Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 50

extreme than the result of Pagani et al. By contrast, Brierley et al. (2009) claimed
that ''a vast poleward expansion of the ocean tropical warm pool'' was responsible
for Pliocene warmth. However, Haywood and Williams (2005) concluded from
their study:

''Our results suggest that mid-Pliocene warming was caused by more carbon
dioxide in the atmosphere combined with climate feedbacks associated with
smaller ice sheets. Since the pattern of sea temperature change reconstructed
from alkenones, and predicted by our climate model, is not consistent with that
produced through changes in ocean circulation/ocean heat transport, we also
conclude that there was no major change in thermohaline circulation at that
time.''

Those who believe that CO 2 is the sole arbiter of climate will attribute the
warming of the Pliocene entirely to CO 2 and will therefore conclude that, if we
wait long enough at a fixed CO 2 concentration of 395 ppm, the Earth will slowly
approach Pliocene conditions and that, if we hold CO 2 at 560 ppm and wait long
enough, T G may rise by as much as 9.6 C (see Figure 2.1 2 ). Despite the many
publications on the subject, what seems to be missing is this: We need a picture of
the Earth under Pliocene conditions, particularly the extent of the ice sheets, but
also ocean circulation, the degree of cloudiness, and the plant coverage of the
Earth. Forcings need to be estimated for these factors (and more).

2.3.5 The past 20 million years or so
Pearson and Palmer (2000) described ''the boron-isotope ( d 11 B) approach to pCO 2
estimation that relies on the fact that a rise in the atmospheric concentration will
cause more CO 2 to be dissolved in the surface ocean, causing a reduction in its
pH''. They were able

to estimate the pH of ancient sea water by measuring the boron-isotope
composition of calcium carbonate ( d 11 B CC ) precipitated from it. This is because
boron in aqueous solution occurs as two species, B(OH) 3 and B(OH) 4 , between
which the equilibrium is strongly pH-dependent over the natural acidity range of
sea water. Furthermore, there is a pronounced isotopic fractionation between the
species

'' ...

, so that the ( d 11 B) of each species is highly dependent on pH. Because
boron incorporation into marine carbonates is predominantly from B(OH) 4 ,
( d 11 B CC ) is a sensitive pH indicator. The pH of seawater is governed by the
carbonate equilibria, such that for a given pH value it is possible to calculate
the aqueous CO 2 concentration and thereby make quantitative estimates of
atmospheric pCO 2 . The pH and aqueous CO 2 concentration of the surface ocean
vary spatially because of factors such as deep-water upwelling, local productivity
regimes and freshwater inflows. To arrive at pH estimates that most closely
reflect atmospheric pCO 2 , it is necessary to measure the ( d 11 B) of carbonates
that were precipitated far from coastal influences and sources of upwelling. The

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