Page 23 - Applied Photovoltaics

1.6 THE GREENHOUSE EFFECT
To maintain the earth’s temperature, energy reaching the earth from the sun must
equal energy radiated back out from the earth. As with incoming radiation, the
atmosphere interferes with outgoing radiation. Water vapour absorbs strongly in the
4–7 ȝm wavelength band and carbon dioxide in the 13–19 ȝm wavelength band. Most
outgoing radiation (70%) escapes in the ‘window’ between 7 and 13 ȝm.
If we had no atmosphere, as on the moon, the average temperature on the earth’s
surface would be about –18°C. However, a natural background level of 270 ppm CO 2
in the atmosphere causes the earth’s temperature to be about 15°C on average, 33°C
greater than the moon’s. Fig. 1.9 shows the wavelength distribution of incoming and
outgoing energy if the earth and the sun were ideal blackbodies.

Figure 1.9. Spectral distribution of incoming and outgoing radiation at the earth’s
surface if both earth and sun are treated as black bodies. (Note that the peaks of
the two curves have been normalised and the scale of the horizontal axis is
logarithmic.)

Human activities are increasingly releasing ‘anthropogenic gases’ into the
atmosphere, which absorb in the 7–13 Pm wavelength range, particularly carbon
dioxide, methane, ozone, nitrous oxides and chlorofluorocarbons (CFCs). These gases
are preventing the normal escape of energy and are widely accepted to be causing
observed increases in average terrestrial temperatures. According to McCarthy et al.
(2001), ‘Globally-averaged surface temperatures have increased by 0.6 r 0.2°C over
the 20th century, and the globally-averaged surface air temperature is projected by
models to warm 1.4–5.8°C above 1990 levels by 2100. These projections indicate that
the warming would vary by region, and be accompanied by increases and decreases in
precipitation. In addition, there would be changes in the variability of climate, and
changes in the frequency and intensity of some extreme climate phenomena.’ There
are already indications of increased floods and droughts, and a wide range of serious
impacts on human and natural systems are predicted.

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