an uncomfortable one," says Gerald Stanhill, who published many of
these early papers and coined the phrase global dimming. "The first
reaction has always been that the effect is much too big, I don't believe it
and if it's true then why has nobody reported it before."
That began to change in 2001, when Stanhill and his colleague Shabtai
Cohen at the Volcani Centre in Bet Dagan, Israel collected all the available
evidence together and proved that, on average, records showed that the
amount of solar radiation reaching the Earth's surface had gone down by
between 0.23 and 0.32% each year from 1958 to 1992.
This forced more scientists to sit up and take notice, though some still
refused to accept the change was real, and instead blamed it on inaccurate
Solar radiation is measured by seeing how much the side of a black plate
warms up when exposed to the sun, compared with its flip side, which is
shaded. It's a relatively crude device, and we have no way of proving how
accurate measurements made 30 years ago really are. "To detect temporal
changes you must have very good data otherwise you're just analysing the
difference between data retrieval systems," says Ohmura.
Stanhill says the dimming effect is much greater than the possible errors
(which anyway would make the light levels go up as well as down), but what
was really needed was an independent way to prove global dimming was real.
Last year Farquhar and his group in Australia provided it.
The 2001 article written by Stanhill and Cohen sparked Farquhar's
interest and he made some inquiries. The reaction was not always positive
and when he mentioned the idea to one high-ranking climate scientist (whose
name he is reluctant to reveal) he was told: "That's bullshit, Graham.
If that was the case then we'd all be freezing to death."
But Farquhar had realised that the idea of global dimming could explain
one of the most puzzling mysteries of climate science. As the Earth warms,
you would expect the rate at which water evaporates to increase. But in
fact, study after study using metal pans filled with water has shown that
the rate of evaporation has gone down in recent years. When Farquhar
compared evaporation data with the global dimming records he got a perfect
match. The reduced evaporation was down to less sunlight shining on the
water surface. And while Stanhill and Cohen's 2001 report appeared in a
relatively obscure agricultural journal, Farquhar and his colleague Michael
Roderick published their solution to the evaporation paradox in the
high-profile American magazine Science. Almost 20 years after it was first
noticed, global dimming was finally in the mainstream. "I think over
the past couple of years it's become clear that the solar irradiance at the
Earth's surface has decreased," says Jim Hansen, a leading climate
modeller with Nasa's Goddard Institute for Space Studies in New York.
The missing radiation is in the region of visible light and infrared -
radiation like the ultraviolet light increasingly penetrating the leaky
ozone layer is not affected. Stanhill says there is now sufficient interest
in the subject for a special session to be held at the joint meeting of the
American and Canadian geophysical societies in Montreal next May.
So what causes global dimming? The first thing to say is that it's
nothing to do with changes in the amount of radiation arriving from the sun.
Although that varies as the sun's activity rises and falls and the Earth
moves closer or further away, the global dimming effect is much, much larger
and the opposite of what would be expected given there has been a general
increase in overall solar radiation over the past 150 years.
That means something must have happened to the Earth's atmosphere to stop
the arriving sunlight penetrating. The few experts who have studied the
effect believe it's down to air pollution. Tiny particles of soot or
chemical compounds like sulphates reflect sunlight and they also promote the
formation of bigger, longer lasting clouds. "The cloudy times are
getting darker," says Cohen, at the Volcani Centre. "If it's
cloudy then it's darker, but when it's sunny things haven't changed
More importantly, what impact could global dimming have? If the effect
continues then it's certainly bad news for solar power, as darker, cloudier
skies will reduce its meagre efficiency still further. The effect on
photosynthesis, and so on plant and tree growth, is more complicated and
will probably be different in various parts of the world. In equatorial
regions and parts of the southern hemisphere regularly flooded with light,
photosynthesis is likely to be limited by carbon dioxide or water, not
sunshine, and light levels would have to fall much further to force a
change. In fact, in some cases photosynthesis could paradoxically increase
slightly with global dimming as the broken, diffuse light that emerges from
clouds can penetrate deep into forest canopies more easily than direct beams
of sunlight from a clear blue sky.
But in the cloudy parts of the northern hemisphere, like Britain, it's a
different story and if you grow tomatoes in a greenhouse you could be seeing
the effects of global dimming already. "In the northern climate
everything becomes light limiting and a reduction in solar radiation becomes
a reduction in productivity," Cohen says. "In greenhouses in
Holland, the rule of thumb is that a 1% decrease in solar radiation equals a
1% drop in productivity. Because they're light limited they're always very
busy cleaning the tops of their greenhouses."
The other major impact global dimming will have is on the complex computer
simulations climate scientists use to understand what is happening now and
to predict what will happen in the future. For them, global dimming is a
real sticking point. "All of their models, all the physics and
mathematics of solar radiation in the Earth's atmosphere can't explain what
we're measuring at the Earth's surface," Stanhill says. Farquhar
agrees: "This will drive what the modellers have to do now. They're
going to have to account for this."
David Roberts, a climate modeller with the Met Office's Hadley Centre,
says that although the issue of global dimming raises some awkward
questions, some of the computer
simulations do at least address the mechanisms believed to be driving it.
"Most of the processes involving aerosols and formation of clouds are
already in there, though I accept it's a bit of a work in progress and more
work needs to be done," Roberts says.
Another big question yet to be answered is whether the phenomenon will
continue. Will our great grandchildren be eating lunch in the dark?
Unlikely, though few studies are up to date enough to confirm whether or not
global dimming is still with us. "There's been so little done that
nobody really understands what's going on," Cohen says. There are some
O hmura says that satellite images of clouds seem to suggest that the
skies have become slightly clearer since the start of the 1990s, and this
has been accompanied by a sharp upturn in temperature. Both of these facts
could indicate that global dimming has waned, and this would seem to tie in
with the general reduction in air pollution caused by the scaling down of
heavy industry across parts of the world in recent years. Just last month,
Helen Power, a climate scientist at the University of South Carolina
published one of the few analyses of up-to-date data for the 1990s and found
that global dimming over Germany seemed to be easing. "But that's just
one study and it's impossible to say anything about long-term trends from
one study," she cautions.
It's also possible that global dimming is not entirely down to air
pollution. "I don't think that aerosols by themselves would be able to
produce this amount of global dimming," says Farquhar. Global warming
itself might also be playing a role, he suggests, by perhaps forcing more
water to be evaporated from the oceans and then blown onshore (although the
evidence on land suggests otherwise). "If the greenhouse effect causes
global dimming then that really changes the perspective," he says. In
other words, while it keeps getting warmer it might keep getting darker.
"I'm not saying it definitely is that, I'm just raising the
Ultimately, that and other questions will have to be considered by the
scientists around the world who are beginning to think about how to prepare
the next IPCC assessment report, due out in 2007. "The IPCC is the
group that should investigate this and work out if people should be scared
of it," says Cohen. Whatever their verdict, at least we are no longer
totally in the dark about global dimming.
Global Dimming: A Review of the Evidence, G Stanhill and S Cohen
Agricultural and Forest Meteorology Volume 107 (2001), pages 255-278
The Cause of Decreased Pan Evaporation Over the Past 50 Years, M Roderick
and G Farquhar Science Volume 298 (2002), pages 1410-1411
Observed Reductions of Surface Solar Radiation at Sites in the US and
Worldwide, B Liepert Geophysical Research Letters Volume 29 (2002), pages