Understanding Heat Budget

The Earth’s climate is a solar powered system. Globally, over the course of the year, the Earth system—land surfaces, oceans, and atmosphere—absorbs an average of about 240 watts of solar power per square meter (one watt is one joule of energy every second). The absorbed sunlight drives photosynthesis, fuels evaporation, melts snow and ice, and warms the Earth system. The state of the global climate depends upon the balance of energy fluxes (flows) into and out of the climate system (made up of the atmosphere, oceans, ice masses, biosphere and geosphere). The most important component in this respect is the atmosphere, and the major energy fluxes of incoming sunlight and outgoing terrestrial radiation from the Earth, which must balance. The nature of the Earth’s atmosphere, with its greenhouse gases, also affects the state of the global climate.

The Sun doesn’t heat the Earth evenly. Because the Earth is a sphere, the Sun heats equatorial regions more than polar regions. The atmosphere and ocean work non-stop to even out solar heating imbalances through evaporation of surface water, convection, rainfall, winds, and ocean circulation. This coupled atmosphere and ocean circulation is known as Earth’s heat engine.

The climate’s heat engine must not only redistribute solar heat from the equator toward the poles, but also from the Earth’s surface and lower atmosphere back to space. Otherwise, Earth would endlessly heat up. Earth’s temperature doesn’t infinitely rise because the surface and the atmosphere are simultaneously radiating heat to space. This net flow of energy into and out of the Earth system is Earth’s energy budget. The Earth’s heat or energy budget is determined by the amount of sunlight that is either absorbed or reflected by the Earth’s varied surfaces.

When the flow of incoming solar energy is balanced by an equal flow of heat to space, Earth is in radiative equilibrium, and global temperature is relatively stable. Anything that increases or decreases the amount of incoming or outgoing energy disturbs Earth’s radiative equilibrium; global temperatures rise or fall in response.

The earth absorbs Insolation (it is a measure of solar radiation energy received on a given surface area in a given time) and re-emits outgoing terrestrial radiation and these two are in balance.

Radiation from the Sun and Earth

The Earth’s atmosphere has an important influence on the heat budget of the global climate system. This is determined by the processes involved in solar (Sun) and terrestrial (Earth) energy transfers. Radiation emitted from the Sun has a temperature of approximately 6000°C. The radiation is emitted over a spectrum of wavelengths, with a specific quantity of energy for each wavelength. Most solar energy is emitted with a wavelength of approximately 0.5 microns. This represents radiation in the visible part of the spectrum. The total energy output of the Sun is approximately 64 million Wm^-2.

The solar radiation disperses uniformly in all directions. After travelling 93 million miles, only a tiny fraction of the energy emitted by the Sun is intercepted by the Earth. Therefore, the energy arriving at the top of the Earth’s atmosphere is many orders of magnitude smaller than that leaving the Sun. The latest satellite measurements indicate a value of 1368Wm^-2 for the energy received at the top of the atmosphere. This is known as the solar constant.

The Earth also emits radiation, but since it is much cooler than the Sun, its radiating energy is in the longer wavelength, invisible infrared or heat part of the spectrum. Sometimes, we can indirectly see heat radiation, for example the heat shimmers rising from a tarmac road on a hot sunny day. The energy received by the Earth from the Sun balances the energy lost by the Earth back into space.

Heat / Energy Budget

Earth’s heat engine does more than simply move heat from one part of the surface to another; it also moves heat from the Earth’s surface and lower atmosphere back to space. This flow of incoming and outgoing energy is Earth’s energy budget. For Earth’s temperature to be stable over long periods of time, incoming energy and outgoing energy have to be equal. In other words, the energy budget at the top of the atmosphere must balance. This state of balance is called radiative equilibrium.

About 29 percent of the solar energy that arrives at the top of the atmosphere is reflected back to space by clouds, atmospheric particles, or bright ground surfaces like sea ice and snow. This energy plays no role in Earth’s climate system. About 23 percent of incoming solar energy is absorbed in the atmosphere by water vapor, dust, and ozone, and 48 percent passes through the atmosphere and is absorbed by the surface. Thus, about 71 percent of the total incoming solar energy is absorbed by the Earth system.

Processes in Atmosphere and surface of Earth

The atmosphere and the surface of the Earth together absorb 71 percent of incoming solar radiation, so together, they must radiate that much energy back to space for the planet’s average temperature to remain stable. However, the relative contribution of the atmosphere and the surface to each process (absorbing sunlight versus radiating heat) is asymmetric. The atmosphere absorbs 23 percent of incoming sunlight while the surface absorbs 48. The atmosphere radiates heat equivalent to 59 percent of incoming sunlight; the surface radiates only 12 percent. In other words, most solar heating happens at the surface, while most radiative cooling happens in the atmosphere. How does this reshuffling of energy between the surface and atmosphere happen?

