[denim_chicken]

The current level of atmospheric CO2 is not unprecedented, but the current rate of change of atmospheric CO2 is. Couple that with massive ecological decimation and biodiversity due to human habitation and it is clear that our current predicament is geologically unprecedented.

[akvadrako]

I don't think we have the resolution from the ages of 4000ppm to determine if the rate of CO2 change was comparable to now.

If you have any links to sources that show otherwise, that would be appreciated.