Energy BuFFER

Melting snow and ice requires a great deal of energy. These phase changes consume sensible heat and radiative energy that would otherwise go into warming up a region. The opposite effect attends the fall freeze-up: latent energy is released to the atmosphere, lakes, rivers, oceans, and soil as ice forms from water and vapor. This delays the seasonal decrease in temperatures. Overall then, latent energy exchanges within the seasonal cryosphere act as a thermal buffer, similar to the way in which ocean heat capacity moderates the climate of marine environments.

The amount of energy involved in these phase changes can be estimated from the extent of the seasonal cryosphere, although the area of the seasonal sea ice and snow cover is better known than the volume. Assuming average thicknesses of 1.5 m and 2 m for the ice cover that melts each summer in the Southern Hemisphere and Northern Hemisphere, respectively, the average (1979-2010) sea-ice contribution to this latent energy budget is approximately 11 X 1021 J per year (11 ZJ), equivalent to 349 TW (table 8.1). This neglects the energy required to warm the ice to the melting point. Compare this with a global energy consumption of 14.8 TW in 2009. This latent energy is drawn from ocean surface waters and the atmosphere.

The depth of the seasonal snowpack has a similar amount of spatial variability to sea-ice thickness, ranging from less than 100 mm w.e. in interior steppe and

Table 8.1

Estimated Latent Energy Required to Freeze/Melt the Seasonal Components of the Cryosphere Based on the Average Minimum and Maximum Sea-Ice Area (1979-2010) and Snow-Covered Area (1967-2010)

Table 8.1

Estimated Latent Energy Required to Freeze/Melt the Seasonal Components of the Cryosphere Based on the Average Minimum and Maximum Sea-Ice Area (1979-2010) and Snow-Covered Area (1967-2010)

Component

Area (106 km2)

(m w.e.)

Em (1021 J yr-1)

Pm (102 W)

Seasonal snow

44.8

0.3

4.50

142

Northern Hemisphere

sea ice

8.8

1.8

5.31

168

Southern Hemisphere

sea ice

12.6

1.35

5.70

181

Active layer

22.8

0.18

1.37

44

Total

16.87

536

Note: H is an estimate of the average snow/ice thickness, Em is the latent melt energy, and Pm is the average annual rate of transfer of latent energy, in TW.

Note: H is an estimate of the average snow/ice thickness, Em is the latent melt energy, and Pm is the average annual rate of transfer of latent energy, in TW.

tundra environments to more than 2000 mm w.e. in high-latitude coastal and mountain regions. Taking 300 mm w.e. as an estimate of the mean and using the area of seasonal snow cover from chapter 1, melting of the seasonal snowpack requires a total of 4.5 X 1021 J per year (142 TW). This is less than half of the energy consumed by melting of seasonal sea ice, but it is all derived from the atmospheric energy budget. This amount of energy is released to the troposphere each year through condensation of water vapor and freezing/deposition of snow crystals, and then consumed during melt.

Taking a similar approach for the seasonal freeze-thaw of the active layer in permafrost, with the assumption of an average active layer depth of 1 m, with 20%

ice content, active layer phase changes involved an additional 44 TW of latent energy. The total latent energy cycled in seasonal snow and ice is therefore about 536 TW. Additional latent energy exchanges are associated with seasonally frozen ground, lake ice, and river ice; these are difficult to estimate but likely contribute an additional 10-20 TW. This is an enormous amount of energy. Only 3% of the total solar energy available annually at the Earth surface is incident in the latitudes 60° to 90° (both hemispheres combined). This represents about 2400 TW in latitudes above 60°, where the majority of the snow/ice melt energy is consumed each summer. The latent energy sink therefore represents about 20% of the available solar radiative energy at these latitudes.

Note that for all of the seasonal cryosphere, the latent energy budget averages to near zero over a year (no net source or sink), depending on the state of the cryosphere. This foreshadows discussions of cryospheric change in chapter 9. Melting of glaciers and ice sheets over the past several decades has introduced an additional, unidirectional energy sink. The energy committed to this is consequential. For the period 2002-2009, for instance, the world's glaciers and ice sheets melted at an average rate of about 750 Gt per year. The energy required for this amounts to 8 X 1012 W (8 TW). Thinning Arctic sea ice and permafrost add to this cryospheric energy sink. These reductions in the cryosphere are acting as a thermal buffer that reduces the severity of atmospheric and oceanic warming. The efficacy of this buffer will diminish as the cryosphere contracts.

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