It is widely accepted that there is a relationship between ice volume and the solar insolation in summer in the northern hemisphere. The Earth''s glacial cycles are driven by cyclic changes in the Earth''s orbital elements. This conclusion is based on the strong coherence between the approx. 20000 and 40000 year spectral components of ice volume and insolation (Milankovitch-curve) records. These frequencies are determined by the variation of the obliquity of the Earth''s axis and by its position relative to the Earth''s orbit around the sun. The degree of sum mer insolation on the Earth''s northern Hemisphere is believed to be relevant to climate because the North Atlantic is where cold saline water is being formed. Present day deep water circulation is driven by salt build-up in due to net evapora tion. In contrast, in the North Pacific precipitation exceeds evaporation. Thus, deep water transfers a surplus of salt from the N. Atlantic to the North Pacific. This surface water delivers also oxygen to the deep ocean. In contrast, upwelling deep water transfers nutrients from the deep ocean to the surface water. Today the time of renewal of deep water is in the order of 1000 years.
