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Sea ice is found in remote polar oceans. On average, sea ice covers about 25 million square kilometers (9,652,553 square miles) of the earth, or about two-and-a-half times the area of Canada. Because most of us do not live in the polar regions, we may live for several decades and never see sea ice. Although it may not directly affect us, it is a critical component of our planet because it influences climate, wildlife, and people who live in the Arctic.

This article offers a glimpse of the characteristics and different forms of sea ice, why it is so important to our environment, and popular scientific methods for studying it. Sample images from data illustrate the type of information scientists seek to learn about sea ice. If you want a more in-depth, scientific discussion on the thermodynamics and physics of sea ice, visit the “Processes” section. Finally, we describe how sea ice affected explorers who struggled to reach the poles.

What is sea ice?

Sea ice is simply frozen ocean water. It forms, grows, and melts in the ocean. In contrast, icebergs, glaciers, ice sheets, and ice shelves all originate on land. Sea ice occurs in both the Arctic and Antarctic. In the Northern Hemisphere, it can currently exist as far south as Bohai Bay, China (approximately 38 degrees north latitude), which is actually about 700 kilometers (435 miles) closer to the Equator than it is to the North Pole. In the Southern Hemisphere, sea ice only develops around Antarctica, occurring as far north as 55 degress south latitude.

Sea ice grows during the winter months and melts during the summer months, but some sea ice remains all year in certain regions. About 15 percent of the world’s oceans are covered by sea ice during part of the year.

Why is sea ice so important, and why do scientists study it?

Even though sea ice occurs primarily in the polar regions, it influences our global climate. Sea ice has a bright surface, so much of the sunlight that strikes it is reflected back into space. As a result, areas covered by sea ice don’t absorb much solar energy, so temperatures in the polar regions remain relatively cool. If gradually warming temperatures melt sea ice over time, fewer bright surfaces are available to reflect sunlight back into space, more solar energy is absorbed at the surface, and temperatures rise further. This chain of events starts a cycle of warming and melting. This cycle is temporarily halted when the dark days of the polar winter return, but it starts again in the following spring. Even a small increase in temperature can lead to greater warming over time, making the polar regions the most sensitive areas to climate change on Earth.

Sea ice also affects the movement of ocean waters. When sea ice forms, most of the salt is pushed into the ocean water below the ice, although some salt may become trapped in small pockets between ice crystals. Water below sea ice has a higher concentration of salt and is more dense than surrounding ocean water, and so it sinks. In this way, sea ice contributes to the ocean’s global “conveyor-belt” circulation. Cold, dense, polar water sinks and moves along the ocean bottom toward the equator, while warm water from mid-depth to the surface travels from the equator toward the poles. Changes in the amount of sea ice can disrupt normal ocean circulation, thereby leading to changes in global climate (for more information, see Sea Ice and Global Climate).

Too much or too little sea ice can be a problem for wildlife and people who hunt and travel in polar regions. In the Arctic, sea ice can be an obstacle to normal shipping routes through the Northern Sea route and Northwest Passage. See the Environment section to learn more about the impact of sea ice changes on people and wildlife.

What is the difference between sea ice and icebergs, glaciers, and lake ice?

The most basic difference is that sea ice forms from salty ocean water, whereas icebergs, glaciers, and lake ice form from fresh water or snow. Sea ice grows, forms, and melts strictly in the ocean. Glaciers are considered land ice, and icebergs are chunks of ice that break off of glaciers and fall into the ocean. Lake ice is made from fresh water and freezes as a smooth layer, unlike sea ice, which develops into various forms and shapes because of the constant turbulence of ocean water.

The process by which sea ice forms is also different from that of lake or river ice. Fresh water is unlike most substances because it becomes less dense as it nears the freezing point. This difference in density explains why ice cubes float in a glass of water. Very cold, low-density fresh water stays at the surface of lakes and rivers, forming an ice layer on the top.

In contrast to fresh water, the salt in ocean water causes the density of the water to increase as it nears the freezing point, and very cold ocean water tends to sink. As a result, sea ice forms slowly, compared to freshwater ice, because salt water sinks away from the cold surface before it cools enough to freeze. Furthermore, other factors cause the formation of sea ice to be a slow process. The freezing temperature of salt water is lower than fresh water; ocean temperatures must reach -1.8 degrees Celsius (28.8 degrees Fahrenheit) to freeze. Because oceans are so deep, it takes longer to reach the freezing point, and generally, the top 100 to 150 meters (300 to 450 feet) of water must be cooled to the freezing temperature for ice to form.