Sea ice and the global climate
Sea ice, formed when ocean surface temperatures reach freezing, plays a critical role in the global climate system. This ice acts as a barrier, insulating warm ocean water from the cold air above and influencing the exchange of heat, moisture, and salinity within the ocean. There are two primary types of sea ice: fast ice, which is stationary and adheres to coastlines, and drift ice, which floats and is moved by winds and currents. Seasonal changes lead to significant variations in sea ice cover, particularly in the Arctic and Antarctic regions, where the extent can fluctuate between 14-16 million square kilometers in the Arctic during winter and 17-20 million square kilometers in the Antarctic.
Sea ice is crucial for regulating the Earth's temperature, as it reflects sunlight and helps to maintain cooler ocean temperatures. However, climate change, driven by human-induced greenhouse gas emissions, has led to a significant decline in sea ice extent and thickness. This reduction affects not only global climate patterns but also polar ecosystems, putting species like polar bears and seals at risk due to the loss of habitat essential for hunting and breeding. If current trends continue, projections suggest that the Arctic Ocean could be ice-free in the summer by 2030, marking a significant shift in the planet's climatic and ecological landscape.
Subject Terms
Sea ice and the global climate
Definition
Sea ice is formed when the surface layer of the ocean reaches the freezing point. For saltwater, this occurs at a temperature of –1.8 Celsius, slightly colder than the freezing point of freshwater. Because water is more buoyant in its solid state, the newly formed ice floats on top of the ocean, forming a barrier between the cool air and the warm water.
There are two types of sea ice. Fast ice (also known as land fast ice) is seawater that has frozen along the shore line, or, in shallow areas, has frozen to the ocean floor. These formations are, by their very nature, stationary. Drift ice is frozen salt water that floats and is affected by winds and currents. First-year ice sheets, or floes, are typically thin and subject to shattering and refreezing, forming rafts and ridges.
A cluster of drift ice is called pack ice, and this is what covers both the Arctic and the Antarctic regions of the planet. The amount of pack ice in the polar regions varies with the season. During winter, usually covers between 14 and 16 million square kilometers in the Arctic and between 17 and 20 million square kilometers in the Antarctic. In the Antarctic, the sea ice melts during the summer and is therefore considered seasonal ice, but in the Arctic it remains year-round.
Sea ice poses a hazard for shipping and has been noted in sailors’ logs going back to the fourth century BCE. Sea ice is commonly confused with icebergs. Although both represent significant shipping hazards, icebergs are made of freshwater, usually from precipitation or snowmelt, and break off from glaciers.
Significance for Climate Change
Sea ice, especially Arctic sea ice, plays a significant role in the overall global climate by regulating the exchange of heat, moisture, and in the ocean. As seawater freezes, its salt content is reduced in a process called brine rejection. The salt rejected from the frozen water increases the salinity of the surrounding ocean and can affect ocean currents. Sea ice insulates the warm ocean water and keeps it from losing heat to the much colder arctic or subarctic air. It also, however, reflects more sunlight than the surrounding ocean, preventing the sunlight from warming the ocean. When more sunlight reaches the water, the heat is absorbed, raising the temperature. Fissures or cracks in the ice release the trapped heat from the warmer ocean water, which can affect precipitation and cloud cover.
Greenhouse gases (GHGs) emitted through human activities and the resulting increase in global mean temperatures are the most likely underlying cause of sea ice decline, but the direct cause is a complicated combination of factors resulting from the warming and from climate variability.
Passive microwave satellite data show that, as of 2006, the Arctic sea ice decreased by 3.6 percent during each of the previous three decades. In 2007, Arctic sea ice was at a record low. Despite a recovery to near-average levels during the following winter, 2008 summer sea ice coverage was nearly as low as the previous year. Sea ice continued to decline over the years. Scientists from NASA reported in 2022 that sea ice was shrinking at a rate of 12.2 percent per decade, according to data from 1981 to 2010. By 2024, the maximum Arctic winter sea ice had shrunk by roughly the size of the state of Alaska. Scientists at the United States National Snow and Ice Data Center have predicted that, if current trends persist, by the year 2030, the Arctic Ocean would see its first ice-free day in more than one million years.
As the global climate changes, the quantity of polar sea ice is diminishing, affecting regional ecosystems. Animals such as polar bears, walruses, and seals rely on ice floes for breeding, shelter, and especially hunting. As the summer ice diminishes, the summer hunting season—crucial for animals that hibernate—grows shorter. The large mammals have to travel farther to find food, and more time passes between kills. When they do not have sea ice to travel on, they must stay on land. A study published by ABC News in 2024 indicated that those on land do not get the nutrients they need from the terrestrial food available. According to the study, most lost 2.2 pounds (1 kilogram) per day.
Bibliography
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Jacobo, Julia. "Climate Change and Loss of Sea Ice Putting Polar Bears at Risk of Starvation, Collar Cameras Show." ABC News, 13 Feb. 2024, abcnews.go.com/International/climate-change-loss-sea-ice-putting-polar-bears/story?id=107158174. Accessed 21 Dec. 2024.
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Schueman, Lindsey Jean. "Why Polar Bears Need Sea Ice to Survive in the Arctic." One Earth, 28 Feb. 2024, www.oneearth.org/species-of-the-week-polar-bear/. Accessed 21 Dec. 2024.
Thomas, David N. Frozen Oceans: The Floating World of Pack Ice. London: Natural History Museum, 2004.
Thomas, David N., and Gerhard S. Dieckmann, eds. Sea Ice: An Introduction to its Physics, Chemistry, Biology, and Geology. Malden, Mass.: Blackwell Science, 2003.