The Greenland and Antarctic ice sheets under 1.5 °C global warming

Abstract

Even if anthropogenic warming were constrained to less than 2°C above pre-industrial, the Greenland and Antarctic ice sheets will continue to lose mass this century, with rates similar to those observed over the last decade. However, nonlinear responses cannot be excluded, which may lead to larger rates of mass loss. Furthermore, large uncertainties in future projections still remain, pertaining to knowledge gaps in atmospheric (Greenland) and oceanic (Antarctica) forcing. On millennial time scales, both ice sheets have tipping points at or slightly above the 1.5-2.0°C threshold; for Greenland, this may lead to irreversible mass loss due to 1 the surface mass balance-elevation feedback, while for Antarctica, this could result in a collapse of major drainage basins due to ice-shelf weakening.Projecting future sea-level rise (SLR, Box 1) is primarily hampered by our incomplete knowledge of the contributions of the Greenland and the Antarctic Ice Sheets (GrIS and AIS, respectively), Earth's largest ice masses. In this paper we review the potential contribution of both ice sheets under a strongly mitigated climate change scenario that limits the rise in global near-surface temperature to less than 2°C above pre-industrial (targeting 1.5°C), as agreed at the 21st UNFCCC climate conference in Paris. We base the review on both present-day observed/modelled changes and future forcings according to the RCP2.6 scenario. We use RCP2.6, the most conservative of the four Representative Concentration Pathways of greenhouse gas concentration trajectories adopted by the IPCC for its Fifth Assessment Report, because it is the scenario in the published literature that best approximates to the above warming range. Ice-sheet mass balance is defined as the net result of all mass gains and losses, and surface mass balance (SMB) as the net mass balance at the ice-sheet surface (where a negative mass balance means mass loss), including the firn layer. Hence, SMB does not include dynamical mass loss associated with ice flow at the ice-sheet margin or melting at the ice-ocean interface. Increased ice flow accounts for about one third of the recent GrIS mass loss 1 . For Antarctica, where mass lost through ice discharge past the grounding line (the limit between the grounded ice sheet and floating ice shelf) is roughly evenly shared between oceanic basal melt before reaching the ice front and iceberg calving, increased ice flow accounts for all of the recent mass loss 2,3 .In the following sections we synthesize: (i) the latest available evidence of GrIS and AIS mass balance changes together with possible climate forcings from the atmosphere/ocean; (ii) the expected responses of the ice sheets under conditions of limited (1.5°C) global warming by 2100. In the concluding section, we highlight outstanding issues that require urgent attention by the research community in order to improve projections. Greenland forcing and mass-balance changesGreenland has warmed by ∼5°C in winter and ∼2°C in summer since the mid-1990s 4 , which is more than d...

ISMIP6 Antarctica: a multi-model ensemble of the Antarctic ice sheet evolution over the 21st century

“…The results presented here do not include any weighting of the ice flow models based on their agreement with observations or the number of simulations submitted. As explained in previous studies (Goelzer et al, 2017(Goelzer et al, , 2018Seroussi et al, 2019), the range of initialization techniques adopted by models leads to varying biases. Some models are initialized with a long paleoclimate spin-up, giving limited spurious trends but an initial configuration further from the observed state, whereas models initialized with data assimilation of present-day observations can capture these conditions accurately but often have non-495 physical trend in their evolution.…”

“…The simulations performed as part of ISMIP6-Antarctica Projections represent a significant improvement compared to previous intercomparisons of Antarctic evolution, especially in terms of the treatment of ice shelves, grounding line evolution, and 500 ocean-induced basal melt (Bindschadler et al, 2013;Nowicki et al, 2013a). These are representative of improvements made to ice flow models over the past decade (Pattyn et al, 2018). However, several limitations remain, regarding both external forcings and ice flow models (Pattyn et al, 2018).…”

“…These are representative of improvements made to ice flow models over the past decade (Pattyn et al, 2018). However, several limitations remain, regarding both external forcings and ice flow models (Pattyn et al, 2018). SMB forcing from AOGCMs generally has a coarse resolution, and no regional model was used to downscale the forcing, unlike what was done for Greenland (Nowicki et al, a control run, is not yet appropriate given the large trend in the historical simulations and ctrl experiments (Fig.…”

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“…The Rossby wave trains connecting the equatorial Pacific to Antarctica are expected to develop within a few weeks in response to ENSO anomalies (e.g. Hoskins and Karoly, 1981;Mo and Higgins, 1998;Peters and Vargin, 2015), so we first use the synchronous (DJF) SOI in Sect. 3.…”

Ice Sheet Elevation Mapping and Change Detection With the Ice, Cloud and Land Elevation Satellite-2

“…The response of the cryosphere to increasing global temperatures has crucial consequences for society. Predictions of the rate of sea level rise through the next century rely on accurate understanding and modeling of glacier and ice sheet behavior (Pattyn et al, 2018). Since the late 1990s observations have revealed dramatic changes of many ice streams and outlet glaciers in Greenland and Antarctica, causing alarming mass loss (Enderlin et al, 2014, van Den Broeke et al, 2016, The IMBIE team, 2018.…”