Abstract. The Oregon Cascades in the Pacific Northwest of the United States contain six glacierized volcanos. Here we document the 2023 glacier extents on five volcanos in the Oregon central Cascades: Broken Top, the Three Sisters (South, Middle, and North), and Mt. Jefferson. Using prior extent data from 2018 (Three Sisters and Broken Top) or 2020 (Mt. Jefferson) and 1990, we show that the glaciers on these summits lost ~ 20 % of their area between 2018/20 and 2023 and ~ 26 % of their area between 1990 and 2018/20. Glacier area loss thus increased 4.5- to 6.1-fold from ~ 0.6–1.9 % yr-1 (1990–2018/20) to ~ 3.2–11.3 % yr-1 (2018/20–2023). These rates of recent glacier area loss are unprecedented in each volcano’s recorded history. When combined with 2015/16 and 2023 glacier extents for the sixth volcano in the Oregon Cascades (Mt. Hood), we find that recent rates of glacier area loss are only correlated with measures of glacier size: glaciers with a smaller elevation span (p = 0.0005) and smaller area (p = 0.0018) are losing area faster. Because the Oregon Cascade summers of 2014–2023 were ~ 0.7 °C warmer than the 1985–1994 mean and ~ 1.5 °C warmer than the 1975–1984 mean, warming magnitudes sufficient to raise equilibrium lines above the headwalls of many of Oregon’s smaller glaciers, we attribute the increased rate of Oregon Cascade glacier area loss to this warming climate.

The Younger Dryas is the last major abrupt climate change event of the last deglaciation occurring ~12 900–11 700 years ago. Large portions of the Northern Hemisphere cooled and much of the Southern Hemisphere warmed during the event in a bipolar seesaw pattern. While changes in net precipitation were more variable at higher latitudes, Northern Hemisphere subtropics and tropics were generally drier and the Southern Hemisphere subtropics wetter from southward migration of the Intertropical Convergence Zone. Many of the climate changes related to the Younger Dryas were likely a response to increased freshwater discharge to the North Atlantic and the attendant reduction in Atlantic meridional overturning strength. Although multiple freshwater forcing hypotheses have been proposed, the existing terrestrial and marine records indicate that the northward retreat of the southern margin of the Laurentide Ice Sheet from the Great Lakes caused a routing of freshwater from the western Canadian Plains from the Mississippi River to the St. Lawrence River, with the increased freshwater discharge to the North Atlantic slowing ocean circulation and ultimately causing the Younger Dryas.

Abstract. As part of the southern Cascades, Mt. Hood is the tallest and most glaciated peak in Oregon, U.S.A. Despite alpine glaciers being one the clearest indicators of human-caused climate change, the 21st century behavior of glaciers on Mt. Hood has not been directly documented. Here we directly measure changes in Mt. Hood’s glacier extents from 2003 to 2023 and find dramatic retreat of all glaciers, with one glacier disappearing, another two nearing this status, and a third retreating towards this status. The seven largest glaciers on the volcano lost ~2.8 km2, or ~40 % of their area in the 21st century. Comparison to historic records of glacier area back to 1907 shows that this 21st-century retreat is unprecedented with respect to the previous century and has outpaced modeled glacier changes. The rate of retreat over the last 23 years is more than double the fastest rate documented in the last century from 1907 to 1946. We demonstrate that this century-scale retreat strongly correlates with regional 30-year-average climate warming of ~1.1 ºC since the early 1900s, but not with regional changes in precipitation. We conclude that Mt. Hood’s glaciers are retreating in response to a warming climate and that this recession has accelerated in the 21st century, with attendant consequences for water resources.

