchments. 

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4. Similar Holocene glaciation histories in tropical South America and Africa

   https://doi.org/10.1130/G48059.1  

   Vickers, Anthony C.; Shakun, Jeremy D.; Goehring, Brent M.; Gorin, Andrew; Kelly, Meredith A.; Jackson, Margaret S.; Doughty, Alice; Russell, James (September 2020, Geology)

   null (Ed.)

   Abstract Tropical glaciers have retreated alongside warming temperatures over the past century, yet the way in which these trends fit into a long-term geological context is largely unclear. Here, we present reconstructions of Holocene glacier extents relative to today from the Quelccaya ice cap (Peru) and the Rwenzori Mountains (Uganda) based on measurements of in situ14C and 10Be from recently exposed bedrock. Ice-extent histories are similar at both sites and suggest that ice was generally smaller than today during the first half of the Holocene and larger than today for most, if not all, of the past several millennia. The similar glaciation history in South America and Africa suggests that large-scale warming followed by cooling of the tropics during the late Holocene primarily drove ice extent, rather than regional changes in precipitation. Our results also imply that recent tropical ice retreat is anomalous in a multimillennial context. 

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5. Early Holocene cold snaps and their expression in the moraine record of the eastern European Alps

   https://doi.org/10.5194/cp-17-2451-2021  

   Braumann, Sandra M.; Schaefer, Joerg M.; Neuhuber, Stephanie M.; Lüthgens, Christopher; Hidy, Alan J.; Fiebig, Markus (January 2021, Climate of the Past)

   Abstract. Glaciers preserve climate variations in their geologicaland geomorphological records, which makes them prime candidates for climatereconstructions. Investigating the glacier–climate system over the pastmillennia is particularly relevant first because the amplitude andfrequency of natural climate variability during the Holocene provides theclimatic context against which modern, human-induced climate change must beassessed. Second, the transition from the last glacial to the currentinterglacial promises important insights into the climate system duringwarming, which is of particular interest with respect to ongoing climatechange. Evidence of stable ice margin positions that record cooling during the past12 kyr are preserved in two glaciated valleys of the Silvretta Massif in theeastern European Alps, the Jamtal (JAM) and the Laraintal (LAR). We mappedand dated moraines in these catchments including historical ridges usingberyllium-10 surface exposure dating (10Be SED) techniques andcorrelate resulting moraine formation intervals with climate proxy recordsto evaluate the spatial and temporal scale of these cold phases. The newgeochronologies indicate the formation of moraines during the early Holocene (EH), ca. 11.0 ± 0.7 ka (n = 19). Boulder ages along historical moraines (n = 6) suggest at least two glacier advances during the Little Ice Age (LIA; ca. 1250–1850 CE) around 1300 CE and in the second half of the 18th century. An earlier advance to the same position may have occurredaround 500 CE. The Jamtal and Laraintal moraine chronologies provide evidence thatmillennial-scale EH warming was superimposed by centennial-scale cooling.The timing of EH moraine formation coincides with brief temperature dropsidentified in local and regional paleoproxy records, most prominently withthe Preboreal Oscillation (PBO) and is consistent with moraine depositionin other catchments in the European Alps and in the Arctic region. Thisconsistency points to cooling beyond the local scale and therefore aregional or even hemispheric climate driver. Freshwater input sourced fromthe Laurentide Ice Sheet (LIS), which changed circulation patterns in theNorth Atlantic, is a plausible explanation for EH cooling and moraineformation in the Nordic region and in Europe. 

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