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Global Dimming

by Crazy Horse Tue Sep 14th, 2010 at 01:19:23 PM EST

A friend turned me on to the concept of Global Dimming.  While i'd been through some of the accompanying literature a bit, and while i've read some about the effect of jet contrails, i'd never seen it put together like this. Yet i have no idea of where this science stands.

I'm sure there are people at ET who are far more familiar with this research. I'd like to get a handle if there is any validity here. I'm using a BBC show from 2005 as a base for discussion, but have no idea of where this research has gone over the past five years.

NARRATOR: Instead we have to take urgent action to tackle the root cause of both global warming and Global Dimming - the burning of coal, oil and gas. We may have to make very difficult choices, about how we live and how we generate our electricity. We have been talking about such things for 20 years. But so far very little has been done in practical terms. The discovery of Global Dimming makes it clear that we are rapidly running out of time.

the story begins with measurements from the three days after 9/11 when flights were grounded.

DR DAVID TRAVIS: We found that the change in temperature range during those three days was just over one degrees C. And you have to realise that from a layman's perspective that doesn't sound like much, but from a climate perspective that is huge.

Then comes solar measurement from Israel.

NARRATOR: A 22% drop in solar energy was simply massive. If it was true surely Israelis should be freezing. There had to be something wrong. So when Gerry published his results they were ignored. DR GERALD STANHILL: I must say the publications had almost no effect whatsoever on the scientific community.

NARRATOR: But in fact Gerry was not the only scientist who had noticed a fall in sunlight. In Germany a young graduate climatologist called Beate Liepert found that the same thing seemed to be happening over the Bavarian Alps too. DR BEATE LIEPERT (Lamont-Doherty Earth Observatory): I was the same, I was as sceptical as any other climatologist. But then, um, I, I saw the same results er in Germany, so um I believed him.

By now i'm thinking if there were something here, we would be discussing it. or perhaps i'm just lazy, and Dimming is part of the global discussion, i'm just not aware of it.

DR MICHAEL RODERICK: And then one day, just by accident, I had to go to the library to get an article out Nature. As you do, I couldn't find it. And I just glanced at a, through the thing, and there was an article called Evaporation Losing Its Strength. Which reported a decline in pan evaporation over Russia, United States and Eastern Europe. And there in the, in the measurements, they said that the, the pans had on average, evaporated about a hundred millimetres less of water in the last thirty years.
Project INDOEX, as it was called, was a huge multinational effort. For four years every possible technique was used to sample and monitor the atmosphere over the Maldives. INDOEX cost twenty-five million dollars, but it produced results - and they surprised everyone.

PROF VEERABHADRAN RAMANATHAN: The stunning part of the experiment was this pollutant layer which was three kilometre thick, cut down the sunlight reaching the ocean by more than 10%.

So the scientists detect an effect which is 10X what the models predict.

DR PETER COX: We've got two competing effects really, that we've got the greenhouse effect, which has tended to warm up the climate. But then we've got this other effect that's much stronger than we thought, which is a cooling effect that comes from particles in the atmosphere. And they're competing with one another. And we know the climate's moved to a warmer state by about point six of a degree over the last hundred years. So the whole thing's moved this way. If it turns out that the cooling is stronger than we thought then the warming also is a lot stronger than we thought, and that means the climate's more sensitive to carbon dioxide than we originally thought, and it means our models may be under sensitive to carbon dioxide.

Long story short, if there's something here, you'd think this would be getting some attention. So i'm assuming there's been discrediting research done of which i'm unaware. (no time to check, barely found time to do this diary, so let's use the resources of ET.)

But then...

NARRATOR: At this point, whatever we did to curb our emissions, it would be too late. Ten thousand billion tons of methane, a greenhouse gas eight times stronger than carbon dioxide, would be released into the atmosphere. The Earth's climate would be spinning out of control, heading towards temperatures unseen in four billion years. But this is not a prediction - it is a warning. It is what will happen if we clean up pollution while doing nothing about greenhouse gases. However, the easy solution - just keep on polluting and hope that Global Dimming will protect us - would be suicidal.

