Environmental Engineering Reference
In-Depth Information
Chapter
9
Climate change
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 million 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 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, with no long-term trend. Unfortunately the length of
instrumental temperature measurements at most sites is short and so it is impossible to
reach clear conclusions about whether temperature 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.
Initially we will examine the types of evidence that have been used to find out whether
there have been changes in climate over time. We will then look at what this shows in
terms of climate change, concentrating on the recent geological period. In earlier times
the continents, as plates, have changed their positions on the globe and therefore changes
in climate could be caused by this process rather than by atmospheric effects (see Chapter
10).
