The mean surface temperature on Earth has varied nearly synchronously with the carbon dioxide content of the atmosphere over the last million years. From the air bubbles trapped in ice cores across the globe, we know that glacial periods had an atmospheric concentration of about 200 ppm and interglacial periods had an atmospheric concentration of about 280 ppm. Since 1850, the atmospheric concentration has risen to 370 ppm, and is still rising.

The key process on glacial/interglacial timescales, is a change in the ventilation of the deep Southern Ocean. Poleward-shifted westerlies bring deep water laden with respired CO2 directly up to the surface where the respired CO2 is easily vented to the atmosphere. Equatorward- weakened westerlies shut down this process by allowing a low-salinity cap to develop over the Southern Ocean. The tendency for poleward-shifted westerlies in a warm climate and equatorward-weakened westerlies in a cool climate pushes the climate system toward warm or cold extremes and gives rise to the feedback. Atmospheric CO2 and Antarctic temperatures rise and fall together as part of the feedback (Toggweiler, Russell and Carson, 2006).

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