Environmental Engineering Reference
In-Depth Information
10.2
WHAT IS THE GREENHOUSE EFFECT?
The term greenhouse effect is derived by analogy to a garden greenhouse. There, a glass covered
structure lets in the sun's radiation, warming the soil and plants that grow in it, while the glass cover
restricts the escape of heat into the ambient surroundings by convection and radiation. Similarly,
the earth's atmosphere lets through most of the sun's radiation, which warms the earth's surface,
but certain gases, called greenhouse gases (GHG), trap outgoing
radiative
heat near the surface,
causing elevated surface temperatures.
The warming effect on the earth's surface by certain gases in the atmosphere was first recog-
nized in 1827 by Jean-Baptiste Fourier, the famous French mathematician. Around 1860, the British
scientist John Tyndall measured the absorption of infrared radiation by CO
2
and water vapor, and
he suggested that the cause of the ice ages may be due to a decrease of atmospheric concentrations
of CO
2
. In 1896, the Swedish scientist Svante Arrhenius estimated that doubling the concentration
of CO
2
in the atmosphere may lead to an increase of the earth's surface temperature by 5-6
◦
C.
The major natural GHG are water vapor and carbon dioxide. If not for these gases, the earth
would be quite inhospitable, with a temperature well below freezing. So, why are we concerned
about adding more man-made GHG to the atmosphere? Will not the earth become even more
comfortable to living creatures? The answer is that humans and ecological systems have adapted to
present climatic patterns. Perturbing those patterns may result in unpredictable climatic, ecologic,
and social consequences.
10.2.1
Solar and Terrestrial Radiation
The sun emits electromagnetic radiation ranging from very short wavelength gamma rays, X rays,
ultraviolet through visible, to infrared radiation. A section of the solar spectrum reaching the earth,
which contains the near-UV, visible, and near-IR spectrum up to 3.2
m, is shown in Figure 10.1.
Three spectra are presented. The uppermost curve gives the solar irradiance outside the earth's
atmosphere; the lowest curve is the spectrum of incident radiation at sea level. The dashed curve,
which follows closely the upper curve, is the spectral
radiance
of a black body heated to a tem-
perature of 5900 K, scaled to equal the total solar
irradiance
at the earth.
4
This tells us that the
sun's surface temperature is approximately 5900 K (the interior of the sun is much hotter, by many
millions K, because of the nuclear fusion reactions occurring there). A black body heated to 5900 K
emits its peak radiance in the visible portion of the electromagnetic spectrum, at about 500 nm
(0.5
µ
m). The reason the solar irradiance curve does not follow the black-body radiance curve
exactly is that at some wavelengths there is excess radiance, at others a deficit.
5
The solar irradiance spectrum at sea level (lower curve) is much different from the spectrum
at the top of the atmosphere. This is because gases in the earth's atmosphere absorb or scatter
µ
4
A black body is a perfect radiator. It absorbs all the incident radiation of any wavelength, and it emits
radiation at any wavelength commensurate with its temperature. A radiation field enclosed by a black body
of temperature
T
has an energy density (and distribution of that energy with wavelength) that depends only
upon the temperature and not on any other characteristics of the black body.
5
Excess radiance results from excitation of atoms and ions in the solar corona (e.g., the sodium-D lines around
589 nm). Deficits result from self-absorption of the incident radiation by other gaseous atoms and ions which
are also present in the solar corona. These are called Fraunhofer lines.
