# Thermodynamics: Cycle

Sea ice growth begins during the autumn when incoming solar energy decreases and air temperatures fall below the freezing point. Ice growth continues through the winter, and the ice becomes thicker as heat continually transfers from the relatively warm ocean to the cold atmosphere. As the sun climbs higher in the sky and solar energy increases in the spring and summer, the temperatures rise and the ice begins to melt. If the ice has not grown thick enough during the autumn and winter, it will completely melt during the spring and summer. However, if the ice grows thick enough during the growth season, it will remain through the summer, become thinner through the melt season, and thicken again the following autumn. Such ice may remain for several years, thinning during the summer and regrowing the following autumn and winter. The figure below illustrates this concept, which is called the growth and melt cycle.

This image illustrates sea ice, snow cover, and upper-ocean temperature evolution from winter to summer in the Beaufort Sea in 2014. The top image (a) shows sea ice mass balance, which reached maximum thickness in late May 2014. The bottom image (b) shows water temperature and oceanic heat observed by an Ice Mass Balance Buoy. From Hwang et al. 2017.

Where growth outpaces melt, ice gradually becomes thicker over the years. Does this mean that sea ice becomes more and more thick? No. Sea ice eventually reaches what scientists call a thermodynamic equilibrium state.

Remember that ice grows because of a transfer of heat from the relatively warm ocean to the cold air above. Also remember that ice insulates the ocean from the atmosphere and inhibits this heat transfer. The amount of insulation depends on the thickness of the ice; thicker ice allows less heat transfer. If the ice becomes thick enough that no heat from the ocean can be conducted through the ice, then ice stops growing. This is called the thermodynamic equilibrium thickness. It may take several years of growth and melt for ice to reach the equilibrium thickness. In the Arctic, the thermodynamic equilibrium thickness of sea ice is approximately 3 meters (9 feet). However, dynamics can yield sea ice thicknesses of 10 meters (30 feet) or more. Equilibrium thickness of sea ice is much lower in Antarctica, typically ranging from 1 to 2 meters (3 to 6 feet).