Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 63

Figure 2.23. CO 2 concentration and benthic d 18 O (inverse measure of temperature) over the
past 60 million years (adapted from Ruddiman, 2010).
volcanic CO 2 input to the ocean and atmosphere because of a slowing rate of sea-
floor spreading and increased CO 2 removal by enhanced chemical weathering in
tectonically uplifting regions like Tibet.
In a broad sense, this long-term CO 2 decrease provided some support for the
idea that CO 2 has been the long-term driver of global cooling, but a closer look
revealed major problems. By 22 million years ago, the alkenone and boron
isotope data both showed that estimated CO 2 concentrations were already within
the range typical of the glacial cycles of the past 800,000 years. If CO 2 concentra-
tions of 180 to 300 ppm have played an integral role in allowing glacial cycles in
the past 800,000 years, why did comparably low CO 2 values 22 million years ago
not initiate glacial cycles? And if the average CO 2 trend has not fallen in the past
22 million years, what caused the substantial bipolar cooling during that time?
Other proposed causes seem insucient to explain large-scale cooling. Gradual
plate motions and falling sea level have extended the northern margins of circum-
Arctic continents into cooler near-polar latitudes, but models suggested that
these factors were not enough to explain the major cooling observed.''