As part of the southern Cascades, Mt. Hood is the tallest and most glacierized peak in Oregon, USA. Despite alpine glaciers being one the clearest indicators of human-caused climate change, the 21st century behavior of glaciers on Mt. Hood has not been directly documented at the ground level. Here we directly measure changes in the extents of Mt. Hood's glaciers from 2003 to 2023 and find dramatic retreat of all glaciers, with one glacier ceasing to flow (joining another glacier that ceased flowing before 2003) and another three glaciers retreating towards this status. By 2023, Mt. Hood glaciers had lost similar to 17 % of their 2015-2016 area and similar to 39 % of their 1981 area. The rate of area loss from 2015-2016 to 2023 (similar to 2.10 % yr-1) was similar to 2.6 times faster than the rate from 1981 to 2015-2016 (similar to 0.81 % yr-1). The seven largest glaciers on the volcano lost similar to 25 % of their area between 2000 and 2023. Comparison to historic records of glacier area back to 1907 shows that this 21st century retreat is unprecedented relative to the previous century. The rate of area loss over the last 23 years (similar to 1.07 % yr-1) was similar to 1.9 times faster than the fastest rate documented in the last century from 1907 to 1946 (similar to 0.56 % yr-1) and similar to 3.5 times faster than the 20th century average (similar to 0.31 % yr-1). This unprecedented rate of retreat corresponds to regional summer warmth reaching 1.7-1.8 degrees C (2013-2023 average) relative to the early 1900s, but not with regional changes in winter precipitation. We conclude that Mt. Hood's glaciers are retreating in response to a warming climate and that this recession has accelerated in the 21st century.

During the last deglaciation, collapse of the saddle between the North American Cordilleran and Laurentide ice sheets led to rapid ice-sheet mass loss and separation, with meltwater discharge contributing to deglacial sea-level rise. We directly date ice-sheet separation at the end of the saddle collapse using 64 Be-10 exposure ages along an similar to 1200-km transect of the ice-sheet suture zone. Collapse began in the south by 15.4 +/- 0.4 ka and ended by 13.8 +/- 0.1 ka at similar to 56 degrees N. Ice-sheet model simulations consistent with the Be-10 ages find that the saddle collapse contributed 6.2-7.2 m to global mean sea-level rise from similar to 15.5 ka to similar to 14.0 ka, or approximately one third of global mean sea-level rise over this period. We determine 3.1-3.6 m of the saddle collapse meltwater was released during Meltwater Pulse 1A similar to 14.6-14.3 ka, constituting 20-40% of this meltwater pulse's volume. Because the separation of the Cordilleran and Laurentide ice sheets occurred over 1-2 millennia, the associated release of meltwater during the saddle collapse supplied a smaller contribution to the magnitude of Meltwater Pulse 1A than has been recently proposed.

Significance The Ice-Free Corridor (IFC) has long played a key role in hypotheses about the peopling of the Americas. Earlier assessments of its age suggested that the IFC was available for a Clovis-first migration, but subsequent developments now suggest a pre-Clovis occupation of the Americas that occurred before the opening of the IFC, thus supporting a Pacific coastal migration route instead. However, large uncertainties in existing ages from the IFC cannot preclude its availability as a route for the first migrations. Resolving this debate over migration route is important for addressing the questions of when and how the first Americans arrived. We report cosmogenic nuclide exposure ages that show that the final opening of the IFC occurred well after pre-Clovis occupation.

We present a portable photon-counting LiDAR that uses a bistatic geometry to measure pulse broadening in the multiple-scattering regime. A diffusion model allows us to extract optical scattering and absorption coefficients of glacier ice.

We present new Be-10 surface exposure ages from boulders on bedrock to directly date northwest Laurentide ice-sheet deglaciation through a wide swath of the western Canadian Shield that had no previous reliable temporal constraints on ice-margin retreat. Uplift-corrected boulder Be-10 surface exposure ages are 13.9 +/- 0.2 ka (n = 6) at a site on the western edge of the Slave Craton and 12.4 +/- 0.2 ka (n = 5, 1 outlier) at a second site similar to 110 km up-ice to the east. These direct Be-10 ages for ice-margin retreat are similar to 2.4 kyr and similar to 1.6 kyr older, respectively, than the canonical deglacial chronology for the northwest Laurentide ice sheet that is based on minimum-limiting C-14 dates. We infer an ice-margin retreat rate of 60-100 m yr(-1) between the two sites over an interval spanning the transition from the Allerod warm period into the Younger Dryas cold period. This is significantly slower than the rapid >800 m yr(-1) retreat rate for the northwest Laurentide ice sheet inferred from earlier deglacial chronologies, which has been hypothesized as a potential source of meltwater forcing for the Younger Dryas cold period. These first direct ages on spatio-temporal patterns of deglaciation in this data-poor region suggest that additional refinement of the deglacial chronology is needed to test hypotheses on the relation between ice-sheet retreat, associated meltwater discharge, abrupt climate change, and rapid sea-level rise. (C) 2022 Elsevier Ltd. All rights reserved.

