On Sea Level - Ice Sheet Interactions

This thesis focuses on the physics of static sea-level changes following variations in the distribution of grounded ice and the influence of these changes on the stability and dynamics of marine ice sheets. Gravitational, deformational and rotational effects associated with changes in grounded ice mass lead to markedly non-uniform spatial patterns of sea-level change. I outline a revised theory for computing post-glacial sea-level predictions and discuss the dominant physical effects that contribute to the patterns of sea-level change associated with surface loading on different timescales. I show, in particular, that a large sea-level fall (rise) occurs in the vicinity of a retreating (advancing) ice sheet on both short and long timescales. I also present an application of the sea-level theory in which I predict the sea-level changes associated with a new model of North American ice sheet evolution and consider the implications of the results for efforts to establish the sources of Meltwater Pulse 1A. These results demonstrate that viscous deformational effects can influence the amplitude of sea-level changes observed at far-field sea-level sites, even when the time window being considered is relatively short (≤ 500 years).Earth and Planetary Science

Atmospheric gravitational tides of Earth-like planets orbiting low-mass stars

Temperate terrestrial planets orbiting low-mass stars are subject to strong tidal forces. The effects of gravitational tides on the solid planet and that of atmospheric thermal tides have been studied, but the direct impact of gravitational tides on the atmosphere itself has so far been ignored. We first develop a simplified analytic theory of tides acting on the atmosphere of a planet. We then implement gravitational tides into a general circulation model of a static-ocean planet in a short-period orbit around a low-mass star -- the results agree with our analytic theory. Because atmospheric tides and solid-body tides share a scaling with the semi-major axis, we show that there is a maximum amplitude of the atmospheric tide that a terrestrial planet can experience while still having a solid surface; Proxima Centauri b is the poster child for a planet that could be geophysically Earth-like but with atmospheric tides more than 500$\times$ stronger than Earth's. In this most extreme scenario, we show that atmospheric tides significantly impact the planet's meteorology -- but not its climate. Two possible modest climate impacts are enhanced longitudinal heat transport and cooling of the lowest atmospheric layers. The strong radiative forcing of such planets dominates over gravitational tides, unlike moons of cold giant planets, such as Titan. We speculate that atmospheric tides could be climatologically important on planets where the altitude of maximal tidal forcing coincides with the altitude of cloud formation and that the effect could be detectable for non-Earth-like planets subject to even greater tides

Post-Glacial Isostatic Adjustment and Global Warming in Subarctic Canada: Implications for Islands of the James Bay Region

