The Role of Ice Streams in the Demise of the British-Irish Ice Sheet

Accurate projection of the future evolution of the Greenland and Antarctic ice sheets is a key challenge of glaciology and climate science. Ice sheet flow is dominated by ice streams; narrow corridors of fast flow bounded laterally by slower flowing ice, discharging the majority of ice from an ice sheet. These ice streams are particularly vulnerable to retreat, and their behaviour evolves along with the ice sheet as a whole. These interactions contribute to uncertainty in projections of ice sheet evolution. In part, this uncertainty can be addressed by examining the palaeo record, providing information on ice stream behaviour over thousands of years. This thesis presents a series of simulations of the British-Irish Ice Sheet using the latest generation BISICLES ice sheet model. Model simulations are used to determine the role of ice streams in the demise of the British-Irish Ice Sheet. First, BISICLES is used to examine the dynamical processes that control the retreat of a major ice stream of the British-Irish Ice Sheet, and this demonstrates vulnerability of the ice stream to Marine Ice Sheet Instability. Then a new basal sliding scheme is implemented coupled with thermo-mechanics, and this successfully models the placement and spacing of the majority of British-Irish Ice Sheet ice streams. Finally, simulations of the deglaciation style of the North Sea demonstrate the significant influence of the Norwegian Channel Ice Stream. Through these simulations, in combination with novel model-data comparison techniques, the considerable role of ice streams in the demise of the British-Irish Ice Sheet is shown. Ice stream evolution and interaction with other factors driving deglaciation needs to be adequately considered in the aim of projecting the future evolution of the Antarctic and Greenland ice sheets. Research such as presented here, modelling and reconstructing palaeo ice sheets, continues to advance this aim

Growth and retreat of the last British–Irish Ice Sheet, 31 000 to 15 000 years ago: the BRITICE-CHRONO reconstruction

The BRITICE-CHRONO consortium of researchers undertook a dating programme to constrain the timing of advance, maximum extent and retreat of the British–Irish Ice Sheet between 31 000 and 15 000 years before present. The dating campaign across Ireland and Britain and their continental shelves, and across the North Sea included 1500 days of field investigation yielding 18 000 km of marine geophysical data, 377 cores of sea floor sediments, and geomorphological and stratigraphical information at 121 sites on land; generating 690 new geochronometric ages. These findings are reported in 28 publications including synthesis into eight transect reconstructions. Here we build ice sheet-wide reconstructions consistent with these findings and using retreat patterns and dates for the inter-transect areas. Two reconstructions are presented, a wholly empirical version and a version that combines modelling with the new empirical evidence. Palaeoglaciological maps of ice extent, thickness, velocity, and flow geometry at thousand-year timesteps are presented. The maximum ice volume of 1.8 m sea level equivalent occurred at 23 ka. A larger extent than previously defined is found and widespread advance of ice to the continental shelf break is confirmed during the last glacial. Asynchrony occurred in the timing of maximum extent and onset of retreat, ranging from 30 to 22 ka. The tipping point of deglaciation at 22 ka was triggered by ice stream retreat and saddle collapses. Analysis of retreat rates leads us to accept our hypothesis that the marine-influenced sectors collapsed rapidly. First order controls on ice-sheet demise were glacio-isostatic loading triggering retreat of marine sectors, aided by glaciological instabilities and then climate warming finished off the smaller, terrestrial ice sheet. Overprinted on this signal were second order controls arising from variations in trough topographies and with sector-scale ice geometric readjustments arising from dispositions in the geography of the landscape. These second order controls produced a stepped deglaciation. The retreat of the British–Irish Ice Sheet is now the world’s most well-constrained and a valuable data-rich environment for improving ice-sheet modelling.publishedVersio

Exploring the ingredients required to successfully model the placement, generation, and evolution of ice streams in the British-Irish Ice Sheet

Ice stream evolution is a major uncertainty in projections of the future of the Greenland and Antarctic Ice sheets. Accurate simulation of ice stream evolution requires an understanding of a number of “ingredients” that control the location and behaviour of ice stream flow. Here, we test the influence of geothermal heat flux, grid resolution, and bed hydrology on simulated ice streaming. The palaeo-record provides snapshots of ice stream evolution, with a particularly well constrained ice sheet being the British-Irish Ice Sheet (BIIS). We implement a new basal sliding scheme coupled with thermo-mechanics into the BISICLES ice sheet model, to simulate the evolution of the BIIS ice streams. We find that the simulated location and spacing of ice streams matches well with the empirical reconstructions of ice stream flow in terms of position and direction when simple bed hydrology is included. We show that the new basal sliding scheme allows the accurate simulation for the majority of BIIS ice streams. The extensive empirical record of the BIIS has allowed the testing of model inputs, and has helped demonstrate the skill of the ice sheet model in simulating the evolution of the location, spacing, and migration of ice streams through millennia. Simulated ice streams also prompt new empirical mapping of features indicative of streaming in the North Channel region. Ice sheet model development has allowed accurate simulation of the palaeo record, and allows for improved modelling of future ice stream behaviour

