Ice is the substance that defines Antarctica more fundamentally than any other, and it reaches its most spectacular form anywhere on the planet in the White Continent’s enormous ice sheets and associated ice shelves.

Ice sheets constitute the largest size category of glaciers, which are masses of ice that move under their own weight and the force of gravity. Bodies of glacial ice larger than 20,000 square miles (50,000 square kilometers) are classified as ice sheets. Like any glacier, ice sheets form when snow doesn’t melt, continues to accumulate, and—through compaction and various thermodynamic processes—begins densifying into ice.

All glaciers flow, but unlike smaller mountain and valley glaciers, which tend to advance in one direction from upslope accumulation zones, ice sheets are usually domed and flow out in radiating fashion from the central height. They’re drained by outlet glaciers and ice streams, and often reach a seacoast in the form of tidewater glaciers, ice tongues, and the floating ice-sheet extensions known as ice shelves.

Currently, only two ice sheets are found on Earth: the Greenland Ice Sheet and the Antarctic Ice Sheet. However, in colder times in the planet’s history, other ice sheets formed covering parts of North America (the Laurentide Ice Sheet), Northern Europe (the Weichselian Ice Sheet), Western Europe (the Scandinavian Ice Sheet), Northern Asia (the Barents-Kara Ice Sheet), and southern South America (the Patagonian Ice Sheet). However, these other ice sheets have since melted and only the Antarctic and Greenland ice sheets remain today.

The Greenland Ice Sheet, which has likely existed the better part of three million years, covers about 660,000 square miles (1.71 million square kilometers) of that predominantly Arctic land (the biggest island in the world). That’s a massive body of ice, no question, but it pales in comparison with the vastness of the Antarctic Ice Sheet, which covers some 5.4 million square miles (14 million square kilometers).

With a total volume of about 6.5 million cubic miles (27 million cubic kilometers) compared to the Greenland Ice Sheet’s 0.7 million cubic miles (2.9m cubic kilometers), Antarctica’s ice sheet is by far the largest single reservoir of ice on the planet: the greatest component of the cryosphere, Earth’s frozen environment.

The Antarctic Ice Sheet, while often referred to as a singular feature—the “polar ice cap” at the bottom of the world—is in fact comprised of three separate components, confusingly often themselves referred to as ice sheets in their own right. Defined by distinctive rates of flow, topographic controls, and other variables, these are the East Antarctic Ice Sheet, the West Antarctic Ice Sheet, and the Antarctic Peninsula Ice Sheet.

The Transantarctic Mountains, which run better than 2,000 miles (3,200 kilometers) between the Weddell and Ross seas, divide the West and East Antarctic ice sheets (and West, or “Lesser,” Antarctica and East, or “Greater,” Antarctica, in general). The Antarctic Peninsula Ice Sheet is the comparatively much smaller ice body, also technically situated in West Antarctica, which cloaks much of that northernmost finger of the White Continent, the Antarctic Peninsula.

The East Antarctic Ice Sheet is the king of all ice sheets, and—given its size and at least partial resilience to the changing climate we’re living in today—the true throne of the cryosphere. About 10 times larger than the West Antarctic Ice Sheet, it accounts for more than 90 percent of all of the White Continent’s ice.

The average thickness of the East Antarctic Ice Sheet is about 7,303 feet (2,226 meters). That’s an average, mind you: This colossal ice sheet reaches a thickness of about 15,670 feet (4,776 meters) at its maximum depth. The largest lake in Antarctica, Lake Vostok, one of numerous subglacial lakes on the White Continent, lies a staggering 2.5 miles (4 kilometers) below the surface of the East Antarctic Ice Sheet.

Unlike much of the West Antarctic Ice Sheet, most of the bedrock underlying the East Antarctic Ice Sheet is above sea level. But not, by any means, all of it.  Denman Canyon, for example, a sub-ice landform of the East Antarctic Ice Sheet, attains the greatest depth of any terrestrial canyon on Earth, plunging more than two miles below sea level. And a quite significant zone of below-sea-level bedrock underlies the East Antarctic ice of the Wilkes Subglacial Basin.

It is the high, dry, basically trackless, wind-scoured height of the East Antarctic Ice Sheet that creates the vast Antarctic Polar Plateau. Ice domes and ice rises form the loftiest elevations of this ice-sheet wilderness, which tops out at about 13,428 feet (4,093 meters) on Dome Argus: the highest ice elevation in Antarctica (though exceeded in height by the bedrock summits of the Ellsworth and Transantarctic mountains).

This high-standing icy realm is also the coldest place on Earth. A verified temperature of -93.2 degrees Celsius (-135.8 degrees Fahrenheit) was recorded on the ice ridge between Dome Argus and Dome Fuji, and it’s speculated that subtle hollows of the ice divide here may, due to the pooling of chilled air in depressions, get colder yet: maybe down to -98 degrees C (-144.4 degrees F) or below.

