Geoscience Reference
In-Depth Information
5 per cent is scattered by the atmosphere. A further 22 per cent is reflected back to
space from the clouds, and 3 per cent is reflected back from the ground. (The global
albedo, or reflectivity, therefore amounts to about 30 per cent).
The shortest path for solar radiation to reach the earth's surface is when it is
aligned at right angles to the earth, which is at the equator. With distance away from
the equator, solar radiation has to penetrate an increasingly thick layer of atmosphere,
because the sun's rays are now aligned obliquely to the surface of the earth. This results
in minimum warming in polar latitudes and maximum warming in tropical latitudes.
The outcome of this differential heating, combined with the earth's rotation, results in
two primary circulation patterns: the Hadley Cell and the Polar Cell (
Chapter 2
,
Figure
2.1
). Latitude is thus the primary factor responsible for global temperature distribution
and the location of major frontal zones. Any changes in the latitudinal position of
land-masses will have an impact on long-term climatic changes, as we saw in the case
of the Cenozoic displacements of Africa and Australia (Chapters
18
and
22
).
The seasonal displacement of these atmospheric circulation cells and frontal zones
(
Chapter 2
,
Figure 2.2
) is a direct consequence of the tilt of the earth's axis, which
causes an apparent northward displacement of the sun during the boreal summer and
a southward displacement during the austral summer. Here again, long-term changes
in the tilt of the earth's axis will cause long-term changes in seasonal climate (see
Chapter 3
).
Altitude is another important factor controlling temperature. Temperature decreases
with increasing elevation at a rate of about 6
C/1,000 m, which is known as the lapse
rate. The temperature difference between the lowest and highest points on earth
amounts to about 55
°
C, which is about the same as the January mean sea level
potential temperature variation with latitude (from
°
C).
Over long time scales, tectonic uplift and subsidence will have an impact on local
temperatures.
Finally, ocean circulation and distance inland ('continentality') determine the pre
sent-day temperature range on land, as well as moisture availability. Once again, any
long-term changes in continental configuration will be apparent as long-term changes
in precipitation and temperature. However, our concern in this chapter is not with
the long-term changes, since we have already considered these in detail for all of the
major deserts and their margins in Chapters
18
-
22
. Our concern here is with the more
recent changes of the past few centuries.
+
20
°
Cto
−
40
°
C, or about 60
°
25.3 The 'greenhouse effect'
We saw in the previous section that only about half of all solar radiation is actually
absorbed at the earth's surface; the rest is either absorbed or scattered by the atmo-
sphere and clouds, or else it is reflected back to space (
Figure 25.1
). The earth's
atmosphere absorbs about 20 per cent of the outgoing long-wave (infrared) terrestrial
