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Abstract Atmospheric downward longwave radiation at the surface ( L d ) varies with increasing CO 2 and other greenhouse gases. This study quantifies the uncertainties of current estimates of global L d at monthly to decadal timescales and its global climatology and trends during the past decades by a synthesis of the existing observations, reanalyses, and satellite products. We find that current L d observations have a standard deviation error of ~3.5 W m −2 on a monthly scale. Observations of L d by different pyrgeometers may differ substantially for lack of a standard reference. The calibration of a pyrgeometer significantly affects its quantification of annual variability. Compared with observations collected at 169 global land sites from 1992 to 2010, the L d derived from state‐of‐the‐art satellite cloud observations and reanalysis temperature and humidity profiles at a grid scale of ~1° has a bias of ±9 W m −2 and a standard deviation of 7 W m −2 , with a nearly zero overall bias. The standard deviations are reduced to 4 W m −2 over tropical oceans when compared to L d observations collected by 24 buoy sites from 2002 to 2011. The −4 W m −2 bias of satellite L d retrievals over tropical oceans is likely because of the overestimation of L d observations resulting from solar heating of the pyrgeometer. Our best estimate of global means L d from 2003 to 2010 are 342 ± 3 W m −2 (global), 307 ± 3 W m −2 (land), and 356 ± 3 W m −2 (ocean). Estimates of L d trends are seriously compromised by the changes in satellite sensors giving changes of water vapor profiles.
Wang et al. (2013) studied this question.