Energy Balance

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The energy balance of any object, be it a small piece of ice floating on the ocean or an entire planet, refers to the net rate at which energy is absorbed or lost. If the object receives more energy than it loses, the balance is positive; the extra energy is generally stored as thermal energy, increasing the object's temperature. The energy could also be used to evaporate water or to melt ice or snow, so that the extra energy is converted to latent heat (energy needed for melting or evaporation). The energy balance of several objects will be described first for the Earth as a whole, secondly for the Arctic regions, and finally for the surface of the ground or ocean in the Arctic.

There are only two important elements in the energy balance of the Earth as a whole: the incoming solar shortwave radiation and the outgoing thermal longwave radiation. The amount of solar radiation that is absorbed depends on the energy received from the sun, and is also strongly affected by the reflectivity, or albedo, of the planet. The albedo is the fraction of the solar radiation which is reflected. Clouds, snow, and sea ice are all highly reflective and reduce the total solar energy absorbed. The thermal energy emitted by the planet is a strong function of the temperature of the emitting surfaces. Cold objects emit much less energy than warmer objects. The thermal radiation is also dependent on the nature of the surface (the emissivity); most objects in nature have a high emissivity (close to 1.0, meaning they emit as much as is possible). The energy balance of the planet as a whole is close to zero, since the mean temperature is not changing rapidly. The average temperature of the planet and the thermal energy lost have adjusted to just the value needed to balance the absorbed solar energy; the nature of the energy balance is not uniform over the planet.

Because of the spherical shape of the Earth and the inclination of the Earth's axis, more solar radiation, on average, is absorbed near the equator than at the poles. The tropics are consequently warmer and emit more thermal energy to space than the polar regions, but not enough more to balance the solar input. Additional energy passes out from the tropics toward the poles, not by radiation, but by movement of the atmosphere and oceans. This transfer of energy occurs in three ways. One, referred to as sensible heat, is achieved by warm air moving toward the poles and cool air moving toward the tropics. A second is similar, but occurs in the ocean and is accomplished by global-scale ocean currents. The third is in the form of water vapor, and is called latent heat. Tropical water absorbs energy in the process of evaporation, the moist air moves toward the poles, and energy is released again when the water condenses to form new clouds. These processes of energy transfer, from the tropics to the poles, are the primary causes of most of the Earth's wind and weather.

In the Arctic regions, the energy balance has a pronounced seasonal cycle, with strong solar radiation in the summer and almost none in the winter. The outgoing thermal radiation increases in the summer to about the same level as the absorbed solar flux, and in the winter drops by about 25% from its summer values. This gives a tremendous net radiative loss of heat during the winter, amounting to about 150 Wm-2 (think of the heat that would be produced by a 150 W light bulb on every square meter). This loss is balanced by the cooling of the air, land, and sea as well as by the transport of sensible and latent heat from lower latitudes to higher latitudes in the atmosphere and oceans. Of these, the transport of sensible heat by the atmosphere is the most important.

At the local scale, the energy balance depends on the nature of the surface (vegetation, soil, snow, ice, or water) and where it is located. At the surface of the Earth, the terms of the energy balance are the down-welling solar flux, the reflected solar flux, the absorbed downwelling thermal flux (from the atmosphere and clouds above), the emitted thermal flux, the sensible heat passed from or to the atmosphere, the latent heat produced by evaporation or condensation of moisture on the surface, the latent heat of melting of ice or snow at the surface, and, finally, the heat conducted to or from the substrate below. The energy balance at the surface must be close to zero, since no energy can be stored in the thin layer right at the surface, and so the conducted heat will compensate for the other terms of the energy balance and also warm or cool the substrate. One factor that greatly influences the energy balance is the albedo. Snow- or ice-covered surfaces are much more reflective than bare soil, vegetation, or open water, and they reduce the absorbed solar energy during the months when the sun is up. Clouds are also very important in terms of their role in obscuring the sun and reducing the downwelling solar flux. Moisture in the air can play a very important role in melting snow. When warm, moist air passes over snow, there is a rapid condensation of moisture on the cold snow surface and the release of large quantities of latent heat, which can melt the snow. The conductivity of the surface is important, because it determines how rapidly heat can be brought to the surface to compensate a net surface energy loss. Snow has low conductivity and reduces the rate of heat conduction, allowing the surface temperature to drop faster than for bare soil, water, or ice surfaces.

The energy balance over the oceans differs from that over land; the oceans can produce or absorb a large amount of energy without substantially changing the water temperature, and because the latent heat term is often very large. The range of temperatures seen over oceans is generally much smaller than seen over land. In the Arctic Ocean, the summer temperature never rises much above the freezing point because the large absorbed solar flux is balanced, to a large extent, by the latent heat used for melting ice. In the winter, the surface temperature drops to -30°C to -40°C, which is cold, but not as cold as the -70°C sometimes seen over land in Greenland, Siberia, Alaska, or Canada. The large net loss of radiative thermal energy over the ice is balanced mostly by the latent heat released by freezing sea water, heat that is then conducted through the sea ice to the surface. This freezing and ice growth is largest where the ice is thin and where there is little snow. The albedo of the ice-covered oceans and snow-covered land plays an important role in climate change, because as ice extent and snow cover are reduced by warming, the much lower albedo of the open water or bare land allows for increased absorption of solar energy, leading to yet more warming. This cycle is called the ice-albedo feedback.

Understanding the energy balance of a planet, region, or locale is fundamental for understanding its climate and how it might change.

R.W. Lindsay

See also Albedo Further Reading

Curry, J.A., J. Schramm & E.E. Ebert, "On the sea ice albedo climate feedback mechanism." Journal of Climate, 8: (1995): 240-247 Peixoto, J.P. & A.H. Ort, Physics of Climate, New York: American Institute of Physics, 1992

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