Global Warming and its possible impacts on Essex
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Another argument is that it is only when there are land masses in the polar regions that continental-
sized glaciers can occur. Thus when Britain was in the latitudes of the Sahara, Africa was
correspondingly further south, parts of it extending into southern polar regions. Africa experienced
a major glaciation in the Permo-Carboniferous, evidence for which can be found in the Sahara
today. There is little evidence for glaciation in the northern hemisphere at this time, but there were
no major land masses near the north pole.
A further complication is that moist air masses must be able to penetrate the polar regions. At the
time of the Alpine orogenic mountain building period, about 60 million years ago, North America,
Europe and Asia were joined together forming an enormous land mass extending into northern polar
regions. However, the land mass was so extensive that the air masses were dry by the time they
reached the north polar area. Periglacial, rather than glacial, conditions prevailed. However, when
America began to split away from Eurasia, the Atlantic formed and moist air could reach the polar
latitudes and major continental glacier build-up could occur.
Again, the time scale of these events is so long that we discount them when considering present day
climate change.
Astronomical controls on climate
The major ice ages seem to have a periodicity of 150 million years. As collisions should be random,
the periodicity may indicate another cause for ice ages, possibly astronomical. This, however, is
speculation at this stage. It is important to realise that 'ice age' is a misnomer as each comprises a
sequence of alternating warm (interglacial) and cold (glacial) stages. The last ice age started about
2.6 million years ago and it is quite possible that we are in an interglacial.
Climate variation over the last 2.6 million years shows that we have had over 50 cycles of glacial
and interglacial conditions. It is now accepted that the control for these is at least partly astronomical,
related to variations in the earth's position in relation to the sun and, hence affecting the amount of
the solar heating we receive.
The path of the earth round the sun varies from being elliptical to almost circular, this known as the
eccentricity of the orbit. The change from one to the other and back again is orderly, taking 96,000
years to complete a cycle. Currently the earth is tilted at 23.5°, but the angle of tilt varies from 21.5°
to 24.5°, this variation is known as the obliquity of the ecliptic. This cycle takes 42,000 yrs. Finally,
the axis of the earth, currently tilted at 23.5°, 'wobbles' taking 21,000 yrs to complete a cycle, the
precession of the equinoxes. Each cycle has a different impact on the solar heating received by the
earth and they work in combination, known as orbital forcing (Fig. 1). When the various cycles are
combined, the result compares well with the record of the heavy oxygen isotope, 018, variation over
the last million years (Fig. 1). Oxygen18 occurs as 1 in 500 of the oxygen atoms in the atmosphere
and is preferentially locked into glaciers. As glaciers expand, so the atmosphere becomes depleted
in O18. Thus variations (denoted by A or 5) in the amount O18 can be used to indicate cold and warm
stages.
This leads scientists to think that orbital forcing is the fundamental control on climatic change, with
smaller-scale factors causing deviation from the pattern. How the process is translated into action at
the earth's surface is far from understood. One view is that it is impact of orbital forcing at 60°N that
is crucial. During interglacials, this area is free of glaciers, but during cold stages it is covered by ice
which reaches to c.550N, i.e. it is this area that is alternately hospitable and inhospitable to ice.
Essex Naturalist (New Series) 17 (2000)