Global Dimming…(and its effects on global climate change)

[Aro]

A recent BBC documentary called Global Dimming (programme transcript) discussed the possibility of the existence of a recently discovered phenomenon – that of Global Dimming.

The measure of the effect Global Dimming required corroboration by multiple measurement methods because it was initially considered very surprising, The effect was so large that scientists found it difficult to believe nobody had mentioned it before (extraordinary claims require extraordinary evidence) and it initially seemed to be in direct contradiction to the accepted parameters of Global Warming.

Two kinds of measurements, light-meter, and water evaporation rates, have been made for over 50 years, and both have been shown to indicate that the amount of sunlight reaching the Earth’s surface has declined by 10 to 30 percent, depending on location, from the 1950s to the early 1990s. They say we are getting less sunlight thanks the visible pollution in the atmosphere which encourages clouds to form in a fashion which reflects much more sunlight (than would normally happen with clouds formed around natural air pollutants such as pollen.)

It stated (generally) that the presence of the effect of global dimming has acted as a counterweight to the ravages of global climate change (Nee: warming) in an uneasy yet dangerous equilibrium. It then explained that the process we are currently endeavouring upon, in cleaning up our air pollution and reducing pollutant emissions (whilst NOT reducing Co2 levels), will upset this delicate balance and allow the full force of global climate change to endure. With devastating results.

One finding was very interesting:

NARRATOR: For 15 years Travis had been researching an apparently obscure topic, whether the vapour trails left by aircraft were having a significant effect on the climate. In the aftermath of 9/11 the entire US fleet was grounded, and Travis finally had a chance to find out.

DR DAVID TRAVIS: It was certainly, you know, one of the tiny positives that may have come out of this, an opportunity to do research that hopefully will never happen again.

NARRATOR: Travis suspected the grounding might make a small but detectable change to the climate. But what he observed was both immediate and dramatic.

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.

NARRATOR: One degree in just three days no one had ever seen such a big climatic change happen so fast. This was a new kind of climate change. Scientists call it Global Dimming. Two years ago most of them had never even heard of it, yet now they believe it may mean all their predictions about the future of our climate could be wrong…

It also blamed Global Dimming as a possible unrealised contributing factor to the famine crises on the mid 1980’s , and that this is just a taste of what we could face in future years if the issue goes unaddressed or unchecked.

NARRATOR: The 1984 Ethiopian famine shocked the world. It was partly caused by a decade’s long drought right across sub-Saharan Africa - a region known as the Sahel. For year after year the summer rains failed. At the time some scientists blamed overgrazing and poor land management. But now there’s evidence that the real culprit was Global Dimming. The Sahel’s lifeblood has always been a seasonal monsoon. For most of the year it is completely dry. But every summer, the heat of the sun warms the oceans north of the equator. This draws the rain belt that forms over the equator northwards, bringing rain to the Sahel. But for twenty years in the 1970s and 80s the tropical rain belt consistently failed to shift northwards - and the African monsoon failed. For climate scientists like Leon Rotstayn the disappearance of the rains had long been a puzzle. He could see that pollution from Europe and North America blew right across the Atlantic, but all the climate models suggested it should have little effect on the monsoon. But then Rotstayn decided to find out what would happen if he took the Maldive findings into account.

DR LEON ROTSTAYN (CSIRO Atmospheric Research): What we found in our model was that when we allowed the pollution from Europe and North America to affect the properties of the clouds in the northern hemisphere the clouds reflected more sunlight back to space and this cooled the oceans of the northern hemisphere. And to our surprise the result of this was that the tropical rain bands moved southwards tracking away from the more polluted northern hemisphere towards the southern hemisphere.

NARRATOR: Polluted clouds stopped the heat of the sun getting through. That heat was needed to draw the tropical rains northwards. So the life giving rain belt never made it to the Sahel.

Now, I know we have some major keen and knowledgeable environmental scientists on this board and would be interested in learning how much of the programme is considered hard science and how much is still currently speculation and hearsay. Obviously the conclusions inferred from the data may be fairly scare-mongering worst-case scenarios, but is the raw data and underlying ideas effectively incontrovertible? (MODS: I placed this in GD as I’m quite sure there is a difference of opinion on the matter)

Thanks.

[Lemur866]

Nitpick.

Climate change factors may (or may not) have caused crop failures in east africa. But crop failures don’t cause famines. War causes famine. If Ethiopia wasn’t undergoing a civil war at the same time as the crop failures it would have been simple to ship them food.

I remember at the time people were asking things like “Why are they all living in the desert? Of course they’re starving!” But they were living in the desert refugee camps because they had to abandon their farms because of the civil war.

Famine is not an agricultural problem, it is a social/political problem.

[yojimbo]

Very good point Lemur866.

As a saying in Ireland goes “God sent the blight, but the English made the famine”

[Kimstu]

I think what the OP is asking about is the status of research on the role played in climate change by various changes to the earth’s albedo, or reflectivity. Clouds, including artificial clouds such as contrails, have varying effects on albedo depending on their composition and height, etc., as discussed in the above link.

These discoveries aren’t all that terribly recent, and they are AFAIK discussed in the climate literature—in fact, the above-linked Wikipedia article even references the post-9/11 contrails study that the OP is talking about. But the science is still very uncertain, and AFAICT nobody really knows yet what the overall impact of the various albedo effects, including cloud albedo, will be on the global climate.

I think jshore has mentioned this issue in previous climate change threads such as this one. You can find lots of discussions of it by googling “albedo contrails” or “albedo pollution”. But my guess would be that anybody who claims to know the net effect of albedo changes on climate change, or the net consequences to albedo from reducing pollution, is talking through their hat. If jshore comes by here he may have some more specific information.

[jshore]

Ackkk!!! I haven’t really kept up on this “global dimming” stuff (besides which I have sworn off “doping” to much over the next few months).

But fortunately there is an excellent new site out there, realclimate.org, put together by a bunch of real climate scientists with strong publication records. And, it so happens that they have taken on the subject of global dimming here and here. While you are at that site, I’d strongly recommend snooping around and reading other topics too. This site fills a cavernous vacuum in good public discussion of climate change science.