The
Globe Grows Darker As Sunshine Diminishes 10% to 37%
By Kenneth Chang
15 May, 2004
New
York Times
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 the
effect.
"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 no dimming.
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 it.
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 unbelievable."
After an analysis,
he was convinced that the figures were reliable and presented his findings
at a scientific conference.
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.
© Copyright
2004 The New York Times Company