Abstract The production of meltwater from glacier ice, which is exposed at the margins of land ice during the summer, is responsible for a large proportion of glacier mass loss. The rate of meltwater production from glacier ice is especially sensitive to its physical structure and chemical composition which combine to determine the albedo of glacier ice. However, the optical properties of near-surface glacier ice are not well known since most prior work has focused on laboratory-grown ice or deep cores. Here, we demonstrate a measurement technique based on diffuse propagation of nanosecond-duration laser pulses in near-surface glacier ice that enables the independent measurement of the scattering and absorption coefficients, allowing for a complete description of the processes governing radiative transfer. We employ a photon-counting detector to overcome the high losses associated with diffuse optics. The instrument is highly portable and rugged, making it optimally suited for deployment in remote regions. A set of measurements taken on Crook and Collier Glaciers, Oregon, serves as a demonstration of the technique. These measurements provide insight into both physical structure and composition of near-surface glacier ice and open new avenues for the analysis of light-absorbing impurities and remote sensing of the cryosphere.

The Antarctic Ice Sheet (AIS) response to past warming consistent with the 1.5–2°C “safe limit” of the United Nations Paris Agreement is currently not well known. Empirical evidence from the most recent comparable period, the Last Interglaciation, is sparse, and transient ice‐sheet experiments are few and inconsistent. Here, we present new, transient, GCM‐forced ice‐sheet simulations validated against proxy reconstructions. This is the first time such an evaluation has been attempted. Our empirically constrained simulations indicate that the AIS contributed 4 m to global mean sea level by 126 ka BP, with ice lost primarily from the Amundsen, but not Ross or Weddell Sea, sectors. We resolve the conflict between previous work and show that the AIS thinned in the Wilkes Subglacial Basin but did not retreat. We also find that the West AIS may be predisposed to future collapse even in the absence of further environmental change, consistent with previous studies.

During the last deglaciation, North Atlantic climate abruptly warmed at the Belling (similar to 14.7 ka), cooled into the Younger Dryas (similar to 12.9 ka) and abruptly warmed again into the Holocene (similar to 11.7 ka). While these events are defined by Greenland ice cores, there is still considerable uncertainty on Greenland ice-sheet margin responses to abrupt climate change. To refine the ice sheet's deglacial history, we present new cosmogenic nuclide surface exposure ages from boulders on bedrock at five sites in southernmost Greenland fjords located midway between the coast and inland ice margin. We find ice-sheet thinning below three local topographic highs at 12.7 +/- 0.3 ka (n = 3),13.1 +/- 0.4 ka (n = 1, 2 outliers), and 12.3 +/- 0.2 ka (n = 3), with up-fjord retreat at 12.5 +/- 0.3 ka (n = 3) and 12.7 +/- 0.2 ka (n = 4) based on two sites just above the mid-fjord marine limit. These mid-fjord Be-10 ages therefore show southernmost Greenland ice-sheet thinning and retreat during the Younger Dryas. We hypothesize that this thinning and retreat was a response to ocean warming prior to the Holocene and/or summer shortwave radiative forcing during the Younger Dryas due to peak boreal summer insolation. Our results also support a previously hypothesized winter bias in proxy records of Younger Dryas atmospheric cooling, since a large summer cooling during the Younger Dryas could have counteracted the effects of ocean warming and direct radiative forcing, inhibiting ice-sheet retreat. (C) 2021 Elsevier Ltd. All rights reserved.

