![]() Two of the three cells would be considered “ice covered,” which means that the full grid cell area counts toward the extent. Example: Let’s say you have three 10 by 10 kilometer grid cells covered by 16 percent ice, 2 percent ice, and 90 percent ice. The most common threshold (and the one NSIDC uses) is 15 percent, meaning that if the data cell has greater than 15 percent ice concentration, the cell is considered ice covered less than that and it is said to be ice free. A more precise explanation of extent versus area gets more complicated.Įxtent defines a region as “ice-covered” or “not ice-covered.” For each satellite data cell, the cell is said to either have ice or to have no ice, based on a threshold. That is why, if you compare extent and area in the same time period, extent is always bigger. Area would be the measure of where there is cheese only, not including the holes. Extent would be a measure of the edges of the slice of cheese and all the space inside it. Extent is always a larger number than area, and there are pros and cons associated with each method.Ī simplified way to think of extent versus area is to imagine a slice of Swiss cheese. Some organizations report ice area, but NSIDC primarily reports ice extent. What is the difference between sea ice area and extent?Īrea and extent are different measurements, providing slightly different information about sea ice conditions. Scientists use the 15 percent cutoff because it generally provides the most consistent agreement between satellite and ground observations. Usually, scientists define a threshold of minimum concentration to mark the ice edge the most common cutoff is at 15 percent. Sea ice extent is a measurement of the ocean where there is at least some sea ice. Sea ice comes in various sizes and thicknesses depending on its age. Learn more about how Arctic and Antarctic sea ice differ. By contrast, the Arctic Ocean is intimately linked with the climate systems around it, making it more sensitive to changes in climate. Wind and ocean currents around Antarctica isolate the continent from global weather patterns, keeping it cold. It also tends to nearly completely melt out during the summer months of the Southern Hemisphere. ![]() Sea ice can spread out more freely around Antarctica, and is often much thinner than Arctic sea ice. Part of the difference is between their geographic layout: the Arctic is an ocean surrounded by land, while Antarctica is a continent surrounded by vast oceans. Sea ice extent surrounding Antarctica fluctuates much more than the Arctic, in both space and time. Scientists monitor both Arctic and Antarctic sea ice, but Arctic sea ice is more significant to understanding global climate because much more Arctic ice remains through the summer months, reflecting sunlight and cooling the planet. Credit: Tom Prater/Carbon Brief Why is there greater focus on Arctic sea ice? This graphic demonstrates the difference in solar heat absorption on a white surface versus the darker ocean surface. The loss of sea ice also has the potential to accelerate global warming trends and to change climate patterns.įor more on the ways sea ice interacts with other Earth systems, including global ocean circulation, people, and animals, see Why sea ice matters. This is consistent with observations of a warming Arctic. According to scientific measurements, both the thickness and extent of summer sea ice in the Arctic have shown a dramatic decline since the 1980s. Thus, a small temperature increase at the poles leads to still greater warming over time, making the poles the most sensitive regions to climate change on Earth. So, oceans absorb about 90 percent of the sun’s energy, causing them to heat while surrounding temperatures rise further. Instead of reflecting 80 percent of the sunlight, the ocean reflects only about 10 percent. This serves to insulate the sea ice, maintaining cold temperatures and delaying ice melt in the summer.Īs sea ice melts in the summer, it exposes the dark ocean surface. Snow-covered sea ice reflects as much as 90 percent of the incoming solar radiation. Snow has an even higher ability to reflect solar energy than sea ice. The darker ocean reflects only 6 percent of the sun’s energy and absorbs the rest, while sea ice reflects 50 to 70 percent of the incoming energy. ![]() Sea ice has a much brighter surface compared to many other Earth surfaces, particularly the surrounding ocean. Sea ice keeps the polar regions cool and helps moderate global climate. Credit: US Geological Survey Why is sea ice important?
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