From Global Warming Art
This figure shows the absorption bands in the Earth's atmosphere (middle panel) and the effect that this has on both solar radiation and upgoing thermal radiation (top panel). Individual absorption spectrum for major greenhouse gases plus Rayleigh scattering are shown in the lower panel.
Both the Earth and the Sun emit electromagnetic radiation (e.g. light) that closely follows a blackbody spectrum, and which can be predicted based solely on their respective temperatures. For the sun, these emissions peak in the visible region and correspond to a temperature of ~5500 K. Emissions from the Earth vary following variations in temperature across different locations and altitudes, but always peak in the infrared. The bulk of emissions from the Earth radiate from within the colder regions of the atmosphere rather than from the surface directly, and give the Earth an average emission temperature of about 250 K (-20 °C) (Kiehl and Trenberth 1997).
The position and number of absorption bands are determined by the chemical properties of the gases present. In the present atmosphere, water vapor is the most significant of these greenhouse gases, followed by carbon dioxide and various other minor greenhouse gases. In addition, Rayleigh scattering, the physical process that makes the sky blue, also disperses some incoming sunlight. Collectively these processes capture and redistribute 25-30% of the energy in direct sunlight passing through the atmosphere. By contrast, the greenhouse gases capture 70-85% of the energy in upgoing thermal radiation emitted from the Earth surface. This disparity is a major factor in creating the greenhouse effect, whereby thermal energy is trapped near the Earth's surface warming the planet.
The greenhouse effect is an essential physical process, mediated primarily by water vapor, that warms the Earth approximately 32 °C (Lashof 1989), allowing the planet to be habitable and maintain abundant liquid water. By contrast global warming, involves a small increase in average temperatures mediated in large part by a few percent increase in the greenhouse effect. This increase is primarily caused by large increases in carbon dioxide (+35% concentration since 1700), methane (+150%) and nitrous oxide (+20%) . These gases, in combination with a water vapor feedback , provide the small increase in the greenhouse effect blamed for anthropogenic global warming.
It should be noted that while some bands are saturated (i.e. 100% of radiation in that band is absorbed), that does not imply that further increases in greenhouse gas concentration have no effect on that band. Rather additional greenhouse gases will cause the radiation to be captured closer to the Earth's surface which still increases warming. However rather than doubling as concentrations double, the overall effect proceeds only by small increments giving rise to a logarithmic progression rather than a linear one.
Data sources and notes
The data used for these figures is based primarily on Spectral Calculator of GATS, Inc. which implements the LINEPAK system of calculating absorption spectra (Gordley et al. 1994) from the HITRAN2004 (Rothman et al. 2004) spectroscopic database. To aid presentation, the absorption spectra were smoothed. Features with a bandwidth narrower than 0.5% of their wavelength may be obscured.
Calculations were done on the assumption of direct vertical transmission through an atmosphere with gas concentrations representative of modern day averages. In particular, absorption would be greater for radiation traveling obliquely through the atmosphere as it would encounter more gas.
The total scattering and absorption curve includes only the components indicated in the lower panel. These represent the vast majority of absorption contributing to the greenhouse effect and follow the treatment of Peixoto and Oort (1992), but other minor species such as carbon monoxide, nitric oxide and chloroflourocarbons (CFCs) have been omitted. Also omitted was scattering due to aerosols and other sources besides Rayleigh scattering.
The peaks in the blackbody spectra were adjusted to have the same height for ease in presentation.
Images that include only the top or bottom portion of this figure are also available:
This figure was prepared by Robert A. Rohde.
- [abstract] Gordley, Larry L., Benjamin T. Marshall, Allen D. Chu (1994). "LINEPAK: Algorithms for modeling spectral transmittance and radiance". Journal of Quantitative Spectroscopy & Radiative Transfer 52 (5): 563-580.
- [abstract] [ Kiehl, J. T. and Trenberth, K. E. (1997). "Earth's Annual Global Mean Energy Budget". Bulletin of the American Meteorological Association 78: 197-208.
- Daniel A. Lashof (1989). "The dynamic greenhouse: Feedback processes that may influence future concentrations of atmospheric trace gases and climatic change". Climatic Change 14 (3): 213-242.
- [abstract] [ L.S. Rothman, D. Jacquemart, A. Barbe, D. Chris Benner, M. Birk, L.R. Brown, M.R. Carleer, C. Chackerian Jr., K. Chance, L.H. Coudert, V. Dana, V.M. Devi, J.-M. Flaud, R.R. Gamache, A. Goldman, J.-M. Hartmann, K.W. Jucks, A.G. Maki, J.-Y. Mandin, S.T. Massie, J. Orphal, A. Perrin, C.P. Rinsland, M.A.H. Smith, J. Tennyson, R.N. Tolchenov, R.A. Toth, J. Vander Auwera, P. Varanasi, G. Wagner (2004). "The HITRAN 2004 molecular spectroscopic database". Journal of Quantitative Spectroscopy & Radiative Transfer 96: 139-204.
- Peixoto, Jose P. and Abraham H. Oort (1992). Physics of Climate. Springer. ISBN 0883187124.
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