The Polar Plateau of the East Antarctic Ice Sheet is, incidentally, host to some of the best-known research bases in Antarctica, including Russia’s Vostok Station and the U.S. Amundsen-Scott South Pole Station. To call these outposts remote—well, that’s an understatement!

The heights of the Antarctic Polar Plateau are also, roughly speaking, the main source of the ferocious katabatic winds—driven by cold, dense air moving downslope—which rake much of coastal Antarctica.

Much smaller than its counterpart in East Antarctica—it accounts for “only” about a tenth of the White Continent’s ice—the West Antarctic Ice Sheet is still, by any other measure, a glorious cryospheric mass. It covers, after all, an impressive 760,000 square miles (1.97 million square kilometers), quite a bit larger as a standalone ice body than the Greenland Ice Sheet. Thinner though it may be than the East Antarctic Ice Sheet, the West Antarctic Ice Sheet still reaches a respectable 4,285 feet (1,306 meters) of thickness.

We’ve already hinted that much of the bedrock beneath the West Antarctic Ice Sheet lies below sea level. That makes the bulk of West Antarctic ice an example of a marine ice sheet (which would also descripe, for example, the aforementioned Wilkes Subglacial Basin in East Antarctica). Were you to strip away West Antarctica’s ice cover, you’d reveal a complex mosaic of rugged islands and seaways.

The West Antarctic Ice Sheet drains to the Bellingshausen, Weddell, Amundsen, and Ross seas.

The network of ice caps and glaciers dominating much of the mountainous terrain of the Antarctic Peninsula—not only the most-northerly, but also the most-visited part of the Continent, its tip a “mere” 600 miles (1,000 kilometers) from the southern extremity of South America—is sometimes considered a component of the West Antarctic Ice Sheet.

Given its unique geography, topography, glacial flow, and climate, however, many scientists treat it separately as its own distinct feature. A first-of-its-kind 2022 analysis that ranked the largest glaciers in the world excluded the Antarctic and Greenland ice sheets, but identified the ice body of the Antarctic Peninsula—at some 31,217 square miles (80,852 square kilometers)—as the biggest standalone glacier in the world.

Because the Antarctic Peninsula’s maritime-influenced climate encourages heavier snowfall rates than most of the rest of the White Continent, and because of its northerly position, its ice sheet, uniquely here, experiences significant seasonal gain-loss fluctuation across the year.

The Antarctic Ice sheet is more than a mile thick in places! At its thickest point the ice sheet is 4,776 meters deep, but on it averages 2,160 meters thick. Overlying a rugged bedrock landscape of buried peaks, canyons, and basins—some of them, especially in West Antarctica, well below sea level—the Antarctic Ice Sheet gives Antarctica the highest average elevation of any continent on Earth: about 7,200 feet (2,200 meters). The weight of the overlying ice actually depresses the buried land surface—by as much as 1,640 feet (500 meters).

The Antarctic Ice Sheet likely began forming some 35 or so million years ago. It’s worth emphasizing that this thick mantle of ice has accumulated due to the deep cold of the polar climate, not because of exceptional amounts of snowfall. Most of Antarctica is a polar desert with very low precipitation and an extremely dry atmosphere; its ice sheets built up because the white stuff laid down by the continent’s only modest amounts of snowfall didn’t melt, and continued to deepen and densify over the eons.

Three quarters of the Antarctic coastline is composed of ice shelves, which are floating, land-attached masses of ice. They’re predominantly the seaward extensions of ice sheets and their draining outlet glaciers and ice streams, though it’s also possible to produce an ice shelf from a combination of sea ice and direct accumulating snowfall.

The point where a sea-reaching ice sheet nudges off the bedrock and starts floating, thus turning into an ice shelf, is called the grounding line. Ice shelves play a critical role in stabilizing the ice sheets they buttress.

Antarctica hosts by far the greatest collection of ice shelves on the planet—the White Continent comes fringed by 162 ice shelves (15 major ice shelves and 147 minor ones)—with only a few smaller examples found elsewhere, namely in the Arctic Canada, Russia, and Greenland.

The largest is the Ross Ice Shelf, which covers some 182,000 square miles (472,000 square kilometers) in the Ross Sea; this is the world’s biggest ice shelf. In the Heroic Age of Antarctic Exploration, the Ross Ice Shelf and its wall of ice cliffs were called “the Barrier,” obstructing as they did the Ross Sea route to the South Pole. Next in size is the 166,024-square-mile (430,000-square-kilometer) Ronne-Filchner Ice Shelf in the Weddell Sea, on the opposite side of the White Continent.

