This blog post examines in detail how high sea temperatures and ocean currents in the West Antarctic region concentrate warm water beneath small ice shelves, accelerating melting, and what this phenomenon signifies for ice shelf changes across the entire Antarctic.
The Antarctic continent holds an enormous amount of ice, enough to raise global sea levels by approximately 57 meters if it were to melt entirely. Among this ice, ice shelves refer to structures where the massive ice sheet covering the land, several kilometers thick, moves toward the coast under the influence of gravity, with part of it extending over land and floating on the sea. Approximately 75% of the Antarctic coastline is covered by such ice shelves, varying in thickness from 100 meters to 1,000 meters depending on the region. The changing mass of ice shelves over time is closely linked to global warming and is treated as a key element in climate change research. Among the factors causing ice loss from ice shelves, the amount of ice breaking off as icebergs is relatively well-observed. However, data on how much ice melts and disappears from the underside of the ice shelf due to the influence of warm seawater has long been scarce. This is because the underside of the ice shelf is extremely difficult to access. Recently, changes in wind patterns around Antarctica have allowed warmer deep water to penetrate beneath the ice shelves, increasing the need for accurate measurements of this process. Melting at the ice shelf base directly impacts sea level rise around Antarctica.
The total mass of an ice shelf is determined by four factors. First, the supply of ice flowing from the landmass to form the ice shelf is a key factor increasing its mass. Second, the amount of snow accumulating on the ice shelf surface also contributes to its growth. Conversely, the amount of ice that breaks off as icebergs and drifts into the ocean reduces the ice shelf’s mass. Similarly, the amount of ice that melts away from the ice shelf’s base due to the influence of warm seawater also causes the ice shelf to diminish. The total change in the ice shelf’s mass is calculated based on the interaction of these four factors. Only recently, with the accumulation of satellite data, has it become possible to analyze these elements precisely.
The supply of ice flowing from land to form the ice shelf is calculated by measuring the ice flow velocity and thickness at the boundary where the ice shelf meets the land. Ice flow velocity can be calculated with centimeter-level precision by comparing two radar images captured by satellites at regular time intervals. Ice thickness is calculated by measuring the height of ice floating on the water surface using satellite altimeters, then accounting for buoyancy arising from the density difference between ice and seawater.
The amount of snow accumulating on the ice shelf is calculated by combining ice cores obtained through drilling with climate prediction models, achieving relatively high accuracy. The amount of ice lost as icebergs can be measured based on the iceberg’s area and thickness. However, in practice, tracking is difficult because icebergs move rapidly and melt quickly in seawater, making precise quantification challenging. Therefore, to ensure long-term reliability, researchers indirectly calculate mass loss from iceberg calving by measuring ice velocity and thickness at a reference line several kilometers inland from the ice shelf’s edge. Changes in the overall area and thickness of the ice shelf are used to determine the total increase or decrease in ice shelf mass, which also allows for the estimation of the amount of ice melting and disappearing from the ice shelf’s base.
Research findings indicate that the amount of ice supplied from land to the entire Antarctic ice shelf over one year was approximately 2.049 trillion tons, while the snow accumulated on the ice shelf surface amounted to about 444 billion tons. Conversely, ice lost as icebergs separated from the ice shelves amounted to about 1.321 trillion tons, while ice lost due to melting at the ice shelf base, influenced by warm seawater, was approximately 1.454 trillion tons. Ultimately, the entire Antarctic ice shelf system experienced an annual mass loss of about 282 billion tons. Among these losses, ice shelf base melting accounted for an average of 52%, but showed significant regional variation, ranging from a minimum of 10% to a maximum of 90%.
Analysis dividing Antarctic waters into four zones based on longitude revealed that in small ice shelves like the Pine Island Ice Shelf and the Croz Ice Shelf in the West Antarctic region, ice shelf base melting accounted for an average of 74%. In contrast, other regions generally hovered around 40%. Notably, the Filchner–Ronne Ice Shelf (spanning the North and Southwest Antarctic) and the Ross Ice Shelf (South Antarctic), which together produce about one-third of all Antarctic icebergs, showed a very low proportion of ice shelf bottom melt, at just 17%.
The top 10 largest ice shelves, which cover 91% of Antarctica’s ice shelf area, account for only about half of the total Antarctic ice shelf bottom melt volume. The remaining half occurs on small ice shelves covering just 9% of the total area. This phenomenon arises because small ice shelves are concentrated in the relatively warmer waters of the West Antarctic Sea. Therefore, the approach used in past studies—simply extrapolating data centered on large ice shelves proportionally by area—likely significantly overestimated the total Antarctic ice shelf bottom melt volume.
Examining the ice shelf bottom melt volume per unit area, the Antarctic average was approximately 0.81 meters annually. However, regional variations were substantial, ranging from 0.07 meters to a maximum of 15.96 meters. Small ice shelves in West Antarctica exhibited very high melt rates, while large ice shelves in other regions showed relatively low rates. These differences result from the complex interaction of the topography beneath the ice shelves and the seawater temperature structure. Melting is particularly intense beneath ice shelves close to land, while seawater freezing actually increases with distance from land. These findings reaffirm that sub-ice melting in Antarctic ice shelves varies significantly depending on local conditions and the marine environment, and that it has a crucial impact on global sea level rise and the climate system.