The century-long 8.2 ka cold event interrupted early Holocene boreal warmth and may have halted retreat of Greenland ice-sheet margins during the last deglaciation. Here, we synthesize new and existing glacial geological data to assess the behavior of the southwest Greenland ice sheet during the 8.2 ka cold event. In southwest Greenland near the town of Kangerlussuaq, existing and new Be-10 surface exposure ages demonstrate that deposition of the Keglen moraines ended at 8.0 +/- 0.1 ka (n = 10, 1 outlier), with prior studies arguing that these moraines represented an ice-margin stillstand in response to the 8.2 ka cold event. However, new Be-10 ages show that the southwest Greenland ice-sheet margin retreated from the Umivit moraines, which lie 5-10 km outboard of the Keglen moraines, at 8.2 +/- 0.2 ka (n = 11). Accordingly, we suggest that the southwest Greenland ice-sheet margin in the Kangerlussuaq region retreated 5-10 km during the 8.2 ka cold event. Farther south and inland from Nuuk, new Be-10 boulder-on-bedrock ages adjacent to the modern ice-sheet margin demonstrate that the ice-margin was retreating with no moraine deposition at 8.2 +/- 0.1 ka (n = 4, 1 outlier), with this retreat continuing up to at least similar to 7.5 ka according to an existing threshold lake record. Therefore, we propose that the southwest Greenland ice-sheet margin underwent continued retreat during the 8.2 ka cold event in response to elevated early Holocene boreal summer insolation that overwhelmed the impacts from century-scale cooling. Caution should be used in assuming climatic causation for moraine deposition based on temporal correlation. (C) 2021 Elsevier Ltd. All rights reserved.

During the last interglaciation (LIG;-129-116 ka), global mean sea level (GMSL) was 6-9 m above present. However, the source, or sources, of the higher-than-present sea level are only partially confirmed with far fewer geologic constraints than GMSL itself. Because of modest LIG Greenland Ice Sheet retreat that raised sea level by < 2.5 m, Antarctic ice sheets are hypothesized to have contributed significantly to the LIG GMSL highstand, but direct evidence is limited. Here we infer ice-sheet presence or absence on West Antarctica and the Antarctic Peninsula using sediment geochemistry in the Bellingshausen Sea at Ocean Drilling Program (ODP) Site 1096. In particular, a combination of Sr-NdPb isotopes and trace-element ratios allows differentiation between silt sourced from West Antarctica and the Antarctic Peninsula. From the Holocene (<11.7 ka) back through Marine Isotope Stage (MIS) 5d (-116 ka), we find in-50% of our samples glacially eroded silt sourced from under the West Antarctic Ice Sheet that was transported to the Bellingshausen Sea in the Antarctic Circumpolar Current, in addition to silt sourced from under the proximal Antarctic Peninsula Ice Sheet. However, West Antarctic-sourced silt is absent during the LIG (MIS 5e) when only Antarctic Peninsula-sourced silt was deposited at ODP Site 1096. This lack of West Antarctic-sourced silt is consistent with the two ice-core constraints on West Antarctic Ice Sheet LIG size, which combined with our record point towards the absence of the West Antarctic Ice Sheet during the LIG. We therefore provide the first marine-based sedimentary evidence that supports West Antarctic Ice-Sheet absence during the LIG. 0 2021 Elsevier Ltd. All rights reserved.

The northern hemisphere experienced an abrupt cold event ~ 8200 years ago (the 8.2 ka event) that was triggered by the release of meltwater into the Labrador Sea, and resulting in a weakening of the poleward oceanic heat transport. Although this event has been considered a possible analogue for future ocean circulation changes due to the projected Greenland Ice Sheet (GIS) melting, large uncertainties in the amount and rate of freshwater released during the 8.2 ka event make such a comparison difficult. In this study, we compare sea surface temperatures and oxygen isotope ratios from 28 isotope-enabled model simulations with 35 paleoproxy records to constrain the meltwater released during the 8.2 ka event. Our results suggest that a combination of 5.3 m of meltwater in sea level rise equivalent (SLR) released over a thousand years, with a short intensification over ~ 130 years (an additional 2.2 m of equivalent SLR) due to routing of the Canadian river discharge, best reproduces the proxy anomalies. Our estimate is of the same order of magnitude as projected future GIS melting rates under the high emission scenario RCP8.5.

