A record of past oceanic temperatures and ice-cap growth can be extracted from measurement of oxygen-isotope ratios (expressed as d¹⁸O above) in marine fossils. Data collected from surface-dwelling and bottom-dwelling critters allows us to infer the thermal structure of the oceans through time. This information helps us to determining the rates and patterns of oceanic circulation, which in turn are critical parameters in determining global and regional climate. Today, seawater that is cooled at the North and South poles sinks and flows along the bottom toward the equator. Deep waters are thus very cold even at the tropics. In the figure above, modern deep-water oxygen-isotope ratios are quite heavy (cold temperatures) compared to the considerably warmer temperatures of the mid-latitudes (say, 45º N or S) or the hot temperatures of the tropics (equatorial latitudes). The fatness of the lines in recent times covers the rapid oscillations in climate stemming from the frequent coming and going of the Northern Hemisphere ice sheets.

Over the past 70 million years, it is clear that the temperatures in the tropical surface oceans have stayed roughly the same. Meanwhile, the mid-latitude surface and bottom water data show similar cooling trends up to about 35 million years ago. From 35 million years to the present, the bottom waters continue a strong cooling trend. As the later reflect polar climates most directly, let's focus on this record. From 70 to 50 million years ago the bottom waters were as warm as the modern mid-latutide surface oceans. But then started a long period of declining temperatures punctuated by a sudden plunge at about 38 million years. This marks the onset of continent-scale glaciation in Antarctica; there are as yet no major ice caps in the Northern Hemisphere. It is worth noting that from this point on the changing volume of water locked up in the ice caps also impacts that oxygen isotope record; thus, the continuing decline over time is a combination of cooling polar temperatures and growing ice caps.

At about 18 million years there is a sudden warming. What caused this? Some have speculated that the massive eruptions of the Columbia River Flood Basalts added enough CO₂ to the atmosphere to temporarily warm global climate. The warming is followed by a decrease in temperatures that lead to the onset of continent-scale Northern Hemisphere glaciation about 2.6 million years ago. From this time to the present, the glaciar have been coming and going with a 40,000 to 100,000 year frequency that is modulated by the wobbling orbit of the Earth.

The causes of this general cooling over the last 50 million years are not fully understood. One important factor is probably the opening of the Drake Passage between Antarctica and South America. This allowed continuous flow of the Circumpolar Current around Antarctica and thereby thermally isolated this vast continent. The uplift of the Himalayas is proposed to have accelerated rates of chemical weathering such that excess burial of marine carbonates (CaCO₃) removed CO₂ from the atmosphere and thereby caused global climate cooling. The onset of Northern Hemisphere glaciation may have been related to volcanic activity in the Kurile and Kamchatka arcs (eastern Siberia). The idea is that they put enough ash into the upper atmosphere to cool the Northern Hemisphere by reflecting back a significant proportion of incoming sunlight. Each of these mechanisms still requires more evidence related to the relative timing of these events plus a detailed understanding of the relalative magnitude of climatic change that each event could cause. There is still a lot to do!