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 Historical
evidence shows that the climate of the world since the planet
was formed more than 4,000 million years ago has fluctuated
greatly, and on many occasions has borne little resemblance
to what we experience today.
For example, we are currently
in the middle of an ice epoch (longer than an ice age) which
has lasted millions of years and is likely to continue for
millions more. Yet, in the context of the history of the planet,
this is not a normal period. More average conditions would
be significantly warmer, producing the lush vegetation and
hot conditions that prevailed aeons ago when dinosaurs walked
the Earth for millions of years.
The reason why we are in the
middle of such a cold epoch has a great deal to do with the
positioning of the land masses. Almost imperceptibly, the
great continents are constantly moving and changing location.
Throughout the history of the Earth it has been unusual to
have one polar ice-cap; it is unique for us now to have two
of them.
Normally the circulation of the
warm currents of the oceans, as I mentioned earlier, helps
distribute the heat and ensure a general uniformity of temperatures.
However, the land mass that has become known to us as Antarctica
has temporarily (in global terms!) positioned itself over
the South Pole thereby blocking off warm currents. This has
allowed ice some 1,800 metres (6,000 feet) thick to form a
sheet over that now most inhospitable of continents.
Similarly, by another quirk of
the slow drift of the continents, land has encircled the North
pole, causing the waters there to be largely cut off from
the worldwide drift of oceanic currents. This too has allowed
an ice-cap to develop at the North Pole. Eventually – and
by this I mean in tens of millions of years – the land masses
of North America and Europe, which are moving away from each
other, will be far enough apart to allow the warmer currents
from the Atlantic to warm up the Arctic and melt the ice-cap.
However, as far as humans are concerned at present the existence
of two polar ice-caps is, for all practical purposes, a permanent
one.
Within an ice epoch there are
ice ages which alternate with shorter warmer periods
known as interglacials. At the moment the Earth is
passing through an interglacial which has lasted for around
10,000 years following the last Ice Age, which in turn went
on for some 100,000 years. It would appear from historical
climatic evidence that this ice age/interglacial pattern was
established at the beginning of this ice epoch. Perhaps ominously
for man, the pattern suggests that ice ages last around 100,000
years on average and the shorter, warmer interglacials around
10,000 – so we are nearing the end of our current warmer period.
However, there is no need for
any alarm at this thought. The next ice age could be up to
1,000 years or more away – a short period in climatology but
a comfortingly lengthy one for us. And in any case no one
can yet predict what effect the greenhouse effect may have
on the overall pattern of global cooling and in arresting
a return to glacial conditions.
What does seem apparent is that
within the current interglacial, starting some 10,000 years
ago, there have been smaller patterns emerging – periods of
warmer weather, followed by colder weather and so on. These
have been broken down by climatologists into four main periods.
The first followed the end of the last Ice Age, indeed it
caused it to end, and probably reached its warmest about 5,000
or 6,000 years ago. At this time the temperature would have
been on average about 2C (3.6F) warmer than the present day.
This period has acquired the
name the Optimum period as a result, and was followed by a
much colder spell which more or less coincides with the historical
period called the Iron Age, which reached its coldest around
2,500 years ago. (It should be remembered that these changes
are gradual and do not occur overnight).
Useful link
Ice Age part 2
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