Water is Earth’s principal greenhouse molecule: It’s responsible for well over half of the atmosphere’s absorption of solar and terrestrial radiation. So it’s a crucial ingredient in any model of Earth’s radiation balance and climate. But the study of atmospheric water’s absorption spectrum has for decades been fraught with mystery and controversy. In addition to the expected spectral lines—corresponding to transitions between discrete rotational, vibrational, and electronic quantum states—the spectrum includes a broad continuum absorption that has yet to be fully explained.1 

Beginning in the late 1960s, and especially since the late 1990s, some researchers have suspected that at least part of the continuum could be due to water dimers. As gas-phase molecules collide with one another, every so often two of them form a hydrogen bond (as shown in the figure), remain together for a time, and then go their separate ways. The dimer would have a...

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