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The land ice contribution to global mean sea level rise has not yet been predicted1using ice sheet and glacier models for the latest set of socio-economic scenarios, nor using coordinated exploration of uncertainties arising from the various computer models involved. Two recent international projects generated a large suite of projections using multiple models2–8, but primarily used previous-generation scenarios9and climate models10, and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios11,12using statistical emulation of the ice sheet and glacier models. We find that limiting global warming to 1.5 degrees Celsius would halve the land ice contribution to twenty-first-century sea level rise, relative to current emissions pledges. The median decreases from 25 to 13 centimetres sea level equivalent (SLE) by 2100, with glaciers responsible for half the sea level contribution. The projected Antarctic contribution does not show a clear response to the emissions scenario, owing to uncertainties in the competing processes of increasing ice loss and snowfall accumulation in a warming climate. However, under risk-averse (pessimistic) assumptions, Antarctic ice loss could be five times higher, increasing the median land ice contribution to 42 centimetres SLE under current policies and pledges, with the 95th percentile projection exceeding half a metre even under 1.5 degrees Celsius warming. This would severely limit the possibility of mitigating future coastal flooding. Given this large range (between 13 centimetres SLE using the main projections under 1.5 degrees Celsius warming and 42 centimetres SLE using risk-averse projections under current pledges), adaptation planning for twenty-first-century sea level rise must account for a factor-of-three uncertainty in the land ice contribution until climate policies and the Antarctic response are further constrained.

Worldwide glacier retreat and associated future runoff changes raise major concerns over the sustainability of global water resources1–4, but global-scale assessments of glacier decline and the resulting hydrological consequences are scarce5,6. Here we compute global glacier runoff changes for 56 large-scale glacierized drainage basins to 2100 and analyse the glacial impact on streamflow. In roughly half of the investigated basins, the modelled annual glacier runoff continues to rise until a maximum (‘peak water’) is reached, beyond which runoff steadily declines. In the remaining basins, this tipping point has already been passed. Peak water occurs later in basins with larger glaciers and higher ice-cover fractions. Typically, future glacier runoff increases in early summer but decreases in late summer. Although most of the 56 basins have less than 2% ice coverage, by 2100 one-third of them might experience runoff decreases greater than 10% due to glacier mass loss in at least one month of the melt season, with the largest reductions in central Asia and the Andes. We conclude that, even in large-scale basins with minimal ice-cover fraction, the downstream hydrological effects of continued glacier wastage can be substantial, but the magnitudes vary greatly among basins and throughout the melt season. The future of glaciers and associated runoff is projected for 56 large drainage basins globally, with glacier wastage impacting on runoff and water resources even in basins with limited glacier cover.

Melt from debris‐covered glaciers represents a regionally important freshwater source, especially in high‐relief settings as found in central Asia, Alaska, and South America. Sub‐debris melt is traditionally predicted from surface energy balance models that determine heat conduction through the supraglacial debris layer. Convection is rarely addressed, despite the porous nature of debris. Here we provide the first constraints on convection in supraglacial debris, through the development of a novel method to calculate individual conductive and nonconductive heat flux components from debris temperature profile data. This method was applied to data from Kennicott Glacier, Alaska, spanning two weeks in the summer of 2011 and two months in the summer of 2020. Both heat flux components exhibit diurnal cycles, the amplitude of which is coupled to atmospheric conditions. Mean diurnal nonconductive heat flux peaks at up to 43% the value of conductive heat flux, indicating that failure to account for it may lead to an incorrect representation of melt rates and their drivers. We interpret this heat flux to be dominated by latent heat as debris moisture content changes on a diurnal cycle. A sharp afternoon drop‐off in nonconductive heat flux is observed at shallow depths as debris dries. We expect these processes to be relevant for other debris‐covered glaciers. Debris properties such as porosity and tortuosity may play a large role in modulating it. Based on the present analysis, we recommend further study of convection in supraglacial debris for glaciers across the globe with different debris properties. Glacier melt under a surface layer of rocky debris is typically predicted by determining the energy available at the debris surface, then calculating heat transferred via conduction through the debris layer. However, heat can also be transferred in the debris layer through other processes such as air movement and the evaporation of melt water. We analyzed temperature data in the debris to show that these processes are significant, accounting for up to 43% of heat transferred via conduction. These must thus be investigated further and incorporated into models of glacier melt. We developed a new method to constrain supraglacial debris thermal properties and nonconductive heat fluxes from temperature profile dataNonconductive heat flux can be as much as 43% the conductive heat flux, thus it is important to understand its driving processesWe interpret nonconductive heat flux to be dominated by convection of moisture and latent heat flux as the debris diurnally wets and dries We developed a new method to constrain supraglacial debris thermal properties and nonconductive heat fluxes from temperature profile data Nonconductive heat flux can be as much as 43% the conductive heat flux, thus it is important to understand its driving processes We interpret nonconductive heat flux to be dominated by convection of moisture and latent heat flux as the debris diurnally wets and dries

