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"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.
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Is global dimming masking the full impact of
global warming? Some climate experts worry that it is, with the possible
consequence that as we reduce pollution, the climate will heat up to
unprecedented levels.
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