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The Cenozoic Evolution of Atmospheric Carbon Dioxide

"One approach toward assessing the role of pCO2 in forcing climate change is to evaluate records of isotopic fractionation that occurred during marine photosynthetic carbon fixation. The isotopic composition of photosynthetic marine organic carbon is primarily a function of [CO2aq], growth rate, and cell geometry of the organism. By sampling sedimentary alkenones from oligotrophic-type settings, the effect of growth rate and cell geometry is presumably minimized, thereby leaving [CO2aq] as the major control on alkenone isotopic compositions.

Our results show that pCO2 ranged between 1000 to 1500 ppmv in the middle to late Eocene, and then decreased in several steps during the Oligocene, and reached modern levels by the latest Oligocene. The fall in pCO2 likely allowed for a critical expansion of ice sheets on Antarctica, and promoted conditions that forced the onset of terrestrial C4 photosynthesis."

Google: it's good for your brain.

Now it's up to other scientists to come up with alternative Eocene-Oligocene climate scenarios where the radiative forcing effects of CO2 are not the major cause of significant climatic cooling, because the researchers have shown a) that CO2 levels were dropping significantly when the ice sheets were established and expanding, and b) the likeliest cause of the cooling was the reduced greenhouse effect due to lower levels of atmospheric CO2 (climate modeling, dontcha know).

For Sheppard to say there's "no evidence"; I repeat the statement of "ridiculous".

Google: it's good for your brain.

I do it often. That's how I usually find the latest stories on global cooling.

So what accounts for the decrease in atmospheric CO2, i.e. what caused it besides a prior decrease in temperature? What flora, which consume CO2, and fauna, which produce it, in the biosphere would prosper with the decreasing temperature that would consume atmospheric CO2? What microbes prosper at lower temperatures that consume CO2 are there? What other sinks do you have besides water and carbonate formation, which is exothermic?

However, pCO2 records derived from stomata densities of terrestrial plant leaf remains suggest that pCO2 during the Eocene was not substantially higher than modern concentrations. In either case, it would appear that changes in post-middle Eocene climates were driven by factors other than pCO2, such as changes in continental elevations, oceanic circulation, and possibly sea level.

How about the earth got cooler?

My results indicate that following the expansion of ice on East Antarctica (ca. 14.5-12 Ma), pCO2 steadily increased until about 9 Ma and stabilized at pre-industrial values (ca. 290 ppmv; generally below the threshold level required by the pCO2-C4 hypothesis).pCO2 remained relatively constant throughout the late Miocene and therefore provide no evidence that changes in pCO2 forced ecological change during this time.

I believe you have the cart before the horse. Colder temperatures takes the CO2 out of the atmosphere. Warmer temperatures degas the oceans. The CO2 levels in the late Eocene were about twice to over five times current levels. Other eras have had more than ten times current levels, IIRC. Thanks for the graph & link.

I wrote, "Warmer temperatures degas the oceans."

Degas is only an artist's surname so far. I should have written warmer temperatures will cause the degasification of the oceans dissolved carbon dioxide.

Colder temperatures increase the solubility of carbon dioxide in the oceans. The carbon in the atmosphere has to go somewhere. It can go into the oceans with cooler temperatures. Most forms of living things in the biosphere, except those adapted to living at or near polar regions, fair less well in cooler temperatures, i.e. their metabolisms slow down.

The question is why did the temperature drop? The atmospheric CO2 decrease was a natural result. It's not that people stopped burning wood and eating plants. It is from one or more natural phenomena, i.e. Milankovitch Cycles, decreased sunspot activity like that being observed now, increased volcanic activity spewing sulphates in the air or a catastrophic impact with an object from space putting a massive amount of dust in the atmosphere.