Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints

@article{Som2012AirD2,
  title={Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints},
  author={Sanjoy M. Som and David C. Catling and Jelte P. Harnmeijer and Peter M. Polivka and Roger Buick},
  journal={Nature},
  year={2012},
  volume={484},
  pages={359-362}
}
According to the ‘Faint Young Sun’ paradox, during the late Archaean eon a Sun approximately 20% dimmer warmed the early Earth such that it had liquid water and a clement climate. Explanations for this phenomenon have invoked a denser atmosphere that provided warmth by nitrogen pressure broadening or enhanced greenhouse gas concentrations. Such solutions are allowed by geochemical studies and numerical investigations that place approximate concentration limits on Archaean atmospheric gases… 

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