LETTERS TO THE EDITOR
Fig. 1 - Effect of doubling the atmospheric content of CO2 on the width and depth of the CO2 15µm absorption trough. The broad arrows emphasize the widening of the feature at higher CO2 levels. Prepared by JLBerry after figures in Zhong and Haigh, 2013.
Sometimes I see statements that increasing levels of CO2 and methane will not have any further effect on
Earth’s climate, because the major CO2 absorption band near the wavelength of
the peak of Earth’s emitted radiation, 15 µm, is fully saturated (i.e. can absorb no more energy).
The average albedo of the earth is about 30%, so 70 percent of the sun’s radiation is absorbed, directly and indirectly (through plants) heating the Earth’s surface. This is re-radiated at longer wavelengths, in the infra-red part of the spectrum, with a peak at 15 (microns, 10-6m). Carbon Dioxide in the atmosphere absorbs radiation in several bands near this peak (the bands are centered at 9.4µm, 10.4µm, and 11.4- 20.0µm, thus heating the atmosphere.
Fig.1 is a schematic rendering of the spectrum of outgoing fluxes at the top of the atmosphere with current concentra-
tions of O3, CH4 and N2O (long-dashed curve). The area under the curve is pro- portional to the flux of outgoing energy. The solid curve shows the shape and
size of the CO2 absorption band at cur- rent CO2 levels. Note that it has a ‘W’
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Editor’s Note: The connection between rising CO2 levels and climate warming References
shape with a narrow central peak at wavenumber 680 cm-1 or 15 µm. The
absorption of CO2 in the atmosphere is saturated in the center of the absorption (the twin bottoms of the ‘W’). The effect
of increasing levels of CO2 is to increase absorption in the wavelengths of the outer limbs of the ‘W’, regions in which
CO2 is less strongly absorbed. As that happens, even though the ‘W’ cannot get deeper, it gets wider as more and more energy is absorbed at the wavelengths of its outer limbs. Its area on the graph
is thus increased, meaning that CO2 is absorbing more of the Earth’s outgoing
radiation (8% if CO2 doubles versus 6% at present levels of CO2) hence warming the atmosphere more than before.
Of course, this is a gross oversimplifi- cation. The absorption band structure of
CO2 is more complicated. There also for- ward feedback mechanisms (a warming planet means a more moist atmosphere, which further speeds things up because water vapor is a better absorber than
CO2). Cloud cover may be expected to increase, and we do not yet understand the effect of clouds on the Earth’s radia- tion budget very well.
Jul.Aug.Sep 2019 • TPG 7
Wolfe, WL, and George J. Zissis, 1978, The InfraRed Handbook, Revised Edition, 1985. Prepared for Office of Naval Research, Department of the Navy, by Environmental
Research Michigan. P. 5-74. Institute of This book pre-dates
most of the controversy about CO2 and was produvced for the Navy. However its
discussion of the absorption of CO2 by the atmosphere is complicated, and the use of
units in the illustrations is inconsistent.
Zhong, Wenyi, and Joanna D. Haigh, 2013, The Greenhouse Dioxide.
Effect and Carbon Royal Meteorogical Society:
Weather, v.68, no.4, pp. This article gets into the gory guts of the topic: the languag- es is clear but it requires a lot of patience to understand many of the figures.
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