Atmospheric Emission of Infrared Radiation

the earth's climate system is characterized by the energy balance of the Earth, the distribution of energy in space (or Earth's atmosphere), and temporal energy variation. The Earth's radiative energy balance is governed by the balance between the solar radiation and absorption by the Earth, and subsequent radiation from the Earth to outer space. The absorbed solar shortwave radiation by Earth is emitted back to outer space in thermal infrared or longwave radiation to maintain the Earth's heat energy balance. The imbalance (surplus) in the Earth's energy budget leads to global warming.

In the 20th century, according to global temperature records, the near surface of the Earth warmed by nearly 1.0 degree F (0.6 degrees C). This global warming is due to anthropogenic climate-forcing agents, such as greenhouse gases (GHG) including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3), water vapor, and other gases. These climate-forcing agents block, trap, or absorb the thermal infrared energy radiated from the Earth to space. These agents also have energy-scattering ability. The absorbed thermal infrared energy is later radiated back to the Earth making the Earth warmer (the greenhouse effect). These climate-forcing agents also absorb solar shortwave energy and radiate that to the Earth in a later stage, thus warming the Earth's surface.

According to climatologic research, water vapor (without clouds), CO2, CH4, and O3 cause about 36-70 percent, 9-26 percent, 4-9 percent, and 3-7 percent of the greenhouse effect on the Earth, respectively. The higher ends of the ranges quoted here are for the gas alone and the lower ends are for overlapping. Water vapor is a profound contributor to the greenhouse effect. Unfortunately, based on evaporation feedback, the amount of water vapor in the atmosphere is increasing with passing years.

Climate forcing is an imposed, natural, or anthropogenic perturbation of the Earth's energy balance with space. Anthropogenic greenhouse gases such as CO2, CH4, O3, N2O, chlorofluorocarbons (CFCs), and black carbon particles cause the largest positive climate forcing, warming up the Earth. CO2 is the greatest climate-forcing agent as its absorptive and radiative power of atmospheric infrared energy is more than any other greenhouse gases. Black carbon produced by forest burning, emissions from thermal power plants, and volcanic activities has the next highest climate-forcing effect. CH4 and O3 also contribute significantly to global warming.

Climate forcing agents' ability to absorb and emit back to earth thermal infrared or longwave is also measured in terms of global warming potential (GWP). GWP is a relative scale: GWP of a climate-forcing agent is compared to the same mass of CO2. GWP is mostly based on the radiative properties of the gases and how they impact the climate system, especially the warming of the surface of the Earth. Of course, each greenhouse gas does not absorb the Earth's radiated heat with similar potential. It also depends upon the rate of decay of each gas relative to CO2. GWP is the measure of relative radiative effect of a given gas compared to another (CO2) over a particular time horizon, such as 50, 100, 500, or 1,000 years. Hydrofluorocarbons (HFCs) and Perfluorocar-bons (PFCs) are the most heat-absorbent. Methane and N2O are more than 21 times and 270 times more heat-absorbent than CO2.

It is essential to reduce the amount of positive climate-forcing agents, or GHGs, from the atmo sphere to at least slow down the global warming process. The proposed Kyoto Protocol is designed to slow emissions of several of these positive climate-forcing agents.

SEE ALSO: Greenhouse Effect; Greenhouse Gases; Kyoto Protocol; Radiation, Absorption; Radiation, Infrared.

BIBILIOGRAPHY. American Geophysical Union, "Water Vapor in the Climate System," www.agu.org (cited November 2007); James E. Hansen and Makiko Sato, "Trend in Measured Climate Forcing Agents," Proceedings of the National Academy of Sciences of the United States of America (v.98/26, 2001); J.T. Houghton et al., eds., Climate Change 2001: The Scientific Basis (Cambridge University Press, 2001); Intergovernmental Panel on Climate Change (IPCC), "Radiative Forcing of Climate Change," www.grida .no (cited August 2007); Avi Singhal, "Stochastic Mathematical Modeling of Thermodynamics of Global Warming Related to Greenhouse Effects," www.eas.asu.edu (cited November 2007).

SUDHANSHU SEKHAR PANDA Gainesville State College

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