global dimming
n. The gradual reduction in the amount of sunlight reaching the earth's surface.
People may be oblivious to so-called global dimming, which tends to occur during cloudy periods.

Shabtai Cohen, who works at the Vocani Centre in Israel, said: "The cloudy times are getting darker. If it's cloudy then it's darker, but when it's sunny things have not changed much." Experts say the impact of reduced solar radiance may be greater in overcast areas of the northern hemisphere, such as Britain and Europe.
—“Black Carbon Is Plunging The World Into Darkness,” Western Daily Press, December 19, 2003
In 1985, a geography researcher called Atsumu Ohmura at the Swiss Federal Institute of Technology got the shock of his life. As part of his studies into climate and atmospheric radiation, Ohmura was checking levels of sunlight recorded around Europe when he made an astonishing discovery. It was too dark. Compared to similar measurements recorded by his predecessors in the 1960s, Ohmura's results suggested that levels of solar radiation striking the Earth's surface had declined by more than 10% in three decades. Sunshine, it seemed, was on the way out.

The finding went against all scientific thinking. By the mid-80s there was undeniable evidence that our planet was getting hotter, so the idea of reduced solar radiation — the Earth's only external source of heat — just didn't fit. And a massive 10% shift in only 30 years? Ohmura himself had a hard time accepting it. "I was shocked. The difference was so big that I just could not believe it," he says. Neither could anyone else. When Ohmura eventually published his discovery in 1989 the science world was distinctly unimpressed. "It was ignored," he says.

It turns out that Ohmura was the first to document a dramatic effect that scientists are now calling "global dimming". Records show that over the past 50 years the average amount of sunlight reaching the ground has gone down by almost 3% a decade.
—David Adam, “Earth is 20% darker, say experts,” The Guardian (London, England), December 18, 2003
2001 (earliest)
A number of studies show that significant reductions in solar radiation reaching the Earth's surface have occurred during the past 50 years. … Finally the steps needed to strengthen the evidence for global dimming, elucidate its causes and determine its agricultural consequences are outlined.
—Gerald Stanhill & Shabtai Cohen, “Global dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probable causes and possible agricultural consequences,” Agricultural and Forest Meteorology, April 19, 2001
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