How Do Glaciers, Ice Sheets, and Sea Ice Differ? Definitions and Climate Change

The decline of global ice is sparking renewed "ice awareness."

A glacier towers above a hiker at sunset.
Argentina's Perito Moreno glacier towers above a hiker at sunset.

Marco Botticelli / Getty Images

Think snow and ice can't exist outside of the winter season? Think again.

At any given time and season, various forms of ice—including glaciers, ice sheets, and sea ice—cover about 10% of Earth's land and water surfaces. This is a good thing—as climate change so callously reminds us, these frozen landscapes play a vital role in Earth's global climate. Here we explore what specifically that role looks like for each major form of ice.

Definitions of Ice Forms

Glaciers, ice sheets, and sea ice are a part of Earth’s cryosphere—the portions of the earth where water lives in its solid form.

Glaciers

Aerial view of a Franz Josef glacier
New Zealand's Franz Josef Glacier.

Matt Palmer / Getty Images

Glaciers are fields of land ice that form when perennial accumulations of snow compress over one hundred or more years, forming massive layers of ice. So massive, in fact, that they move under their own weight, flowing downhill like a very slow river. However, if you didn't know this, you'd likely never notice it. Most glaciers creep along at such a snail's pace (one foot per day, for example) their movement can't be detected with the naked eye.

While today's glaciers have existed since the last ice age (the Pleistocine Epoch) when ice covered about 32% of land and 30% of oceans, they've dwindled significantly since then. These ice forms are now limited to regions experiencing high snowfall in winter and cool temperatures in summer, such as Alaska, the Canadian Arctic, Antarctica, and Greenland.

Glaciers not only draw millions of visitors to these locations every year (think Montana's Glacier National Park); they also serve as a major freshwater resource. Their meltwater feeds into streams and lakes, which are then used for crop irrigation. Glaciers also provide drinking water for people living in mountainous yet arid climates. For example, in South America, Bolivia's Tuni glacier provides at least 20% of the annual water supply for the people of La Paz.

Ice Sheets

A researcher pulls his gear across a vast snow-covered ice sheet.

Alex Hibbert / Getty Images

If glacial ice blankets an area of land more than 20,000 square miles (50,000 square kilometers) in size, it's known as an ice sheet.

What's In an Icy Name?

Ice sheets go by different names depending on their characteristics. For example, some of the smallest-sized ice sheets are called "ice caps." If an ice sheet extends out over water, it's known as an "ice shelf." And if a pieces of an ice shelf breaks off, an infamous "iceberg" is born.

Although they resemble snow-covered ground, ice sheets don't form from a single blanket of snow. They're made up of countless layers of snow and ice that collect over thousands of years. During the last glacial period, ice sheets covered North America, northern Europe, and the tip of South America. Today, however, there are only two: The Greenland and Antarctic Ice Sheets. Together, the pair contains 99% of Earth's freshwater ice.

Ice sheets also store vast amounts of carbon dioxide and methane, keeping these greenhouse gases out of the atmosphere where they'd otherwise be contributing to global warming. (The Antarctic ice sheet alone stores roughly 20,000 billion tons of carbon.)

Sea Ice

Polar bear walking on partly-melted Arctic sea ice.

zanskar / Getty Images

Unlike glaciers and ice sheets which form on land, sea ice—frozen ocean water—forms, grows, and melts in the ocean. Also unlike its sister ice forms, sea ice extent changes on a yearly basis, expanding in winter and declining somewhat every summer.

In addition to being a critical habitat for arctic animals, including polar bears, seals, and walruses, sea ice helps to regulate our global climate. Its bright surface (high albedo) reflects roughly 80% of the sunlight that strikes it back out into space, which helps keep the polar regions where it resides cool.

How Climate Change Impacts These Ice Forms

Just like ice cubes eventually succumb to the sun on a hot summer's day, the world's ice is retreating in response to global warming.

As of the writing of this article, an estimated 400 billion metric tons of glacier ice has been lost each year since 1994; the Antarctic and Greenland Ice Sheets are losing mass at a rate of 151 and 277 billion metric tons per year, respectively; and 99% of the oldest and thickest sea ice in the Arctic has been lost to global warming. Not only is this melting a grave disadvantage in and of itself, but it's also negatively impacting our overall environment.

Ice Loss Encourages More Warming

One of the implications of the loss of global ice is what scientists call the "ice-albedo feedback loop." Because ice and snow are more reflective (have a higher albedo) than land or water surfaces, as global ice cover shrinks, the reflectivity of Earth's surface does too, meaning more incoming solar radiation (sunlight) is absorbed by these newly-revealed darker surfaces. Because these darker surfaces absorb more sunlight and heat, their presence further contributes to warming.

Meltwater Contributes to Sea Level Rise

Melting glaciers and ice sheets pose an additional problem: sea-level rise. Because the water they contain is normally stored on land, the runoff from glaciers and melt is significantly increasing the amount of water in the world's oceans. And similarly to an overfilled bathtub, when too much water is added to too small of a basin, water inundates the surrounding environment.

Scientists at the National Snow & Ice Data Center (NSIDC) estimate that if the Greenland and Antarctic Ice sheet were to melt completely, the global sea levels would rise by 20 feet and 200 feet, respectively.

Too Much Freshwater Destabilizes Our Oceans

The runoff from ice melt is also contributing to the dilution or "desalinization" of the ocean's saltwater. In 2021, news broke that the Atlantic Meridional Overturning Circulation (AMOC)—an ocean conveyor belt responsible for carrying warm water from the tropics northward into the North Atlantic Ocean—was the weakest it had been in over one thousand years, likely due to freshwater inflow from melting ice sheets and sea ice. The problem stems from the fact that freshwater has a lighter density than saltwater; because of this, water currents tend not to sink, and without sinking, the AMOC ceases to circulate.

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