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and arachnids confined to this same time span, and these organisms also
metabolize by passive diffusion. Thus, animal fossils provide further evidence for
the hypothesized high O 2 concentrations during the Permo-Carboniferous''
(Berner, 2004). The principal cause of the high O 2 values was the rise of large
vascular land plants that brought about increased O 2 production due to the
increased global burial of microbially resistant
lignin-rich organic matter in
sediments.
2.3.10.8 Phanerozoic summary
As with almost every area of climatology, data on the Phanerozoic climate and
CO 2 concentrations are sparse and noisy, and the interpretation of the data in
terms of climatological parameters requires complex models and various assump-
tions. Various investigators have arrived at different interpretations regarding the
connection between CO 2 concentrations and climate change during the
Phanerozoic. As Figure 2.35 shows, Shaviv and Veizer (2003) found a periodic
variation in the climate that seemed to match variations in cosmic ray flux, and
CO 2 variations that did not seem to be highly relevant to climate change.
However, a different interpretation of the data suggests that CO 2 variability was
associated with climate change and cosmic rays play at most a secondary role.
The evidence suggests that the Earth was relatively warm from about 550 to
400 million years ago, although the temperature may have varied considerably
within that time frame. Around 400 million years ago, the Earth began to cool,
and the cooling bottomed out with extensive glaciation for about 40 to 50 million
years approximately 330 million years ago. Subsequently, the Earth warmed again,
and finally cooled again during the most recent 100 million years. Variations of
the climate on shorter time scales within that general scope are subject to consider-
able uncertainty. The data and models for CO 2 suggest that R(CO 2 ) was very
large prior to about 300 million years ago, peaking about 550 million years ago.
R(CO 2 ) declined slowly from 550 million years ago and very rapidly from 400 to
350 million years ago. The cold period centered around 300 million years ago was
associated with very low values of R(CO 2 ). Temperature rose significantly after
about 280 million years ago but CO 2 rose only very moderately. Over the last 100
million years, temperatures and R(CO 2 ) both declined. There seems to be very
little doubt that major changes in the Earth's climate are at least sometimes
associated with large changes in R(CO 2 ).
Boucota and Gray (2001) provided a very lengthy detailed review of
Phanerozoic climatic models and the relationship to the CO 2 content of the
atmosphere. They concluded:
''
...
considerable disparity exists between the curves generated by the varied
models
. The wide disparities between the various published curves suggest
that the presently published models are inadequate. Considerable disparity also
exists between all the models and the geological climatic evidence indicating
changing climatic gradients through the Phanerozoic. This indicates, based on
...
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