Here @theFosterlab we use the boron pH proxy to reconstruct CO2 in the geological past beyond the reach of the ice core record (the last 800 thousand years, kyr). What the figure above shows is a compilation of the latest boron data including some data we published this year compared to actual measuresments of the atmosphere and bubbles of ancient atmosphere trapped in the ice cores. The first time we made a plot like this was a couple of years ago when CO2 was still just below 400 ppm. Given the relentless rise in atmospheric CO2 it looks like it will be around 20 years or so until even the highest values in the last 3 million years are exceeded. Pliocene - here we come!
The R-script to make the above figure can be downloaded here, the data here and the relevent references here. If you are interested in coming and working with us to reconstruct CO2 in the past we have several fully funded PhD studentships on offer (see here).