To characterize the spatiotemporal variations of glacier surface speed on the Kenai Peninsula, Alaska (∼3,900 km2), we derived 92 surface speed fields between October 2014 and December 2019 using intensity offset tracking on Sentinel‐1 data. On average, speeds are 50% greater in spring (March‐May) than the annual mean (69 m a−1) while winter speeds are close to the annual mean. While marine‐terminating glaciers have their maximum speed near the terminus, both land‐ and lake‐terminating glaciers flow fastest around the median glacier elevation. On average, the lake‐terminating and tidewater glaciers flow 1.7 and 2.3 times faster than the land‐terminating glaciers, respectively. Monthly variations over the 5‐year period are strikingly synchronous regardless of terminus type suggesting that regional‐scale meteorological drivers govern the temporal variability. Mean annual speeds fluctuate roughly ±10% of the period mean without an apparent trend. At lake‐terminating Bear Glacier, a short‐term tripling in ice speed in fall 2019 over the area below an ice‐dammed lake coincides with an observed glacier lake outburst flood (GLOF). An earlier GLOF caused a persistent breach of the beach barrier between the proglacial lake and ocean which likely led to overall speed‐up of the lower glacier part throughout 2019. A significant speedup was also observed at the lower part of the lake‐terminating Ellsworth Glacier and attributed to rapid glacier retreat and lake expansion, probably further amplified by the terminus area becoming buoyant and a large tabular iceberg breaking off. Our results highlight the impact of GLOFs and proglacial characteristics in spatial and temporal glacier speed variations. Glaciers constantly move but the speed varies with time and space. We use satellite data to calculate the surface ice speed variations of all glaciers on the Kenai Peninsula, Alaska (∼3,900 km2), between October 2014 and December 2019. On average, the glacier speed is 50% higher in spring than the annual average. Overall, the lake‐terminating glaciers flow 70% faster, and marine‐terminating glaciers more than 100% faster than the land‐terminating glaciers. Over the 5‐year period, all glaciers regardless of terminus type tended to speed up or slow down simultaneously, probably driven by large‐scale variations in weather governing the water supply to the drainage system. The lower part of Bear Glacier tripled in speed in fall 2019 when an ice‐dammed lake suddenly drained through the glacier. Another glacier outburst flood in 2018 led to a breach of the beach barrier between the large lake at the glacier terminus and the ocean which might have caused the considerable speedup of the lower glacier part throughout 2019. Lake‐terminating glaciers flow 1.7 times and tidewater glaciers 2.3 times faster than the land‐terminating glaciersMonthly ice speed variations are largely synchronous across the Peninsula irrespective of terminus typeA glacier outburst flood at Bear Glacier caused both a short‐term and a longer‐term speed‐up of the glacier's lower part Lake‐terminating glaciers flow 1.7 times and tidewater glaciers 2.3 times faster than the land‐terminating glaciers Monthly ice speed variations are largely synchronous across the Peninsula irrespective of terminus type A glacier outburst flood at Bear Glacier caused both a short‐term and a longer‐term speed‐up of the glacier's lower part

AbstractAlaska’s arctic glaciers have retreated and thinned during recent decades, and glaciers in the central Brooks Range are no exception. Digital elevation models (DEMs) reconstructed from topographic maps (from 1970 and 1973) were differenced from a 2001 interferometric synthetic aperture radar DEM to calculate the volume and mass changes of 107 glaciers covering 42 km2(1970/1973) in the central Brooks Range, Alaska, U.S.A. For each glacier the 1970/1973 DEM was 3-D co-registered (horizontal and vertical) to maximize agreement between the non-glacierized terrains of both DEMs. Over the period 1970–2001, total ice volume loss was 0.69 ± 0.06 km3corresponding to a mean (area-weighted) specific mass balance rate of -0.54 ± 0.05 m w.e. a-1(± uncertainty). The arithmetic mean of all glaciers’ specific mass balance rates was -0.47 ± 0.27 m w.e. a-1(± 1 std. dev.). A value of -0.52 ± 0.36 m w.e. a-1(± 1 std. dev.) was found when 3-D coregistration is performed over the entire domain instead of individually for each glacier, indicating the importance of proper co-registration. Glacier area, perimeter, boundary compactness, mean elevation, and mean slope were correlated with specific balance rates, suggesting that large, low-elevation, elongated and shallow sloped glaciers had more negative balance rates than small, high-elevation, circular, and steep glaciers. A subsample of 36 glaciers showed a mean area reduction of 26 ± 16% (±1 std. dev.) over ∼35 years.

