Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 65

currently defies a mechanistic explanation, or climate sensitivity to CO 2 was
extremely high.''
As Royer et al. (2011) pointed out, ''the PETM is considered a paleo-analog
of present day climate change in terms of rate and magnitude of carbon release''
although, as Kump (2011) emphasized, the annual release of carbon during the
PETM was far less than today's, but it was sustained over a much longer time.

2.3.9 100 to 300 million years ago

Royer et al. (2011) compared crude estimates of CO 2 concentration with estimates
of benthic d 18 O and tropical sea surface temperatures (SST) over the time range
from 125 million years ago to 50 million years ago. The CO 2 and SST data show
considerable scatter. There was a warm period at around 55 million years ago but
it does not seem to have been accompanied by higher CO 2 . In this regard, Royer
et al. (2011) chose to ignore multiple CO 2 measurements near 55 million years ago
that were low and, instead, accepted one outlier measurement that was four times
higher. From this, they derived a high sensitivity of D T G to CO 2 concentration.
This result does not seem credible to this writer.

2.3.10 Estimates of climate sensitivity based on CO 2 and climate in
the Phanerozoic

2.3.10.1 Introduction

During the Phanerozoic (the past 540 million years or so), the Earth experienced
significant changes. These included redistribution of continents via continental
drift, the emergence of vascular plants driving up oxygen content in the atmo-
sphere, changing CO 2 concentrations (as high as 20 times current levels in some
periods), and many other changes, as discussed by Berner (2004). One particular
time period, the so-called Permo-Carboniferous between about 330 and 280
million years ago, was marked by extensive world glaciation, low CO 2 levels, and
high oxygen content (30-35%). In addition, the brightness of the Sun increased
with time across this eon.
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 a number of
assumptions. Various investigators have arrived at different interpretations
regarding the connection between CO 2 concentrations and climate change during
the Phanerozoic. Most have concluded that changing CO 2 concentrations was the
main factor producing long-term climate change. Others claim that the effect of
CO 2 was secondary and galactic cosmic ray variability was the important factor.

2.3.10.2 Climate during the Phanerozoic

Veizer (2005) pointed out: ''in the Phanerozoic, some organisms secreted their
shells as the mineral calcite (CaCO 3 ), which often preserves the original oxygen