Schmitty Thompson wears glasses and a sweater, and smiles at the camera while standing in front of a vast field.

What ice sheets can teach us about ancient ocean shorelines

Around 80,000 years ago, the Earth was in the middle of the late Pleistocene era, and much of Canada and the northern part of the United States was blanketed in ice. The massive Laurentide Ice Sheet covered millions of square miles, and in some places, up to 2 miles thick. Over vast timescales this ice sheet advanced its way across the continent slowly, gouging out what we now know as the Great Lakes, carving the valleys, depositing glacial tills, and transforming the surface geology of much of the southern part of Canada and northern US. Further west, the Cordilleran ice sheet stretched across what is now Alaska, British Columbia, and the northern parts of the Western US, compressing the ground under its massive weight. As these ice sheets depressed the land beneath them, the Earth’s crust bulged outwards, and as the planet warmed and the ice sheets began to melt, the pressure was released, returning the crust underneath to its previous shape. As this happened, ocean water flowed away, resulting in lower sea levels locally, but higher levels across the other side of the planet.

The effects of massive bodies of ice forming, moving, and melting are far from negligible in their impact on the overall geology of the region, the sea level throughout history, and the patterns of a changing climate. Though there are only two ice sheets on the planet today, deducing the ancient patterns and dynamics of ice sheets can help researchers fill the geological record and even make predictions about what the planet might look like in the future. Our guest on Inspiration Dissemination this week is PhD candidate and researcher Schmitty Thompson, of the Department of Geology in CEOAS. Thompson is ultimately trying to answer questions about ice distribution, sea levels, and other unknown parameters that the geologic record is missing during two different ice age warming periods. Their research is very interdisciplinary – Thompson has degrees in both math and geology, and also uses a lot of data science, computer science, and physics in their work. They are using computer modeling to figure out just what the shorelines looked like during this time period around 80,000 years ago. 

Schmitty Thompson, fourth year PhD candidate with Jessica Creveling in the Geology Department.

“I use models because the geologic record is pretty incomplete – the further back you go, the less complete it is. So by matching my models to the existing data, we can then infer more information about what the shoreline was like,” they explain. To do this accurately, Thompson feeds the model what the ice sheets looked like over the course of around 250,000 years. They also need to incorporate other inputs to the model to get an accurate picture – variables such as the composition of the interior of the Earth, the physics of Earth’s interior, and even the ice sheets’ own gravitational pull (ice sheets are so massive they exert a gravitational pull on the water around them!)

Using math to learn about ice

The first equation to describe global changes in sea level was published in 1976, with refining throughout the 90s and early 2000s. Thompson’s model builds on these equations in two versions: one which can run in about 10 minutes on their laptop, and another which can take multiple weeks and must run on a supercomputer. The quicker version uses spherical harmonics as the basis function for the pseudospectral formulation, which is basically a complex function that does math and incorporates coefficient representations of the earth’s radius, meridional wave numbers, variation across north/south and east/west, and a few other variables. The short of it is that it can perform these calculations across a 250k time span relatively quickly, but it makes assumptions about the homogeneity of the earth’s crust and mantle viscosity. Think of it like a gumball: a giant, magma-filled gumball with a smooth outer surface and even layers. So while this method is fast, the assumptions that it makes means the output data is limited in its usefulness. When Thompson needs a more accurate picture, they turn to collaborators who are able to run the models on a supercomputer, and then they work with the model’s outputs.

While the model is useful for filling in gaps in the historical record, Thompson also points out that it has uses in predicting what the future will look like in the context of a changing climate. After testing out these models and seeing how sensitive they are, they could be used by researchers looking at much smaller time scales and more sensitive constraints for current and future predictions. “There are still lots of open questions – if we warm the planet by a few degrees, are we going to collapse a big part of Antarctica or a small part? How much ice will melt?”

To learn more about ice sheets, sea levels, and using computer models to figure out how the shoreline looked thousands of years ago, tune in to Schmitty Thompson’s episode on Inspiration Dissemination this upcoming Sunday evening at 7 PM PST. Catch the show live by streaming on, or check out the show later wherever you get your podcasts!

Thompson was also recently featured on Alie Ward’s popular podcast Ologies. You can catch up with all things geology by checking out their episode here.

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