In this article, we first evaluate two widely accepted methods to estimate global atmospheric downward longwave radiation (L d ) under both clear and cloudy conditions, using meteorological observations from 1996 to 2007 at 36 globally distributed sites, operated by the Surface Radiation Budget Network (SURFRAD), AmeriFlux, and AsiaFlux Projects. The breakdown of locations is North America (20 sites), Asia (12 sites), Australia (2 sites), Africa (1 site), and Europe (1 site). Latitudes for these sites range from 0° at the equator to ±50°; elevation ranges from 98 to 4700 m, and six different land cover types are represented (deserts, semideserts, croplands, grasslands, forests, and wetlands). The evaluation shows that the instantaneous L d under all-sky conditions is estimated with an average bias of 2 W m−2 (0.6%), an average standard deviation (SD) of 20 W m−2 (6%), and an average correlation coefficient (R) of 0.86. Daily L d under all-sky conditions is estimated with a SD of 12 W m−2 (3.7%) and an average R of 0.93. These results suggest that these two methods could be applied to most of the Earth's land surfaces. Accordingly, we applied them to globally available meteorological observations to estimate decadal variation in L d . The decadal variations in global L d under both clear and cloudy conditions at about 3200 stations from 1973 to 2008 are presented. We found that daily L d increased at an average rate of 2.2 W m−2 per decade from 1973 to 2008. The rising trend results from increases in air temperature, atmospheric water vapor, and CO2 concentration.
