Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 64

The problem as Ruddiman explained is ''persistently low CO 2 concentrations
estimated for the past 22 million years'' during which the climate cooled substan-
tially. Although Ruddiman did not consider this, it appears that there are
inconsistencies between CO 2 and climate prior to 22 million years ago as well,
although, to some extent, higher CO 2 was roughly associated with warmer
climates. Ruddiman said: ''Paleoclimatologists were left with three possibilities.''
These were:

(1) They ''might have overlooked something crucial.''
(2) Effects other than CO 2 ''could have had a much stronger effect than thought.''
(3) ''The proxy methods used to reconstruct CO 2 concentrations prior to ice-core
records could be invalid.''

Ruddiman discounted the first possibility and leaned toward the third. The
argument seems to come down to this. Ruddiman doubts that there is a missing
factor, but known factors do not seem to explain the variability of climate. There-
fore, the only thing that he can think of that could be the cause of long-term
climate change is variable CO 2 . Since the data on CO 2 do not agree with this
precept, the data must be wrong. This type of argument has been used a number
of times recently in climatology. If the data do not agree with theory, throw the
data out! However, that seems antithetical to the scientific method.
One interesting event during this era was the so-called Paleocene-Eocene
Thermal Maximum (PETM) that occurred about 55 million years ago. There is
good evidence that T G rose by at least several degrees (some estimates range from
4to9 C) in as little as 10,000 to 30,000 years. It is widely believed that this could
only result from a sudden massive input of greenhouse gases. However, Zeebe
(2011):

...

estimated the size of the PETM carbon input based on sediment records
of deep-sea carbonate dissolution and showed that the subsequent rise in
atmospheric CO 2 alone was insucient to explain the full amplitude of global
warming. We concluded that in addition to direct CO 2 forcing, other processes
must have caused a portion of the PETM warming

''

. Our study showed that
there were processes in addition to CO 2 forcing that caused part of the warming,
not that CO 2 was irrelevant. The processes are as yet unidentified—some may
have operated independently, others as a response or feedback to the CO 2
release.''

...

In contrast to Zeebe's indication of uncertainty regarding the PETM, Kump
(2011) asserted that he understands the whole process. The initial release of CO 2
provided warming that added CH 4 to amplify the effects of CO 2 . In fact, Kump
(2011) provided a detailed description of the Earth during the PETM. Most of this
seems to be subjective cloth woven from invisible thread. The methane hydrate
hypothesis was discussed by Higgins and Schrag (2006) who concluded that
analysis of the PETM leads to ''a high climate sensitivity''. Pagani et al. (2006)
concluded: '' ... the PETM either resulted from an enormous input of CO 2 that