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(A)
Periodic variation
(B)
Impulsive change
of central tendency
Quasi-periodic variation
(C)
Downward trend
of central tendency
Stable central tendencies
(stationarity)
Increasing variability
(D)
Time
Figure 13.2
Different types of climatic variation. The scales are arbitrary.
Source:Hare (1979). Courtesy World Meteorological Organization.
variability. On the other hand, while one can
correctly say (for example) that the major glacial
and interglacial cycles of the Pleistocene are
expressions of climate variability within the past
two million years, it is also appropriate to consider
the evolution from full glacial to interglacial
conditions as an expression of climate change.
Similarly, while we usually view the global tem-
perature rise over the past 100 years as
climate change, reserving the term variability for
embedded shorter timescale features, the century-
long warming could also be viewed as an aspect of
climate variability over the past 1000 years. The
distinction between variability and change is
hence dependent on the time frame over which
one considers the climate statistics.
The United Nations Framework Convention
on Climate Change (UNFCCC) offers a different
definition that can help to resolve some of these
problems. They define climate change as 'a change
of climate which is attributed directly or indirectly
to human activity that alters the composition
of the atmosphere and which is in addition to
natural climate variability observed over compar-
able timescales'. This definition is useful in that
it makes a clear distinction between natural
processes and anthropogenic influences. The
remainder of this chapter will view climate change
in this context. Variability, in turn, will be viewed
as associated with natural processes.
B CLIMATE FORCING,
FEEDBACK AND RESPONSE
The most fundamental measure of the earth's
climate state is the global mean, annually averaged
surface air temperature. Year-to-year and even
decadal-scale variations in this value can occur
due to processes purely internal to the climate
system. The warm phase of ENSO, for example,
may be viewed as an internal process in which heat
in the ocean reservoir (i.e., heat already within the
