A significant amount of climatic variance is distributed over periods
longer than 100,000 to 400,000 years, but the exact patterns and causes of
climatic change over these long time scales has been difficult to establish
due to the incomplete and altered nature of the geological record [e.g.,
Crowley and North, 1991; Wright et al., 1992; Zachos
et al., 1993].
Shackleton and Imbrie [1990] combine empirical and theoretical arguments
to support the possibility that quasi-periodic changes in the climate system
related to variations in the rate of extinctions or to major extraterrestrial
perturbations may be a significant component of natural climate variability,
just as tectonic activity may explain much of the variance over
10
-10
year time scales.
The last decade has also seen an intensified focus on the processes by which variable tectonic activity may have influenced climate of the last several million years. Building on earlier work [ Hahn and Manabe, 1975], recent climate model experiments have illustrated how the development of expansive mountain belts and high plateaus can impact climate both regionally and globally [ Kutzbach et al., 1993; Ruddiman and Kutzbach, 1989; Broccoli and Manabe, 1992]. Prell and Kutzbach [1992] used a series of model experiments, along with paleoclimatic observations, to suggest that the development of the Tibetan Plateau was the prime factor behind the development of a strong Asian monsoon system. More controversially, several workers have gone further to refine how variations in tectonic activity could influence the carbon dioxide content of the earth, and hence the long-term radiative affects of this trace-gas species on the Earth's climate [ Molnar and England, 1990; Rea et al., 1990; Raymo and Ruddiman, 1992; Kerrick and Caldeira, 1994].
Tectonically-driven changes in climate are not likely in the next few centuries, but the past influences of orography, atmospheric greenhouse gases, paleogeography, and ocean-circulation changes must still be unraveled before the record of pre-Quaternary ``extreme'' climate states can be used to its fullest extent for testing the sensitivity of predictive climate models. Paleoclimatic model validation will be discussed below. A key point for this section, however, is that the details of climatic forcing and boundary condition configuration get progressively more uncertain with increasing geologic age [ Crowley and North, 1991; Rind, 1992; Crowley, 1993].