Geoscience Reference
In-Depth Information
What are these processes which might lead to a change
in Earth's climate? Why does the climate vary so much
over time? These are questions to which we have no
easy answer. There are at least four different time scales
which require explanation: glacial/interglacial, stadial/
interstadial, postglacial oscillations and fluctuations
over the last 150 years. In looking for causes, we must be
aware that the global climate is the product of a complex
system involving the atmosphere/hydrosphere/litho-
sphere/cryosphere. Changes can be forced upon the
system by factors which may be either radiative or non-
radiative, internal or external. In addition we have feed-
back mechanisms which interact within the atmosphere
or between the atmosphere and Earth ( Figure 9.9 ). Let us
look at these in turn.
of only 0·6 C in Earth's mean annual temperature.
Nevertheless, this small figure could be important in
climatically marginal areas.
A more certain link between solar variations and long-
term 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 M km from the sun. The farthest point is
known as the aphelion (Greek ap , 'far' + helios ), which is
approximately 152 M km from the sun.
At present the perihelion occurs around 4 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 1,400 W m -2 is received, whilst
at aphelion the value is 1,311 W m -2 , thus varying by about
7 per cent between perihelion and aphelion. If Earth is at
the perihelion during the northern hemisphere winter it
will receive more energy and therefore be warmer than if
it were at the aphelion at that time. The time of year at
which Earth is nearest the sun does change over time. A
complete cycle takes about 22,000 years and is termed the
precession of the equinoxes ( Figure 9.11 ). It has the effect
of changing the relative warmth of winter and summer
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 9.10 ). From 1100 to 1250, 1460 to 1550 and
1645 to 1715 sunspot maxima were very low and the sun
was less active. It seems unlikely that we should expect
variations of more than 0.1 per cent in solar output during
the sunspot cycle 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
(a)
(b)
Normal to plane of the ecliptic
Equinox
Past Future
5
3
Eccentricity
Plane of earth's
axial tilt
Solstice
1
Wobble
Sun
P
A
Tilt
Solstice
Precession
Spin axis of earth
24.50
24.00
23.50
23.00
Earth
Equinox
Figure 9.11
(a) Geometry of the sun-earth system, showing the
factors causing variation in radiation receipt by the earth.
(b) Changes in eccentricity, tilt and precession for the last
250,000 years and the next 100,000 years.
Source: After Imbrie and Imbrie (1979)
22.50
Tilt
250
200
150
100
50
0
-50
-100
Thousands of years ago
 
 
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