Evidence of Recent Change
Part B: Shrinking Sea Ice
Sea Ice Extent Revisited
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, 2012 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. It's not just summer minimum extents that are decreasing. In 2017, the Arctic sea ice winter maximum also reached a record-setting low of 14.42 million square kilometers (5.57 million square miles). 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.
- Watch this video from NASA showing minimum Arctic sea ice area and trends from 1979-2018. Click on the expand icon at the bottom right corner of the video player to view the video in full screen mode.
- 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?
Sea Ice Volume
Although sea ice extent is the most widely used sea ice measure to study climate and climate change, it is not the only measure scientists use. Sea ice doesn't melt (or grow) in just two dimensions. As you saw in Lab 2B, sea ice thickness is also an important parameter. By combining information about sea ice extent (area) and sea ice thickness, scientists can determine sea ice volume, which is a better indicator of climate than sea ice extent alone. This measure is used less frequently than sea ice extent because it is much harder to determine. Currently, there is no method for getting continuous observations of Arctic sea ice volume. Observations from satellites, Navy submarines, moorings (places where boats or ships are secured), and field measurements all have limitations in coverage over space and time. However, observational data can be used in conjunction with computer models to provide estimates of sea ice volume changes on a continuous basis. One such model is the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS), which was developed at the University of Washington's Polar Science Center).
- Watch this video showing average monthly sea ice volume from 1979-2013, as determined by PIOMAS. Visit Arctic Sea Ice Volume monthly averages to view an updated, static graph of these data. (Note that the color codes for the months are different between the static and video graphs.)
©Andy Lee Robinson. Used with permission. NOTE: The video is large and may take several minutes to load.
Stop and Think
1: What is the overall average trend for monthly sea ice volume between 1979 and 2018? Explain.
- Watch this video showing Arctic sea ice annual minimum volume from 1979-2018, as determined by the PIOMAS model. NOTE: Pause the video around 28 seconds to keep the full graph in view. Click on the expand icon at the bottom right corner of the video player to view the video in full screen mode.
©Andy Lee Robinson. Used with permission.
Stop and Think
2: Compare how Arctic sea ice area and volume have changed since 1979:
- If the result is positive, it is a percentage increase.
- If the result is negative, it is a percentage decrease.
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.
Stop and Think
3: Describe the effect declining ice coverage has on Earth's surface albedo as a function of time.
Optional Extension: Impacts of Declining Sea Ice
Polar Vortex: When Warm = Cold
2012 was the warmest year on record for the United States, with 34,008 daily high temperature records set or tied (Freedman, 2013). However, the winter of 2013-2014 brought equally impressive low temperatures to many parts of the country, including the deep south. If you live in one of the areas that experienced these frigid temperatures (or watched national news coverage during this time), you've probably heard that this extreme cold was attributed to the polar vortexpolar vortex: a large pocket of very cold air, typically the coldest air in the Northern Hemisphere, which sits over the polar region during the winter season..
So what, exactly, is the polar vortex and why does it make it so cold when scientific data show that overall, the planet is warming?
The polar vortex is a band of low pressure Arctic air that is normally centered on the North Pole, keeping things cold way up north where we expect it to be cold. The jet stream (the fast-flowing air current that separates warm and cold air masses) typically holds the polar vortex in place over the Arctic, but under the right conditions, cold air from the Arctic can make its way south to the mid-latitudes. As global temperatures rise and more and more sea ice melts each summer, more solar energy is absorbed by the Arctic Ocean. As the ocean warms, it radiates excess heat back into the atmosphere during winter. This warm air disrupts the polar vortex swirling around the Arctic and makes it easier for frigidly cold air to travel southward.
in the video below, President Obama's Science and Technology Advisor, Dr. John Holdren, explains the polar vortex and why climate change makes extreme weather more likely in the future.
As a class, discuss:
- Are extremely cold temperatures common where you live?
- What is the coldest temperature you have personally experienced in your home town?
Want to learn more about other ways declining sea ice extent is affecting 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