Earth's Frozen Oceans

Part B: Sea Ice Thickness

Sea ice plays an important role in regulating exchanges of heat, moisture, and salinity (saltiness) in the polar oceans. Although sea ice begins as a thin layer floating on top of the water, it can grow to be several meters thick depending on a number of different conditions involving what's occurring above and below the ice.

Sea ice is typically sandwiched between water on the bottom and a blanket of snow on top. Sea ice insulates the relatively warm ocean water below from the cold atmosphere above except where cracks in the ice allow heat and water vapor to escape.

The diagram below shows one example of how sea ice thickness varies over the course of a year. In any given year, the relative thickness of the ice and snow layers could be very different from what is shown here. Use the diagram to help you answer the Checking In and Stop and Think questions.

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Seasonal growth and melt of arctic sea ice over one year. The left y-axis represents thickness of sea ice and snow, referenced to the top of the ice. Image courtesy of the National Snow and Ice Data Center (NSIDC), University of Colorado, Boulder. Modified from Maykut and Untersteiner (1971).

Provenance: Image courtesy of National Snow and Ice Data Center (NSIDC), University of Colorado, Boulder (http://nsidc.org/cryosphere/seaice/processes/growth_melt_cycle.html). Modified from Maykut and Untersteiner (1971).
Reuse: This item is offered under a Creative Commons Attribution-NonCommercial-ShareAlike license http://creativecommons.org/licenses/by-nc-sa/3.0/ You may reuse this item for non-commercial purposes as long as you provide attribution and offer any derivative works under a similar license.

What determines sea ice thickness?

Ice will become thicker over time if it grows faster than it melts. However, there is a limit to how thick the ice can become. Ice grows when heat from the relatively warm ocean is transferred to the cold air above. Ice also insulates the ocean from the atmosphere and inhibits this heat transfer. As the ice becomes thicker, less and less heat is able to escape into the air. If the ice becomes thick enough that no heat from the ocean can be conducted through the ice, the ice stops growing. This is called thermodynamic equilibriumthermodynamic equilibrium: the state of a system in which its energy distribution is such that all parts of the system have the same temperature and no heat flows.. It can take several annual cycles of growth and melt before sea ice reaches this equilibrium thickness. In the northern hemisphere, the thermodynamic equilibrium thickness is about 3 meters, whereas in the southern hemisphere, equilibrium thickness is between 1 and 2 meters. Sea ice that is thicker than the thermodynamic equilibrium thickness is a result of dynamic (motion-related) processes.

Close your eyes and think about the ocean. Envision yourself swimming, sailing, or surfing. What does it feel like? Are you stationary or are you moving—bobbing, drifting, or getting pushed around? Depending on a number of different conditions, the ocean can be fairly calm or quite rough, but either way it is always in motion. Sea ice, like anything else afloat in the oceans, is constantly subjected to a number of different forces from things like wind, ocean currents, the Coriolis effectCoriolis effect: deflection caused by Earth's rotation , internal ice stressinternal ice stress: measure of the compactness, or strength, of the ice, and sea surface tiltsea surface tilt: a force, determined by differences in sea surface level, that influences ice motion; several factors contribute to differences in the ocean surface level, including uneven heating, salinity variations, and currents, especially near coastal regions or ice shelves. (Optional: Learn more about these dynamic forces). Ice floating out in the open sea is constantly in motion as a result of these forces. Even fast icefast ice: ice that is anchored to the shore or ocean bottom, typically over shallow ocean shelves at continental margins; fast ice is defined by the fact that it does not move with the winds or currents. , which doesn't move around, is continuously pushed, pulled, and pummeled by wind, waves, ocean currents, and other ice. The image below shows how tumultuous life can be for sea ice.

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Various types of sea ice congregated along the east coast of Greenland in late March 2010. Image credit: NASA Earth Observatory

Provenance: NASA image by Jeff Schmaltz, MODIS Rapid Response Team at NASA GSFC. (http://earthobservatory.nasa.gov/IOTD/view.php?id=43387)
Reuse: This item is in the public domain and maybe reused freely without restriction.

Sea ice motion isn't two dimensional. In addition to the lateral (side to side) motion you see on the surface, there is also a vertical (up and down) component. Watch the short YouTube video below to get a feel for the undulating motion of sea ice caused by the rise and fall of the ocean water.

Sea ice thickness is related to age

Scientists typically classify sea ice by stages of development that relate to both thickness and age. New icenew ice: a general category of ice representing the earliest stages of sea ice growth. is less than 10 centimeters thick. As the ice builds up and gets thicker (10-30 cm thick), it becomes young iceyoung ice: a general category of ice that is less than one year old.. First-year icefirst-year ice: floating ice of no more than one year's growth developing from young ice; first-year ice is characteristically level (flat) where it is undisturbed by pressure, but can be rough and angular where ridges occur. is thicker than 30 centimeters, but has not made it through a summer melt season. Multiyear icemultiyear ice: ice that has survived at least one melt season; thickens as more ice grows on its underside. has survived one or more summer melt seasons and is considerably thicker than younger ice, typically between 2 and 4 meters thick. To put into perspective how thick that is, the average adult male in the U.S. is 1.76 meters tall and the average adult female in the U.S. is 1.63 meters tall (Fryar, Gu, & Ogden, 2012).

The images below show the average age of sea ice in the Arctic during the month of February from 1985-2000 (left) and the age of sea ice in February of 2008 (right). The colors indicate the age of the sea ice in years, ranging from light blue (1 year old) to dark blue (8+ years).

Carefully examine the two images and compare the ages of the sea ice.

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Average age of Arctic sea ice. Image credit: NASA Earth Observatory

Provenance: Image source: NASA Earth Observatory (http://earthobservatory.nasa.gov/IOTD/view.php?id=8596). NASA image by Robert Simmon, based on data from Charles Fowler, James Maslanik, Sheldon Drobot, and William Emery, Arctic Regional Ice Forecasting System (http://ccar.colorado.edu/arifs/), University of Colorado-Boulder.
Reuse: This item is in the public domain and maybe reused freely without restriction.

How Do We Know What We Know?

Measuring sea ice thickness can be tricky business. Here is a brief introduction to some of the tools and techniques scientists use, along with some resources to help you launch your own investigation into how sea ice thickness is measured.

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