Geoscience Reference
In-Depth Information
and describe its consequences with much confidence. In
recent centuries human activity has almost certainly
begun to have an impact on climate. Two questions
remain. What is the effect of these climatic fluctuations?
And where is our climate going now?
Some of the effects are all too apparent to us. In those
areas which are marginal to agriculture, like parts of the
Sahel, minor changes in climate may have appalling
consequences, bringing crop failure, soil erosion and
famine. Some of the effects are more subtle, but none
the less significant. As the pattern of climate changes
people tend to move if they are able; new areas become
favourable, others may become unfavourable. It has been
suggested that the stimulus to the Viking invasions and
settlement of Iceland, Greenland and Britain was climatic
deterioration in Scandinavia. Nomadic tribes today
respond to similar stimuli as grazing levels vary, though
political factors make international migration much more
difficult.
The effects of climatic change are not confined to
agriculture. As we shall see in later chapters, fluctuations
also influence landscape processes. Throughout the
temperate regions of the world the imprint of past climatic
change is clear within the landscape. Glacial landforms lie
hundreds of kilometres beyond the limits of the present
ice caps; lakes which were at one stage huge inland seas
are now small pools in comparison; river valleys that once
carried vast torrents of water are now occupied by small,
generally placid streams; fine, wind-blown silt and former
sand dunes in currently moist areas testify to the former
strength of winds and presence of aridity. Effects of similar
magnitude can be detected in the vegetation. In many
areas the range of plants that we find today is a result of
the migration and mixing of vegetation in response to
climatic changes. The global system, as we have noted
before, is intricately interrelated. Changes in one part
affect others, and the effect is nowhere more apparent than
through the influence of a changing climate.
KEY POINTS
1
Climate has been changing since Earth's atmosphere first formed. We can find evidence of this in the rocks
and unconsolidated sediments which have accumulated over millions of years.
2
Some of this change has apparently been the result of the movement of continental crust. More recently
Earth has experienced much colder periods, interspersed with warmer phases when continental movement
has been so small that its effects would be insignificant. There is evidence of warmer and cooler periods
even during the last millennium.
3
For earlier periods we have to use indirect or proxy evidence, but since the seventeenth century instruments
have been used to measure the elements of climate.
4
The causes of the changes are not fully understood and are likely to vary, depending upon the time scale.
We can distinguish external and internal factors. The orbital variations of Earth about the sun are important
external factors and do appear to have been significant in triggering the recent Ice Ages. Internally, the
changing nature of Earth's surface, both land and sea, and atmosphere can have major effects, amplified
through positive feedback. Human impacts on climate have now become so marked that we cannot be
sure that the natural cycle of Ice Ages, which has lasted for the last 5 million years or so, will continue.
FURTHER READING
Drake, F. (2000) Global Warming, London: Arnold. A text at undergraduate level for those interested in both an explanation
of the physical principles behind the greenhouse effect and the policy implications of global warming. Written for non-
mathematicians but goes into reasonable depth.
Goodess, C. M., Palutikof, J. P. and Davies, T. D. (1992) The Nature and Causes of Climate Change, London: Belhaven
Press. Intermediate to advanced text about the causes of climatic change. It also offers suggestions on future climate,
taking natural and anthropogenic factors into account.