When Rupert’s Land and the North-Western Territory became a part of Canada as the Northwest Territories in 1870, the islands of James Bay were included within the new territorial boundaries. These same islands became a part of Nunavut in 1999, when the new territory was created from the eastern region of the Northwest Territories. Although the James Bay islands remain part of Nunavut, the western James Bay Cree assert that the western James Bay islands, including Akimiski Island, were part of the Cree traditional territory and that these islands have never been surrendered through treaty. This land-claim issue is further complicated by the fact that glacial isostatic adjustment (GIA) is occurring in the James Bay region and that the islands of James Bay may one day become part of mainland Ontario or Quebec. We used numerical models of the GIA process to predict how shorelines in James Bay will migrate over the next 1000 years as a result of post-glacial sea-level changes. These predictions, which were augmented by an additional contribution associated with sea-level rise due to global warming, were used to determine whether the islands in James Bay will ever become part of the mainland. The predictions for the islands are sensitive to the two primary inputs into the GIA predictions, namely the models for the geometry of the ancient Laurentide ice sheet and the viscoelastic structure adopted for the solid earth, as well as to the amplitude of the projected global warming signal. Nevertheless, it was found that many of the smaller and larger islands of James Bay will likely join the mainland of either Ontario or Quebec. For example, using a global warming scenario of 1.8 mm sea-level rise per year, a plausible range of GIA models suggests that the Strutton Islands and Cape Hope Islands will join mainland Quebec in ~400 years or more, while Akimiski Island will take at least ~700 years to join mainland Ontario. Using the same GIA models, but incorporating the upper boundary of global warming scenarios of 5.9 mm sea-level rise per year, the Strutton Islands and Cape Hope Islands are predicted to join mainland Quebec in ~600 years or more, and Akimiski Island is predicted not to join mainland Ontario. Since Akimiski Island is already being prospected for diamonds and the future ownership of emergent land remains an issue, these findings have great economic importance.Quand la Terre de Rupert et le Territoire du Nord-Ouest ont joint les rangs du Canada sous le nom de Territoires du Nord-Ouest en 1870, les îles de la baie James ont été intégrées aux nouvelles frontières territoriales. Ces mêmes îles font maintenant partie du Nunavut depuis 1999, lorsque le nouveau territoire a été créé à partir de la région est des Territoires du Nord-Ouest. Bien que les îles de la baie James fassent toujours partie du Nunavut, les Cris de l’ouest de la baie James soutiennent que les îles du côté ouest de la baie James, dont l’île Akimiski, faisaient partie du territoire traditionnel cri et que ces îles n’ont jamais été cédées par l’intermédiaire d’un traité. Cette revendication territoriale est davantage compliquée par le fait qu’un ajustement isostatique glaciaire est en train de se produire dans la région de la baie James au point où un de ces jours, les îles de la baie James pourraient faire partie de la partie continentale de l’Ontario ou du Québec. Nous avons employé des modèles numériques du processus d’ajustement isostatique pour prédire de quelle manière les littoraux de la baie James migreront au cours des 1000 prochaines années en raison des changements postglaciaires caractérisant le niveau de la mer. Ces prévisions, qui ont été enrichies de données supplémentaires se rapportant à l’élévation du niveau de la mer attribuable au réchauffement climatique, ont été utilisées pour déterminer si les îles de la baie James feront un jour partie du continent. Les prévisions relatives aux îles sont sensibles à deux intrants principaux en matière de prévisions d’ajustement isostatique, notamment les modèles de géométrie de la nappe glaciaire du Laurentien ancien ainsi que la structure viscoélastique adoptée pour la croûte terrestre, de même qu’à l’amplitude du signal projeté relativement au réchauffement climatique. Néanmoins, nous avons déterminé que grand nombre des îles plus petites et plus grosses de la baie James se rattacheront vraisemblablement à la partie continentale de l’Ontario ou du Québec. Par exemple, en s’appuyant sur un scénario de réchauffement climatique donnant lieu à une élévation du niveau de la mer de 1,8 mm par année, une étendue plausible pour les modèles d’ajustement isostatique laisse entendre que les îles Strutton et les îles du cap Hope rejoindront la partie continentale du Québec dans environ 400 ans ou plus, tandis que l’île Akimiski mettra environ 700 ans à s’intégrer à la partie continentale de l’Ontario. À l’aide des mêmes modèles d’ajustement isostatique, mais en tenant compte de la borne supérieure des scénarios de réchauffement climatique qui correspond à une élévation du niveau de la mer de 5,9 mm par année, les îles Strutton et les îles du cap Hope devraient rejoindre la partie continentale du Québec dans environ 600 ans ou plus, tandis que l’île Akimiski ne rejoindrait pas la partie continentale de l’Ontario. Puisque l’île Akimiski fait déjà l’objet de l’exploration de diamants et que l’appartenance future des terres émergentes constitue toujours un enjeu, ces observations revêtement une grande importance du point de vue économique

Sea Level Change in the Western James Bay Region of Subarctic Ontario: Emergent Land and Implications for Treaty No. 9