Behavioural tendencies of the last British-Irish Ice Sheet revealed by data-model comparison

Integrating ice sheet models with empirical data pertaining to palaeo-ice sheets promotes advances in the models used in sea-level predictions, and can improve our understanding of past ice sheet behaviour. The large number of empirical constraints on last British-Irish Ice Sheet make it ideal for model-data comparison experiments. Here, we present an ensemble of 600 model simulations, which are compared to data on former ice flow extent, flow geometry and deglaciation timing. Simulations which poorly recreate data were ruled out, allowing us to examine the remaining physically realistic simulations which capture the ice sheets’ behavioural tendencies. Our results led to a novel reconstruction of behaviour in the data poor region of the North Sea, insights into ice stream, potential ice shelf, and readvance dynamics, and potential locations of periphery ice caps. We also propose that the asynchronous behaviour of the British-Irish Ice Sheet is a consequence of the geography of the British Isles and the merging and splitting of different bodies of ice through saddle merger and collapse. Furthermore, persistent model-data mismatches highlight the need for model development, especially regarding the physics of ice-ocean interactions. Thus, this work highlights the power of integrating models and data, a long-held aim of palaeoglaciolog

De-tuning Albedo Parameters in a coupled Climate Ice Sheet Model to simulate the North American Ice Sheet at the Last Glacial Maximum

The Last Glacial Maximum extent of the North American Ice Sheets are well constrained empirically, but have proven to be challenging to simulate with coupled Climate Ice Sheet models. Coupled Climate-Ice Sheet models are often too computationally expensive to sufficiently explore uncertainty in input parameters, and it is unlikely that values calibrated to reproduce modern ice sheets will reproduce the known extent of the ice at the Last Glacial Maximum. To address this, we run an ensemble with a coupled Climate-Ice Sheet model (FAMOUS-ice), simulating the final stages of growth of the last North American Ice Sheets’ maximum extent. Using this large ensemble approach, we explore the influence of numerous uncertain ice sheet albedo, ice sheet dynamics, atmospheric, and oceanic parameters on the ice sheet extent. We find that ice sheet albedo parameters determine the majority of uncertainty when simulating the Last Glacial Maximum North American Ice Sheets. Importantly, different albedo parameters are needed to produce a good match to the Last Glacial Maximum North American Ice Sheets than have previously been used to model the contemporary Greenland Ice Sheet, due to differences in cloud cover over ablation zones. Thus calibrating coupled climate-ice sheet models on one ice sheet may produce strong biases when the model is applied to a new domain

Large ensemble simulations of the North American and Greenland ice sheets at the Last Glacial Maximum with a coupled atmospheric general circulation-ice sheet model

The Last Glacial Maximum (LGM) is characterised by huge ice sheets covering the Northern Hemisphere, especially over North America, and by its cold climate. Numerical simulations of the climate and ice sheets of the LGM have been performed to better understand these systems, however the inherent uncertainty and sensitivity in the simulations to the selection of model parameters remain uncertain. Here, we perform a 200-member ensemble of simulations of the North American and Greenland ice sheets and climate of the LGM with an ice sheet-atmosphere-slab ocean coupled model (FAMOUS-BISICLES) to explore sensitivities of the coupled climate-ice system to 16 uncertain parameters. In the ensemble of simulations, the global temperature is primarily controlled by the combination of parameters in the large-scale condensation scheme and the cumulus convection scheme. In simulations with plausible LGM global temperatures, we find that the albedo parameters have only a small impact on the Greenland ice volume due to the limited area of surface ablation associated with the cold climate. Instead, the basal sliding law controls the ice volume by affecting ice transport from the interior to the margin. On the other hand, like the Greenland ice sheet in future climate change, the LGM North American ice sheet volume is controlled by parameters in the snow and ice albedo scheme. Few of our simulations produce an extensive North American ice sheet when the global temperature is above 12 °C. Based on constraints on the LGM global temperature, the ice volume and the southern extent of the North American ice sheet, we select 16 acceptable simulations. These simulations lack the southern extent of ice compared to reconstructions, though show reasonable performance on the ice sheet configuration and ice streams facing the Baffin Bay and the Arctic Ocean. The strong sensitivities of the North American ice sheet to albedo at the LGM may imply a potential constraint on the future Greenland ice sheet by constraining the albedo schemes