The major ice ages in Earth history and when they occurred

The causes of the long-term past and recent ice ages are foimded on two factors. First, there are the astronomical factors which relate to the way the Earth revolves aroimd the Sim and, second, there are factors involving the carbon cycle. The important astronomical factors are now considered to 

where did all the carbon go What mechanisms caused the drawdown of carbon dioxide and the removal of a substantial volume of carbon from the actively circulating part of its biogeochemical cycle These questions have not yet been satisfactorily explained but it is likely that life forms increased their uptake of atmospheric carbon dioxide substantially. Possibly, algae in the oceans were responsible. Alternatively or additionally, increased productivity by tropical and semitropical ecosystems may have been stimulated when they increased in extent as land emerged from the continental shelf sea-levels due to ice accumulation and sea-level fall. The drop in temperature caused by the depletion of atmospheric carbon dioxide combined with the astronomical position of the Earth at the furthest point of its elliptical path around the sun conspired to generate an ice age. Why ice ages of the past (Table 1.3) came to an end is another question that must relate to the carbon cycle, at least to some extent equally enigmatic are the issues of when and why the current ice age will end. 

The last three million years are unique in geological history for another reason the emergence of humans and their transformation into a major agent of environmental change. It is likely that the environmental 

Another significant threshold in the people - carbon - environment relationship began with the great era of exploration in the fifteenth and sixteenth centuries. The primary objective of European colonists was the acquisition of riches. They came, not always in the form of sought-after precious metals, but as biomass or carbon-based resources. For example, wood was prized by British colonists and traders because of a dearth of tall 

The age of exploration of the sixteenth and seventeenth centmies was made possible through wood. European adventurers, chmch and royalty 

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Table 1.3. The major ice ages in Earth history and when they occurred...

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