Earth Sciences / 37a result of what may be a general climatic warming. Many climatologists predict that such a
warming is likely to continue, but the consequences for sea levels are difficult to estimate and
predictions vary widely.11. Energy from the Sun
Tides are driven by gravitational energy and plate tectonics by the heat generated by the radioactive
decay of elements in the Earth’s mantle, but the energy driving the atmosphere, oceans, and living
organisms is supplied by the Sun. To a limited extent this energy can also be harnessed directly to
perform useful work for humans. Solar heat can be used directly to warm buildings and water,
desalinate water, and cook food. Sunlight can be converted into electricity. Electrical power can also
be generated from wind and sea waves and, because the atmospheric circulation responsible for
wind and wind-driven waves is driven by solar heat, these are also forms of solar energy.
The outer layer of the Sun, which is what we see and the region from which the Sun radiates, is at a
temperature of about 6000 K and it radiates energy at 73.5×10^6 W from every square metre of its
visible surface (the photosphere; being entirely gaseous, the Sun has no solid surface). The figure
can be calculated because the Sun behaves as a ‘black body’. This is a body that absorbs all the
energy falling on it and radiates energy at the maximum rate possible; the rate is calculated by using
Stefan’s law^9 and is proportional to the absolute temperature raised to the fourth power.
The Sun radiates in all directions and the Earth, being a very small target at a distance of 150 million
km, intercepts 0.0005 per cent of the total. At the top of the Earth’s atmosphere this amounts to about
1360 W m-2, a value known as the ‘solar constant’.
Solar output is not as constant as this name suggests. Between 1981 and 1984, it decreased by 0.07
per cent (HIDORE AND OLIVER, 1993, p. 166). This is a small deviation, but a decrease of about
0.1 per cent sustained over a decade would be sufficient to produce major climatic effects and a 5 per
cent decrease might trigger a major glaciation. Cyclical variations in the Earth’s rotation and orbit
also alter the solar constant. These are believed to be the major cause of large-scale climatic change,
and variations in solar output, marked by changes in sunspot activity, are linked to less dramatic
changes, such as the Little Ice Age, a period when average temperatures were lower than at present
which lasted from about 1450 to 1880. Some scientists believe that the recent climatic warming and
rise in atmospheric carbon dioxide concentration are both wholly due to the marked increase in
energy output of the Sun since about 1966 (CALDER, 1999).Figure 2.9 The development of a sea cliff, wave-cut platform, and wave-build terrace