Decadal Changes in Surface Radiative Fluxes

The aim of this presentation is to give an overview on the state of our knowledge on decadal changes in surface radiative fluxes, with a focus on both shortwave and longwave changes. With respect to changes in the shortwave, the focus will be on recent studies on the solar dimming and brightening phenomenon. This phenomenon refers to the increasing evidence that solar radiation at the Earth’s surface is not stable over time, but undergoes substantial multidecadal variations (i.e., dimming and brightening). An inadequate representation of these effects in climate models can for example lead to an inaccurate reproduction of the observed decadal warming rates, as evidenced in different studies.  A growing number of studies assess the magnitude of dimming and brightening also in remote areas. Artificial intelligence methods have recently been applied in attempts to expand station data information over larger areas. Studies using methods to filter out clouds in the observational data records allow for more insight into the relative contribution of direct aerosol and cloud effects to the dimming and brightening trends. Modelling studies help to quantify the contribution of unforced climate system-inherent variability to this phenomenon, and provide projections of future availability of the solar resource e.g. for energy applications.   

In the longwave, climate models simulate an increase in downward longwave radiation at the surface at a rate of  approx. 2 Wm^-2 / decade under current forcings globally, due to the enhanced greenhouse effect. Current model projections suggest that this rate will increase or decrease over the coming decades depending on future emission scenarios.  Reanalyses suggest an increase of around 1.5 Wm^-2 over recent decades. This is supported by station observations from the Baseline Surface Radiation Network (BSRN), which overall also indicate an increase in downward longwave radiation over recent decades.