Sunlight reaching the Earth's surface has been quietly declining for decades, and this BBC Horizon film tracks the scientists who found it and argued about why. Gerald Stanhill, who coined the term after comparing Israeli sunlight records from the 1950s to the 1980s, opens the case, followed by researchers who found the same drop from Siberia to Antarctica. The suspect is airborne pollution: particulates from burning fossil fuels that reflect sunlight back into space and seed thicker, longer-lived clouds. Veerabhadran Ramanathan's Indian Ocean Experiment measures a haze layer over the Arabian Sea dense enough to cut solar radiation by ten percent. The film's sharpest piece of evidence comes from the three days after September 11, 2001, when US airspace was grounded, contrails vanished, and the average diurnal temperature range measurably widened. That leads to the film's real warning: dimming has been masking the true extent of greenhouse warming, and cleaning up pollution without addressing carbon emissions could let hidden heating through all at once.