An AI Counted 39,619 Icebergs Stuck to the Seafloor Around Antarctica. Nine in Ten Are Smaller Than a Square Kilometer, and Together They Form a 'Picket Fence' That Holds the Coastal Sea Ice in Place.

Around the edge of Antarctica, thousands of icebergs are not drifting at all. They have run aground on the shallow seafloor and sit there, sometimes for days, sometimes for decades, acting as anchors for the sea ice that forms around them. Until now nobody had counted them. A new dataset built with artificial intelligence has, and the total is 39,619.

The map, published in the journal Earth System Science Data, is the work of Kaihong Jiao, who began the project as a master's student with the Australian Antarctic Program Partnership at the University of Tasmania, along with colleagues from the Institute for Marine and Antarctic Studies, the Australian Antarctic Division and Geoscience Australia. The team trained a deep-learning model to pick out stationary icebergs in synthetic aperture radar images from the European Sentinel-1 satellites, which see through cloud and polar darkness, and then ran it around the entire continent for 2025.

"Traditionally, iceberg monitoring has relied heavily on manual analysis of satellite imagery," Jiao said. "The aim of my project was to develop accurate, continentwide automated mapping of grounded iceberg distribution and size, and that's what we did." Before the project, no complete large-scale map of grounded icebergs existed.

The 39,619 icebergs in the dataset cover a combined 13,430 square kilometers, an area roughly the size of Connecticut, and are spread along 57% of the Antarctic coastline. But they are heavily clustered: just 14% of the coast holds 80% of them. The model can detect icebergs as small as 1.6 hectares, about four acres, and that resolution turned out to matter. Icebergs smaller than a square kilometer make up 93% of the total count and 54% of the total grounded area.

"We discovered that tiny, frequently overlooked icebergs less than 1 square kilometer in size actually dominate both the total number and area," Jiao said.

Those small icebergs are the key to what the authors call the "picket fence effect." Dense clusters of grounded bergs in shallow water act like fence posts: sea ice forming between them catches and holds, building up stationary "landfast" ice that stays attached to the shore through the season. Landfast ice in turn shields the floating ice shelves behind it from ocean swell, and the ice shelves hold back the glaciers that drain the continent's interior. Pull the posts and the fence goes; lose the fence and the shelves are exposed.

That chain is where the climate risk enters. Small icebergs have more surface area relative to their volume and are more sensitive to warming water than large ones, so they are the first to melt enough to lift off the seafloor and drift away. The authors write that this may indicate "a unique sensitivity of the Antarctic coastal system to climate change," in which the ungrounding of tiny icebergs "could trigger abrupt changes in the stability of landfast ice."

The dataset is public, and the team intends it as a baseline. Repeating the analysis year over year would show where the picket fence is thinning, and coupling it to ocean models would let researchers test how much landfast ice depends on it. "Our public dataset offers a crucial new baseline for modeling coastal ice stability and understanding broader environmental changes," Jiao said.

The upshot is simple. Antarctica's coastline is held together in part by tens of thousands of small chunks of ice nobody had bothered to count, and the ones doing most of the work are the ones most likely to disappear first.