Environmental Engineering Reference
In-Depth Information
radiative or non-radiative, internal or external. In addition we have feedback mechanisms
which interact within the atmosphere or between the atmosphere and Earth (Figure 9.8).
Let us look at these in turn.
EXTERNAL FACTORS
The most important external radiative factor is the sun. The sun may appear to us as a
stable star but satellite observations of the solar beam intensity suggest small variations
of output only partly connected with the well known eleven-year sunspot cycle. Long-
term observations of sunspot numbers indicate that the cycle is varied in terms of the
frequency of sunspots at the peaks of the cycles (Figure 9.9). From 1100 to 1250, 1460 to
1550 and 1645 to 1715 sunspot maxima were very low. It seems unlikely that we should
expect variations of more than 1 per cent in solar output as a result of the sunspot or other
natural changes; simple calculations of Earth's radiative balance suggest that even a 1 per
cent difference in output would lead to a change of only 0·6° C in mean annual
temperature. Nevertheless, this small figure could be important in climatically marginal
areas.
A more certain link between solar variations and longterm climate change has been
established through the work of Milankovich, a Yugoslavian mathematician. He
determined the changes in solar radiation reaching Earth's surface as a result of orbital
variations. Three interacting variations are known to occur, involving regular changes in
(1) the shape of Earth's orbit around the sun, (2) the tilt of Earth's axis of rotation and (3)
the time of year when Earth is closest to the sun. The present-day orbit of Earth around
the sun is approximately elliptical. The nearest point of this orbit to the centre of the orbit
is known as the
perihelion
(Greek
peri
, 'near' +
helios
, 'sun'), and is about 147·1 million
km from the sun. The farthest point is known as the
aphelion
(Greek
ap
, 'far' +
helios
),
which is approximately 152 million km from the sun.
At present the perihelion occurs around 3 January, while the aphelion is around 5 July.
The difference in distance of Earth from the sun at these times affects the amount of solar
radiation reaching the atmosphere. At perihelion a maximum of 1400 W m
−2
is received,
whilst at aphelion the value is 1311 W m
−2
, thus varying by about
