Global Dimming

[Theben]

This is a topic which I haven't heard much about until today when flipping thru TV channels. So, like any good intarweb wank, I Wiki'ed it.

Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4% reduction over the three decades from 1960–1990. However, since 1990, the trend has reversed.[1]

It is thought to have been caused by an increase in particulates such as sulphate aerosols in the atmosphere due to human action. The switch from a "global dimming" trend to a "brightening" trend in 1990 happened just as global aerosol levels started to decline.

Global dimming has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas. Global dimming also creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming.

At 1st I thought, "great, a counter to global warming." But it wasn't that easy.

Some scientists now consider that the effects of global dimming have masked the effect of global warming to some extent and that resolving global dimming may therefore lead to increases in predictions of future temperature rise.[17] According to Beate Liepert, "We lived in a global warming plus a global dimming world and now we are taking out global dimming. So we end up with the global warming world, which will be much worse than we thought it will be, much hotter."[42] The magnitude of this masking effect is one of the central problems in climate change with significant implications for future climate changes and policy responses to global warming.[43]

But it's much more complicated than an either warming or dimming issue. Global warming and global dimming are not mutually exclusive or contradictory. In a paper published on March 8, 2005 in the American Geophysical Union's Geophysical Research Letters, a research team led by Anastasia Romanou of Columbia University's Department of Applied Physics and Mathematics, New York, also showed that the apparently opposing forces of global warming and global dimming can occur at the same time.[44] Global dimming interacts with global warming by blocking sunlight that would otherwise cause evaporation and the particulates bind to water droplets. Water vapor is one of the greenhouse gases. On the other hand, global dimming is affected by evaporation and rain. Rain has the effect of clearing out polluted skies.

So basically the models indicating a 2C-3C rise in temp by 2100 could now look like 3-5C. Add on top of this the higher risk of methane clathrate being released from the seabed and global temps could rise higher than 10C.

Methane is a powerful greenhouse gas. Despite its short atmospheric half life of 7 years, methane has a global warming potential of 62 over 20 years and 21 over 100 years (IPCC, 1996; Berner and Berner, 1996; vanLoon and Duffy, 2000). The sudden release of large amounts of natural gas from methane clathrate deposits has been hypothesized as a cause of past and possibly future climate changes. Events possibly linked in this way are the Permian-Triassic extinction event, the Paleocene-Eocene Thermal Maximum.

So that's it. Discuss.

[Lancer]

There's a really extended solar min (no sunspots) so add iceage into the pot and see how it tastes.

http://www.popularmechanics.com/science/earth/4248062.html

[Theben]

Yeah, the solar minimum is probably our one chance to get warming under wraps.

[LotC]

Originally posted by Lancer
There's a really extended solar min (no sunspots) so add iceage into the pot and see how it tastes.

http://www.popularmechanics.com/scie...h/4248062.html

The solar cycle is going pretty much as planned. It's quite normal to have no sunspots during the minimum. Next solar maximum is still scheduled for 2012. It's predicted to be a whopper too. Expect some brilliant auroras, maybe some blackouts, and perhaps even a few knocked out satellites.

Insert Mayan calendar end of the world prophecies here:

[Theben]

Okay, nevermind. Apparently solar cycles don't have a great impact on GW, unless, of course, we actually have a mini ice-age.

[Berzerker]

"Global dimming interacts with global warming by blocking sunlight that would otherwise cause evaporation and the particulates bind to water droplets. Water vapor is one of the greenhouse gases. On the other hand, global dimming is affected by evaporation and rain. Rain has the effect of clearing out polluted skies."

I understand particulates can serve as the "nuclei" for rain drops, that water vapor condenses around the particulate so that even if evaporation declines the pollution would promote rain. So many factors...

[Theben]

Originally posted by Berzerker
I understand particulates can serve as the "nuclei" for rain drops, that water vapor condenses around the particulate so that even if evaporation declines the pollution would promote rain. So many factors...

The program I watched also noted in the Maldives study, that with pollutants available as 'nuclei' the rain droplets that formed were much smaller but more numerous, further causing localized dimming...

[Maniac]

[notyoueither]

Originally posted by Theben
Okay, nevermind. Apparently solar cycles don't have a great impact on GW, unless, of course, we actually have a mini ice-age.

That's a pretty large unless.

[Theben]

Well solar cycles are about 11 years each; apparently that's not long enough to make much of a difference. 400 years is another story. Some people are predicting this minimum will go on a bit longer, but I don't know what evidence there is to show that.

It gets even better! Besides the ice-albedo effect (ice reflects light/heat: more ice, exponential cooling- less ice, exponential warming), we get this gem:

Large scale changes in weather patterns may also have been caused by global dimming. Climate models speculatively suggest that this reduction in sunshine at the surface may have led to the failure of the monsoon in sub-Saharan Africa during the 1970s and 1980s, together with the associated famines such as the Sahel drought, caused by Northern hemisphere pollution cooling the Atlantic.[38] Because of this, the Tropical rain belt may not have risen to its northern latitudes, thus causing an absence of seasonal rains. This claim is not universally accepted and is very difficult to test.

[sic]

A natural form of large scale environmental shading/dimming has been identified that affected the 2006 northern hemisphere hurricane season. The NASA study found that several major dust storms in June and July in the Sahara desert sent dust drifting over the Atlantic Ocean and through several effects caused cooling of the waters — and thus dampening the development of hurricanes

So pollutants from Europe may have contributed to drought and desertification in sub-sahara, which may have contributed to dust drifting over the Atlantic, which may have cooled the waters, which may have dampened the 2006 hurricanes. Yay variables!

[rmsharpe]

Hooray for manufactured crises!

[dannubis]

Yes, let us burn even more oil we could use to make polymers!

[Elok]

Originally posted by LotC
Insert Mayan calendar end of the world prophecies here:

Sorry, I don't believe in the prophecies of anyone dumb enough to run thorn-studded strings through his own penis.

[Felch]

More solar radiation and a warmer planet will have an amazing effect on plant life, and should greatly increase crop yields. Sounds like good news.

[dannubis]

Tell that to the Sahel countries