Abstract
The changes of the outgoing longwave radiation (OLR) in clear-sky conditions have been calculated using High Resolution Infrared Radiation Sounder (HIRS) observations from 1979 to 2004. After applying corrections for satellite orbital drift and intercalibration of the HIRS/2 data from the NOAA satellites, the OLR is calculated from a multivariate regression over the tropical ocean region. The clear-sky OLR retrievals compare well with the observed top-of-atmosphere radiation measurements, although the precision and stability uncertainties are larger. While the tropical ocean surface temperature has risen by roughly 0.2 K from 1982 to 2004, the reconstructed OLR remains stable over the ocean. Consequently, there is an increase in the clear-sky greenhouse effect (GHE) of 0.80 W m−2 decade−1. This trend is shown to be larger than the uncertainty in the stability of the HIRS retrievals.
The observations are compared with two phase 3 of the Coupled Model Intercomparison Project model ensembles: one ensemble includes both natural and anthropogenic forcings [the twentieth-century (20C) ensemble] and the other ensemble only contains natural climate variability (the control ensemble). The OLR trend in the 20C simulations tends to be more negative than observed, although a majority is found to be within the observational uncertainty. Conversely, the response of the clear-sky OLR to SST is shown to be very similar in observations and models. Therefore, the trend differences between the 20C simulations and observations are likely because of internal climate variability or uncertainties in the external forcings. The observed increase in GHE is shown to be inconsistent with the control ensemble, indicating that anthropogenic forcings are required to reproduce the observed changes in GHE.
