The energy balance of the Earth refers to the interception of solar radiation by the Earth, its distribution by the ocean, atmosphere, land, and biosphere, and the ultimate release of heat energy back to space. Among many other vital functions, the Earth's energy balance heats the Earth to temperatures far above the minus 454 degrees Fahrenheit (3 degrees Kelvin) of deep space. An understanding of the Earth's energy balance is necessary to understand how our climate is determined, and the role that human activity plays in the climate system.

The process is relatively simple in concept. The Sun provides the energy that drives the climate system. Different parts of the climate system absorb the solar energy-clouds, ocean, land, etc. The absorbed energy is converted to heat that warms the Earth and makes it habitable. The absorption of solar energy is uneven in both space and time, producing the distinctive pattern and seasonal variation of our climate.

Energy from the Sun

A thermonuclear fusion reaction in the Sun fuses hydrogen into helium and releases huge quantities of energy-about 4 x 10²⁶ watts, which is about 1.8 x 10¹⁶ times greater than the power generated by the Three Gorges Dam in China, the world's largest power plant. The energy created by the fusion reaction is converted into thermal energy (heat) and raises the temperature of the Sun to about 6,000°C (11,000°F). The energy flow at the surface of the Sun is about 63 x 10⁶ W/m². The solar heat energy travels through space in the form of electromagnetic waves that the transfer the heat through a process known as radiation. Hence the term "solar radiation" given to the energy released by the Sun.

Some of the various types of electromagnetic radiation as defined by wavelength. Visible light has a spectrum that ranges from 0.40 to 0.71 micrometers (µm). (Source: PhysicalGeography.net) Solar radiation arrives at the Earth in a wide range of wavelengths. The incoming solar radiation is centered on the wavelength band of 0.2-2 micrometers (1 micrometer=one millionth of a meter). The principal range of received radiation includes ultraviolet radiation (UV, 0.001-0.4 micrometers), visible radiation (light, 0.4-0.7 micrometers), and infrared radiation (IR, 0.7-100 micrometers).

Energy Transfer from the Sun to the Earth

The rate of solar energy flow diminishes considerably as it travels through space towards Earth. When it reaches the outer portion of the Earth's atmosphere, the energy flow is about at 1,366 watts per square meter, averaged over the entire year. This quantity is known as the solar constant. The term "constant" is used because this quantity has changed relatively over the last few hundred years. However, over longer periods of time-so-called "geologic time"-there are larger variations in the quantity of solar energy reaching the Earth, which in turn have had major impacts on climate.

The Earth has a cross section of about 127,400,000 km², which means the total solar power intercepted by the Earth is about 1.7 x 10¹⁷ W. By comparison, the capacity of all the electric power generation stations in the world is about 3.7 terawatts (TW), or 3.7 x 10¹² watts. Thus, the Sun delivers nearly 46,000 times more energy than the world's electric power consumption.

Earth's Orbit. Source: NASA

Equatorial regions receive more solar energy than do the polar regions because the Earth is a sphere, and because the Earth's axis tilts at an angle of 23.5° with respect to its plane of orbit. The amount of solar heating of the polar latitudes also varies greatly throughout the year. In the summer, polar latitudes receive almost as much solar energy as do the tropics, while in the winter they receive no solar heat at all. Meanwhile, the tropics receive a relatively constant amount of solar energy. The large difference in solar heating between the equator and the pole is the driving force behind the massive poleward transfer of heat by global atmospheric and ocean circulation. This pattern of heating of the Earth and its atmosphere drives large-scale atmospheric circulation patterns, and even the seasons themselves. The difference in solar heating between day and night also drives the strong diurnal (daily) cycle of surface temperature over land.

Energy from Earth and Earth's Temperature

Approximately 30% of incoming shortwave solar radiation energy is scattered or reflected back to space by molecules, tiny airborne particles (known as aerosols), and clouds in the atmosphere or by the Earth's surface. This reflected light is what enabled the Earth to be seen from space, just as we can see the moon. The total reflection by the Earth is a quantity known as the planetary albedo. The term has its origins from a Latin word albus, meaning "white," and is defined as the proportion, or percentage of solar radiation of all wavelengths reflected by a body or surface to the amount incident upon it. An ideal white body has an albedo of 100% and an ideal black body, 0%. Albedo values for the Earth's surface range between 3% for water to over 95% for fresh snow.

Earth's energy balance. Source: Physcisworld.com

The remaining 70% of incoming shortwave solar radiation energy is absorbed by the Earth's surface and atmosphere. The absorbed energy amounts to approximately 235 W^-2. All heated objects release electromagnetic radiation, especially so if, like, the Earth, they are surrounded by empty space. This energy is referred to as outgoing radiation. An object will continue to warm if the incoming radiation exceeds the outgoing radiation. In turn, this will increase the outgoing radiation according to an important principle in physics known as the Stefan-Boltzman law. This law tells us that the release of heat from a body increases much faster than its temperature. An equilibrium temperature is reached when the outgoing radiation equals the incoming radiation.

The Greenhouse Effect

If the Earth were a blackbody (a body that absorbs all radiation that falls on it), its equilibrium temperature would be -18 °C. Scientists call this Earth's effective temperature. In reality, the Earth is much warmer than that. Why?

Greenhouse effect.Source: Ocean World, Texas A and M University

The dominant gases of the atmosphere (nitrogen and oxygen) are transparent to outgoing longwave radiation in the infrared range. However, water vapor, carbon dioxide, and methane are so-called "greenhouse gases" that absorb infrared energy. This energy is then held as thermal energy, increasing the temperature between the ground and the lower 10 kilometers of the atmosphere. But the absorption slows the passage of the outgoing infrared radiation and warms the atmosphere, analogous to the effects of a greenhouse. Hence the term "greenhouse effect." Without the greenhouse effect, the Earth would be 33ºC cooler; that is, the average temperature of the Earth would be about -18ºC as opposed to 15ºC.

Note that the greenhouse effect does not put the Earth's energy budget "out of balance." The heat is held temporarily in the atmosphere, and then eventually is released from the atmosphere as long wavelength infrared. In effect, the greenhouse effect causes the planet to raise its surface temperature until the amount of heat radiated from the top of the absorbing layer of the atmosphere is equal to the solar radiation at the top of the atmosphere. The effective temperature (-18°C) is reached at the top of the absorbing layer, while down at the surface of the Earth it is much warmer.

The greenhouse effect and other components of the Earth's energy balance is central to climate change science. Human activity such as land use change and the combustion of fossil fuels affect the albedo of the planet, release greenhouse gases, and other wise perturb the climate system. Understanding the important details of the complex system is one of the great challenges facing the world's scientific community.

Sources

• Columbia University, Department of Earth and Environmental Sciences, Solar Radiation and the Earth's Energy Balance, Accessed 20 August 2008.

• Pidwirny, Michael (Lead Author); Dagmar Budikova (Topic Editor). 2008. Earth's energy balance. In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth October 18, 2006; Last revised July 10, 2008; Retrieved August 21, 2008]. 

• Trenberth, Kevin E., Earth's Energy Balance. In: Cutler J. Cleveland, Editor(s)-in-Chief, Encyclopedia of Energy, Elsevier, New York, 2004, Pages 859-870.