Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 37

Figure 2.7. Estimated mean deep-sea temperature change over the past 60 million years. The
curve marked Zachos et al. (2001) includes the effects of ice buildup, while the curve marked
Hansen and Sato is attributed only to deep-sea temperature change.
tion at Antarctica and Greenland, the isotope ratios in sediments are partly due to
deep-ocean temperature change and partly due to the isotope effect in evaporation
as ice sheets form. They assumed that half of the isotope ratio was due to tem-
perature change during the last 35 million years.
Given Hansen and Sato's rough estimate of deep-ocean temperature change
over the past 50 million years, they raised the question of the relation of deep-
ocean temperature change to global mean surface temperature change. Here, they
developed an argument that makes some sense, but is quite subjective and not
nearly as ironclad as they implied. Their argument was:
''Deep ocean temperature depends on sea surface temperature at high
latitudes in winter, the location and season at which surface water is most dense
and sinks to the deep ocean. This leads us to infer that deep ocean temperature
change is a useful approximation of global mean surface temperature change on
millennial time scales. This fortuitous result is a consequence of substantial offset
between the two principal factors that would make temperature change at the
sites of deep-water formation differ from global mean surface temperature
change. First, temperature change at high latitudes is amplified relative to global
mean temperature change. But, second, temperature change is smaller over ocean
than over land. These two competing factors substantially offset one another.
Both of these tendencies (polar temperature change amplification and ocean
versus land temperature change diminution) are present in observational data
and models, and are well understood.''
First of all, it is not clear that the notion of a single global average
temperature has much utility. Second, while it may well be true that deep-ocean
temperatures are related to global average temperatures, it is by no means clear