Summary of climate variations - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 204

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Summary of climate variations
Cooling over the past 50 million years Zachos et al. (2001) studied oxygen isotope
ratios in ancient sedimentary deposits and thereby estimated deep-sea temperatures
over the past 60 million years. Hansen et al. (2010) modified their results to take into
account the fact that isotope data represent a combination of deep-sea temperature
and ice sheet volume and, during periods when there were ice sheets, the data had to
be corrected for this (se e Figure 2. 7 ). Deep-sea temperature dropped by about 12 C
over the past 50 million years.
Hansen et al. (2010) claimed that deep-sea temperature is closely related to
global average temperature. They roughly estimated that changes in deep-water
temperature represent two thirds of global average temperature changes, which
implies that the change in global average temperature was perhaps 12 C over
the past 50 million years. That represents a huge cooling effect.
Origin of glaciation—past 34 million years This topic is discussed in Section
2.5.2. About 55 mybp , Australia began to drift northward away from Antarctica.
Evidence suggests that by 34 mybp , a circumpolar ocean current developed around
Antarctica that thermally isolated it, allowing glaciation to proceed. In addition,
there may have been a reduction in atmospheric CO 2 . Mountain building may have
hastened glaciation. Since about 34 mybp , the temperature of the Earth has been on a
generally downward path, permeated by significant oscillations.
The past 3 million years Around 3 mybp the Isthmus of Panama closed
isolating the waters of the Atlantic and Pacific Oceans. It seems likely that this
may have fundamentally changed global ocean circulation. Evaporation in the
tropical Atlantic and Caribbean left those ocean waters saltier. As this salinity
increased, the water transported northward in the Atlantic became warmer and
saltier. As this water reached high North Atlantic latitudes, it transferred heat and
moisture to the atmosphere, leaving behind cold salty dense water that sank toward
the ocean floor. This water flowed at depths, southward and beneath the Gulf
Stream, to the Southern Ocean. Hence, this thermohaline circulation was enhanced