Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Love, R., Andres, H. J., Condron, A., & Tarasov, L. Freshwater routing in eddy-permitting simulations of the last deglacial: the impact of realistic freshwater discharge. Climate of the Past, 17(6), (2021): 2327–2341. https://doi.org/10.5194/cp-17-2327-2021.

Description: Freshwater, in the form of glacial runoff, is hypothesized to play a critical role in centennial- to millennial-scale climate variability, such as the Younger Dryas and Dansgaard–Oeschger events, but this relationship is not straightforward. Large-scale glacial runoff events, such as Meltwater Pulse 1a (MWP1a), are not always temporally proximal to subsequent large-scale cooling. Moreover, the typical design of hosing experiments that support this relationship tends to artificially amplify the climate response. This study explores the impact that limitations in the representation of runoff in conventional “hosing” simulations has on our understanding of this relationship by examining where coastally released freshwater is transported when it reaches the ocean. We particularly focus on the impact of (1) the injection of freshwater directly over sites of deep-water formation (DWF) rather than at runoff locations (i.e. hosing), (2) excessive freshwater injection volumes (often by a factor of 5), and (3) the use of present-day (rather than palaeo) ocean gateways. We track the routing of glaciologically constrained freshwater volumes from four different inferred injection locations in a suite of eddy-permitting glacial ocean simulations using the Massachusetts Institute of Technology General Circulation Model (MITgcm) under both open and closed Bering Strait conditions. Restricting freshwater forcing values to realistic ranges results in less spreading of freshwater across the North Atlantic and indicates that the freshwater anomalies over DWF sites depend strongly on the geographical location of meltwater input. In particular, freshwater released into the Gulf of Mexico generates a very weak freshwater signal over DWF regions as a result of entrainment by the turbulent Gulf Stream. In contrast, freshwater released into the Arctic with an open Bering Strait or from the Eurasian ice sheet is found to generate the largest salinity anomalies over DWF regions in the North Atlantic and GIN (Greenland–Iceland–Norwegian) seas region respectively. Experiments show that when the Bering Strait is open, the Mackenzie River source exhibits more than twice as much freshening of the North Atlantic deep-water formation regions as when the Bering Strait is closed. Our results illustrate that applying freshwater hosing directly into the North Atlantic with even “realistic” freshwater amounts still overestimates the amount of terrestrial runoff reaching DWF regions. Given the simulated salinity anomaly distributions and the lack of reconstructed impact on deep-water formation during the Bølling–Allerød, our results support that the majority of the North American contribution to MWP1a was not routed through the Mackenzie River.

Description: This research has been supported by the Bundesministerium für Bildung und Forschung (Research for Sustainability initiative, FONA, through the PalMod project), the Natural Sciences and Engineering Research Council of Canada Discovery Grant, and by the NSF (grant no. OCE-1903427).

Description: The land ice contribution to global mean sea level rise has not yet been predicted1 using 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 models2,3,4,5,6,7,8, but primarily used previous-generation scenarios9 and climate models10, and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios11,12 using 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.

Description: Projections of the sea level contribution from the Greenland and Antarctic ice sheets (GrIS and AIS) rely on atmospheric and oceanic drivers obtained from climate models. The Earth System Models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) generally project greater future warming compared with the previous Coupled Model Intercomparison Project phase 5 (CMIP5) effort. Here we use four CMIP6 models and a selection of CMIP5 models to force multiple ice sheet models as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We find that the projected sea level contribution at 2100 from the ice sheet model ensemble under the CMIP6 scenarios falls within the CMIP5 range for the Antarctic ice sheet but is significantly increased for Greenland. Warmer atmosphere in CMIP6 models results in higher Greenland mass loss due to surface melt. For Antarctica, CMIP6 forcing is similar to CMIP5 and mass gain from increased snowfall counteracts increased loss due to ocean warming.