What is global dimming?
84Dimming or warming, or both?
We all know what is meant by global warming: the trend for the temperature of the planet to rise. We also know the cause, human activity mostly through the increased production of atmospheric carbon dioxide. But another phenomenon which is related to global warming is a source of some confusion: global dimming.
In 2001, a UK scientist called Gerry Stanhill looked at the records of sunlight reaching the earth over the previous fifty years and noticed that it had dropped by around 22%. Something was stopping light reaching the earth's surface. Particles in the air, caused by industrial pollution cause the formation of water droplets which in turn reflect more of the sunlight back away from earth so less energy actually gets to the surface. And if the oceans don't get the same amount of heat as previously, the evaporation rate will be different, so less water is cycled back into the atmosphere. And that's bound to affect the weather, particularly the distribution of rainfall and therefore drought.
How does dimming affect global warming?
With the light comes heat so the dimming effect reduced the amount of heat the earth gets from the sun. Nevertheless, we know that the land, air and sea temperatures are rising despite the dimming effect of the industrial pollutants in the air. So what is going on?
The dimming effect is opposite to the greenhouse effect. The greenhouse effect works through the reflected heat energy from the earth's surface being captured by the greenhouse gases in the atmosphere and the trapped heat then warms up the earth. The dimming effect reduces the amount of light, and therefore heat, that hits the earth's surface in the first place.
That means two important things. Firstly, the rise in temperatures would have been more if it wasn't for the dimming effect caused by the pollutants. Reducing the pollutants actually increases the warming effect of the carbon dioxide in the atmosphere. Secondly, it meant that the climate was actually more sensitive to carbon dioxide levels than was previously thought. If the projections of temperature rises were based on a continuing dimming effect, then there is an even greater urgency to deal with the carbon dioxide emissions.
Then NASA discovered in 2007 that the actions to reduce the impact of CFCs (chlorofluoruocarbons) from aerosols on the atmosphere was paying off in the form of increased light hitting the surface. The measures taken by the Montreal agreement to limit these ozone-damaging chemicals, was actually paying off. The further evidence is the gradual closing of the hole in the ozone layer over Antarctica.
Can we play one against the other?
A sensible question though is whether these two effects can cancel each other out, but unfortunately the question isn't quite so simple. An increase in the particles in the atmosphere will cause water droplets to form, but water vapour is itself the major greenhouse gas. But rain, evaporation, and cloud formation, all affect global dimming too because when it rains, the pollutant particles are washed out of the sky, increasing the amount of light that hits the surface again, and that reduces the effect of global dimming.
So it's not a simple case of pairing one against the other. They are complex and have related causes. It was, at one time, suggested that a controlled increase in pollutants into the upper atmosphere might be the lesser of two evils - we'd reduce global warming at the expense of the quality of the atmosphere. But it's not at all realistic. At the very least, we'd massively increase acid rain and weather systems would be disastrously affected. And worst of all, we'd be committed to continuing the polluting strategy as a means of survival.
How does all this affect climate models?
Global dimming is one more factor that is included in computerised climate models and it's effect is used to adjust the predictions of temperature rise. Of course, if governments commit to reducing air pollution and are effective, that will cause a change in the dimming effect, which will affect the predictions of global warming. These models are dynamic, and allow for the testing of various scenarios so it's possible to relate policy actions to expected outcomes.
The evidence is clear that global dimming does not compensate for the effects of atmospheric carbon dioxide and can't be considered as a palliative. Representatives of the fossil fuel industries have occasionally referred to global dimming as a mitigating factor but the science is clear that although dimming and warming are related effects, the major problem for humanity is global warming. Reducing atmospheric carbon dioxide is not just an option, it's a necessity.
See also William R. Wilson's excellent hubs on global warming.
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Certainly after the fiasco of Copenhagen, the political will seems to have dimmed. It's almost as if the politicians are so scared that anyone else might get some minor competitive advantage, that they are willing to compromise humanity's future. That's dim in all senses of the word.
Enjoyed reading your article Bob. I first heard of 'Global Dimming' on a TV documentary.
Interesting fact it explained was that when US aircraft were all grounded in the immediate aftermath of 911 the dimming effect was reduced in the USA.
Yes, it's an effect that can change very rapidly and that makes it regional. Thanks for reading the article - glad you enjoyed it.
