Global Dimming vs. Global Warming - The Revelation of 9/11

[Mephistopheles]

The following BBC article is about two corresponding phenomena of environment pollution: The global warming (caused by greenhouse gases, mainly methane and carbon dioxide emissions) and the global dimming (caused by dirt particles and vapor trails of aircrafts that reduce the amount of sunlight reaching the earth).

Basically, the global dimming reduces the effects of global warming. If we reduced the causes for global dimming we would drastically suffer more from global warming.

The effects of (a reduced) global dimming were observed shortly after 9/11 when (nearly) no planes had been flying in US airspace (thus leaving no vapor trails in the sky). During the grounding the temperature range (!) jumped by over a degree Celsius.

I saw the BBC documentary about global dimming on TV and I have to admit that the theory was quite new to me.


SOURCE1 (short version, see below)
SOURCE2 (full version)

Global Dimming
Horizon producer David Sington on why predictions about the Earth's climate will need to be re-examined.



Questions and answers about global dimming
Programme transcript

We are all seeing rather less of the Sun. Scientists looking at five decades of sunlight measurements have reached the disturbing conclusion that the amount of solar energy reaching the Earth's surface has been gradually falling. Paradoxically, the decline in sunlight may mean that global warming is a far greater threat to society than previously thought.

The effect was first spotted by Gerry Stanhill, an English scientist working in Israel. Comparing Israeli sunlight records from the 1950s with current ones, Stanhill was astonished to find a large fall in solar radiation. "There was a staggering 22% drop in the sunlight, and that really amazed me," he says.

Intrigued, he searched out records from all around the world, and found the same story almost everywhere he looked, with sunlight falling by 10% over the USA, nearly 30% in parts of the former Soviet Union, and even by 16% in parts of the British Isles. Although the effect varied greatly from place to place, overall the decline amounted to 1-2% globally per decade between the 1950s and the 1990s.

Gerry called the phenomenon global dimming, but his research, published in 2001, met with a sceptical response from other scientists. It was only recently, when his conclusions were confirmed by Australian scientists using a completely different method to estimate solar radiation, that climate scientists at last woke up to the reality of global dimming.

Dimming appears to be caused by air pollution. Burning coal, oil and wood, whether in cars, power stations or cooking fires, produces not only invisible carbon dioxide (the principal greenhouse gas responsible for global warming) but also tiny airborne particles of soot, ash, sulphur compounds and other pollutants.

This visible air pollution reflects sunlight back into space, preventing it reaching the surface. But the pollution also changes the optical properties of clouds. Because the particles seed the formation of water droplets, polluted clouds contain a larger number of droplets than unpolluted clouds. Recent research shows that this makes them more reflective than they would otherwise be, again reflecting the Sun's rays back into space.

Scientists are now worried that dimming, by shielding the oceans from the full power of the Sun, may be disrupting the pattern of the world's rainfall. There are suggestions that dimming was behind the droughts in sub-Saharan Africa which claimed hundreds of thousands of lives in the 1970s and 1980s. There are disturbing hints the same thing may be happening today in Asia, home to half the world's population. "My main concern is global dimming is also having a detrimental impact on the Asian monsoon," says Prof Veerhabhadran Ramanathan, one of the world's leading climate scientists. "We are talking about billions of people."

But perhaps the most alarming aspect of global dimming is that it may have led scientists to underestimate the true power of the greenhouse effect. They know how much extra energy is being trapped in the Earth's atmosphere by the extra carbon dioxide (CO2) we have placed there. What has been surprising is that this extra energy has so far resulted in a temperature rise of just 0.6°C.

This has led many scientists to conclude that the present-day climate is less sensitive to the effects of carbon dioxide than it was, say, during the ice age, when a similar rise in CO2 led to a temperature rise of 6°C. But it now appears the warming from greenhouse gases has been offset by a strong cooling effect from dimming - in effect two of our pollutants have been cancelling each other out. This means that the climate may in fact be more sensitive to the greenhouse effect than thought.

If so, then this is bad news, according to Dr Peter Cox, one of the world's leading climate modellers. As things stand, CO2 levels are projected to rise strongly over coming decades, whereas there are encouraging signs that particle pollution is at last being brought under control. "We're going to be in a situation, unless we act, where the cooling pollutant is dropping off while the warming pollutant is going up. That means we'll get reduced cooling and increased heating at the same time and that's a problem for us," says Cox.

