Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 49

warmth? Or was this aided and abetted by other factors such as more intense
ocean circulation transporting heat from the tropics to the higher latitudes?''
Schneider and Schneider (2010) reviewed the work of several investigators
regarding the relationship of CO 2 concentration to climate in the early Pliocene
(3-5 million years ago). There is considerable evidence that about 3 million years
ago, the global average temperature was several degrees warmer than it is today.
Haywood and Valdes (2004) provide numerous references to previous work on
''sea surface temperatures (SSTs) reconstructed from planktonic foraminifera,
ostracods, siliceous microfossil records, diatom records, terrestrial vegetation
records and numerous records of higher than present sea levels.'' These investiga-
tors used ''the alkenone CO 2 method to reconstruct Pleistocene-Pliocene pCO 2
histories from six ocean localities.'' However there was a wide diversity in inferred
CO 2 concentration at the six sites, which lends some doubt as to their accuracy.
Nevertheless, there were some common features. All of the sites indicated a signifi-
cant decrease in CO 2 concentration from 5 million years ago toward 1 million
years ago. According to Pagani et al. (2010), CO 2 concentrations were between
365 and 415 ppm about 4.5 million years ago when temperatures were 3-4 C
warmer than pre-industrial values. Seki et al. (2010) arrived at even lower CO 2
concentrations. If these estimates are correct, CO 2 concentrations were comparable
with those of today, yet the Earth was considerably warmer. Alarmists such as
Pagani et al. (2010), who assume that CO 2 is the primary forcing for climate
change, concluded that the longer term Earth system climate sensitivity is much
higher than the fast feedback climate sensitivity (using the political definition of
climate sensitivity). In fact, Pagani et al. (2010) suggested values as high as 9.6 C
for a CO 2 doubling from the pre-industrial value. It is not totally clear what they
meant by this, but apparently they believe that if we hold the CO 2 concentration
at 560 ppm and wait long enough, T G will gradually rise by up to 9.6 C.
Haywood and Valdes (2004) pointed out:
''Numerous proposals exist within the literature to account for the relative
climatic warmth of the middle Pliocene. These include increased concentrations
of CO 2 , enhanced thermohaline circulation, a more vigorous flow of surface
ocean gyres, alterations in the outflow of Antarctic deep water, and changes
in the elevations of mountain chains. All of these explanations have weaknesses
when examined in detail and there may have been numerous contributing factors
to middle Pliocene warmth. For example, it has been suggested that the warmth
was generated through a combination of enhanced atmospheric CO 2 and an
increase in thermohaline circulation.''
Their modeling led them to conclude that the main forcing that produced
higher temperatures during the Pliocene was reduced land ice cover, ''but with
strong positive feedbacks from clouds.'' Lunt et al. (2010) analyzed the Pliocene
with a climate model and concluded that the Earth system climate sensitivity is
30-50% greater than the fast-feedback sensitivity, which is considerably less