Each year less light reaches the surface of the Earth. No one is sure
what's causing 'global dimming' - or what it means for the future. In fact
most scientists have never heard of it.
By David Adam
Thursday December 18,
In 1985, a geography
researcher called Atsumu Ohmura at the Swiss Federal Institute of
Technology got the shock of his life. As part of his studies into climate
and atmospheric radiation, Ohmura was checking levels of sunlight recorded
around Europe when he made an astonishing discovery. It was too dark.
Compared to similar measurements recorded by his predecessors in the
1960s, Ohmura's results suggested that levels of solar radiation striking
the Earth's surface had declined by more than 10% in three decades.
Sunshine, it seemed, was on the way out.
The finding went
against all scientific thinking. By the mid-80s there was undeniable
evidence that our planet was getting hotter, so the idea of reduced solar
radiation - the Earth's only external source of heat - just didn't fit.
And a massive 10% shift in only 30 years? Ohmura himself had a hard time
accepting it. "I was shocked. The difference was so big that I just
could not believe it," he says. Neither could anyone else. When
Ohmura eventually published his discovery in 1989 the science world was
distinctly unimpressed. "It was ignored," he says.
It turns out that
Ohmura was the first to document a dramatic effect that scientists are now
calling "global dimming". Records show that over the past 50
years the average amount of sunlight reaching the ground has gone down by
almost 3% a decade. It's too small an effect to see with the naked eye,
but it has implications for everything from climate change to solar power
and even the future sustainability of plant photosynthesis. In fact,
global dimming seems to be so important that you're probably wondering why
you've never heard of it before. Well don't worry, you're in good company.
Many climate experts haven't heard of it either, the media has not picked
up on it, and it doesn't even appear in the reports of the
Intergovernmental Panel on Climate Change (IPCC).
extraordinary thing that for some reason this hasn't penetrated even into
the thinking of the people looking at global climate change," says
Graham Farquhar, a climate scientist at the Australian National University
in Canberra. "It's actually quite a big deal and I think you'll see a
lot more people referring to it."
That's not to say
that the effect has gone unnoticed. Although Ohmura was the first to
report global dimming, he wasn't alone. In fact, the scientific record now
shows several other research papers published during the 1990s on the
subject, all finding that light levels were falling significantly. Among
them they reported that sunshine in Ireland was on the wane, that both the
Arctic and the Antarctic were getting darker and that light in Japan, the
supposed land of the rising sun, was actually falling. Most startling of
all was the discovery that levels of solar radiation reaching parts of the
former Soviet Union had gone down almost 20% between 1960 and 1987.
The problem is that
most of the climate scientists who saw the reports simply didn't believe
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 recording
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 analyzing 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
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.
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
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.
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 clues
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
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
of Surface Solar Radiation at Sites in the US and Worldwide, B Liepert
Geophysical Research Letters Volume 29 (2002), pages 1421-1433