Antarctic Earth structure foretells future sea-level rise

It may seem counterintuitive to look to the rock beneath the ice of Antarctica to calculate how much ice will melt and how fast sea levels will rise due to climate change, but for the first time, scientists have completed a detailed study of this important interaction among land, ice and sea.   

The Antarctic ice sheet is the largest ice mass on Earth, and understanding how it responds to climate change is key to predicting future sea levels. Researchers, including Colorado State University Professor Rick Aster, found that solid Earth uplift – caused by land rising as heavy glaciers melt away – would limit Antarctica's contribution to sea-level rise by up to 40% under low-warming scenarios but amplify it under high-warming scenarios.  

“With nearly 700 million people living in coastal areas and the potential cost of sea-level rise reaching trillions of dollars by the end of the century, understanding the domino effect of Antarctic ice melt is crucial,” Natalya Gomez, lead author of the study published in Science Advances and an associate professor at McGill University, said in a statement. 

Scientists, including Aster, spent more than 15 years collecting new seismological and uplift data in Antarctica to produce a state-of-the-art computational model for predicting land, ice and global sea-level interactions at unprecedented detail and complexity. It is the first model to incorporate Earth structure to forecast Antarctic ice melt, and it estimates how quickly – and differently – sea levels will rise across the world’s coastlines. 

Authors used the model to simulate future ice sheet and sea-level outcomes under various amounts of warming. Results show that reducing greenhouse gas emissions could slow melting Antarctic ice enough to allow Earth uplift to partially stabilize the Antarctic ice sheet and prevent some future sea-level rise.  

“Our findings show that while some sea-level rise is inevitable, swift and substantive action to lower emissions could prevent some of the most destructive impacts of climate change, particularly for coastal communities,” Gomez said.   

Earth and ice dynamics 

Little was known about the Earth structure underlying Antarctica until scientists launched a project called POLENET, the Polar Earth Observing Network, more than 15 years ago. They scanned the Earth beneath the frozen continent to depths of hundreds of kilometers using highly sensitive seismographs, which measure how fast seismic waves travel through the Earth. They also recorded how the land surface was rising with specialized GPS receivers.   

Knowing the shape and composition of the solid Earth beneath the Antarctic ice sheet is important because it influences how the ice flows to the ocean. As an ice sheet melts, the underlying Earth decompresses, flows and rises, and this uplift causes the ice moving from the grounded inland ice sheet to the ocean to slow and get stuck on the bedrock topography. 

POLENET scientists found that Earth’s crust and underlying mantle vary greatly across the Antarctic continent. The viscosity of the Earth’s mantle, or how easily it flows, along with the thickness of the lithosphere – the elastic layer that makes up the tectonic plates – determine how quickly the land rebounds. A warmer and softer mantle and thinner lithosphere increase how much and how quickly the planet rebounds.   

Researchers found that West Antarctica has very low mantle viscosity, and the land is rebounding very quickly in response to melting ice, while East Antarctica, with a thick elastic layer and high mantle viscosity, responds much more slowly. 

Aster, a seismologist and professor of geophysics in the Warner College of Natural Resources, likens the process to the way a foam mattress slowly springs back to its original shape after weight is removed. The same thing happens with the planet but on scales of decades to millennia, depending on the properties of the underlying Earth. 

“Remarkably, we found that West Antarctica is one of the fastest uplifting places on the planet, because the mantle viscosity there is quite low,” Aster said, “so uplift turns out to be an important factor in understanding and modeling the system and how fast it's going to potentially collapse and raise global sea level under the influences of climate change.” 

Antarctica is largely isolated from plate tectonic boundaries and forces, so the observed uplift rates are mainly caused by ice mass loss. In some places, the coast of West Antarctica is uplifting by up to 4 centimeters, or about 1.5 inches, per year, Aster said. This process, called glacial isostatic adjustment, is an important factor for ice sheet stability. 

Thwaites Glacier in West Antarctica is particularly susceptible to collapse, where warming ocean water is melting the ice shelf from below. Thwaites Glacier is so massive that if it were to melt completely, global sea levels would rise approximately 60 centimeters, or about 2 feet, submerging many coastal communities, which is why it has earned the nickname “Doomsday Glacier.” 

"If the ice sheet melts like it is today but the melting doesn't accelerate, then the uplift of the solid Earth is rapid enough to slow the loss of ice, and slow global sea-level rise that will affect hundreds of millions of people around the world,” Aster said. 

However, high greenhouse gas emissions would cause the ice sheet to melt so quickly that the land would not sufficiently rebound to slow ice flow to the ocean, he added. In fact, bedrock uplift beneath the ocean with no ice atop it would eventually expel water from near Antarctica and cause additional sea-level rise across the rest of the world. 

The gravity of the situation 

The immense Antarctic ice sheet’s gravitational attraction draws water toward the continent, but as the ice sheet loses mass as it melts, that attraction will lessen, exacerbating sea-level rise elsewhere. 

Because of gravity and Earth structure and rotation, some areas of the world will experience greater sea-level rise than others. The simulations showed that small island developing nations near the equator will see the most dramatic sea-level rise under all future scenarios. 

“If the entire West Antarctic ice sheet collapsed, as it did prior to the last Ice Age, then we're contemplating eventual sea-level rise globally on average of around 3.5 meters (about 11.5 feet) from just this sector of Antarctica, and that's truly world changing,” Aster said. 

Sea-level rise combined with storm surges makes a bad situation worse for low-lying communities, which is why it’s important to understand all the processes affecting the future of Earth’s coasts, coastal populations and ecosystems. 

"With this study, we incorporate all of the important components of the Earth system and begin to quantify how much greenhouse gas drawdown will be necessary for the Earth uplift restraining mechanism to benefit humanity in terms of sea-level rise,” Aster said. 

The study was a collaboration among researchers at CSU, McGill, Pennsylvania State, Cambridge, Columbia, Ohio State, the University of Massachusetts Amherst, the University of Washington and the Union of Concerned Scientists. It was funded by the Canadian Natural Sciences and Engineering Research Council, the U.S. National Science Foundation and the Canada Research Chairs program.