Similarities between the timing of orbitally driven changes in seasonality and extent of ice sheets makes clear that there must be a causal link. The long-standing view is that glaciers respond to summer insolation. However, this link cannot explain the hemispheric synchronicity of glacial response. Dr. Broecker proposes that, instead, orbital cycles drive reorganizations of the ocean's thermohaline circulation. These, in turn, generate changes in the atmosphere's CO₂ content. Hence, CO₂, rather than summer insolation, is the dominant driver of glacial cycles.

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