Atmospheric absorption of near infrared and visible solar radiation by the hydrogen bonded water dimer

Based on the physico‐chemical properties of water dimers, their near infrared and visible absorption of solar radiation in the earth's atmosphere is calculated. The calculation uses equilibrium constants determined by statistical mechanics, and a vibrational absorption spectrum determined by a coupled oscillator quantum mechanics model and ab initio quantum chemistry. The resulting total atmospheric absorption was calculated using a line‐by‐line radiative‐transfer model, and depends significantly on the dimer abundance, as well as on the frequency and line width given to vibrational transitions. The best estimate achieved for the possible range of total absorption from 400 nm to 5000 nm by water dimer in the tropics is 1.6–3.3 W m−2. In a global‐warming scenario, the increased temperature and water vapour partial pressure result in a nonlinear increase in the absorption of solar radiation by water clusters. Most of the energy from water dimer absorption is deposited in the lower troposphere, particularly in the tropics, tending to make it more convective.