The Greenhouse Effect
Imagine placing the Earth in a glass bottle where the only possible energy transfers are through electromagnetic radiation. Solar radiation either passes through the bottle without change; bounces off the bottle, changing direction but not energy; or is absorbed by the content of the bottle. The radiation absorbed stimulates the motion of molecules in the bottle, increasing the temperature of the bottle and its contents.
At a higher temperature, the bottle and its contents emit more electromagnetic radiation than at a cooler temperature. Radiation absorbed by the bottle will warm it and its contents, and therefore will emit still more electromagnetic radiation. Eventually, all of the solar energy that enters the bottle escapes. The conservation of energy law dictates that the energy coming in must equal the energy going out. Most of the energy enters as visible light (shortwave radiation), while most of the energy escapes as infrared wavelengths (longwave radiation) that are not absorbed by the bottle or its contents.
An annual energy budget, which divides the planet into the atmospheric components of atmosphere, clouds and surface, takes into account these major energy transfers. On average, Earth receives 341 W m-2 of electromagnetic radiation from the sun annually. Of this radiation, about 30% is reflected. Atmospheric components and the Earth’s surface absorb 78 W m-2 and 161 W m–2, respectively, of the remaining 239 W m–2.
Without greenhouse gases such as carbon dioxide (CO2), methane (CH4), or nitrous oxide (N2O), Earth’s temperature would cool to an average of about –18°C (255 K), rather than the 14°C (287 K) that we currently enjoy. Such a greenhouse-gasless world would approximate a perfect blackbody having a maximum energy emission at a wavelength of 11.4 µm, in contrast to the 10.1 µm actually observed. Greenhouse gases also differ in their concentrations and chemistry in Earth’s atmosphere. All greenhouse gases participate in chemical reactions that limit their residence time in the atmosphere. For example, CO2 dissolves in water and forms carbonic acid. We can compare the “warming potential” of greenhouse gases relative to CO2 based on their mean residence time and absorption characteristics. The total radiative effect of a greenhouse gas thus depends on its concentration and warming potential.
In summary, greenhouses gases do not influence the amount of electromagnetic energy that Earth emits to outer space. Rather, they alter the temperature of the surface and the wavelengths that escape. Higher concentrations of greenhouse gases warm the surface and shift outgoing radiation to wavelengths at which the greenhouse gases are transparent. For example, higher concentration of CO2 in the atmosphere would increase the depth of the “notch” at 15 µm, but this would be compensated by a warmer planet that would emit more radiation at shorter wavelengths.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents
CitationBloom, A. (2012). The Greenhouse Effect. Retrieved from http://www.eoearth.org/view/article/51cbf0407896bb431f6a0e71
That is a nice summary. However, the 18 C figure to which the Earth would cool without a greenhouse effect should have a minus sign in front of it - i.e., 255 K = -18 C. Also, while it's true that at radiative equilibrium, the radiation the Earth emits to space must equal the amount of solar energy it absorbs, it's not correct that changes in greenhouse gases have no influence on the radiation emitted to space. An increase in CO2, for example, will temporarily reduce emissions to space, allowing the climate to warm. At equilibrium, the incoming absorbed and outgoing emitted radiation will once again be equal as a warmed climate causes emitted radiation to rise back toward a balance with incoming radiation, but no longer at the same level as before (currently about 239 W m-2), because the warmer temperature will have melted some reflective ice and snow, so that the absorbed radiation will exceed the 239 W m-2. Changes in clouds will also affect the level at which incoming absorbed and outgoing emitted radiation are in balance.