Geoscience Reference
In-Depth Information
9
So far in our discussions concerning the atmosphere it has
been the understanding and description of the nature and
controls of our
present
climate system that have been
stressed. However, there is abundant evidence that Earth's
climate has rarely, if ever, been the same as that of today.
From the distant geological past through to the most
recent millennium, we can find evidence that our climates
have been different. In the British Isles we can find
signs that ice has built up over our islands, that desert
conditions have prevailed and that about 60 M years ago
warm tropical seas deposited the clays of the London
basin (
Chapter 10).
If the climate changes, then it is almost
certain that all other aspects of the environmental system,
such as the geomorphology, hydrology and biogeography,
will change in response; we are very much part of a
dynamic system.
What is meant by a change of climate? If we plot annual
temperature or rainfall values at a particular site through
time (
Figure 9.1
),
it is apparent that values vary from year
to year. At some sites the pattern may be entirely random,
or we may find oscillations between warmer periods and
cooler periods, or wetter and drier years, with no long-
term trend. Unfortunately the length of instrumental
measurements at most sites is short and so it is impossible
to reach clear conclusions about whether the climate
values have changed in a statistical sense or whether they
merely demonstrate a very variable climate. Over longer
periods of time, it is apparent from various lines of
evidence that major changes of climate have taken place.
Recent evidence from ice cores and oceanic sediments
indicate that these can take place over very short periods
of time.
We will first look at what has been found in terms
of climate change, concentrating on the most recent
geological period and into the instrumental record.
Second we will look at some of the suggested causes of
these changes. Their impacts on the environment will be
examined in
Chapters 23
and
28.
Glacial periods
From a variety of lines of evidence it is now possible to
determine with reasonable certainty the environmental
conditions that have prevailed over Earth within the recent
geological past. Land and oceanic sediments record clear
evidence of numerous alternations between warmer and
colder conditions over the last 2 M years. At least eight
such cycles have occurred in the last million years, with
the warm part of the cycle lasting only a relatively short
time before another gradual encroachment of ice and
then a rapid melt period (
Figure 9.2
).
During the cold phases, ice advanced across much of
north-west Europe from Scandinavia and across much of
North America from centres over northern Canada. It
appears that ice occupied these areas for only a short
period of time. For the remainder of the cold phase the
climate on the tropical side of the ice sheets was cold and
dry, and provided a source area for much of the wind-
blown sediments called
loess
. In the tropics, lake levels
indicate that the glacial periods were generally arid, as is
indicated by the wider spread of active sand dunes. Former
