Effects of increased near-infrared absorption by water vapor on the climate system

Recent improvements in the spectroscopic data for water vapor have significantly increased the near‐infrared absorption in models of the Earth's atmosphere. The climatic effects of increased near‐infrared absorption have been simulated with the latest Community Atmosphere Model (CAM3). The shortwave parameterization in CAM3 has been updated to minimize differences between CAM3 and line‐by‐line (LBL) calculations based upon the High Resolution Transmission (HITRAN) spectroscopic database issued in 2001. The new model reproduces LBL calculations of the near‐infrared absorption to within 0.9% ± 1.4% and the near‐infrared heating rates to within 0.02 ± 0.02 K d−1. Estimates of the global annual mean shortwave absorption by water vapor have been calculated from the editions of the AFGL and HITRAN databases issued in 1982 and 2001, respectively. The main changes in water vapor spectroscopy during this period are the addition of many missing weak lines and increased estimates of line strength in near infrared wavelengths. The clear‐sky and all‐sky shortwave absorption increase by 4.0 W m−2 and 3.1 W m−2, respectively, in calculations replacing the old with the new spectroscopic parameters. The atmosphere becomes warmer, moister, and more stable with the increased absorption in simulations with sea surface temperatures either prescribed from observations or predicted using a slab‐ocean model. The latent heat flux and precipitation both decrease by approximately 2%. Hence the additional absorption has the effect of weakening the hydrological cycle in the atmospheric model.