Principle 4. Carbon dioxide is a greenhouse gas.

This entry was compiled, edited and written by: Cutler Cleveland

The redistribution of fossil carbon (from fossil fuels) among the atmosphere, oceans, and land is significant from a climate perspective because carbon dioxide is a greenhouse gas. A greenhouse gas is any of the gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of infrared radiation emitted by the Earth's surface, the atmosphere, and clouds. This causes the so-called "greenhouse effect."

The greenhouse phenomenon is part of the energy balance of the Earth, which 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, and thus makes life possible.

Energy from Earth and Earth's Temperature

 Physcisworld.comEarth's energy balance. Source: Physcisworld.com

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.

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?

 Ocean World, Texas A and M UniversityGreenhouse 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. 
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