Multidecadal Variability of Solar Radiation at the Earth's Surface and its Causes

Multidecadal variation of solar radiation at Earth’s surface has been documented since the 1950s. Slow decreasing radiation on the decadal scale is referred to as dimming, and increasing radiation is termed brightening. Generally, dimming from the 1950s through the middle 1980s followed by brightening through the first decade of the 2000s has been documented over all continents. The most comprehensive analyses were done for places with the highest density of measurements, i.e., Europe, Japan, China and the United States, although trends have been published for India, Israel, several countries in Africa, New Zealand, Antarctica, and other locations. Most have observed both long-term dimming and brightening, however, India has only experienced dimming. Generally, brightening trends range mostly between 2 and 10 Wm^-2 per decade. Dimming trends show about the same magnitudes but are negative. These changes are significantly large considering that doubling CO₂ since preindustrial times accounts for an increase in downwelling longwave radiation of about 4 Wm^-2. Attribution to clouds, the direct effect of aerosols, and the second indirect effect of aerosols (effect of aerosols on cloud cover) have been described. However, an explanation of the primary forcing for these trends has been lacking, although, variability of solar output over its 11-year cycle can be discounted as too small.

Observed multidecadal variability of surface solar radiation over the continents of the Northern Hemisphere is similar and somewhat in phase, i.e., when brightening occurs in Europe, it also occurs in east Asia and North America. This implies that the primary forcing for these multidecadal trends is likely meteorological in nature and large scale, although aerosols dominate surface radiation in some parts of the globe such as India, West Africa, and industrial parts of China. Recent work has shown that slow variations of ocean sea surface temperature (SST) patterns are similar to observed decadal-scale surface solar variability over adjacent continents. For example, the reversal of the Pacific Decadal Oscillation (PDO) index in the mid-1980s coincides with the change from dimming to brightening over North America. A recent study at ETH Zurich used an ensemble of 66 CMIP6 models to confirm the association of slowly changing SST patterns (represented by indexes such as the PDO and AMO) to decadal-scale dimming and brightening trends over the continents, globally. That work and former studies show no link between long-term surface solar trends and greenhouse gas (GHG) warming, however, GHG warming’s indirect effect on increasing marine heat wave number and intensity, and their positioning by internal variability processes of the oceans may be changing that paradigm.