CO2 levels at the present time appear unusual, based on the current data.
Ice cores show lower CO2 levels 200-280 ppm but as a contrast paleo leaf stomata counts (reacting to CO2 fertilisation) suggest much higher fluctuations and peak levels throughout the investigated parts of the Holocene
Kouwenberg L.L.R., et al (2005). Atmospheric CO2 fluctuations during the last Millennium reconstructed by stomatal frequency analysis of Tsuga heterophylla needles. Geology, 33 (1), 33-36.
Wagner, F et al (1996) A natural experiment on plant acclimation: Lifetime stomatal
frequency response of an individual tree to annual atmospheric CO2 increase. Proc. Natl. Acad. Sci. USA Vol. 93, pp. 11705–11708, October 1996 Ecology
Wagner, F et al (1999), Century-Scale Shifts in Early Holocene Atmospheric CO2 Concentration Science 18 June 1999; 284: 1971-1973
Wagner, F et al. (2004). Reproducibility of Holocene atmospheric CO2 records based on stomatal frequency analysis. Virtual Journal Geobiology, volume 3, Issue 9, September 2004, section 2B.
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Even if some evidence were presented that demonstrates that historic CO2 levels were either the same as or greater than the present time's, it would lead to a nagging question, 'Why was CO2 so high during such and such epoch?' With today's CO2 increases, there is a clear source for it: burning of organic matter
Considering the global carbon budget the atmospheric carbon content has increased with 161 Pentagram from the original 590 Pg whilst the oceanic CO2 has increased with about 110 Pg at around 38,000 Pg, a quarter percent. Consequently what happens in the oceans is important for the carbon in the atmosphere. Change in vertical currents have a mega impact. Think of a beer bottle. If the pressure is realized the CO2 comes out of solution, Same happens at an oceanic convection current but in orders of magnitude that can seriously disrupt atmospheric CO2. Biota play little role in that.
For the numbers:
Sabine, Christopher L, Richard A. Feely et al. 2004. The Oceanic Sink for Anthropogenic CO2.. Science Vol. 305, No 5682, pp. 367-371, July 16, 2004
This is what I find in my home, PL, not oil compagnies.