April 11, 2011

What about sea ice in winter?

Researchers take measurements of the physical properties of sea ice in Fram Strait, in the Canadian Arctic. Thinner sea ice melts away faster in the summer compared to older, thicker ice. Credit: Angelika Renner
Arctic sea ice has most likely frozen to its maximum extent for the winter. This event marks the turning point between winter and spring for sea ice, and may affect the amount of ice that will remain by end of summer. In early April, NSIDC scientists discuss what each year’s maximum signifies for the upcoming summer. Meanwhile, readers are asking how the ice cover in recent years is different from the ice cover in years past.

Arctic sea ice grows each winter, hitting its annual maximum sometime in March. When the sun returns to the Arctic in the spring, the air warms up and the ice starts to melt. The ice retreats through the warmer summer months, hitting its lowest point sometime in September. Although scientists watch the sea ice year-round, they pay attention to the highest and lowest extents of the year as indicators of the overall health of the sea ice.

Have the dates of the maximum/minimum shifted?
Reader Sandy Briggs asked Icelights whether the dates of the sea ice minimum and maximum extents have shifted since 1979. Since 1979, the average date of the winter maximum has not changed very much, explains NASA research scientist Walt Meier, who tracks sea ice data year-round. “But we see a lot of year-to-year variation in the date of both the maximum and the minimum dates,” Meier said. The date of the maximum has varied by more than 30 days since 1979. In the summer, in contrast, Arctic sea ice has started freezing up a little later each year. The ice now hits its lowest annual extent an average of six days later than it did in 1979. The minimum, or the amount of sea ice remaining at the end of summer, has declined more than the maximum, the peak of winter sea ice. The minimum ice extent has dropped by more than 30 percent since satellite records started in 1979, while the maximum has declined by only about 8 percent over that same time period. Meier said, “In winter, there are only limited areas where the extent can vary—most of the Arctic Ocean is surrounded by land.” The ice can only expand as far as the ocean extends. In addition, he said, winter ice extent depends more on water temperature than on air temperature, and ocean temperature changes more slowly than air temperature.

Arctic sea ice appears to have reached its maximum extent for the year on March 7, 2011. The maximum extent signals the start of the melt season in the Arctic Ocean.

How important is the thickness of Arctic sea ice?
Reader James Waligora of League City, Texas asked about ice thickness and measurements in a recent email to us. Ice extent measures the surface of the ice, but does not indicate how much ice is beneath the surface. To really gauge the health of the ice cover, researchers need to know how thick it is. Thickness is very important to the stability of the ice cover. Thicker ice can better withstand heat, winds, and currents, to survive the summer months, while thinner ice responds more quickly to changes in weather.

In recent years, ice thickness has also declined, according to several measurements. Researchers use several satellites, along with field measurements, to look at ice thickness. From 2003 to 2009, the Ice, Cloud and land Elevation Satellite (ICESat) orbited the Earth and took measurements of ice thickness. A 2009 study based on ICESat data showed that ice thickness had declined dramatically during that period. That satellite mission ended in 2009, but the next ICESat mission is slated to launch in 2018. For now, researchers are using another method that allows them to estimate ice thickness based on the age of the ice. Older ice that has survived for several winters grows thicker than new ice that has only had one winter to freeze up. Researchers determine the ice age using the same satellite data that researchers use to measure extent. Although ice age measurements are an indirect way to measure ice thickness, this data matches up well with the ICESat ice thickness data. Updated ice age data for each spring are usually available on Arctic Sea Ice News & Analysis in early April.