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Research during the last 20 years has led to a major expansion in knowledge about long-term climatic variability and dynamics. Two developments in particular have advanced the theoretical understanding of the major environmental changes that induce continuous changes in ecosystems. The first development was a recognition that the alternation of glacial and interglacial climates has been paced by the variations in solar radiation generated by periodic variations in the Earth's orbit. The second development involved an increased understanding of the hierarchical controls of regional climatic variations. Studies of marine plankton from deep-sea cores were critical to the first development, whereas the second development arose from regional to global syntheses of paleoclimatic data combined with analyses of paleoclimatic simulations from climate models. These two sets of information illustrate a theoretical …