Overview

The Milankovitch theory of regularly changing earth orbital characteristics became widelyaccepted as the primary cause for ice ages after the 1930s. This was despite the lack of asatisfactory theoretical framework linking the periodic orbital characteristics to changes in theearth s temperature. The Milankovitch theory gained stronger support when deep cores from the oceanfloor and polar ice sheets were extracted and analysed. Although other more random variations weresuperimposed, it was possible to identify signals of about 100,000 years, 40,000 years and 22,000years associated with the respective orbital characteristics of eccentricity, obliquity andprecession.

Despite the prominence gained by Milankovitch during the 20th century, Arrhenius greenhousetheory, which had drifted out of favour, was far from forgotten. Feedback between atmosphericcarbon dioxide and the biosphere was proposed as a possible amplification factor for theMilankovitch orbital forcing. In addition, the quantity of coal being burned at the beginning ofthe 20th century in support of industry had led to suggestions, even at that time, that it might bea cause for global warming, although the magnitude was not then quantified. Arrhenius, beingsensitive to the cold winters experienced in his native Sweden, is reputed to have commented thatwarming may not be altogether a bad thing.

By the early 1950s a carbon dioxide-climate linkage was again under investigation. Evidencewas mounting that the earth had been warming and that atmospheric carbon dioxide concentration wasalso increasing. As a component of the 1957 58 International Geosphere Year, an internationallycoordinated program was established to gather observations of atmospheric carbon dioxideconcentration from a worldwide network. Mauna...