Ice age initiation by an ocean-atmospheric circulation change in the Labrador Sea

The contrast between the large Greenland ice sheet and the nearly ice-free area of Canada to the west exemplifies the critical role of the moisture supply in glaciation. The trigger for new ice-sheet growth in Canada at the end of the last interglacial was probably an increase in the moisture supply rather than regional cooling. Triggering is attributed to stronger storm activity in the Labrador-Baffin area, caused by warming in the Labrador Sea and cooling in the Greenland-Norwegian Sea. The key to this temperature and circulation change may have been an increase in the hydrologic deficit of the Mediterranean Sea. A larger hydrologic deficit would have increased Mediterranean salinity and, consequently, the volume of Mediterranean Overflow Water. A resulting increased upwelling of Mediterranean Overflow Water from the lower thermocline off Scotland would have diverted additional warm North Atlantic Drift water westward via the Irminger Current, thus warming the Labrador Sea. The underlying cause of the hydrologic deficit increase was diminishing Milankovitch summer insolation in lower latitudes. This reduced African monsoon intensity and Nile River discharge. In modern times nearly all Nile discharge to the Mediterranean has been lost due to irrigation practices, culminating with the completion of the Aswan High Dam in 1968. The total increase in the hydrologic deficit due to Nile discharge loss is now 8%. In the next 70–100 years the Nile loss, added to larger evaporative losses due to CO2 warming, may increase the deficit as much as 22% above the inferred deficit at the beginning of the last glaciation thus further warming the Labrador Sea. Consequently, the triggering of major ice-sheet growth in Canada ∼ 100 years from now is possible.