Global Changes during the Last 3 Million Years: Climatic Controls and Biotic Responses

INTRODUCTION 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 and in the distributions of plant and animal taxa. 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 provide a theoretical framework for understanding climatic variations through time at the subcontinental scale and lead to a recognition that elements of the biosphere, such as vegetation and marine plankton, have experienced large periodic variations in climate for millions of years. In this paper, we describe the major global climatic variations for the last 20 ka, the last 175 ka, and the last 3 Ma. We combine a map view of changes from the last 20 ka with a time-series perspective for the two longer time intervals. The order of our discussion of these time periods allows us to interpret the longer records in terms of lessons learned from studies of the last 20 ka. Large spatial data sets and climate-modeling experiments for the last 20 ka show how global and regional controls of climate govern the regional patterns of environmental change. They also illustrate the biospheric response a) to the change from full-glacial to interglacial conditions and b) to a full cycle of moisture-balance changes in the northern tropics from dry to wet to dry conditions. Records from the last 175 ka, which span almost two full glacial/interglacial cycles, help us evaluate how vegetation and marine plankton populations track these long-term environmental changes. Records spanning the last 3 million years show the antiquity of the large periodic variations and how selected ecosystem and biogeographic changes are embedded within longer-term non-reversing trends in climate. From studying these three periods of large climate changes we have concluded that major elements of the biosphere track the long-term environmental changes fairly closely. In 1976, Davis (42) used …