## 3 Results
### MERRA-2 GDB (SSR Trends) and Its Evaluation
#### 3.2.2 The MERRA-2 GDB on Global, Hemispherical and Regional Scales
The decreasing SSR of MERRA-2 during 1991-2000 is not in agreement with most measurements and modelling studies (10; 20; 21; 23), while during 2000-2009 this negative trend is in line with other studies (12; 34; 36). MERRA-2 reproduces strong decreases of SSR, namely solar dimming, caused by the great volcano eruptions of El-Chichon (1982) and Pinatubo (1991), as well as by strong El-Nino's (e.g., 1998, 2004 and 2012). Years characterized by the El-Chichon and Pinatubo recovery, as well as La-Nina's (e.g., 2008, 2015 or 2018), are characterized by positive anomalies corresponding to brightening. These findings prove that MERRA-2 Reanalysis appropriately reproduces physical phenomena, such as volcanic eruptions and ENSO (El Nino Southern Oscillation), which drive the Earth's climate. Figure 11: Global distribution of GDB, namely changes of MERRA-2 deseasonalized SSR anomalies, over the 40-year period January 1980–December 2019. Areas shaded with reddish/bluish colors are those with positive/negative trends (brightening/dimming). Areas (pixels) with statistically significant trends are indicated by black dots. World areas (Europe, India and East Asia) enclosed by red rectangles are characterized by homogeneous SSR trends (reference is made to them below). The MERRA-2 Reanalysis offers the advantage of covering a quite long (climatological) time period (1980-2019, i.e., 40 years) which overlaps with the recent anthropogenic global warming. There have been speculations, mostly based on surface station measurements of SSR, about how GDB interplays with this recent rapid global warming, either accelerating or counteracting it [8,18,22,103,104]. Therefore, the determination of GDB at these large spatial (global) and temporal (40-year) scales enables a better quantification and understanding of the role of GDB for the global warming, in relation to the predominant role of anthropogenic greenhouse gases, according to the MERRA-2 Reanalysis. To this aim, the changes of SSR, i.e., GDB, have been estimated not only over the entire 40-year period (1980-2019), but also over three sub-periods, January 1980-December 1999, January 2000-December 2009 and January 2010-December 2019, which have been characterized by different phases of GDB, according to the literature [11,12,24]. The results are given in Table 4 on global and hemispherical scales, as well as separately for land and oceans of the globe and the two hemispheres. According to MERRA-2, the SSR change during 1980-2019 is equal to \(-6.31\pm 0.19\) W/m\({}^{2}\) or \(-0.158\pm 0.005\) W/m\({}^{2}\)/year on global scale (dimming), with dimming observed in both hemispheres (\(-5.72\pm 0.28\) and \(-6.16\pm 0.38\) W/m\({}^{2}\) or \(-0.143\pm 0.007\) W/m\({}^{2}\)/year and \(-0.154\pm 0.01\) W/m\({}^{2}\)/year over the North and South Hemispheres, respectively). Stronger dimming took place over oceans (\(-7.81\pm 0.25\) W/m\({}^{2}\) or \(-0.195\pm 0.006\) W/m\({}^{2}\)/year) than continents (\(-2.58\pm 0.25\) W/m\({}^{2}\) or \(-0.065\pm 0.006\) W/m\({}^{2}\)/year), which are in line with the stronger global warming observed over land than oceans (+0.29 \({}^{\circ}\)C decade\({}^{-1}\) and +0.13 \({}^{\circ}\)C decade\({}^{-1}\), [105]). The overall 40-year dimming over the globe resulted from a persisting dimming from the early 1980\({}^{\prime}\)s through to the late 2000\({}^{\prime}\)s, being stronger in the 20 th than in the 21 st century, and a subsequent slight brightening from 2010 to 2019. The changing phases of GDB in the two hemispheres are the same as those already described for the globe, except for the last sub-period, i.e., 2010-2019, when brightening is found in MERRA-2 in the NH against dimming in SH. It is not the same with the land and oceanic areas of the two hemispheres, which have been characterized by different changing phases of GDB.