When was CO2 last at 400 ppm? And what was the climate like?

May 13, 2013   //   by Athena   //   Blog, Earth System  //  Comments Off on When was CO2 last at 400 ppm? And what was the climate like?
Paul N. Pearson, School of Earth and Ocean Sciences, Cardiff University CF10 3AT, UK.
Email: pearsonp@cardiff.ac.uk
Summary Atmospheric CO2 is approaching the 400 ppm mark for the first time in human history which begs the question: when was it last that high? A recent high profile suggestion is that CO2 was that high in the Pliocene epoch (approximately 2.6-5.3 million years ago) and this is now being repeated in the press and around the internet. Here I point out that this claim is based on a few extreme estimates, mostly from sites that systematically overestimate more recent CO2 levels, while the majority of published Pliocene CO2 values are in the 250-400 ppm range. The last time we have consistent evidence for pCO2 over 400 ppm is in the Early Oligocene epoch more than 26 million years ago. This post presents the key graphs and comments on some of the methods used to calculate past pCO2.
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