In 1905 and 1906, the Cree of the southwestern James Bay region signed Treaty No. 9 whereby they relinquished to the Canadian government their claim to the lands south of the Albany River (the northern boundary of the province of Ontario at the time). The official text of Treaty No. 9 made no mention of land submerged below water cover, and thus the Cree did not relinquish such regions at that time. By contrast, the Cree of the northwestern James Bay and southwestern Hudson Bay region who signed the 1929–30 Adhesions to Treaty No. 9 relinquished their claims to “land covered by water” for the area bounded on the south by the northerly limit of Treaty No. 9, as this clause was specifically included in the text of the adhesion. The issue of “land covered by water” is significant because the western James Bay region has been, and will continue to be, subject to sea level changes associated with ongoing adjustments due to the last ice age and modern global warming signals. In the absence of detailed maps, we used models of these processes, constrained by available geophysical and geodetic data sets, to retrodict shoreline changes and the rate of land emergence over the last two centuries within the boundaries specified by Treaty No. 9. We also project shoreline migration to the end of the 21st century within the same region. The rate of land emergence since 1905 in the area south of the Albany River is estimated as ~3.0 km2/yr. Over the next century, land will continue to emerge in this region at a mean rate of ~1.4 km2/yr. This emergent land should be a subject of consideration within any comprehensive land claim put forward by the Cree; in this regard, it will be interesting to see how the Canadian judicial system and the Comprehensive Claims Branch handle the novel issue of emergent land.En 1905 et 1906, les Cris du sud-ouest de la région de la baie James ont signé le Traité no 9, par le biais duquel ils ont cédé au gouvernement du Canada leur droit de revendication des terres au sud de la rivière Albany (la limite nord de la province de l’Ontario à l’époque). Le texte officiel du Traité no 9 ne faisait aucune mention des terres submergées sous l’eau, si bien que les Cris n’ont pas renoncé à ces régions à ce moment-là. En revanche, les Cris du nord-ouest de la baie James et du sud-ouest de la baie d’Hudson qui ont signé les adhésions au Traité no 9 (1929-1930) ont renoncé à leurs revendications aux « terres recouvertes d’eau » dans la zone délimitée au sud par la limite nord du Traité no 9, puisque cette clause était expressément incluse dans le texte de l’adhésion. La question des « terres recouvertes d’eau » est importante parce que l’ouest de la région de la baie James a été et continuera d’être assujettie aux variations du niveau de la mer liées aux ajustements continus découlant de la dernière période glaciaire et des récents signes de réchauffement planétaire. En l’absence de cartes détaillées, nous avons utilisé des modèles de ces processus, limités par les ensembles de données géophysiques et géodésiques disponibles, pour déterminer de façon rétrospective les changements du littoral et le taux d’émergence des terres au cours des deux derniers siècles dans les limites précisées dans le Traité no 9. Nous faisons également une projection de la migration du littoral jusqu’à la fin du XXIe siècle dans cette même région. Le taux d’émergence des terres depuis 1905 dans la région au sud de la rivière Albany est estimé à ~3,0 km2/année. Au cours du prochain siècle, les terres continueront d’émerger dans cette région au taux moyen de ~1,4 km2/année. Ces terres émergées devraient être prises en compte dans toute revendication territoriale globale présentée par les Cris. À cet égard, il sera intéressant de voir comment le système judiciaire canadien et la Direction générale des revendications globales traiteront cette nouvelle question des terres émergées

Evolution of a Coupled Marine Ice Sheet–Sea Level Model

We investigate the stability of marine ice sheets by coupling a gravitationally self-consistent sea level model valid for a self-gravitating, viscoelastically deforming Earth to a 1-D marine ice sheet-shelf model. The evolution of the coupled model is explored for a suite of simulations in which we vary the bed slope and the forcing that initiates retreat. We find that the sea level fall at the grounding line associated with a retreating ice sheet acts to slow the retreat; in simulations with shallow reversed bed slopes and/or small external forcing, the drop in sea level can be sufficient to halt the retreat. The rate of sea level change at the grounding line has an elastic component due to ongoing changes in ice sheet geometry, and a viscous component due to past ice and ocean load changes. When the ice sheet model is forced from steady state, on short timescales (<∼500 years), viscous effects may be ignored and grounding-line migration at a given time will depend on the local bedrock topography and on contemporaneous sea level changes driven by ongoing ice sheet mass flux. On longer timescales, an accurate assessment of the present stability of a marine ice sheet requires knowledge of its past evolution.Earth and Planetary Science

Quantifying the Uncertainty in Ground-Based GNSS-Reflectometry Sea Level Measurements

Global Navigation Satellite System reflectometry (GNSS-R) tide gauges are a promising alternative to traditional tide gauges. However, the precision of GNSS-R sea-level measurements when compared to measurements from a colocated tide gauge is highly variable, with no clear indication of what causes the variability. Here, we present a modeling technique to estimate the precision of GNSS-R sea-level measurements that relies on creating and analyzing synthetic signal-to-noise-ratio (SNR) data. The modeled value obtained from the synthetic SNR data is compared to observed root mean square error between GNSS-R measurements and a colocated tide gauge at five sites and using two retrieval methods: spectral analysis and inverse modeling. We find that the inverse method is more precise than the spectral analysis method by up to 60 for individual measurements but the two methods perform similarly for daily and monthly means. We quantify the contribution of dominant effects to the variations in precision and find that noise is the dominant source of uncertainty for spectral analysis whereas the effect of the dynamic sea surface is the dominant source of uncertainty for the inverse method. Additionally, we test the sensitivity of sea-level measurements to the choice of elevation angle interval and find that the spectral analysis method is more sensitive to the choice of elevation angle interval than the inverse method due to the effect of noise, which is greater at larger elevation angle intervals. Conversely, the effect of tropospheric delay increases for lower elevation angle intervals but is generally a minor contribution