Figure 1. Source: National Snow & Ice Data Center.

Arctic sea ice apparently reached its annual maximum extent on March 17, 2018, and it was the second lowest in the record, according to a report from the National Snow and Ice Data Center.

Each summer the arctic warms, and as it does, the sea ice covering the Arctic Ocean melts, reaching an annual low-point in late summer. Then, each winter the arctic cools, the surface of the ocean freezes, and the area covered by sea ice expands. The sea ice reaches its maximum extent in late winter, this year on March 17.

The National Snow and Ice Data Center tracks the extent of the sea ice using satellite images, as shown in Figure 1. The map is a polar view, with the North Pole in the center, the sea ice in white, and the ocean in blue. The land forms are in gray, with North America at lower left, and Eurasia running from Spain at lower right to the Russian Far East at the top. The magenta line shows the 1981-2010 average extent of the ice for the month of March. It doesn’t look like much on the map, but the anomaly in 2018 amounts to 436,300 square miles less than average.

(Click on figure for larger view.)

Figure 2. Source: National Snow & Ice Data Center.

Figure 2 shows the trend in Arctic sea ice from 1979-2018. The declining trend is easy to see. (The y-axis does not extend to zero to better show the change.) The National Snow and Ice Data Center applied a linear regression trend line to the data (blue line), and the trend shows an average loss of 16,400 square miles per year.

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What about the annual minimum? That has been shrinking, too. Figure 3 shows the Arctic sea ice minimum in 1980, and Figure 4 shows it in 2012. The prevailing winds tend to blow the ice up against Greenland and the far northern islands of Canada, but you can see that in 1980 most of the sea, from the Canadian islands, to Greenland, to the Svalbard Islands, to Severnaya Zemla (anybody remember the Bond movie “GoldenEye?”), to the north of Far Eastern Russia, was covered by ice. In 2012, however, more than half of the Arctic Sea was ice-free, from north of the Svalbard Islands right around to the Canadian Islands. Even the famed Northwest Passage, a channel through the Canadian Islands, was open.

Figure 3. Minimum Extent of Arctic Sea Ice, 1980. Source: NASA Scientific Visualization Studio.

Figure 4. Minimum Extent of Arctic Sea Ice, 2012. Source: NASA Scientific Visualization Studio.

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Figure 5. Minimum Extent of Arctic Sea Ice, 1979-2017. Source: NASA Global Climate Change.

Figure 5 charts the trend in the annual minimum. At its low in 2012, it was less than half of what it was in 1980.

The volume of the polar ice cap also depends on how thick the ice is. Satellites can photograph the entire ice cap, but data on thickness come to us from on-site measurements at a limited number of points. I don’t have a chart to share with you, but the data seem to indicate that compared to the years 1958-1976, in 2003-2007 the thickness had declined about 50% to 64%, depending on where the measurement was taken. (This change is approximate, being read off of a graph by Kwok and Rothrock, 2009.)

Thus, the decline in the arctic ice cap is actually much larger than suggested by the change in its extent.

Why does arctic sea ice matter? First, Arctic sea ice does not form primarily from snowfall, as does the snowcap in the western United States. Arctic sea ice forms because the temperature is low enough to cause the surface of the water to freeze, just as the your local pond or lake freezes if it gets cold enough. Thus, declining Arctic sea ice is a sign that the Arctic is warming. The Arctic seems to be the part of the planet that is warming the most from climate change, and this is a clear and graphic sign of that change.

Oddly, the warming arctic is one reason for the bizarre weather we have had in Missouri this winter. As noted in a post on 1/22/2015, the warming arctic weakens the polar vortex, which allows arctic cold to escape and travel south, impacting us in Missouri. Figure 6 shows the anomaly in Arctic temperatures from December, 2017 through February, 2018, in C. While it was warm over the entire Arctic, as much as 7°C above average (12.6°F), it was 2-3°C cooler than average over North America (3.6-5.4°F).

Second, it matters because ice is white, but the ocean is blue. That means that sunlight hitting ice reflects back towards space, and is not absorbed. Being blue, however, the ocean absorbs the light, and converts the energy to heat. This reflective capacity is called “albedo,” and the albedo of ocean is less than that of ice. Thus, the ice is melting because of global warming, but then, the melting contributes to even more global warming through the change in albedo. People are fond of saying that the earth has buffering mechanisms that tend to inhibit large climate changes, and such mechanisms do exist, but not everywhere in all things. This is one example where the earth shows positive feedback that destabilizes the climate even further.

Melting Arctic ice is not a major factor in the rising sea level. The reason is that the ice is already in the water. When the ice in your glass of iced tea melts, it doesn’t make the glass overflow. In the same way, as this ice melts, it has only a small effect on sea level. On the other hand, the Greenland Ice Cap and the Antarctic Ice Cap are not already in the water, and as they melt, they do affect sea level.

One final word: the data above are not computer models of future events. They are the best data available of what has already been happening, and what is happening now. To deny the reality of climate change is like denying that a river will flood, even as its water already swirls around your knees.

Sources:

Kwok, R., and D./A. Rothrock. 2009. “Decline in Arctic Sea Ice Thickness from Submarine and ICESat Records: 1958-2008. Beophysical Research Letters 36:L15501. Cited in National Snow & Ice Data Center. State of the Cryosphere. Viewed online 4/12/2018 at http://nsidc.org/cryosphere/sotc/sea_ice.html.

NASA Global Climate Change. Arctic Sea Ice Minimum. Downloaded 4/12/18 from https://climate.nasa.gov/vital-signs/arctic-sea-ice.

NASA Scientific Visualization Studio. Annual Arsctic Sea Ice Minimum 1979-2015 with Area Graph. Downloaded 4/12/18 from https://svs.gsfc.nasa.gov/4435.

NASA Scientific Visualization Studio. Annual Arsctic Sea Ice Minimum 1979-2015 with Area Graph. Downloaded 4/12/18 from https://svs.gsfc.nasa.gov/4435.

National Snow & Ice Data Center. “2018 Winter Arctic Sea Ice: Bering Down. Arctic Sea Ice News & Analysis. 4/4/2018. Downloaded 4/12/2018 from http://nsidc.org/arcticseaicenews.