Evidence of Recent Change

Part B: Shrinking Sea Ice

In August and September 2012, sea ice covered less of the Arctic Ocean than at any other time since at least 1979, when the first reliable satellite measurements began. The National Snow and Ice Data Center (NSIDC) and NASA announced in mid-September that the extent of Arctic sea ice had dropped to 3.41 million square kilometers (1.32 million square miles)—well below the previous record of 4.17 million square kilometers (1.61 million square miles) set in 2007. Loss of polar bear habitat, altered shipping routes, and shifts in global weather patterns are just a few of the side effects that may result from dwindling Arctic sea ice.

1. Watch this video from NASA showing minimum Arctic sea ice area and trends from 1979-2011.


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Checking In

1. How would you describe the graph of Arctic sea ice area?

   steady decline in area from 1979-2011 Not quite. Take another look.
   

   steady increase in area from 1979-2011 Try again.
   

   ups and downs with an overall increase in area from 1979-2011 Oops!
   

   ups and downs with an overall decrease in area from 1979-2011 You got it!
   
2. Scientists at the National Snow and Ice Data Center (NSIDC) keep a constant watch over sea ice. Go to their Sea Ice Index page to explore the most current monthly sea ice extent data available. How does the data for this month compare to the 1979-2000 mean?

Albedo Revisited

Recall from Lab 1 that albedo is a non-dimensional, unitless quantity that indicates how well a surface reflects solar energy. Albedo (α) varies between 0 and 1. Something with an albedo of 0 absorbs all incoming energy, which can be used to heat the surface or, when sea ice is present, melt the surface. A value of 1 means the surface reflects 100% of incoming energy.

Sea ice has a much higher albedo than most other surfaces on Earth, including the surrounding ocean. A typical ocean albedo is approximately 0.06, while sea ice albedo varies from approximately 0.5 to 0.7 for bare ice and up to 0.9 for sea ice covered with snow. This means that the ocean reflects only 6 percent of the incoming solar radiation and absorbs the rest, while sea ice reflects as much as 90 percent of the incoming energy. The sea ice absorbs less solar energy and keeps the surface cooler.

As you saw in Lab 2B, snow also helps insulate the sea ice, maintaining cold temperatures and delaying ice melt in the summer. After the snow does begin to melt, albedo drops to about 0.75 because of darker shallow melt ponds that form on the surface. As melt ponds grow and deepen, the surface albedo can drop down to about 0.15. As a result, melt ponds instigate more rapid ice melt.

Optional Extension: Impacts of Declining Sea Ice

Want to learn more about the impacts declining sea ice extent has on life around the globe? Check out these resources:

• Arctic: Read the NASA article Arctic sea ice on the wane: Now what?.
• Antarctic: Learn more on the site Antarctic Penguins: Bellwethers of Environmental Change