In recent years, the continuous decline in levels of the greenhouse gas CO2 in the atmosphere and a concomitant global cooling have been considered to be one likely cause for glaciations (Ruddiman 1997, Broecker 1995). Computer simulations by Berger et al. (1999) show that, to trigger ice ages in this way, atmospheric CO2 must have decreased from more than 320 parts per million by volume (ppmv) to 200 ppmv during the past 3 Ma.
Several authors such as Pearson and Palmer 1999, Pagani et al. 1999 aim at estimating past atmospheric CO2 contents from chemical (alkenones) and isotope data. A crucial point is the transfer between measured data and atmospheric gas concentrations. As long as one step involves greenhouse experiments the applicability is restricted to the Quaternary. The reason is that biological systems discriminate well in the data-range for which they are adopted. This is for plants often between 100 and 1200 ppm. Data well beyond this range, such as 10%, CO2 can normally not be resolved well by Holocene biological systems. Such high percentages occur if for example as first lower estimate the known Tertiary coal and oil deposits are recalculated to moles and then to respective CO2 contents.
Thus, at present, the precise value of the CO2 levels during the Teriary is, until new methods appear, regarded as unknown. Given the amount of Tertiary coal deposits it is expected to be higher than at present. The author thinks therefore that cooling towards the Quaternary may or may not be linked to a reduction of CO2 concentrations. Oceanographic, orographic (Tibet Plateau) and factors generated by the system itself, such as high temperature gradients, contributed to climate change.
For the Eem (MIS 11) CO2 concentrations might have been slightly above the pre-industrial value of 270 ppm around > 300 ppm (Fischer et al. 1999). For selected times of the Tertiary temperatures might have been even lower than at present.
The fine-scale records of the GRIP ice core confirm that during the last three glaciations the CO2 values lagged climatic change by as much as several thousands of years (Fischer et al. 1999, Mudelsee 2001). This reverses the previously accepted causal nexus:
Based on principles of physics elevated CO2 concentration contribute to elevated temperatures until upper limits are reached. It is however also possible that, depending on the boundary conditions that apply, atmospheric changes can be followed by elevated atmospheric CO2 concentrations.
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