Environmental Engineering Reference
In-Depth Information
Figure 2.5
The earth's energy budget
This 30 per cent of incoming radiation bounced
back unaltered into space is a measure of the
earth's reflectivity or albedo. The remaining 20
per cent of the original incoming radiation is
absorbed mainly by oxygen, ozone and water
vapour in the atmosphere. Thus, of every 100
units of solar energy arriving at the outer edge
of the atmosphere, 70 are absorbed into the
earth/ atmosphere system and 30 are returned to
space having made no contribution to the
system. Most of the 50 units absorbed by the
earth are reradiated as long-wave, terrestrial
radiation. Some energy is also transferred into
the atmosphere by convective and evaporative
processes at the earth's surface. The greenhouse
gases trap the bulk of the outgoing terrestrial
energy, but the atmosphere is transparent to
wavelengths between 8 µm and 11 µm, and this
allows 5 units of radiation to escape directly to
space through the so-called atmospheric
window. Some of the terrestrial radiation
absorbed by the atmosphere is emitted to space
also, but sufficient accumulates to allow 95
units to be reradiated back to the surface. The
exchange of energy between the atmosphere
and the earth's surface involves amounts
apparently in excess of that provided by solar
radiation. This is a direct function of the
greenhouse effect, and its ability to retain energy
in the lower atmosphere. Eventually all of the
long-wave radiation passes out into space also,
but not before it has provided the energy
