Contributors to multidecadal dimming and brightening of surface solar radiation over Northern Hemisphere continents
Authors: Augustine John 
Published: February 2023

Multidecadal dimming and brightening of solar radiation at Earth&#8217;s surface has been shown to occur over all continents. Trends have been especially well documented over the Northern Hemisphere (NH) with dimming from the 1950s through the middle 1980s followed by brightening through the first decade of the 2000s in the United States, Europe, and parts of Asia. Trends in Europe and China have been attributed to both aerosols and clouds, but in the U.S. cloud variability has been dominant. A recent analysis shows that U.S. brightening of 7.4 Wm-2/decade peaked in 2012 and then dramatically dropped to near normal values in 2013. Since then, surface solar radiation in the U.S. has remained within 1 Wm-2 of the long-term average. However, in Europe surface solar radiation has remained high, at least through 2017.&#160;It has been shown that the direct effect of aerosols cannot account for the magnitude of the latest brightening in the U.S. It has also been shown that the second indirect effect of aerosols may explain brightening into the first decade of the 2000s, but is in opposition to the observed dimming after 2012. High aerosol content does explain perpetual dimming in India and industrial parts of China, but, given that the magnitude and period of dimming and brightening trends from the 1950s through the first decade of the 2000s are similar over North America, Europe, and parts of Asia, I speculate the primary cause is meteorological. A recent study documents a strong association between multidecadal surface solar radiation trends over NH continents and long-term North Pacific and North Atlantic sea surface temperature (SST) patterns. For example, the reversal of the Pacific Decadal Oscillation (PDO) index in the mid-1980s is nearly simultaneous with the change from dimming to brightening over NH continents. A similar association is shown between Atlantic SST patterns and continental surface solar radiation trends but with a decade lag. Using reanalysis and observed SST patterns it is demonstrated that persistent warm SST anomalies support overlying semipermanent geopotential height ridges at tropospheric mid-levels that dynamically induce persistent troughs downstream over adjacent continents, if positioning is favorable. Semipermanent troughs over the continents cause greater than average cloud cover and dimming. Conversely, long-term cool SSTs produce the opposite scenario and yield less clouds and brightening downstream over the continents. Further, marine heat waves on either side of North America are shown to be associated with the recent dimming in the midcontinent from 2013 to the present, and warm SSTs in the Mediterranean and North Seas in the last decade are likely responsible for a persistent midlevel geopotential ridge pattern and continued high surface solar radiation there. Recent studies present evidence that the observed increase in frequency and variability of marine heat waves in the past few decades may be associated with global warming, possibly linking warming to trends in surface solar radiation.