I heard some time ago on George Noory's coasttocoastam radio show something about (I think) a book maybe called "Not by fire but by ice", where the author said that the increased output of greenhouse gasses has prevented, so far, another ice age... ;-)
I've heard that viewpoint expressed a number of times and sometimes they draw on the world's previous cold periods of 34 and 23 million years ago. Both times, there was a very sudden drop in temperature and glaciers advanced for hundreds of thousands of years, and they were associated with drops in the level of greenhouse gases.
But that's a long way from claiming that there is a simple 1:1 correspondence. There isn't. There are many factors that interact including the albedo (the reflectivity of the earth), the concentration of carbon dioxide in the oceans (and hence the acidity), the flow of cold currents in the oceans, etc. Also during ice ages, the actual land mass area is increased (because the sea level is lower) and so that increases the albedo and consequent absorption of heat. So it's a multifactor problem with very many interacting variables. In addition, there are methane deposits beneath the sea which were released at important points, for example in the North Sea.
When broadcasters pick on one factor or another, they often do not realise the complexity of the interaction and the necessity for simulation models which can predict consequences.
Having said that, it is certainly true that greenhouse gases maintain the earth's temperature in a range which supports life. But that's at a carbon dioxide level around 350ppm maximum whereas we are already at 387ppm and Copenhagen agreed to discuss later a level of 770ppm! Allowing it to remain over 350ppm means that the heating pump is in a positive feedback loop, with insufficient constraint from the opposing factors.
Even with a small rate of growth in carbon dioxide, there is sufficient to create major instability in weather systems.
Climate and weather is too complicated as far as I'm concerned to discern which faction is correct, or even mostly so.
Anyway, to me, the climate/greenhouse debate is a huge "smokescreen" to divert attention from the major problem. That problem will have more negative effects on humanity than any warming (or cooling). And, any negative effects from warming/cooling will be greatly exacerbated by that problem.
The problem is: Resource depletion caused by humongous OVERPOPULATION. The oceans are running out of fish. Arable land is diminishing. Too many people are chasing the same amount of fresh water. Energy (oil), despite all the efforts to promote alternatives, remains the main energy source. All the starry-eyed alternative source promoters neglect one tiny little problem. If it costs more in energy to produce and distribute energy than you get, GAME OVER.
"Whatever your cause, it is a lost cause without population control."
Overpopulation is certainly another major problem facing humanity, but solving that one and leaving global warming unsolved is still catastrophic. But I don't think it's any kind of smokescreen. No-one is trying to hide the overpopulation issue by discussing global warming. They are different issues.
I don't see it as a question of choosing which one to deal with: I think both have to be addressed. Certainly many governments don't want to talk about overpopulation though.
Maybe another hub should deal with the overpopulation issue.
Bob, Sorry to disagree, but the two problems are one. If it were not for massive overpopulation, there would be little or no effect of humans on the environment (including on the atmosphere).
My position is: I and my family, for several generations, have not contributed to the population explosion. So I am not going to go out of my way to "be green". Let those who drop litters of spawn -- with no more thought of consequence than cats, rats, dogs -- let them sacrifice.
Isn't it interesting that -- it seems to me -- so many who are concerned about the ecology have not contributed to the overpopulation problem. And those who unconcernedly pump out more sub-humans remain blissfully unaware.
How about some spaying and neutering, and let the existing starving children continue to starve, so that they don't pump out more litters of starving children?
dabeaber:
I agree with you that the problems are related. Not only does population growth exacerbate the environental problems, but changes in the environment also affect habitation, social arrangements, etc.
But I think your approach is paradoxically separating them and applying blame to the causes of just one of the factors. It is true that those who have larger families are proportionately increasing population more, but that in no way reduces the urgency for all of use to help reduce carbon dioxide in the atmosphere.
It is something of an easy option to blame "sub-humans" for dropping "litters of spawn" but in many parts of the world, economic survival remains based on large families who can contribute manual labour to family production. Allowing starving large families to die, whilst refusing to contribute to limiting carbon growth is pushing all the blame onto people who mostly have no alternative course of action.
Awareness of the population problem is low at present, largely because the advanced economies have been able to produce sufficient surplus to maintain the same or a growing standard of living. That surplus is very unevenly distributed around the world, and without addressing the economics in poorer countries, no effort to restrict population has much chance of success.
So it's laudable that you're doing your bit, but not a reason to blame those who are in a position to do the same. And certainly the suggestion of leaving starving people to die in a world which dumps its surplus food, seem morally unjustifiable.
userjavid 2 years ago
Global dimming is literary word in contrast to global warming. It may symbolise political temperature prevailing in the world.