I've found two previous diaries

Cooling the Earth: CO2, SO2, and The Sunscreen Fix (Oct 2006) by technopolitical

Global Dimming (sept 2007) by poemless

Judging by what you've quoted, there hasn't been much more work since that quoted in the diaries above.

keep to the Fen Causeway

by Helen (lareinagal at yahoo dot co dot uk) on Tue Sep 14th, 2010 at 02:49:32 PM EST
A review from elsevier 2001


A number of studies show that significant reductions in solar radiation reaching the Earth's surface have occurred during the past 50 years. This review analyzes the most accurate measurements, those made with thermopile pyranometers, and concludes that the reduction has globally averaged 0.51±0.05 Wm−2 per year, equivalent to a reduction of 2.7% per decade, and now totals 20 W m−2, seven times the errors of measurement. Possible causes of the reductions are considered. Based on current knowledge, the most probable is that increases in man made aerosols and other air pollutants have changed the optical properties of the atmosphere, in particular those of clouds. The effects of the observed solar radiation reductions on plant processes and agricultural productivity are reviewed. While model studies indicate that reductions in productivity and transpiration will be proportional to those in radiation this conclusion is not supported by some of the experimental evidence. This suggests a lesser sensitivity, especially in high-radiation, arid climates, due to the shade tolerance of many crops and anticipated reductions in water stress. Finally the steps needed to strengthen the evidence for global dimming, elucidate its causes and determine its agricultural consequences are outlined.

Another one in GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L04702, 4 PP., 2007

Speculations on the impact of variations in surface solar radiation on global warming range from concerns that solar dimming has largely masked the full magnitude of greenhouse warming, to claims that the recent reversal from solar dimming to brightening rather than the greenhouse effect was responsible for the observed warming. To disentangle surface solar and greenhouse influences on global warming, trends in diurnal temperature range are analyzed. They suggest that solar dimming was effective in masking greenhouse warming, but only up to the 1980s, when dimming gradually transformed into brightening. Since then, the uncovered greenhouse effect has revealed its full dimension, as manifested in a rapid temperature rise (+0.38°C/decade over land since mid-1980s). Recent solar brightening cannot supersede the greenhouse effect as main cause of global warming, since land temperatures increased by 0.8°C from 1960 to 2000, even though solar brightening did not fully outweigh solar dimming within this period.

Seems the effect is real, has stopped offsetting global warming in the last 30 years, and was due to atmosheric contamination by particles.

by Xavier in Paris on Tue Sep 14th, 2010 at 03:17:34 PM EST
[ Parent ]
OK, I am not a climatologist, but I use their data.

The second quote by Xavier is from Martin Wild et al. I heard a talk last week by Wild, and he kindly sent me some of his papers. I'll try to summarise them here tomorrow. The basic gist of the talk was:

Europe and North America: dimming until the 1980's, brightening since then, continuing in the last decade.

India and East Asia (especially China): dimming in the last 20 years, probably more strongly lately.

Both these effects could be due to changes in the level of pollution.

I'm not sure if it is possible to say much about the rest of the planet, as there are very few solar radiation measurements there. It is possible to use satellite data to estimate solar radiation, but for long-term trends it can lead to problems. The first is a lack of data before ~1980. The second is that satellite estimates depend on knowledge about the amount of aerosols in the atmosphere. Sometimes this knowledge is so sparse that you have to use long-term averages in aerosol concentrations, and then you mask any trend. If there is a change in cloud cover (which could be caused by a change in aerosols), then you may be able to see that.

Real capricorns don't believe in astrology.

by tomhuld (thomas punkt huld at jrc punkt it) on Tue Sep 14th, 2010 at 04:04:26 PM EST
[ Parent ]
So basically, if you drastically reduce airplane travel the cooling from CO2 reductions you get might be offset by the warming effect of the absence of con-trails and carbon particulates?

