Scientists model ice sheets to better understand the future of global sea level rise

Melting ice sheets in Greenland and Antarctica are major sources of rising global sea levels and are likely to become even larger contributors in the coming decades. Despite this, scientists have only recently begun to incorporate ice sheets into climate models, an expert says. And the impact is not yet fully understood.

Bill Lipscomb, a senior scientist at the National Center for Atmospheric Research, spoke to science writers Oct. 5 about the current state of ice sheet modeling during the virtual ScienceWriters2021 conference. Substantial progress has been made, especially in modeling sea level rise due to melting in Greenland, Lipscomb said. However, uncertainty remains as scientists work to understand the risk of ice sheet collapse in West Antarctica.

How fast and how much might ice sheets contribute to sea level rise in the years to come? It’s a simple question with a complex answer, he said. Ice sheets are one of many factors that play a part in sea level rise, and the effects can vary from place to place. That’s where the atmospheric research center’s Community Earth System Model comes in.

In development since 2010, the model simulates the complex physics of ice sheets and their interactions with the Earth’s oceans, atmosphere and land. It predicts sea level rise more precisely than older models, which treated ice sheets as static.

“It was conventional wisdom that ice sheets were too sluggish to be important for climate change,” Lipscomb said. While warming oceans and mountain glaciers were recognized as contributing to sea level rise, ice sheets were understood to be “in balance,” receiving about the same amount of snowfall as the ice they lost to the surrounding ocean. But since the 1990s, ice sheets have been losing more mass than they gain, and now account for more than a third of sea level rise.

Climate researchers have begun the work of incorporating ice sheets into their models, but it is no easy task. Earlier models were built on the assumption that the border between land and atmosphere and ocean is fixed. Melting ice sheets, however, transport mass between the land and ocean, breaking that assumption.

“Consequently, you break the climate model in hundreds if not thousands of places,” Lipscomb said. “So it’s a huge software and technical challenge to make a model that can accommodate those changing boundaries.”

Capturing the complex physics of ice sheets

Ice sheets gain mass when snow falls on them and gradually turns to ice. Then, under the force of gravity, ice from the sheets can start flowing to the ocean. At the edge of the ocean, ice sheets form what is called an ice shelf, a floating platform that protrudes from ice attached to the bedrock. Once an ice shelf forms it can enter the ocean in two ways: either calving off into icebergs, or melting away via warm ocean water that flows underneath the ice shelf.

One of the goals of Lipscomb’s team for the earth system model was to make ice sheet dynamics more realistic by more precisely modeling interactions between them and the earth’s physical, chemical, and biological systems. This part of the model incorporates a process called two-way coupling, where an ice sheet can interact with another element of the surrounding environment.

“For example, you might have an iceberg in the model that goes into the ocean and affects the subsequent ocean circulation, which potentially could affect how much ice melts in the future,” Lipscomb said. Two-way coupling also allows the model to capture the fact that Greenland can go from being covered by ice to being covered by vegetation, which changes climate patterns and affects the rate of ice sheet melting.

“With Greenland, we have good agreement across models that sea level rise will mainly happen from increased surface melting,” Lipscomb said. As the atmosphere warms, melting surface ice will lead to more exposed ground, which is darker than ice. Darker surfaces can absorb more sunlight, which leads to longer and more intense summertime melt seasons and more water going into the ocean.

In Antarctica, however, the situation is harder to model. That’s because the ocean has contributed more to Antarctic ice sheet melting than the atmosphere has, and the sensitivity of ice shelf melting to ocean warming is less well understood.

“The biggest uncertainties we have surround the extent to which Western Antarctic ice shelves are vulnerable to warming ocean water,” Lipscomb said. The modeling has revealed more about all components of sea level rise, but not enough to quantify the risk of Antarctic ice sheet collapse.

Lipscomb said he remains hopeful. “We’re always seeing progress in what the models can do,” he said in an interview. “Our community continues to grow, and a lot of young scientists are bringing new ideas from all over the world.”