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1 Introduction
We really don't understand what causes decadal-to-centennial climate variability although we have some rudimentary ideas about the possible mechanisms. Which mechanism operates implies the existence and range of
possible predictability. These lectures, therefore, have two purposes: 1) to
look at the evidence for the various proposed mechanisms and see if any can
be chosen as most likely (or if any can be rejected and thereby eliminated
from further consideration) and 2) to see which of the mechanisms imply
predictability of the decadal variability and which are intrinsically unpredictable. The variety of mechanisms is diverse, and,. to save the reader
from turning to the end to see how the story turns out, none can be totally
eliminated at this stage. Indeed, more than one mechanism may apply
at any given time at various places on the globe. It is impossible to be
exhaustive in lectures of this type, so it is hoped that these lectures will
serve as a guide to the ideas and literature of a field that is rapidly catching
on as the realization takes hold that natural variability of the climate is
omnipresent and important, and may be predictable.
We will begin by looking at the nature of decadal-to-centennial variability in the observational record to see what it is we wish to explain
and predict. We will move on to the paleo-record and then to the possible mechanisms that may explain decadal-to-centennial variability in these
records.
1.1 The Instrumental Record
Fig. 1a shows the historical record of averaged air temperature over N orthern Hemisphere land for the last 100 or so years. While the trend is clearly
upward (unless that trend is part of a still longer cycle which clearly cannot be determined from this record alone) we will be interested, in these
lectures, more in the wiggles of the curve than its trend. Clearly visible are
variations of multi-decadal time scale. If the warming since the mid 1970's
are part of a natural fluctuation, then the amplitude of such a fluctuation
is of order 0.5 0 • If the warming in the last twenty years or so is taken as
part of the trend, then there are modulations on the trend of order O.l°C.
The ocean's sea surface temperature (SST), Fig. 1 b, is equally variable
on decadal time scales but the large warming of the 1970's and beyond
is clearly less, at least in the Northern Hemisphere SST. While there are
1 Introduction
We really don't understand what causes decadal-to-centennial climate variability although we have some rudimentary ideas about the possible mechanisms. Which mechanism operates implies the existence and range of
possible predictability. These lectures, therefore, have two purposes: 1) to
look at the evidence for the various proposed mechanisms and see if any can
be chosen as most likely (or if any can be rejected and thereby eliminated
from further consideration) and 2) to see which of the mechanisms imply
predictability of the decadal variability and which are intrinsically unpredictable. The variety of mechanisms is diverse, and,. to save the reader
from turning to the end to see how the story turns out, none can be totally
eliminated at this stage. Indeed, more than one mechanism may apply
at any given time at various places on the globe. It is impossible to be
exhaustive in lectures of this type, so it is hoped that these lectures will
serve as a guide to the ideas and literature of a field that is rapidly catching
on as the realization takes hold that natural variability of the climate is
omnipresent and important, and may be predictable.
We will begin by looking at the nature of decadal-to-centennial variability in the observational record to see what it is we wish to explain
and predict. We will move on to the paleo-record and then to the possible mechanisms that may explain decadal-to-centennial variability in these
records.
1.1 The Instrumental Record
Fig. 1a shows the historical record of averaged air temperature over N orthern Hemisphere land for the last 100 or so years. While the trend is clearly
upward (unless that trend is part of a still longer cycle which clearly cannot be determined from this record alone) we will be interested, in these
lectures, more in the wiggles of the curve than its trend. Clearly visible are
variations of multi-decadal time scale. If the warming since the mid 1970's
are part of a natural fluctuation, then the amplitude of such a fluctuation
is of order 0.5 0 • If the warming in the last twenty years or so is taken as
part of the trend, then there are modulations on the trend of order O.l°C.
The ocean's sea surface temperature (SST), Fig. 1 b, is equally variable
on decadal time scales but the large warming of the 1970's and beyond
is clearly less, at least in the Northern Hemisphere SST. While there are