Timing of cirque formation and the climate necessary to initiate glaciation are fundamental to the understanding of the landscape of the northern Scandinavian mountains. Empty cirques in the Rassepautasjtjåkka massif are located near a glaciated area and thus appear near the glaciation limit. In order to investigate the climate conditions necessary for glacier formation in the cirques, we applied a spatially distributed temperature index melt model. After calibration under present climate conditions, the model was run with different combinations of increased initial winter snow cover and lowered summer air temperatures to assess the climate conditions needed for snow to survive summer and hence form a base for glaciation. Results indicate that a significant increase in precipitation or decrease in summer air temperature or a combination of both is necessary to initiate glaciation. Thus current climate conditions are far from favorable for glaciation. If summer temperature is decreased by 4°C or winter snow cover is more than doubled, only 10% of cirque areas remain snow covered, which is considered as a minimum condition for glacier formation. According to climate reconstructions such conditions have not occurred during the Holocene suggesting that the cirques have not been glaciated during this period. Consequently glaciation of the cirques must have occurred during other parts of the glacial cycles.

To better capture the air‐snow‐ice interaction, a snow/ice enhanced Weather Research and Forecasting (WRF‐ice) model has been developed. This study examines the performance of WRF‐ice and its blowing snow component during a strong cyclone event from October 23 to 27, 2017 over the Antarctic Peninsula, which is characterized by a synoptic cyclone crossing the northern part of the Peninsula and an embodied mesoscale cyclone over the Weddell Sea. Evolution of the cyclone is accurately reproduced in the 5‐km resolution WRF‐ice simulation, and the simulated near‐surface conditions agree well with station and satellite observations. Numerical simulations show that the process of blowing snow sublimation can be prominent within the lower atmosphere when the air is dry, and produces moistening and cooling effects. Over relatively warm and humid areas, cloud enhancement by blowing snow can lead to either colder or warmer surfaces because of competing effects of longwave and shortwave cloud radiative forcings. In particular, additional moisture from blowing snow sublimation can slightly intensify precipitation over the mountains. Surface energy budget analysis indicates that absorbed shortwave (Sa), incoming longwave (Ld), and outgoing longwave (Lu) are dominant components of surface energy budget. When increases in Ld, Lu, and sensible heat flux are combined with decreases in Saand latent heat flux due to blowing snow effects, a negative surface net heat flux (∼0.5 W/m2) occurs during daytime. A positive domain‐total surface mass balance (∼0.43 Gt) is generated during the simulated cyclone event due to increases in precipitation, decreases in runoff, and decreases in sublimation. Snow/ice enhanced Weather Research and Forecasting (WRF‐ice) model reasonably captures surface conditions of ice sheet, ice shelf, and sea ice when verified against the in situ and satellite observationsBlowing snow effects within WRF‐ice include moistening and cooling of the air, cloud enhancement, and intensified precipitationBlowing snow can contribute to positive surface mass balance by increasing precipitation and reducing surface runoff and sublimation Snow/ice enhanced Weather Research and Forecasting (WRF‐ice) model reasonably captures surface conditions of ice sheet, ice shelf, and sea ice when verified against the in situ and satellite observations Blowing snow effects within WRF‐ice include moistening and cooling of the air, cloud enhancement, and intensified precipitation Blowing snow can contribute to positive surface mass balance by increasing precipitation and reducing surface runoff and sublimation

Global sea level provides an important indicator of the state of the warming climate, but changes in regional sea level are most relevant for coastal communities around the world. With improvements to the sea‐level observing system, the knowledge of regional sea‐level change has advanced dramatically in recent years. Satellite measurements coupled with in situ observations have allowed for comprehensive study and improved understanding of the diverse set of drivers that lead to variations in sea level in space and time. Despite the advances, gaps in the understanding of contemporary sea‐level change remain and inhibit the ability to predict how the relevant processes may lead to future change. These gaps arise in part due to the complexity of the linkages between the drivers of sea‐level change. Here we review the individual processes which lead to sea‐level change and then describe how they combine and vary regionally. The intent of the paper is to provide an overview of the current state of understanding of the processes that cause regional sea‐level change and to identify and discuss limitations and uncertainty in our understanding of these processes. Areas where the lack of understanding or gaps in knowledge inhibit the ability to provide the needed information for comprehensive planning efforts are of particular focus. Finally, a goal of this paper is to highlight the role of the expanded sea‐level observation network—particularly as related to satellite observations—in the improved scientific understanding of the contributors to regional sea‐level change. This review paper addresses three important questions: (1) What do we currently know about the processes contributing to sea level change? (2) What observations do we use to gain this knowledge? and (3) Where are these gaps in our knowledge and the need for further improvement in our understanding of the drivers of regional sea level? By answering these specific questions in a focused manner, this paper should be a useful resource for other scientists, sea‐level stakeholders, and a broader audience of those interested in sea level and our changing climate. An overview of the current state of understanding of the processes that cause regional sea‐level change is providedAreas where the lack of understanding or gaps in knowledge inhibit the ability to assess future sea‐level change are discussedThe role of the expanded sea‐level observation network in improving our understanding of sea‐level change is highlighted