Even the most pessimistic forecasts of global warming may now have to be drastically revised upwards. That means a temperature rise of 10°C by 2100 could be on the cards, giving the UK a climate like that of North Africa, and rendering many parts of the world uninhabitable. That is unless we act urgently to curb our emissions of greenhouse gases.

[Mast3rofPuppets]

Disturbing, yet interesting. I've never heard of this Global Dimming before.

[Tas]

Thats interesting.. but i seriously doubt aircraft vapor trails block out that much sun, its just a thin strand that dissapated after like.. a few km's? and look at all those huge clouds formed naturally..

[KoЯsakoff]

Global dimming? It makes such a difference? Heard of global warming, but thats it.

[Mephistopheles]

Originally Posted by The 13th Raptor

Thats interesting.. but i seriously doubt aircraft vapor trails block out that much sun, its just a thin strand that dissapated after like.. a few km's? and look at all those huge clouds formed naturally..

Actually, it was hard for me to believe, too. But the report was quite convincing. Here is a citation from the movie (second source):

DR DAVID TRAVIS: Here are some examples of what we call outbreaks of contrails. These are large clusters of contrails. And here's a particularly er good one from Southern California. Here's the west coast of the United States. And you can see here this lacing network of contrails covering at least fifty per cent, if not seventy five per cent or more of the sky in that area. It doesn't take an expert to realise that if, if you look at the satellite picture and see this kind of contrail coverage that they've got to be having an effect on temperature at the surface.

NARRATOR: But the problem Travis faced was to establish exactly how big an effect the contrails were actually having. The only way to do that was to find a period of time when, although conditions were right for contrails to form, there were no flights. And, of course, that never happened. Until September 2001. Then, for three days after the 11th virtually all commercial aircraft in the US were grounded. It was an opportunity Travis could not afford to miss. He set about gathering temperature records from all over the USA.

DR DAVID TRAVIS: Initially data from over 5,000 weather stations across the 48 united states, the areas that was most dominantly affected by the grounding.

NARRATOR: Travis was not looking just at temperature - that varies a lot from day to day anyway. Instead he focused on something that normally only changes quite slowly: the temperature range. The difference between the highest temperature during the day and the lowest at night. Had this changed at all during the three days of the grounding?

DR DAVID TRAVIS: As we began to look at the climate data and the evidence began to grow I got more and more excited. The actual results were much larger than I expected. So here we see for the 3 days preceding September 11th a slightly negative value of temperature range with lots of contrails as normal. Then we have this sudden spike right here of the 3 day period. This reflects lack of clouds, lack of contrails, warmer days cooler nights, exactly what we expected but even larger than what we expected. So what this indicates is that during this 3 day period we had a sudden drop in Global Dimming contributed from airplanes.

NARRATOR: During the grounding the temperature range jumped by over a degree Celsius. Travis had never seen anything like it before.

DR DAVID TRAVIS: This was the largest temperature swing of this magnitude in the last thirty years.

NARRATOR: If so much could happen in such a short time, removing just one form of pollution, then it suggests that the overall effect of Global Dimming on world temperatures could be huge.

DR DAVID TRAVIS: The nine eleven study showed that if you remove a contributor to Global Dimming, jet contrails, just for a three day period, we see an immediate response of the surface of temperature. Do the same thing globally we might see a large scale increase in global warming.

[Blood n Guts]

I guess it is plausible; clouds can block out up to 25% the sunlight. The problem I have with this is that (IIRC) in order for clouds to block out 20% of the sunlight, it must be a very cloudy day with no blue skies. Even with all the jet traffic and industry that we have, we haven't quite been able to completely blanket the sky with contrails or pollution to create that effect. Although I find it likely that these particles cause some degree of dimming, and probably are much more opaque than condensed water droplets (so much less are needed to cause the same effect), IMO there just can't be enough of them to block out that much sunlight because it would be gray skys all day.

[Billy3384]

I dont know where you live but the light sure as hell isnt dimming over here in Texas

[Phoenix_22]

Hmm...i never knew about this, good find.

Its definetly a good topic, as global warming prevention requires global dimming, yet both are harmful to the ozone, and the air in general.

[Nordicvs]

Yeah, I've read about stuff like this (back in 1996), but it wasn't called "global dimming" back then. Volcanoes blowing up were reducing regional temperates and eventually it had a (slight) global effect.

[Σl.Ðestructo]

I think it would still be better to just make less pollution.

Global dimming, if it does work is still not a solution to the problem of global warming, it is just another side effect of artificial causes that evens the playing field a bit. Not enough.

Nuclear Winter would be a better option.