Climate varies on all timescales and space scales, from interannual climatic variability to very long-period variations related to the evolution of the atmosphere and changes in the lithosphere. Examples of known climatic fluctuations are shown in Fig. 2.14. In this diagram, each row represents an expansion, by a factor of ten, of each interval on the row above it. Thus, one can envisage short-term (high-frequency) variations nested within long-term (lower-frequency) variations (Webb, 1991). However, in the paleoclimatic record, as we delve farther and farther back in time, it is increasingly difficult to resolve the higher-frequency variations. As climatic variations on the timescale of decades to centuries are of the utmost importance to modern society, increasing attention must be focused on paleoclimatic data pertinent to this problem (Bradley and Jones, 1992a).
Climatic fluctuations on different timescales may be brought about by internal or external mechanisms that operate at different frequencies (Fig. 2.15). Changes in the Earth's orbital parameters, for example, are likely candidates for climatic variations on the timescale of glacials and interglacials during the late Quaternary but cannot account for climatic variations that have occurred over the last thousand years. For fluctuations on that timescale, other factors such as volcanic dust loading of the atmosphere, solar variability or internal adjustments between different subsystems in the climate system, are more likely to be involved (Jones et al., 1996). Of course, different forcing factors may have operated together to cause climatic fluctuations of varying magnitude at different times in the past, though individual factors may account for the variance of climate at a particular frequency. Mitchell (1976) pointed out that much of the variance of the climate record results from stochastic processes internal to the climate system. This includes short-period atmospheric processes (e.g., turbulence) with time constants on the scale of minutes or hours, to slower-acting processes or feedback mechanisms that add to climatic variance over longer timescales. However, these factors only contribute
10' yr ago
FIGURE 2.14 Schematic diagram illustrating climatic fluctuations at timescales ranging from decadal (the last 100 yr, lowest panel) to centennial (the last 1000 yr, second panel) to millennial (the last 10,000 yr), and so on, to the last million yr (top panel). Each successive panel, from the back to the front, is an expanded version (expanded by a factor of 10) of one-tenth of the previous column.Thus, higher-frequency climatic variations are "nested" within lower-frequency changes. Note that the temperature scale (representing global mean annual temperature) is the same on all panels.This demonstrates that temperature changes over the last 100 yr (lower panel) have been minor compared to changes over long periods of time. Such changes have occurred throughout history, but they are lost in the noise of the longer-term climatic record; only the larger amplitude changes are detectable as we look far back in time.
white noise to the climate spectrum on timescales longer than the timescale of the process in question (i.e., they contribute to the variance of climate in a random, unpredictable manner, with no effects concentrated at a particular frequency). Superimposed on this background noise are certain peaks in the variance spectrum of
Earth orbital parameter variations
Pole wandering, continental drift i i
-Continental uplift, mountain building, sea-level changes
Mass and composition of atmosphere (except C02, H20, 03) I I I
Volcanic activity - production of stratospheric aerosols
Atmosphere - ocean- cryosphere - biosphere - lithosphère autovariation ■
— Atmosphere - ocean - cryosphere autovariation
Atmosphere - ocean autovariation ■ I
Atmosphere autovariation -
Man's activities land-use; gas, aerosol, heat pollution; etc.)
Interval between ice ages
Duration of ice s
Glacial interglacial fluctuations of present ice age Duration of recent interglacials
Major fluctuations of present interglacial — Fluctuations of past thousand years Inter-annual variability
10' 10' 10' 10' Probable range of time scales involved (years, power of ten)
FIGURE 2.1 5 Examples of potential processes involved in climatic fluctuations and their characteristic timescales (Kutzbach, 1974).
climate that correspond to external forcing mechanisms operating over a restricted time domain (i.e., they are periodic or quasi-periodic phenomena). Such temporal variability may be associated with characteristic spatial variability (Mann et al., 1996). For example, El Nino-Southern Oscillation (ENSO) events recur on timescales of 3-7 yr and have distinct spatial anomaly patterns (Diaz and Kiladis, 1992).
Deterministic forcing mechanisms are only known to operate at a few relatively narrow frequencies, and although very important to climatic variance at those frequencies their contribution to overall climatic variation is minor compared to the role of stochastic processes. This presents problems for both climatic predictability and the interpretation of past climatic changes (as seen in the paleo-climatic record) in terms of particular causative factors (Mitchell, 1976). Nevertheless, certain external forcing mechanisms have often been called upon to account for features of the paleoclimatic record. The most important of these for climatic fluctuations in the Quaternary Period are variations in the Earth's orbital parameters, which are the underlying cause of glacial-interglacial cycles over at least the last million years (Berger and Loutre, 1991). This is discussed further in the next Section and in Section 6.8.
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