Other examples of Antarctic ice shelves are the Larsen Ice Shelf on the Antarctic Peninsula and the Amery Ice Shelf, West Ice Shelf, and Shackleton Ice Shelf in East Antarctica.

Antarctica’s ice sheets and ice shelves are enormously significant not only in shaping the physical environment of the White Continent, but on a global scale.

The sprawling reaches of snow-covered ice in Antarctica—including not only the continental ice sheets and coastal ice shelves, but the seasonally fluctuating expanse of encircling sea ice in the Southern Ocean—reflect tremendous amounts of shortwave radiation from the Sun: about 85 or so percent of that solar insolation. That reinforces the polar cold of Antarctica and also contributes to keeping global temperatures lower than they otherwise would be, playing an important role in Earth’s energy balance.

Antarctica’s ice sheets also store about 70 percent of all of the planet’s freshwater, so understandably melting of those ice sheets would therefore raise sea levels considerably. Indeed, estimates suggest ice loss from Antarctica could contribute better than 3.3 feet (1 meter) of sea-level rise by the end of this century alone. Worse yet, if the Antarctic Ice Sheet melted off entirely, it is thought it would cause a potentially catastrophic rise in global sea levels by some 187 feet (57 meters). For context, about 10 percent of the world’s population reside in coastal areas 33 feet (10 meters) or less above sea level! (Incidentally, to see what Antarctica would look like without ice, check out our guide here.)

An enormous amount of research and monitoring is underway in Antarctica to decipher possible impacts of anthropogenic climate change on the White Continent’s ice sheets and ice shelves, given their major potential impact on sea level and the planet’s radiation balance.

Of paramount concern in the face of our warming climate is the potential for calving and disintegration—including by so-called “ocean forcing,” whereby warm seawater circulates underneath ice shelves and hastens basal melting and fracturing—to diminish and dismantle ice shelves. That in turn could then increase rates of glacial flow off the Antarctic Ice Sheet and thus raise sea levels.

With the current climate change concerns one might expect the answer to be shrinking, right? Surprisingly, the current picture continent-wide isn’t exactly clear-cut. Warming temperatures, hastening glacial flow, and significant ice-shelf loss have been observed in West Antarctica and the Antarctic Peninsula, but the East Antarctic Ice Sheet has, overall, appeared more stable thus far, with recent periods of net growth even observed in places.

A 2023 study suggested 40 percent of Antarctic ice shelves have lost significant volume over the last quarter-century, but documented significant differences between western and eastern Antarctica. It showed, for example, that the Getz Ice Shelf and Pine Island Ice Shelf in West Antarctica lost 1.9 trillion tonnes and 1.3 trillion tonnes over the past 25 years, while the Amery Ice Shelf of East Antarctica gained 1.2 trillion tonnes during the same period.

Greatest ice loss appears to be occurring where ice sheets overlie elevations below sea level—i.e., marine ice sheets—and at particular ice shelves. An Intergovernmental Panel on Climate Change (IPCC) report in 2021 showed that basal melt of ice shelves due to warming seawater temperatures is the main contributor to increased rates of mass loss from outlet glaciers of the West Antarctic Ice Sheet.

Greater instability of West Antarctica’s Thwaites Ice Shelf and the associated Thwaites Glacier, a major outlet glacier of the West Antarctic Ice Sheet, has been ringing alarm bells among the scientific community for years. Because of its central drainage of the West Antarctic Ice Sheet, loss of the Thwaites Glacier—the so-called “Doomsday Glacier” of Antarctica—could significantly destabilize the entire ice sheet, possibly representing a tipping-point event.

Its higher elevation, and more extensive flanking by colder waters, seem to be at least partly buffering East Antarctica from the kind of increased ice-flow rates, melt, and glacial calving going on in West Antarctica. Given the East Antarctic Ice Sheet alone contains enough freshwater to raise global sea levels by some 170 feet (53 meters), that’s at least somewhat encouraging.

But recent research suggests East Antarctica may be more vulnerable to a warming climate than previously realized. A number of East Antarctic glaciers, including the single biggest one, the Totten—the ice volume of which roughly equals the entire West Antarctic Ice Sheet, and is equivalent to perhaps 12.6 feet (3.85 meters) of sea-level rise—are experiencing enhanced melt. Whether recent thinning of the Totten Glacier Ice Shelf due to basal melting reflects natural cyclical ice-shelf variability or a global-warming trend is one current topic of great scientific interest.

It’s essential to note that changes to Antarctic ice have effects besides the climatic and sea-level ones that would be so impactful on humanity. Despite their frigid and austere appearance, these cryospheric environments are their own ecological systems: There’s life under the Antarctic Ice Sheet, persisting in subglacial lakes such as Vostok, and also below the coastal ice shelves, which additionally provide surface-level habitat for breeding penguins and seals.

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