Coupling the U.K. Earth System Model to dynamic models of the Greenland and Antarctic ice sheets

<p>In this presentation we describe how models of the Greenland and Antarctic ice sheets have been incorporated in the global U.K. Earth System model (UKESM1) with a two-way coupling that passes fluxes of energy, water and the locations of ice surfaces between the component models. Offline, file-based coupling is used throughout to pass information between the components, which is both physically appropriate and convenient within the UKESM1 structure. Ice sheet surface mass balance is computed in the land surface model using sub-gridscale multi-layer snowpacks. Icebergs calved from the ice sheets are fed into a Langrangian iceberg drift scheme in the ocean. Ice shelf basal melt is explicitly calculated in cavities resolved by the ocean model, and ice sheet and shelf geometries are kept consistent in all components. We demonstrate that our coupled model remains stable when simulating changes in ice sheet height, extent and grounding-line position of hundreds of kilometres.</p>

Automated mapping of supraglacial hydrology using Machine Learning

<p>Mass loss from Greenlandic and Antarctic ice sheets are predicted to be the dominant contribution to global sea level rise in coming years. Supraglacial lakes and channels are thought to play a significant role in ice sheet mass balance by causing the speed-up of grounded ice and weakening, floating ice shelves to the point of collapse. Identifying the location, distribution and life cycle of these hydrological features on both the Greenland and Antarctic ice sheets is therefore important in understanding their present and future contribution to global sea level rise. Supraglacial hydrological features can be easily identified by eye in optical satellite imagery. However, given that there are many thousands of these features, and they appear in many hundreds of satellite images, automated approaches to mapping these features in such images are urgently needed.</p><p> </p><p>Current automated approaches in mapping supraglacial hydrology tend to have high false positive and false negative rates, which are often followed by manual corrections and quality control processes. Given the scale of the data however, methods such as those that require manual post-processing are not feasible for repeat monitoring of surface hydrology at continental scale. Here, we present initial results from our work conducted as part of the 4D Greenland and 4D Antarctica projects, which increases the accuracy of supraglacial lake and channel delineation using Sentinel-2 and Landsat-7/8 imagery, while reducing the need for manual intervention. We use Machine Learning approaches including a Random Forest algorithm trained to recognise water, ice, cloud, rock, shadow, blue-ice and crevassed regions. Both labelled optical imagery and auxiliary data (e.g. digital elevation models) are used in our approach. Our methods are trained and validated using data covering a range of glaciological and climatological conditions, including images of both ice sheets and those acquired at different points during the melt-season. The workflow, developed under Google Cloud Platform, which hosts the entire archive of Sentinel-2 and Landsat-8 data, allows for large-scale application over Greenlandic and Antarctic ice sheets, and is intended for repeated use throughout future melt-seasons.</p>

Long-term future projections for the Greenland and Antarctic ice sheets with the model SICOPOLIS

<p>The Coupled Model Intercomparison Project Phase 6 (CMIP6) is a major international climate modelling initiative. As part of it, the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) was devised to assess the likely sea-level-rise contribution from the Greenland and Antarctic ice sheets until the year 2100. This was achieved by defining a set of future climate scenarios by evaluating results of CMIP5 and CMIP6 global climate models (GCMs, including MIROC) over and surrounding the Greenland and Antarctic ice sheets. These scenarios were used as forcings for a variety of ice-sheet models operated by different working groups worldwide (Goelzer et al. 2020, doi: 10.5194/tc-14-3071-2020; Seroussi et al. 2020, doi: 10.5194/tc-14-3033-2020).</p><p>Here, we use the model SICOPOLIS to carry out extended versions of the ISMIP6 future climate experiments for the Greenland and Antarctic ice sheets until the year 3000. For the atmospheric forcing (anomalies of surface mass balance and temperature) beyond 2100, we sample randomly the ten-year interval 2091-2100, while the oceanic forcing beyond 2100 is kept fixed at 2100 conditions. We conduct experiments for the pessimistic, "business as usual" pathway RCP8.5 (CMIP5) / SSP5-8.5 (CMIP6), and for the optimistic RCP2.6 (CMIP5) / SSP1-2.6 (CMIP6) pathway that represents substantial emissions reductions. For the unforced, constant-climate control runs, both ice sheets are stable until the year 3000. For RCP8.5/SSP5-8.5, they suffer massive mass losses: For Greenland, ~1.7 m SLE (sea-level equivalent) for the 12-experiment mean, and ~3.5 m SLE for the most sensitive experiment. For Antarctica, ~3.3 m SLE for the 14-experiment mean, and ~5.3 m SLE for the most sensitive experiment. For RCP2.6/SSP1-2.6, the mass losses are limited to a two-experiment mean of ~0.26 m SLE for Greenland, and a three-experiment mean of ~0.25 m SLE for Antarctica. Climate-change mitigation during the next decades will therefore be an efficient means for limiting the contribution of the ice sheets to sea-level rise in the long term.</p>

