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.
Checking InAnswer the following questions about the sea ice thickness plot.
Stop and Think
1: What happened to the snow in the months of July and August? How did this affect the thickness of the sea ice? Explain.
2: Explain why the tops of the sea ice and snow are below 0 cm from July through December. Hint: Think about what the 0 mark on the y-axis represents.
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 equilibrium. 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 effect deflection caused by Earth's rotation , internal ice stress measure of the compactness, or strength, of the ice, and sea surface tilt (Optional: Learn more about these dynamic forces). Ice floating out in the open seas is constantly in motion as a result of these forces. Even fast ice (sea ice that grows from and is attached to shore), 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.
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 ice is less than 10 centimeters thick. As the ice builds up and gets thicker, it becomes young ice, which is between 10 and 30 centimeters thick. First-year ice is thicker than 30 centimeters, but has not made it through a summer melt season. Multiyear ice has survived one or more summer melt seasons and is considerably thicker than younger ice, typically between 2 and 4 meters thick.
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.
Stop and Think
3: How does the age of Arctic sea ice in February 2008 compare to the 1985-2000 average? What do you think this means?
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.