Observations of Natural Climate Variability - Climate Dynamics - page 49

3

OBSERVATIONS OF NATURAL
CLIMATE VARIABILITY

Chapter 2 provided a description of the time-mean state of the climate system.
But to characterize this complex system more completely requires considering
more than the climatology, since there is pronounced variability in the system
on a wide range of time and space scales. This is especially important if we
are concerned about isolating and understanding the human-induced climate
change signal. To differentiate climate change from climate variability, we need
to understand the features, processes, and causes of both.
There are two types of climate variability— internally generated and exter-
nally forced . Internally generated variability results from processes within a
system, while externally forced variability arises when some factor outside the
system causes change.
Classification of internally generated and externally forced variability de-
pends on how the boundaries of a system are defined. For example, if we were
to study the atmosphere in isolation from the rest of the climate system, then
changes in sea surface temperatures would be termed external forcing. But in
the context of the coupled ocean/atmosphere system, variability induced in the
atmosphere by variations in sea surface temperatures are internally generated
variability. In the case of external forcing, changes within the system do not
feed back to modify the forcing agent. One example of external climate forc-
ing is climate change caused by variations in the amount and/or distribution of
solar energy incident on the earth due to changes in the solar luminosity or the
earth's orbital parameters.
Climate variations can also be classified according to their characteristic
space and time scales. Typical classifications by space scale are local , regional ,
continental , and global , corresponding roughly to 1-50 km, 50-1000 km,
1000-10,000 km, and 10,000-40,000 km, respectively. Time scales of vari-
ability include diurnal , intraseasonal, seasonal or annual , interannual , decadal ,
and millennial . Any of these variability signatures may be internally generated
or externally forced. This approach to classifying variability is useful because
the causes and processes of climate variability in the climate system vary with
time and space scales.