Box 1 The rough ride through deglaciation

The upper and lower panels here show high-pass filtered 'proxy' temperature records from ice cores in Greenland12 and Antarctica13, respectively, indicating the abrupt coolings and warmings between 20,000 and 10,000 years ago, superimposed on the general warming trend that characterized the transition from glacial to interglacial conditions. The centre panel shows the inferred courses of events in the north and south.

(1) About 18,000 years ago, changes in Earth's orbital parameters initiate the end of the ice age through increased solar radiation. (2) Fresh water from melting northern ice sheets shuts down thermohaline circulation in the North Atlantic. (3) A 'seesaw' mechanism (grey arrow), which connects the polar regions north and south10, enhances warming in the south. This warming turns on the thermohaline circulation in the Atlantic rapidly, by both (4) a surface advection of saline waters2 and (5) a large-scale discharge of melt water in the south14, which reduces the density of deep water masses formed around Antarctica7, 8.

The north is now in a warm phase, and the seesaw produces (6) the cooling seen in Antarctica from around 14,000 years ago15, and also accelerates melting of northern ice sheets, which reduces the Atlantic thermohaline circulation and triggers an abrupt cooling in the north (7). This stimulates, again by the seesaw, southern warming (8). Finally, the southern warming turns on the thermohaline circulation in the North Atlantic, leading to the final warming in the north (9) that marks the beginning of the current interglacial.

The grey vertical arrows indicate the operation of the seesaw. The red and green arrows indicate the two different mechanisms2, 7, 8 that emerge from modelled simulations of climate, and that are proposed to be the southern cause of switching on the thermohaline circulation in the North Atlantic.

Thomas F. Stocker (1)