Past Changes of the Global Climate System

We will start off with a brief review of climate variability over a time span of about the last 2.5 million years to the present. A first analysis of the long-term climate development reveals a quasiperiodic oscillation with dominant periods of about 41,000 years prior to the middle Pleistocene and a shift toward a longer period of about 100,000 years at around 1.2 million years ago (Clark et al., 1999). A closer look at the climate record for the last 400,000 years confirms the periodic/oscillatory behavior of the climate system with a dominant frequency of about 100,000 years (Figure 1). In addition, there are periods of 41,000 and 23,000 years as well as millennial-scale, less periodic changes embedded into the 100,000-year cycle. Apparently, these climate variations occurred on a global scale, that is, on both the Northern and the Southern Hemisphere at approximately the same time.

Evidence for these climate oscillations comes mostly from indirect sources, the so-called proxy records, since no direct measurements of past temperature are available. Paleontological data that link the fossil remains of certain animal and plant species in the geological record of a region to particular environmental conditions provide one such proxy record. The material to be analyzed comes mostly from scientific drillings into the continental or oceanic crust or from sediment cores in lake bottoms. The distribution of pollen species, for example, in old peat deposits, and isotopic and trace chemical analysis on tree rings provide other paleoenvironmental proxy records. Particularly useful proxy records have been derived from the analysis of polar and mountain-glacier ice cores (Figure 1; see Ice Core Record). The analysis of air entrapped in closed-off bubbles reveals past concentrations of atmospheric trace gases, while ratios of hydrogen and oxygen isotopes measured in molten samples of the ice provide a direct measure of paleotemperatures over the past few hundred thousand years (Alley and Bender, 1998).

The analysis of such records and their interpretation provide insights into the underlying causes of climate variability, which will be discussed in the following section.

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