We present new 10Be surface exposure ages from boulders on bedrock to directly date northwest Laurentide ice-sheet deglaciation through a wide swath of the western Canadian Shield that had no previous reliable temporal constraints on ice-margin retreat. Uplift-corrected boulder 10Be surface exposure ages are 13.9 ± 0.2 ka (n = 6) at a site on the western edge of the Slave Craton and 12.4 ± 0.2 ka (n = 5, 1 outlier) at a second site ∼110 km up-ice to the east. These direct 10Be ages for ice-margin retreat are ∼2.4 kyr and ∼1.6 kyr older, respectively, than the canonical deglacial chronology for the northwest Laurentide ice sheet that is based on minimum-limiting 14C dates. We infer an ice-margin retreat rate of 60–100 m yr−1 between the two sites over an interval spanning the transition from the Allerød warm period into the Younger Dryas cold period. This is significantly slower than the rapid >800 m yr−1 retreat rate for the northwest Laurentide ice sheet inferred from earlier deglacial chronologies, which has been hypothesized as a potential source of meltwater forcing for the Younger Dryas cold period. These first direct ages on spatio-temporal patterns of deglaciation in this data-poor region suggest that additional refinement of the deglacial chronology is needed to test hypotheses on the relation between ice-sheet retreat, associated meltwater discharge, abrupt climate change, and rapid sea-level rise.

The timing of northwest Laurentide ice-sheet deglaciation is important for understanding how ice-sheet retreat, and associated meltwater discharge, may have been involved in abrupt climate change and rapid sea-level rise at the end of the last glaciation. However, the deglacial chronology across the western Canadian Shield is poorly understood, with only a handful of minimum-limiting 14C dates and sparse cosmogenic nuclide exposure dates constraining the timing and pattern of northwest Laurentide ice-sheet retreat across >1000 km of ice-sheet retreat to the marine limit west of Hudson Bay. We present cosmogenic 10Be surface exposure dating of glacial erratics at two sites, within a ~160,000 km2 region with no reliable temporal constraints on ice-margin retreat, to directly date the timing of northwest Laurentide ice-sheet deglaciation. Six erratics perched directly on bedrock at a site on the western edge of the Slave Craton have exposure ages between 12.8±0.6 and 12.2±0.6 thousand years ago (ka; ±1sigma). Five erratics on bedrock, sampled at a site 115 km up-ice to the east, yielded exposure ages between 10.8±0.5 and 11.6±0.5 ka. When corrected for decreased atmospheric depth due to isostatic uplift since deglaciation, the error-weighted mean ages for the two sites indicate that the Laurentide ice sheet retreated through this region of the western Canadian Shield between 13.3±0.2 and 11.8±0.2 ka, or at least 1 kyr earlier than inferred from the canonical compilation of minimum-limiting 14C dates for deglaciation and paleo-glaciological models. We tentatively infer a preliminary ice-margin retreat rate of ~0.1 m kyr-1 over this interval spanning much of the Younger Dryas which, compared to earlier estimates, implies a substantially lower volume of meltwater flux from the retreating northwest Laurentide ice sheet at this time. Additional exposure ages on glacial erratics across this data-poor region are needed for validation of existing deglacial ice-sheet models, which can in turn contribute to comprehensive testing of hypotheses related to northwest Laurentide ice-sheet retreat rate, abrupt deglacial sea-level rise, and potential forcing of associated climate change events.

During the last glacial maximum, the Cordilleran and Laurentide ice sheets met just to the east of the Canadian Rocky Mountains, forming an ice-sheet saddle. When this saddle disappeared has implications on deglacial global sea-level rise and abrupt climate change as well as human migration patterns to the Americas. We will present new 10-Be boulder ages from six sites on a ~1100 km transect along the ice-sheet suture zone, to date Cordilleran-Laurentide ice-sheet separation. Results will directly test whether or not Cordilleran-Laurentide separation contributed to abrupt sea-level rise during meltwater pulse 1a (14.6-14.3 ka) in response to abrupt Bølling warming (14.6-14.0 ka).