WATER-VAPOR AND GREENHOUSE TRAPPING - THE ROLE OF FAR-INFRARED ABSORPTION

Few observations have been made of atmospheric absorption across the f ar infra-red. Yet water vapour absorption in this spectral region may significantly effect climate. The impact of far infra-red absorption i s assessed by calculating the spectral variation of the total and wate r vapour greenhouse effects, for the sub-arctic winter (SAW) and tropi cal (TRP) standard atmospheres. Although the calculated efficiency of greenhouse trapping peaks outside of the far infra-red, the low streng th there of the Planck function causes relatively small absolute forci ngs, except in the carbon dioxide and ozone bands. The sensitivity of the normalised greenhouse effect to water vapour concentration is larg est in the far infra-red for the SAW atmosphere, and in the window reg ion for the TRP. The sensitivity differs most between the two atmosphe res in the far infra-red. Maximum water vapour greenhouse trapping ari ses in the far infra-red, over the middle/upper troposphere; in the SA W case the contribution from the water vapour continuum is virtually e liminated. Improved spectral observations and simulations at far infra -red wavelengths thus appear necessary to better understand the contem porary greenhouse effect, and to validate models of climate change.