PerspectiveAtmospheric Science

How Was Early Earth Kept Warm?

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Science  04 Jan 2013:
Vol. 339, Issue 6115, pp. 44-45
DOI: 10.1126/science.1232662

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Why do some gases cause greenhouse warming, whereas others do not? H2O is a greenhouse gas because it has a permanent electric dipole moment (a charge separation within the molecule) that allows it to interact strongly with electromagnetic radiation. CO2 also has an electric dipole moment, but it has to bend or stretch asymmetrically to create it, because, unlike H2O, it is a linear molecule. N2 and O2 are not normally considered to be greenhouse gases, because these symmetric, diatomic molecules have no electric dipole moment and cannot bend or stretch to create one. But as Wordsworth and Pierrehumbert show on page 64 of this issue (1), N2 and molecular hydrogen (H2) can be greenhouse gases under the right conditions; H2 may have been important for Earth's Archean climate (before 2.5 billion years ago).