A recent study shows that there is a lot more sediment trapped beneath Greenland’s ice sheet than we realized, which could speed up the glaciers slipping into the ocean.
With the planet heating up from fossil fuels and polar ice melting, scientists are rushing to figure out exactly how rising oceans will impact coastal communities. However, it is hard to forecast exactly when coastal flooding will happen because we aren’t sure just how quickly the polar ice is melting. Researchers now believe they have cleared up one of those big unknowns: the type of ground sitting right under Greenland’s ice.
A paper that came out in the journal Geology last October explains how extensive seismic scans uncovered a thick layer of soft mud under most of the ice sheet. Consequently, glaciers might be dumping ice into the sea—and driving up water levels—much quicker than earlier estimates suggested.
Greenland’s geological influence on the rest of the planet
“Greenland plays a huge role in rising global sea levels, but we struggle to predict exactly how much ice will be lost because we don’t know enough about the conditions at the very base of the ice,” explains Yan Yang, a geophysicist at UC San Diego’s Scripps Institution of Oceanography. “Protecting coastal towns relies on accurate forecasts, and determining if that bed is solid rock or soft sediment is crucial for refining our predictions of future sea-level changes.”
The makeup of the ground sitting under an ice sheet significantly impacts how quickly melting glaciers add water to the sea. Glaciers travel to their end points by shifting internally, but they also glide across the surface beneath them. If that surface is solid bedrock, the ice moves more slowly. However, if the foundation is made of wet, loose mud, the glacier picks up speed and drives sea levels up at a much faster pace.
Trying to map something under meters of ice
Earlier research had already spotted sediment under certain parts of the Greenland ice sheet, but mapping the whole thing was tough because the environment is so harsh. For a better look, Yang and his team relied on open-source seismic data from groups like the Greenland Ice Sheet Monitoring Network and Columbia University’s XF network. These systems use sensors placed on the ice to track how energy waves from faraway earthquakes travel through the ground. Studying those waves helps scientists figure out exactly what is going on deep underground.
“We used shockwaves from distant quakes to see under the ice without drilling,” says Yang. “Those waves act like X-rays, changing just a little as they pass through different layers like ice, dirt, or rock. Greenland is covered in stations that capture these signals around the clock.”
Combining records from across the entire ice sheet allowed Yang’s team to create the best map we have so far of what lies at the bottom of Greenland. They discovered that sediment deposits were much more common than anticipated, appearing even beneath deep ice located a long way inland. It also looks like sediment levels change over short distances, so we need more sensors packed closer together to get a sharper view of what is happening. Yang plans to continue analyzing the data to track how these bottom sediments shift as time goes on.
What does the future hold for sea levels?
Still, even with the current level of detail, these discoveries matter right now for forecasting how much sea levels will go up.
“Our work indicates that deep layers of soft mud might make sections of Greenland react more intensely to future heating,” says Yang. “If extra meltwater gets down to the base, that mud could become even weaker, making the ice slide faster and dumping more of it into the sea. This implies that certain areas of Greenland might be at greater risk from climate change than our existing models predict.”