Climate Science

Not So Clear

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Science  09 Sep 2011:
Vol. 333, Issue 6048, pp. 1361
DOI: 10.1126/science.333.6048.1361-c

Water vapor is the greenhouse gas with the greatest effect on the radiative balance of Earth's atmosphere, and it amplifies climate warming through positive feedback. Therefore, knowing precisely how much radiative forcing water vapor provides is of great importance for understanding atmospheric physics and climate change. Ptashnik et al. report laboratory measurements of the absorption of radiation by water in the near-infrared, performed at a range of temperatures and pressures. The data show that spectrally broad continuum absorption (as distinct from the better-characterized series of sharper, higher–cross-section resonances) is actually much higher than commonly assumed in atmospheric models. The result of this difference amounts to a globally averaged value of about 0.75 W/m2 of additional radiative forcing, roughly 0.2% of the total solar input at the top of the atmosphere and about 1% of the global mean clear-sky atmospheric absorption. The authors speculate that this extra absorption could be due to the effect of water dimers.

J. Geophys. Res. 116, D16305 (2011).

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