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13
Climate change
Learning objectives
When you have read this chapter you will:
Understand the difference between climate variability and climate change and know the characteristic
features that may constitute a change of climate,
Be aware of the different timescales on which past climatic conditions are studied and the sources of
evidence that may be used,
Recognize the major climatic forcing factors and feedback mechanisms and the timescales over which
they may operate,
Be aware of the anthropogenic contributions to climate change,
Appreciate the possible impacts of climate change on environmental systems and on society.
A GENERAL CONSIDERATIONS
conditions from tree rings, pollen in bog and lake
sediments, ice core records of physical and chemical
parameters and ocean foraminifera in sediments provide
a wealth of paleoclimatic data.
It is first worthwhile to consider the nature of climate
variations. The standard interval for climatic statistics
adopted by the World Meteorological Organization is
thirty years: 1971 to 2000, for example. However, for
historical records and proxy indicators of climate,
longer, arbitrary time intervals may be used to calculate
average values. Tree rings and ice cores can give
seasonal/annual records, while peat bog and ocean
sediments may provide records with only 100- to 1000-
year time resolution. Hence, short-term changes and the
true rates of change may not be identifiable.
The present climatic state is usually described
in terms of an average value (arithmetic mean, or the
median value in a frequency distribution), a measure of
In this final chapter we examine the variation of climate
on different timescales, related climatic forcing factors
and projected future changes. Global climate change in
the twentieth century has reinforced the recognition that
climatic conditions are non-stationary and that human
activities have major impacts on the climate system.
Realization that climate is far from being constant
came only during the 1840s, when indisputable evi-
dence of former ice ages was obtained. Yet, in many
parts of the world, the climate has altered sufficiently
within the past few thousand years to affect the possi-
bilities for agriculture and settlement. Study of past
climate began with a few individuals in the 1920s and
more actively in the 1950s (see Box 13.1). Weather
records for most parts of the world span only the last
hundred years or so. However,
proxy
indicators of past
