Geoscience Reference
In-Depth Information
Figure 25.1. Solar radiation budget showing inputs and outputs. (Adapted from
Anon.,
1989
.)
scales in time and space, so that Flint's overly pessimistic conclusion no longer seems
valid. There is, of course, still scope for disagreement and differing interpretations
of the primary data. (For a thoughtful analysis of the causes of such disagreement,
see Hulme,
2009
). Nevertheless, the quantity and quality of precise and accurate
information available to us does allow some definite conclusions to be drawn about
what we do know, what we do not as yet know and what we will need to know if we
are to adapt intelligently to possible future change. Before we can consider the causes
of climate change, we need to understand what factors govern the present-day climate.
25.2.1 Factors that determine present-day climate
Solar energy is the ultimate agent that determines our climate on earth. Solar energy is
transferred to the earth by what may best be described as oscillating electromagnetic
forces (radiation), which have different wavelengths (Anon.,
1989
). Most of the
solar radiation that reaches the surface of the earth is in the form of visible light,
with a wavelength of 0.4-0.7
m. Solar energy penetrates the earth's atmosphere
and reaches the earth's surface in the form of relatively short- wave solar radiation,
which is then radiated back from the earth's surface as relatively long-wave terrestrial
radiation. Only about 50 per cent of all solar radiation is received at the earth's surface
(
Figure 25.1
). Roughly 20 per cent is absorbed by the atmosphere and by clouds, and
μ
