Globe Grows Darker as Sunshine Diminishes 10% to 30%
May 13, 2004
By KENNETH CHANG
In the second half of the 20th century, the world became,
quite literally, a darker place.
Defying expectation and easy explanation, hundreds of
instruments around the world recorded a drop in sunshine
reaching the surface of Earth, as much as 10 percent from
the late 1950's to the early 90's, or 2 percent to 3
percent a decade. In some regions like Asia, the United
States and Europe, the drop was even steeper. In Hong Kong,
sunlight decreased 37 percent.
No one is predicting that it may soon be night all day, and
some scientists theorize that the skies have brightened in
the last decade as the suspected cause of global dimming,
air pollution, clears up in many parts of the world.
Yet the dimming trend - noticed by a handful of scientists
20 years ago but dismissed then as unbelievable - is
attracting wide attention. Research on dimming and its
implications for weather, water supplies and agriculture
will be presented next week in Montreal at a joint meeting
of American and Canadian geological groups.
"There could be a big gorilla sitting on the dining table,
and we didn't know about it," said Dr. Veerabhadran
Ramanathan, a professor of climate and atmospheric sciences
at the University of California, San Diego. "There are
many, many issues that it raises."
Dr. James E. Hansen, director of the NASA Goddard Institute
for Space Studies in Manhattan, said that scientists had
long known that pollution particles reflected some
sunlight, but that they were now realizing the magnitude of
"It's occurred over a long time period," Dr. Hansen said.
"So it's not something that, perhaps, jumps out at you as a
person in the street. But it's a large effect."
Satellite measurements show that the sun remains as bright
as ever, but that less and less sunlight has been making it
through the atmosphere to the ground.
Pollution dims sunlight in two ways, scientists theorize.
Some light bounces off soot particles in the air and goes
back into outer space. The pollution also causes more water
droplets to condense out of air, leading to thicker, darker
clouds, which also block more light. For that reason, the
dimming appears to be more pronounced on cloudy days than
sunny ones. Some less polluted regions have had little or
The dynamics of global dimming are not completely
understood. Antarctica, which would be expected to have
clean air, has also dimmed.
"In general, we don't really understand this thing that's
going on," said Dr. Shabtai Cohen, a scientist in the
Israeli Agriculture Ministry who has studied dimming for a
decade. "And we don't have the whole story."
The measuring instrument, a radiometer, is simple, a black
plate under a glass dome. Like asphalt in summer, the black
plate turns hot as it absorbs the sun's energy. Its
temperature tells the amount of sunlight that has shone on
Since the 50's, hundreds of radiometers have been installed
from the Arctic to Antarctica, dutifully recording
sunshine. In the mid-80's, Dr. Atsumu Ohmura of the Swiss
Federal Institute of Technology in Zurich sifted through
the data to compare levels in different regions.
"Suddenly," Dr. Ohmura said, "I realized it's not easy to
do that, because the radiation was changing over time."
He recalled his reaction, saying, "I thought it is rather
After an analysis, he was convinced that the figures were
reliable and presented his findings at a scientific
Asked about his colleagues' reaction, Dr. Ohmura said:
"There's no reaction. Very disappointing."
At that time, Dr. Gerald Stanhill of the Israeli
Agriculture Ministry noticed similar darkening in Israel.
"I really didn't believe it," Dr. Stanhill said. "I thought
there was some error in the apparatus."
Dr. Stanhill, now retired and living in New York, also
looked around and found dimming elsewhere. In the 90's, he
wrote papers describing the phenomenon, also largely
ignored. In 2001, Drs. Stanhill and Cohen estimated that
the worldwide dimming averaged 2.7 percent a decade.
Not every scientist is convinced that the dimming has been
that pronounced. Although radiometers are simple, they do
require periodic calibration and care. Dirt on the dome
blocks light, leading to erroneous indications. Also, all
radiometers have been on land, leaving three-fourths of the
earth to supposition.
"I see some datasets that are consistent and some that
aren't," Dr. Ellsworth G. Dutton, who heads
surface-radiation monitoring at the National Oceanic and
Atmospheric Administration, said. "Certainly, the magnitude
of the phenomenon is in considerable question."
Dr. Beate G. Liepert, a research scientist at the
Lamont-Doherty Earth Observatory of Columbia University,
has analyzed similar information and arrives at a smaller
estimate of the dimming than Drs. Stanhill and Cohen. Dr.
Liepert puts it at 4 percent from 1961 to 1990, or 1.3
percent a decade. "It's a little bit the way you do the
statistics," she said.
A major set of measurements from the Indian Ocean in 1999
showed that air pollution did block significant sunlight.
Following plumes of soot and other pollution, scientists
measured sunlight under the plumes that was 10 percent less
bright than in clear air.
"I thought I was too old to be surprised by anything," said
Dr. Ramanathan, who was co-chief scientist of the projects.
Dr. Ohmura said he hoped to finish his analysis of the
numbers since 1990 by late next month or early July.
"I have a very strong feeling that probably solar radiation
is increasing during the last 14 years," he said. He based
his hunch, he said, on a reduction in cloud cover and
faster melting rates in glaciers.
But clearer, sunnier days could mean bad news for global
warming. Instead of cloudiness slowing rising temperatures,
sunshine would be expected to accelerate the warming.