Changes in Shortwave and Longwave Radiative Fluxes as Observed at BSRN Sites and Simulated with CMIP5 Models

The high precision Baseline Surface Radiation Network (BSRN), established in the early 1990s, allows the monitoring of surface radiative fluxes and their changes with unprecedented accuracy. To investigate changes in the longwave spectrum, the longest records of downward longwave radiation currently available from BSRN have been analyzed. From 25 records covering altogether 353 years, an overall mean increase in downward longwave radiation of +2.0 Wm(-2) per decade since the early 1990s was obtained. Thereby, three quarter of these BSRN sites showed increasing trends (19 sites in total, 9 of them significant), while one quarter indicated decreasing trends (6 sites, 3 significant). This change in downward longwave radiation quantitatively agrees very well with the respective change calculated by the latest generation of global climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). In their simulations, including all known relevant climate forcings the downward longwave radiation shows a very similar increase of currently around +2 Wm(-2) per decade. Thus the limited observational evidence on decadal changes in downward longwave radiation matches well with our understanding of the functioning of the greenhouse effect and its representation in climate models. Based on these considerations I argue that the flux of downward longwave radiation at the Earth's surface is currently increasing by around +2 Wm(-2) per decade globally, indicative of an increasing concentration of greenhouse gases in the atmosphere. In the shortwave spectrum, substantial multidecadal changes in surface shortwave radiation have become evident in long-term observational records over the past 50 years, known as "dimming" and subsequent "brightening". The BSRN stations, starting not before the 1990s, only cover the more recent "brightening" period. A composite record based on 37 BSRN sites indicates an average increase of +2.0 Wm(-2) per decade since the early 1990s. From the 23 longest BSRN records, 20 showed positive trends (11 of them significant), while only 3 of them showed negative trends, none of them being significant. The latest update of the BSRN records thus reconfirms the existence of a widespread brightening in recent decades. In the CMIP5 climate models, on the other hand, the brightening has been much smaller, with only 0.5 Wm(-2) per decade averaged over all models. Thus, unlike the longwave radiative changes, the shortwave changes at the Earth surface are not adequately represented in current climate models.