Earth's history is marked by episodes of large-scale continental glaciation. Most recently, beginning 3 million years ago, northern hemispheric glaciation expanded and developed cyclic variations known as the ice age cycles. With the 19th-century discovery of these cycles in ice extent and climate, changes in atmospheric carbon dioxide (CO2) concentration were proposed as a possible cause. Since the 1980s, scientists have produced detailed reconstructions revealing that, during ice ages, atmospheric CO2 was as much as a third lower than its preindustrial concentration—enough to explain almost half of the approximately 5 °C ice age cooling by weakening the Earth’s natural greenhouse effect. The consensus is that the ice age climate cycles result from cyclic changes in Earth’s orbit, which redistribute sunlight between regions and seasons but do not in themselves significantly heat or cool the globe on an annual-average basis. If so, the regional and seasonal effects of orbital change must cause changes in aspects of the Earth system that then induce changes in global annual-average climate. Changes in the reflection of sunlight by the ice sheets are widely believed to have played such a role. Atmospheric CO2 appears to be a second key Earth system property, and one that caused the ice age cycles to be global rather than simply regional phenomena. The ocean was likely the dominant driver of atmospheric CO2 change between warm “interglacial” and cold “glacial” periods, through multiple aspects of its behavior. First, ice age cooling and other changes allowed bulk global seawater to absorb additional CO2 from the atmosphere. Second, during ice ages, the ocean’s “biological carbon pump” was stronger: Ocean plankton and their sinking debris more effectively removed CO2 from surface waters and the atmosphere, sequestering it in the ocean interior. Polar ocean changes were key to this stronger biological pump, involving some combination of changes in biological productivity, ocean circulation, and air–sea gas exchange. Third, the net effect of these ocean changes was to enhance deep ocean CO2 storage and thus to dissolve calcium carbonate sediment off the seafloor, changing the ocean’s acid/base chemistry so that it absorbed additional CO2 from the atmosphere. The specific polar ocean changes that drove the strengthening of the biological carbon pump and the ensuing seafloor calcium carbonate response are a topic of ongoing debate.