Sensitivity of the Antarctic ice sheets to the warming of MIS11c

<p><span><span>The Marine Isotope Substage 11c (MIS11c) interglacial (425 – 395 thousand years before present) is a useful analogue to climate conditions that can be expected in the near future, and can provide insights on the natural response of the Antarctic ice sheets to a moderate, yet long lasting warming period. However, its response to the warming of MIS11c and consequent contribution to global sea level rise still remains unclear. We explore the dynamics of the Antarctic ice sheets during this period using a numerical ice-sheet model forced by MIS11c climate conditions derived from climate model outputs scaled by three ice core and one sedimentary proxy records of ice volume. We identify a tipping point beyond which oceanic warming becomes the dominant forcing of ice-sheet retreat, and where collapse of the West Antarctic Ice Sheet is attained when a threshold of 0.4 </span></span><sup><span><span>o</span></span></sup><span><span>C oceanic warming relative to Pre-Industrial levels is sustained for at least 4 thousand years. Conversely, its eastern counterpart remains relatively stable, as it is mostly grounded above sea level. Our results suggest a total sea level contribution from the East and West Antarctic ice sheets of 4.0 – 8.2 m during MIS11c. In the case of a West Antarctic Ice Sheet collapse, which is the most probable scenario according to far-field sea-level reconstructions, this range is reduced to 6.7 – 8.2 m, and mostly reflects uncertainties regarding the initial configuration of the East Antarctic Ice Sheet. </span></span></p>

Sensitivity of the Antarctic ice sheets to the warming of marine isotope substage 11c

Studying the response of the Antarctic ice sheets during periods when climate conditions were similar to the present can provide important insights into current observed changes and help identify natural drivers of ice sheet retreat. In this context, the marine isotope substage 11c (MIS11c) interglacial offers a suitable scenario, given that during its later portion orbital parameters were close to our current interglacial. Ice core data indicate that warmer-than-present temperatures lasted for longer than during other interglacials. However, the response of the Antarctic ice sheets and their contribution to sea level rise remain unclear. We explore the dynamics of the Antarctic ice sheets during this period using a numerical ice sheet model forced by MIS11c climate conditions derived from climate model outputs scaled by three glaciological and one sedimentary proxy records of ice volume. Our results indicate that the East and West Antarctic ice sheets contributed 4.0–8.2 m to the MIS11c sea level rise. In the case of a West Antarctic Ice Sheet collapse, which is the most probable scenario according to far-field sea level reconstructions, the range is reduced to 6.7–8.2 m independently of the choices of external sea level forcing and millennial-scale climate variability. Within this latter range, the main source of uncertainty arises from the sensitivity of the East Antarctic Ice Sheet to a choice of initial ice sheet configuration. We found that the warmer regional climate signal captured by Antarctic ice cores during peak MIS11c is crucial to reproduce the contribution expected from Antarctica during the recorded global sea level highstand. This climate signal translates to a modest threshold of 0.4 ∘C oceanic warming at intermediate depths, which leads to a collapse of the West Antarctic Ice Sheet if sustained for at least 4000 years.