SEA LEVEL CHANGE, ICE SHEET FLUCTUATIONS AND GLOBAL WARMING: PAST AND PRESENT

Global warming concerns have motivated the study of past and present sea level changes. Although many factors contribute to sea level change, dominant factors are unloading of the Earth's crust and meltwater supplied to the oceans from retreating ice sheets. Thus sea level changes are a function of past and present ice sheet position and thickness fluctuations. Tide gauge, shoreline tilt and past sea level curves are the classic types of sea level measurements we have used. The advent of accurate satellite altimetry, which we have also used, has provided a decade of precise sea level measurements with global coverage. We have created a numerical "forward" model which assumes a radial viscosity profile for the Earth and a prescribed deglaciation history. The model predicts sea level changes of a global tide gauge network, ancient shorelines or recent satellite altimeter data. The results of these calculations indicate that there should be a steep sea level gradient pointing toward the source of any modern meltwater. In contrast, our numerical "inverse" model uses the sea level data as model input and calculates an ice sheet history that best fits the data using a least squares criteria. The inverse calculation is completed for past ice sheets, showing a maximum ice sheet thickness of 3062 meters over southeast Hudson Bay 17,000 years ago. The calculation is yet to be completed for the case of modern ice sheet melting. However, the sea level change magnitude and spatial pattern observed from satellite altimetry is consistent with thinning of the Greenland Ice Sheet at a rate of 63 cm/year. These reconstructions are useful for determining a more ideal deglaciation history for the last ice age and a better model of meltwater contributions from present-day ice sheet fluctuations, possibly caused by global warming.