Most of the ice cliffs at the edge of Antarctica are far shorter than the height scientists believed was needed to set off a runaway collapse of the ice sheet, according to a study published Oct. 15 in Science. The finding weakens one of the most feared pathways to extreme sea level rise this century.

Researchers from MIT, Woods Hole Oceanographic Institution and the University of Michigan measured thousands of cliffs and found an average height of about 30 meters (roughly 100 feet). That is well below the roughly 100-meter (330-foot) threshold that models have used to trigger a process called marine ice cliff instability.

The result is not an all-clear. Antarctica is still losing ice, and the tall cliffs that do exist broke apart faster than models predicted. But the study suggests the specific domino effect behind some worst-case forecasts may be less likely than feared.

Thousands of Cliffs Measured From Space

According to MIT, the team analyzed more than 15,000 Antarctic ice cliffs using NASA's ICESat-2 laser altimeter, with measurements taken about every three months from 2019 to 2023. The AAAS summary of the paper adds that the researchers used airborne and satellite laser data from 2003 to 2023 and also examined Greenland.

Very tall cliffs were rare. In Antarctica, only 5% of cliffs exceeded about 52 meters (171 feet), and in Greenland only 5% topped about 77 meters (251 feet). A handful of cliffs rose above 100 meters, with one reaching 127 meters.

The tallest cliffs were concentrated where glaciers flow quickly toward the ocean. Their heights were most closely tied to water depth, the thickness of the icy slush in front of the glacier, and cracking near the glacier's edge.

The Domino Effect That Did Not Appear

Marine ice cliff instability is the idea that once a floating ice shelf breaks away, the exposed cliff behind it becomes too tall to support its own weight. It collapses, revealing an even taller cliff further inland, which collapses in turn. Because much of West Antarctica sits on bedrock that deepens inland, the process could, in theory, run away.

That scenario helped drive some of the highest sea level projections for this century. The Intergovernmental Panel on Climate Change has treated ice sheet processes like this as a low-likelihood, high-impact possibility that cannot be ruled out under very high emissions.

The new study tested the idea against real cliffs. Lead author Caroline Needell, a graduate student in the MIT-Woods Hole joint program, said in the MIT release: "We don't see this runaway effect."

In four regions with the tallest cliffs, collapsing cliffs retreated faster than models assumed, but the retreat was short-lived. The ice then spread back out toward the ocean and settled into shorter, more stable cliffs.

The team found natural brakes. Thicker slush of icebergs and sea ice in front of tall cliffs appeared to help hold them up. Deeper water helped buoy the ice, and bumps in the bedrock made cliffs thinner and more stable. For Thwaites Glacier, the authors argue that an immediate runaway retreat is unlikely even if its ice shelf failed and exposed tall cliffs.

Limits of Four Glaciers and a Short Record

The study has clear boundaries. Only four glaciers had both repeat observations and measurable retreat, a small sample for judging how cliffs behave everywhere. The observations span about two decades at most, while ice sheet changes unfold over centuries.

Marine ice cliff instability has never been directly observed, and this study does not prove it can never happen. It shows that today's cliffs are mostly too short and that the tall ones observed stabilized rather than ran away. Conditions could change as oceans warm and more ice shelves thin.

The faster-than-expected retreat of tall cliffs is also a caution. It means some glaciers could lose ice quickly in short bursts, even without a runaway chain reaction. The researchers say better maps of bedrock and ice thinning rates are needed to sharpen sea level projections.

The work was funded by NASA, the U.S. National Science Foundation and the International Thwaites Glacier Collaboration's DOMINOS project, according to MIT.

Planning Implications for U.S. Coastal Cities

For coastal residents, the near-term outlook does not change. NOAA's interagency sea level rise technical report projects U.S. sea levels to rise an average of 10 to 12 inches between 2020 and 2050, a rise already driven by warming oceans and melting ice that is largely locked in.

The study bears mainly on the long-range worst cases that shape planning for the late 21st century and beyond. Cities such as Miami, New York, Boston and New Orleans, along with Gulf and Atlantic coast communities, use those upper-end scenarios to design seawalls, drainage and long-lived infrastructure. A lower risk of runaway collapse could narrow the extreme end of those planning ranges over time, but agencies have not revised their guidance based on this paper.

Homeowners in flood-prone areas should keep using current local flood maps and NOAA projections rather than reacting to a single study. Standard homeowners insurance does not cover flood damage, and high-tide flooding is already increasing in many U.S. coastal communities regardless of how Antarctica's cliffs behave.

Nature World News will follow how ice sheet modelers and federal agencies incorporate the findings into future projections.

The takeaway is measured relief. One frightening mechanism looks less likely based on real-world observations, but Antarctica's ice loss continues, and sea level rise along U.S. coasts is still on track to grow in the coming decades.

What Readers Want to Know

What did the ice cliff study find?

Most Antarctic ice cliffs average about 30 meters tall, far below the roughly 100-meter height models used to trigger runaway collapse.

What is marine ice cliff instability?

It is a proposed chain reaction in which tall exposed ice cliffs collapse under their own weight, exposing taller cliffs inland that collapse in turn.

Does this mean sea level rise is no longer a threat?

No. Antarctica is still losing ice, and NOAA projects 10 to 12 inches of average U.S. sea level rise between 2020 and 2050.

What about Thwaites Glacier?

The authors argue an immediate runaway retreat at Thwaites is unlikely even if its ice shelf failed, but Thwaites remains a major source of ice loss.

How was the study done?

Researchers used NASA ICESat-2 and other laser altimetry data to measure more than 15,000 Antarctic ice cliffs and track the tallest ones over time.

Should coastal homeowners change their plans?

Keep using current local flood maps and NOAA projections. Agencies have not changed guidance based on this study.