evidence: - The daily average concentration of atmospheric CO2 at Mauna Loa Observatory first exceeded 400 ppm on 10 May 2013[27][28] although this concentration had already been reached in the Arctic in June 2012.[29] Data from 2013 showed that the concentration of carbon dioxide in the atmosphere is this high "for the first time in 55 years of measurement—and probably more than 3 million years of Earth history."[30] [...] Concentrations in the geologic past [edit]Estimates in 2023 found that the current carbon dioxide concentration in the atmosphere may be the highest it has been in the last 14 million years.[11] However the IPCC Sixth Assessment Report estimated similar levels 3 to 3.3 million years ago in the mid-Pliocene warm period. This period can be a proxy for likely climate outcomes with current levels of CO2.[101]: Figure 2.34