Penguins walking across sea ice by a large iceberg in front of Thwaites Ice Shelf, a large, unstable mass of ice that extends from the West Antarctic ice sheet into the sea.Peter Neff
Most of the Earth's fresh water is locked in the ice that covers Antarctica. As the ocean and atmosphere grow warmer, that ice is melting at a startling pace with sea levels and global currents changing in response. To understand the potential implications, researchers need to know just how fast the ice is disappearing, and what is driving it back.
The West Antarctic ice sheet, an unstable expanse bordering the Amundsen Sea, is one of the greatest sources of uncertainty in climate projections. Records indicate that it has been steadily shrinking since the 1940s, but key details are missing. Using environmental data gathered from ice samples, tree rings and corals, University of Washington researchers tailored a climate model to Antarctica and ran simulations to understand how changing weather patterns dictate ice melt.
The results, published on Sept. 10 in Nature Geoscience, were surprising. For years, researchers have hypothesized that westerly winds were ferrying warm water toward the ice sheet, accelerating ice melt. The new study flips the existing narrative on its head, or rather on its side, pointing toward winds from the north instead.
"We know the Earth is warming up on average, but that alone doesn't explain ice loss in Antarctica," said Eric Steig, a UW professor of Earth and space sciences. "To understand what's going to happen in the future, we need to understand the details of what's happening now, and critically, whether we are connected to it."
The West Antarctic Ice Sheet sits atop West Antarctica, bordered by ice shelves that stabilize the land-borne ice. Glaciers like the Thwaites, pictured above, form where the ice meets the sea. This study suggests that northerly winds, coming from a low pressure center above the Amundsen Sea, are accelerating ice loss.Wikimedia Commons
The Antarctic ice sheet covers an area larger than the U.S. and Mexico combined. If the Western-Hemisphere portion were to melt, global sea levels would rise by as much as 20 feet. The ice sheet is locked in place by ice shelves, fingers of ice that stretch into the sea. Free floating sea ice blankets the surface of the surrounding waters.
To study weather in Antarctica, where there are fewer weather stations than most of the world, scientists use computer simulations that draw from available data sources. Still, these models often lack data that is specific to the region, limiting the accuracy of their outputs.
In the past century, westerly winds blowing over high latitudes of the Southern Hemisphere have grown stronger in response to human-induced climate change. Indirect evidence also suggested that this trend was driving West Antarctic ice loss. But when the researchers dug into that theory, something didn't add up.
"We thought that we were going to support what the climate models showed, which was that the westerly winds were getting stronger near the coast of Antarctica," said Gemma O'Connor, lead author and a UW postdoctoral researcher of oceanography. "But there was no evidence of westerly winds strengthening in this part of Antarctica."
O'Connor's doctoral research explored how proxy data - historical records from ice cores, trees and coral - can reveal past weather patterns, including wind. Her work showed that the force needed to explain accelerating melt rates was still missing from the equation.
In the new study, researchers conducted a suite of high-resolution ice-ocean simulations to identify what climate patterns were driving ice shelf melting in this critical region of Antarctica. They fed the model a wind pattern for five years at a time, measured how much mass the ice lost, and repeated the process 29 times. Each iteration represented a different wind pattern. Data from the 30 simulations showed that northerly winds consistently exacerbated ice loss. Westerlies did not have the same effect.
The northerly winds, which blow with force in Antarctica, were rearranging the sea ice surrounding Antarctica, capping off small but important gaps called polynyas.
"Sea ice is a really good insulator, it keeps the ocean relatively warm compared to the air," said Kyle Armour, a UW professor of oceanography and of atmospheric and climate science. "When northerly winds close the polynyas, it reduces ocean heat loss, which means warmer waters and more melting of ice shelves below the surface."
Polynyas are like pores on the icy surface of the ocean. When they are blocked, excess heat can't escape. As the ice shelf melts, fresh water mingles with salty ocean water. A density gradient forms between the fresher, lighter water and the open ocean. This gradient powers a current that pulls in more warm ocean water from miles away, advancing ice shelf melt.
Under normal conditions, warm salty water melts the ice shelf from below. When winds from the north shift the sea ice, the ice shelf melts faster, increasing the amount of fresh water around the ice and drawing in more warm water from farther away.Gemma O'Connor
Researchers believe greenhouse gas emissions could be fueling the northerly winds. Early studies suggest that human-induced climate change is decreasing air pressure over the Amundsen Sea. This area hosts an influential low-pressure center that drives many of the Antarctic weather patterns. As it gets even lower, wind speed from the north increases.
"This mechanism provides a connection between West Antarctic ice loss and human-induced climate change, albeit a different mechanism than we previously suspected," O'Connor said. Which is important, the researchers added, because if emissions are contributing to ice loss, perhaps cutting them could curtail it.
"I think what Gemma has done is going to lead to a complete revolution in the understanding of what drives Antarctic ice loss," Armour said. "We had all sorts of theories about the winds that blow from west to east, but the northerly winds weren't even on our radar. We were off by 90 degrees."
Other authors include LuAnne Thompson, a UW professor of oceanography; Mira Berdahl, a UW research scientist of Earth and space sciences; Yoshihiro Nakayama, an assistant professor of engineering at Dartmouth College; Shuntaro Hyogo, a graduate researcher of environmental science at Hokkaido University; and Taketo Shimada, a graduate researcher of environmental science at Hokkaido University
This research was funded by the Washington Research Foundation, the University of Washington eScience Institute, the U.S. National Science Foundation, a Calvin professorship in oceanography, the Japanese Ministry of Education, Culture, Sports, Science, and Technology, Inoue Science Foundation, NASA Sea Level Change Team, the John Simon Guggenheim Memorial Foundation and JST SPRING.
