Atmospheric CO2 last stood above 400 ppm about three million years ago, during the mid-Pliocene.

Is it true that atmospheric CO2 last stood above 400 ppm about three million years ago, during the mid-Pliocene?

Partly. Evidence is mixed and only partial: a Pliocene modeling study and a Wikipedia summary citing the IPCC place CO2 near 400 ppm around 3-3.3 million years ago, while other items report CO2 above 400 ppm at roughly 10-16 million years ago, which would push the last such level more recent than the mid-Pliocene.

Verdict: CONTESTED PARTIAL · 2 sources for, 3 against · Last checked 3 Oct 2026

The private library was unavailable for some searches in this check. Missing library results are not evidence against the claim.

Modelling ice sheet evolution and atmospheric CO 2 during the Late Pliocene. (n.d.). cir.nii.ac.jp. https://doi.org/10.5194/cp-15-1603-2019

Are we in a low CO2 period, compared to the last 590 million years?. (2016). skeptics.stackexchange.com. https://skeptics.stackexchange.com/questions/35927/are-we-in-a-low-co2-period-compared-to-the-last-590-million-years

Carbon dioxide in the atmosphere of Earth. (n.d.). en.wikipedia.org. https://en.wikipedia.org/wiki/Carbon_dioxide_in_the_atmosphere_of_Earth

Was the cretaceous the last time atmospheric carbon dioxide was significantly higher than it is today?. (2012). skeptics.stackexchange.com. https://skeptics.stackexchange.com/questions/8904/was-the-cretaceous-the-last-time-atmospheric-carbon-dioxide-was-significantly-hi

A Limited Role for Carbonic Anhydrase in C4 Photosynthesis as Revealed by a ca1ca2 Double Mutant in Maize.. (2014). hal.inrae.fr. https://doi.org/10.1104/pp.114.237602