Australia: The Land Where Time Began

A biography of the Australian continent 

The Cryosphere - As a Latent Energy Buffer

A large amount of energy is required to melt snow and ice, the sensible heat and radiative energy being consumed by this phase change, from snow/ice to water, energy that would otherwise be used to warm up a region. As winter approaches and temperatures drop the opposite effect is involved as latent energy is released from the water of lakes, rivers, oceans and soil to the atmosphere as ice forms from water and vapour, at latitudes and altitudes that get cold enough for ice to form, delaying seasonal temperature decrease. According to the author1 the overall result is that within the seasonal cryosphere the latent energy exchanges act as a thermal buffer, as does the ocean, in which the latent heat capacity of the ocean moderates the climate of marine environments.

It is possible to estimate the amount of energy involved in these phase changes from the extent of the seasonal cryosphere, though the area of the seasonal sea-ice and snow is better known than is the volume. If an average thickness of 1.5 m in the Southern Hemisphere and 2 m for the ice cover that melts every summer in the Northern Hemisphere is assumed, the average sea-ice contribution (1979-2010) to this latent heat energy budget is approximately 11 x1021 J per year (11 ZJ), which is equivalent to 349 TW, though this doesn't include the energy required to warm the ice to melting point. When this is compared to the global energy consumption of human activities in 2009 of 14.8 TW it gives some idea of the vast amounts of energy involved in the natural system, the latent energy being drawn from ocean surface waters and the atmosphere.

There is a similar amount of spatial variability involved with the depth of the seasonal snow pack as occurs with the thickness of the sea-ice that ranges from 100 mm w.e. in the interior steppe and tundra environments to more than 2000 mm w.e. in coastal and mountain regions at high latitudes. If the mean is estimated as 300 mm w.e., and using the area of seasonal snow cover from Chapter 1, it requires a total of 4.5 x 1021 J per year (142 TW) to melt the seasonal snowpack, which is less than half the energy required to melt the seasonal sea ice, all of which is derived from the energy budget of the atmosphere. This amount of energy is released to the troposphere every year through water vapour condensation and freezing/deposition of snow crystals, which is then consumed as the snow melts.

If a similar approach is taken for the seasonal freeze-thaw in the active layer of permafrost, assuming the depth of the active layer to be 1 m, of which 20 % is ice, the latent energy involved is an additional 44 TW. This takes the total energy cycled in seasonal snow and ice to about 536 TW. According to the author1 additional exchanges of latent energy are associated with ground that is frozen seasonally, lake ice and river ice; and these are difficult to estimate, though they are suggested by the author1 to probably contribute an additional 10-20 TW, a very large amount of energy. Only 3 % of the total annually available solar energy at the surface of the Earth is incident in the latitudes of 60o - 90o , both hemispheres combined, totalling about 2,400 TW for latitudes above 60o, the latitudes where most of the snow/ice melt energy is consumed in summer. Therefore about 20 % of the solar radiative energy that is available at these latitudes is represented by the latent energy sink.

The latent energy budget for the seasonal cryosphere over 1 year averages to near zero, therefore there is no net source or sink, which depends on the state of the cryosphere. An additional, unidirectional energy sink has been introduced over the past several decades with the melting of glaciers and ice sheets, the energy committed to this being consequential. The glaciers and ice sheets of the world melted at an average rate of about 750 GT per year in the period 2002-2009, is an example, the energy being required for this is 8 x 1012 W (8 TW). This cryospheric energy sink is added to by the thinning of Arctic sea ice. The reductions of the cryosphere are acting as a thermal buffer, reducing the degree to which the oceans and the atmosphere warms, though the efficacy of this buffer will decline as the extent of the cryosphere is reduced.


Sources & Further reading

  1. Marshall, Shawn J., 2012, The Cryosphere, Princeton University Press.
Author: M. H. Monroe
Last updated 23/04/2013

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