More than a century ago, British scientist John Tyndall argued that increased heating of the oceans was needed to start a glaciation (Tyndall, J., 1872. The forms of water in clouds and rivers ice and glaciers. International Scientific Series. The Werner Company, Akron, OH, 196 pp.). We show that he was essentially right and that the principal cause of Quaternary glaciations was the intensification of the hydrologic cycle by the warming of tropical oceans and increase of equator-to-pole temperature gradient, which led to the growth of land-based ice in the high latitudes. The change was due to decreased obliquity and to the increased intensity of the solar beam in boreal winter and spring at the expense of summer and autumn. This resulted in higher frequency of El Nino compared to La Nina anomalies. Decreased water vapor greenhouse forcing and increased reflection from expanding snowfields were also instrumental in the transition from the last interglacial into the glacial. The current orbital changes, although less extreme, are qualitatively similar. Association of recent positive seasonal anomalies of global mean temperature with El Nino events suggests that the ongoing global warming may have a significant, orbitally influenced natural component. The warming could continue even without an increase of greenhouse gases. (C) 2003 Elsevier B.V. All rights reserved.
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