40 / Basics of Environmental Science
sky colour by counteracting the effect of Rayleigh scattering; it makes the sky a darker blue after rain
has washed out solid particles.
Once warmed, the Earth also behaves as a black body, radiating energy in the long, infra-red
waveband. All the received energy is reradiated. All the portion which is captured by green plants
and subsequently passed to animals that eat the plants is converted back into heat by the process of
respiration and escapes from the Earth. This must be so, because if captured energy were retained
permanently the Earth would grow continually hotter, and it does not. Overall, the amount of
radiation received from the Sun is equal to the amount radiated into space from the surface of the
Earth, but a proportion of the outgoing energy is retained for a time in the atmosphere. This
produces the ‘greenhouse effect’.
Solar energy can be exploited for domestic and industrial use, as a so-called ‘renewable’ energy
source, but none of the exploitive technologies is free from problems (RAVEN ET AL., 1993,
pp. 234–250).
Figure 2.11 Absorption, reflection, and utilization of solar energy