Global Dimming

Joseph S. Barrera III joe at
Thu Dec 18 18:05:46 PST 2003

Damn, I forgot to forward this. It's already been on /. and Htech
but I wanted to fork the whole article as it's just absolutely fascinating.
10% reduction in three decades... wow.

For the record Google only returns 21 hits for
    "global dimming" solar
alltheweb 6 hits, AltaVista 6.

I'm bracing myself for Rob telling me why this is all bullshit...

For anyone who's ever read Rainbow Mars --
I wonder if "global drying" would be a theory that
many would remain skeptical of until the very end...

- Joe

Goodbye sunshine

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.

David Adam
Thursday December 18, 2003
The Guardian

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 

"It's an 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 them.

"It's 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 recording equipment.

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 much."

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 
clues though.

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 question."

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.

*Further reading*

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 1421-1433

Guardian Unlimited © Guardian Newspapers Limited 2003

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