Three processes of scattering, reflection and absorption take place in the atmosphere and surface of Earth

1. Scattering – It accounts for about 7% of insolation
   • Short wavelengths (insolation is of a short wavelength) are most easily scattered by gas and dust molecules in the atmosphere. This process involves redirecting the insolation towards a different direction.
2. Reflection – It accounts for about 27% of insolation, of which clouds 21% and ground 6%
   • Reflected insolation simply returns to space, wavelength unchanged. Clouds are especially effective in reflecting insolation
   • Reflection is dependent on albedo

• Albedo: the ratio of the light reflected by a body to the light received by it. Albedo values range from 0 (pitch black) to 1 (perfect reflector).
  • absorption of short-wave ultraviolet radiation by oxygen and nitrogen in the thermosphere and
  • absorption of longer-wave Ultra Violet radiation by ozone in the stratosphere

1. • Albedo of snow can reach 90%, and when the sun is at a low angle (e.g. sunrise/set) albedo of water can reach 80%. OTOH when the angle > 40° albedo is about 2-4% (see notes)
2. Absorption – It accounts for about 19% of insolation
   • Gases, clouds, dust and haze all absorb certain wavelengths of insolation, which is transformed into molecular motion resulting in a rise in temperature
   • Absorption is especially pronounced in the thermosphere and stratosphere which are warmer than expected because of the

1. • Water vapour is another main absorber of insolation
     

     Energy Budget of Surface of Earth

     For the energy budget at Earth’s surface to balance, processes on the ground must get rid of the 48 percent of incoming solar energy that the ocean and land surfaces absorb. Energy leaves the surface through three processes: evaporation, convection, and emission of thermal infrared energy.

     About 25 percent of incoming solar energy leaves the surface through evaporation. Liquid water molecules absorb incoming solar energy, and they change phase from liquid to gas. The heat energy that it took to evaporate the water is latent in the random motions of the water vapor molecules as they spread through the atmosphere. When the water vapor molecules condense back into rain, the latent heat is released to the surrounding atmosphere. Evaporation from tropical oceans and the subsequent release of latent heat are the primary drivers of the atmospheric heat engine

     An additional 5 percent of incoming solar energy leaves the surface through convection. Air in direct contact with the sun-warmed ground becomes warm and buoyant. In general, the atmosphere is warmer near the surface and colder at higher altitudes, and under these conditions, warm air rises, shuttling heat away from the surface.

     Finally, a net of about 17 percent of incoming solar energy leaves the surface as thermal infrared energy (heat) radiated by atoms and molecules on the surface. This net upward flux is actually the result of two large but opposing fluxes: heat flowing upward from the surface to the atmosphere and heat flowing downward from the atmosphere to the ground. Remember that the peak wavelength of energy a surface radiates is based on its temperature. The Sun’s peak radiation is at visible and near-infrared wavelengths. The Earth’s surface is much cooler, only about 15 degrees Celsius on average. The peak radiation from the surface is at thermal infrared wavelengths around 12.5 micrometers.

   • The Energy Budget of the Atmosphere

     The Earth, however, has an atmosphere, consisting mostly of nitrogen (78%), oxygen (21%) and a number of greenhouse gases, which affect the Sun-Earth energy balance. The average global temperature is in fact 33°C higher than it should be. Certain atmospheric gases absorb radiation at some wavelengths but allow radiation at other wavelengths to pass through unimpeded. The atmosphere is mostly transparent (little absorption) in the visible part of the spectrum, but significant absorption of solar ultra-violet radiation by ozone, and terrestrial infra-red radiation by water vapour, carbon dioxide and other trace gases occurs.

     The absorption of terrestrial infra-red radiation is particularly important to the energy budget of the Earth’s atmosphere. Such absorption by the trace gases heats the atmosphere, and so the Earth stores more energy near its surface than it would if there was no atmosphere. Consequently the temperature is higher by about 33°C.

     This process is popularly known as the greenhouse effect.

     Conclusion

     The fluxes or flows of energy within the Earth’s atmosphere determine the state of our climate. Factors which influence these on a global scale may be regarded as causes of global climate change. Basic principles determine the state of the Earth-Atmosphere energy budget, and consequently the global climate. The atmosphere’s greenhouse gases influence this energy balance by absorbing outgoing energy from the Earth, thereby increasing the global average surface temperature. This process is called the natural greenhouse effect. Through man-made emissions of greenhouse gases since 1765, this natural phenomenon is being enhanced with possible consequences of global warming

     References

     http://en.wikipedia.org/wiki/Earth%27s_energy_budget

     http://okfirst.mesonet.org/train/meteorology/EnergyBudget2.html

     http://earthobservatory.nasa.gov/

     http://www.fas.org/irp/imint/docs/rst/Sect14/Sect14_1a.html

     http://www.solarenergygreenlifestyleforyou.com/2008/04/solar-energy-from-sun-to-earth.html

     http://www.greenpeace.org/international/photosvideos/photos/greenhouse_effect