text: 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. 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 .
  Carbon dioxide is believed to have played an important effect in regulating Earths temperature throughout its 4.54 billion year history. Early in the Earths life, scientists have found evidence of liquid water indicating a warm world even though the Suns output is believed to have only been 70% of what it is today. Higher carbon dioxide concentrations in the early Earths atmosphere might help explain this faint young sun paradox. When Earth first formed, Earth's atmosphere may have contained more greenhouse gases and  concentrations may have been higher, with estimated partial pressure as large as , because there was no bacterial photosynthesis to reduce the gas to carbon compounds and oxygen. Methane, a very active greenhouse gas, may have been more prevalent as well.
  Carbon dioxide concentrations have shown several cycles of variation from about 180 parts per million during the deep glaciations of the Holocene and Pleistocene to 280 parts per million during the interglacial periods. Carbon dioxide concentrations have varied widely over the Earths history. It is believed to have been present in Earths first atmosphere, shortly after Earth's formation. The second atmosphere, consisting largely of nitrogen and  was produced by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids. A major part of carbon dioxide emissions were soon dissolved in water and incorporated in carbonate sediments.
  The production of free oxygen by cyanobacterial photosynthesis eventually led to the oxygen catastrophe that ended Earths second atmosphere and brought about the Earths third atmosphere (the modern atmosphere) 2.4 billion years ago. Carbon dioxide concentrations dropped from 4,000 parts per million during the Cambrian period about 500 million years ago to as low as 180 parts per million 20,000 years ago .