Goodbye Sunshine - Air Pollution is Reducing Light Reaching the Earth's Surface
Posted on Thursday, December 25, 2003 @ 13:49:37 UTC by vlad
|
|
Overtone writes: http://www.guardian.co.uk/life/feature/story/0,13026,1108853,00.html
Thursday December 18, 2003 The Guardian
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.
By David Adam
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 thatlevels of solar radiation striking the Earth's surface had declined by more than 10% in three decades ...
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 ... doesn't even appear in the reports of the Intergovernmental Panel on Climate Change (IPCC) ...
"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 s upposed land of the rising sun, was actually falling. Most startling of all was the discovery t hat 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 Volcanic 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 Australi a 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 ...
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 ...
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. ***
[THIS SOLAR RADIATION INCREASE IS THUS CONFOUNDED WITH INCREASING CO2 CONCENTRATION.] ***
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 Volcanic Centre. "If it's cloudy then it's darker, but when it's sunny things haven't changed much." ...
"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." ...
"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." ...''
|
| |
|
Don't have an account yet? You can create one. As a registered user you have some advantages like theme manager, comments configuration and post comments with your name.
| |
Average Score: 5
Votes: 1
| |
|