"The future is already here -- it's just not very evenly distributed" William Gibson
by ChrisCook (cojockathotmaildotcom) on Tue Sep 14th, 2010 at 04:12:50 PM EST
Errm, no. Again, I'll have to dig up the reference, but the effects of flying on climate are overall warming, even apart from the CO2.

Carbon particulates cause dimming at the ground, but heat the planet because they are so good at absorbing light.(1)

Adding water vapour from the exhaust just below the stratosphere also adds to the greenhouse effect.

Contrails reflect sunlight during the day (cooling), but help keeping the earth's surface warm at night (warming).

So really, really don't fly unless you have to....

(1) The aerosols that help keep the earth cool come mainly from sulphur dioxide (SO2) that turn into tiny droplets of sulphuric acid in the atmosphere. These droplets are very reflective. Great for counteracting global warming if you don't mind a little acid rain.

Real capricorns don't believe in astrology.

by tomhuld (thomas punkt huld at jrc punkt it) on Tue Sep 14th, 2010 at 04:42:23 PM EST
[ Parent ]
I wrote a diary about this subject here a few years ago, including a bit on global dimming.

At face value, this suggests that the condensation trails and the following cloud formation have an overall global dimming effect. Therefore, temperatures that we measure on earth on an ordinary day with a sky full of airplane contrails are an underestimation of the solar irradiance that the surface should truly receive. Effectively, condensation trails form a negative radiative forcing - similarly to extra cloud formation after a catastrophic volcanic eruption.

That, however, is just half of the story. The reflection of solar radiance is a purely daytime effect. During the night, the reflection qualities of contrails contribute to our disadvantage: heat radiating back into space from the surface and lower atmosphere is blocked by similar airplane contrails. The trapping of radiation aids in keeping up temperatures and thus forms a positive radiative forcing - a forcing which is active for 24 hours, unlike the negative forcing. Contrails in the night are not the same ones you can see during the daytime: contrails have a relative quick turnover, yet aviation is now so pervasive there are contrails formed 24 hours a day.

The Nature publication that inspired it was this one.

Hope that contributes.

Now back to lounging in Cape Town.

by Nomad on Wed Sep 15th, 2010 at 01:15:04 PM EST
[ Parent ]
Thanks folks, i'm really trying to get a handle on what this means. That's why i posted without knowing.

"Life shrinks or expands in proportion to one's courage." - Ana´s Nin
by Crazy Horse on Tue Sep 14th, 2010 at 06:16:52 PM EST
are major cooling factors. They are pretty well factored into the global climate models these days.

My intuition, which seems to be the concensus among the specialists, but hasn't had much of a public airing, is this :

  • Industrialization creates pollution (SO2, dust etc) which has a dimming/cooling effect, both regionally and globally
  • Industrialization also increases CO2 output, which has a well-understood global warming effect.
  • Here's the kicker : the CO2 increase is effectively permanent (surplus CO2 stays in the atmosphere forever, given that the CO2 absorbing systems are saturated). SO2 and particles are short-term only : they eventually precipitate out of the atmosphere.

So the warming effect is cumulative. The cooling effect is indeed considerable, and currently probably masks at least half of the CO2 warming. BUT BUT BUT... as soon as you clean up your act (North America, Western Europe in the 80s, industrial collapse in the eastern bloc in the 90s), the cooling effect grinds to a halt.

I have a feeling that this goes a long way towards explaining the surge in global warming in the 90s (diminished dirty-industrial cooling), and the relative flattening out in the last ten years (Chindia etc).

When Chindia get serious about cleaning up their industry... and they will... the warm bucket of crap hits the fan.

My source for most of this is the following, serious climate science site :

It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II

by eurogreen on Wed Sep 15th, 2010 at 12:53:58 PM EST

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