TV Program Description
Original PBS Broadcast Date: April 18, 2006

Dimming the Sun homepage

"Dimming the Sun" investigates the discovery that the sunlight reaching Earth has been growing dimmer, which may seem surprising given all the international concern over global warming. At first glance, less sunlight might hardly seem to matter when our planet is stewing in greenhouse gases. But the discovery of global dimming has led several scientists to revise their models of the climate and how fast it's changing. According to one recent and highly controversial model, the worst-case warming scenario could be worse than anyone has predicted. "Dimming the Sun" unravels this baffling climate conundrum and the implications for Earth's future.

To find out what global dimming means for the fate of the planet, NOVA reports on the findings of the world's top climate detectives, including an American scientist who found a grim but crucial opportunity immediately following September 11, 2001, when the entire U.S. airline fleet was grounded for three days. This presented a unique opportunity to study the effects of airplane vapor trails on the atmosphere (see The Contrail Effect). Comparing changes in the daily temperature range showed that the absence of dimming from aircraft pollution alone made a marked difference to the temperature. This result hints at how much the effects of atmospheric pollution had been underestimated.

Working in Israel, Dr. Gerald Stanhill was one of the first to discover the surprising fact that less solar energy is reaching the Earth's surface. While his measurements were met with skepticism, a review of worldwide data by Stanhill and a German researcher demonstrated that during the 1980s and early '90s, sunlight reaching Earth's surface had dropped just about everywhere. Halfway around the world, independent studies by Australian scientists confirmed this disturbing diagnosis. (For more, see Discoveries in Global Dimming.)

Scientists have long known that increasing air pollution—the smog that clouds urban skies—endangers our respiratory health. But they had underestimated the impact of pollution on the amount of sunlight reaching Earth. Some scientists now believe that global dimming may also disturb rainfall patterns such as the Asian monsoon. If they are right, global dimming may be one of many factors that contributed to severe droughts and famines in Africa during the 1980s.

The good news is that pollution controls have slowed and possibly even halted global dimming during the last decade. The bad news—and the ironic twist in NOVA's story—is that without pollution, more sunlight is reaching Earth, revealing the full impact of global warming. Although all climate models have important uncertainties, the unsettling implication is that, with dimming fading away in many regions, global temperatures may rise even faster than most models have predicted.