THE INITIATION OF NORTHERN HEMISPHERE GLACIATION

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For well over a century, scientists have speculated that variations in the atmospheric concentration of radiatively important trace gases, such as carbon dioxide, could control the Earth’s climate (e.g. Arrhenius 1896). Recent observations of significant glacial-interglacial variation in atmospheric CO2 in ice cores put this idea on a firm observational as well as theoretical footing (e.g. Barnola et al 1987). In this century, as atmospheric CO2 levels creep steadily upwards in response to the burning of fossil fuels and forests, the specter of manmade global climate change looms beforc us (IPCC 1990)--a possibility that has heightened interest in the natural variability of climate in the past. In particular, the ~ 3°C warming predicted for the next century has stimulated interest in past warm climates, such as the Middle Pliocene around 3.0 million years (Ma) ago, an interval that has often been invoked as a greenhouse world (Budyko et al 1985). In this paper, the climate transition from the warm mid-Pliocene (around 3.2 Ma) to the onset of northern hemisphere ice ages around 2.4 Ma is examined. Evidence for the initiation of significant northern hemisphere glaciation is examined as well as how this event affected climate around the globe. While the cause of individual glacial-interglacial oscillations is tied to Milankovitch variations in the Earth’s orbit around the sun (e.g. Imbrie et al 1992), these insolation changes cannot account for the longterm cooling trend which culminated in northern hemisphere glaciation. In the final section of this paper, mechanisms of long-term climate change