Should you be worried about global dimming? - By Brendan I. Koerner - Slate MagazineHome

bit brighter over the past 15 years. But the uptick in sunlight may not be as
positive a development as it seems.
Technically, the term "global dimming" refers to the reduction of solar
radiation hitting the planet's surface, a phenomenon caused by the proliferation
of aerosols in the atmosphere. These aerosols can be both anthropogenic and
natural in origin: Though industrial soot certainly plays a huge role in
dimming, nothing affects sunlight's Earthward journey like an erupting volcano
spewing sulfate particles. When these aerosols reach the clouds, they cause the
formation of smaller-than-normal water droplets; according to David Sington, the
producer of the BBC documentary, an aerosol-affected cloud will contain six
times as many droplets as an unaffected one. Because a bunch of smaller droplets
have more aggregate surface area than fewer, larger droplets, the clouds
polluted with aerosols are more reflective, and thus more effective at repelling
sunlight.
Between 1958 and 1988, the amount of solar radiation reaching the Earth's
surface declined by 10 percent, an estimate that scientists arrived at by
studying readings from more than 1,600 pyranometers worldwide. But since 1991,
when the eruption of Mount Pinatubo caused the Earth to get much dimmer for
about two years, there has been an overall brightening trend. In fact, according
to NASA's Global Aerosol Climatology Project, worldwide aerosol levels in 2005
were 20 percent off their late 1980s peak (though we're not quite to pre-1958
levels yet). It's not entirely clear if this decline is due to mankind's
efforts, particularly the campaign against the emissions responsible for acid
rain. Next year's Glory satellite mission should clear things up; the spacecraft
is equipped with a sensor that can differentiate between anthropogenic and
natural aerosols. If it turns out that we've been somewhat effective at
controlling aerosols, hey, let's call it a minor environmental victory.
Yet the recent brightening trend isn't necessarily cause for celebration. Though
dimming can severely harm crops—less sunlight means less evaporation, which in
turn means less rain—it may also mask the most noticeable effects of global
warming. Some scientists argue that by reducing the amount of solar radiation
hitting the Earth's surface, the long postwar dimming trend helped keep the
planet cooler than it should have been, given the simultaneous escalation in
greenhouse gas emissions. Indeed, since the world has gotten brighter in recent
years, temperatures have risen; the six warmest years on record since 1850 have
all occurred since 1997.
While the world may be getting brighter overall, two rising economic superpowers
are actually getting darker. Over the last half-century, the amount of sunlight
hitting each square yard of Chinese soil has declined by 3.7 watts; India has
experienced a similar decrease in solar radiation. This could be attributable to
the two nations' surge in aerosol-producing economic activity, which hasn't yet
been accompanied by meaningful regulations meant to control emissions. If China
and India crack down on their soot output, however, the attendant brightening
could result in even higher average temperatures.
Still, getting rid of that nasty Asian brown cloud, a huge mass of aerosols that
currently hovers over a chunk of the continent, has to be a good thing—not only
because it's allegedly wreaking lethal havoc on weather patterns, but also
because it actually causes warming at high altitudes. Though aerosols may mask
warming at the planet's surface, recent research shows that the brown cloud is
heating up the lower atmosphere from approximately 6,500 to 16,500 feet. That's
bad news for the Himalayan glaciers, which are melting at a distressingly rapid clip.
(before this was found the g.w did not follow reality.))