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Sea Ice


Sea Ice

One of the most striking examples of climate change is related to the Arctic sea ice; the video below shows the drastic changes in the extent of Arctic sea ice over the last 35 years.  The ocean is now permanently open to shipping in the summer.

Credit: National Aeronautics and Space Administration (NASA)

This visualization shows the age of the Arctic sea ice between 1984 and 2019. Younger sea ice, or first-year ice, is shown in a dark shade of blue while the ice that is four years old or older is shown as white. A graph displayed in the upper left corner quantifies the area covered sea ice 4 or more years old in millions of square kilometers.

The Arctic sea ice undergoes large fluctuations over the course of a year, and, like all aspects of the climate system, there is a good deal of natural variability. So, while one or two extreme years do not necessarily make a trend, they may be part of a trend. Visit NASA's Earth Observatory to see a nice set of maps looking down on the North Pole showing the sea ice extent over the last 12 years; each map shows the long-term mean ice extent in a yellow line. One thing that becomes apparent is that there is much more variability in the end-of-summer minimum ice extent than the end-of-winter maximum; another thing that is apparent is that the reduction in Arctic sea ice is now a long term trend.

In the figure below, we see a summary of data on the ice extent, reported as an anomaly (departure from the mean), and it now becomes apparent that there has been a more or less steady decline since about 1970.

Graph of arctic sea ice extent standardized anomalies, 1953-2011 showing an overall decrease
Sea ice extent departures from monthly means for the Northern Hemisphere. For January 1953 through December 1979, data have been obtained from the UK Hadley Centre and are based on operational ice charts and other sources. For January 1979 through December 2018, data are derived from passive microwave (SMMR / SSM/I).
Credit: Walt Meier and Julienne Stroeve, National Snow and Ice Data Center, University of Colorado, Boulder, State of the Cryosphere.

In addition to the reduced area of coverage, the Arctic ice is also becoming thinner. The thickness of the ice in the Arctic has been monitored for a long time by the US Navy, using submarines. Now, satellites can measure ice thickness. The comparison below shows a 40% to 50% decrease in the thickness of ice from the average over 1958-1976 compared to the present.

Graph comparing ice thickness in the Arctic, average over 1958-1976 compared to the present
Data from Kwok and Rothrock (2009; doi:10.1029/2009GL039035).
Click here for a text alternative to the figure above
Ice Thickness (m) At Different Locations
Thickness at: 1958-1976 1993-1997 2003-2007
Chukchi Cap 1.9m 1.0m 0.7m
Beaufort Sea 1.9m 1.0m 1.0m
Canada Basin 3.4m 2.1m 1.7m
North Pole 3.7m 2.3m  1.8m
Nansen Basin 3.8m 2.0m 2.2m
Eastern Arctic 3.3m 1.3m 1.2m

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

This reduction in the coverage of Arctic sea ice is significant since it means that during the summer months when the sun is at its brightest in the polar region, and there are 24 hours of daylight, the reflective ice cover is being reduced, allowing for the absorption of a much greater quantity of solar energy that can then warm the whole polar region. This change in sea ice coverage is mainly an Arctic phenomenon — in the Antarctic, the sea ice is relatively constant over this same time period (from 1958-1976).

Finally, a word on sea level. The melting of ice sheets and mountain glaciers is partially responsible for a significant rise in sea level---20 cm in the last 100 years. However, thermal expansion of sea water as a result of warming is equally, if not more, important. Also, since sea ice is already at ocean level, its melting does not contribute to sea level rise.

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