Posted on 03/19/2008 2:36:03 PM PDT by blam
Ancient Global Dimming Linked to Volcanic Eruption
Ker Than
for National Geographic News
March 19, 2008
A "dry fog" that muted the sun's rays in A.D. 536 and plunged half the world into a famine-inducing chill was triggered by the eruption of a supervolcano, a new study says.
The cause of the sixth-century global dimming has long been a matter of debate, but a team of international researchers recently discovered acidic sulphate molecules, which are signs of an eruption, in Greenland ice.
This is the first physical evidence for the A.D. 536 event, which according to ancient texts from Mesoamerica, Europe, and Asia brought on a cold darkness that withered crops, sparked wars, and helped spread pestilence.
Scientists had suspected the dry fog was caused by a volcanic eruption or a comet strike, but searches had failed to uncover evidence for either catastrophe—until now.
"There is no need at the moment to invoke a large-scale extraterrestrial event as the cause, because the evidence is conclusive enough to say that it is certainly consistent with it being a large volcano," said study team member Keith Briffa of the University of East Anglia in the United Kingdom.
The discovery is detailed in a recent issue of the journal Geophysical Research Letters.
Global Ashfall
Tests show the Greenland sulphate molecules were deposited sometime between A.D. 533 and 536. This date correlates well with a sulphate peak found in an Antarctic ice core.
The team suspects the eruption occurred near the Equator, since its ash fell on both ends of the globe.
The Greenland evidence is also consistent with tree-ring data from around the Northern Hemisphere that show reduced growth rates lasting more than a decade starting in A.D. 536.
Curiously, the eruption's cooling effect did not extend to the southern hemisphere, the scientists say.
Together, the tree-ring and acid evidence suggest the sixth-century eruption was even bigger than Indonesia's Mount Tambora eruption of 1815, which also dimmed the sun.
Not Definitive
Ken Wohletz, a volcanologist at Los Alamos National Laboratory in New Mexico, said that while the new evidence strongly supports a large volcanic eruption, a space impact can't be ruled out yet.
"Over two-thirds of Earth's surface is covered with water, and because erosion so quickly wipes away evidence of impacts, the knowledge of when large-scale impacts have occurred in the past is still very incomplete," said Wohletz, who was not involved in the study.
To cement their case, volcano advocates will need to find ash layers deposited by the blast, Wohletz said.
William Ryan, an oceanographer at Columbia University's Lamont-Doherty Earth Observatory in New York, believes it is only a matter of time until ash layers are found.
"I suspect we haven't searched adequately, but this paper will start a hunt," Ryan said.
Indelible Mark
According to written records, the dry fog lingered for just over a year—leaving an indelible mark on human history.
Chinese historians recorded famine events and summer frosts for years after the event.
It was also around this time that a band of Mongolian nomads called the Avars migrated westward toward Europe, where they would eventually establish an empire.
The group may have left home when grasslands that their horses grazed on withered under the darkened skies, historians say.
More controversially, some historians claim that drought caused by the fog contributed to the decline of the Mesoamerican city of Teotihuacan.
The spread of bubonic plague throughout Europe and the Middle East, the rise of Islam, and even the fall of the Roman Empire have also been controversially tied to the event.
Still Vulnerable
If a similar volcanic eruption were to occur today, the effects could be just as devastating, experts say.
The reduced sunlight and ashfall would affect agriculture worldwide, and the thick veil of dust and ash could cripple transportation and communication systems.
"Most aircraft cannot fly in [volcanic] dust clouds," Los Alamos's Wohletz said.
"And these dust clouds have a large electrostatic potential that disrupts radio communication."
To make matters worse, there is practically nothing humans can do to prevent such a catastrophe from happening again—or to lessen its effects.
"In today's society, we're no less independent of nature than humankind has ever been," Wohletz said.
"In fact, we might even be more dependent on it."
Increasing concentrations of atmospheric CO2 cause warming. Sulphate aerosols and ash from volcanic eruptions cause cooling. This article is about a cooling event.
A little off topic.
While cruising Google Earth I noticed a circular pattern in Alabama primarily.
Starts around Montgomery extends WNW into Mississippi and up into Tenn.
Almost looks like an ancient asteroid crater.
Can you enlighten me?
Sorry, no.
No hard evidence of that.
Besides, nowadays, man-made global warming caused by increasing CO2 levels are being blamed for taking us into the next ice age.
You did know, didn't you, that global warming causes temperatures to drop?
If you can make sense of that, let me know. The liberals have it figured out but no one else that I know of.
1816 was another “year without summer”
That's not how science works. Science is about data supporting an explanatory framework (colloquially called a "theory", but that gets confusing). Data and analyses of the data are interpreted in terms of how well they support alternate frameworks. A framework that gets considerable support from the observational data is considered a good explanation.
Thus, you get something like this:
Climate sensitivity constrained by CO2 concentrations over the past 420 million years (PDF)
Here's a readable summary:
Long-term geological record puts minimum value on climate sensitivity
Quote: "We were able to accomplish this tuning because one of the factors in the geochemical model is the relationship between carbon dioxide and temperature," said Royer. "We found that the deep-time geological records exclude the possibility of weak climate sensitivities: we conclude that the amount of warming for every doubling of carbon dioxide must be at least 1.5 °C."
Thus, the data and analyses of this study support the explanatory framework -- the relationship of atmospheric CO2 concentrations and global temperatures.
Nowadays, man-made global warming caused by increasing CO2 levels are being blamed for taking us into the next ice age. ... You did know, didn't you, that global warming causes temperatures to drop?
Yes. Read about the Younger Dryas
If you can make sense of that, let me know.
"The dense water masses that sink into the deep basins are formed in quite specific areas of the North Atlantic and the Southern Ocean. In these polar regions, seawater at the surface of the ocean is intensively cooled by the wind. Wind moving over the water also produces a great deal of evaporation, leading to a decrease in temperature, called evaporative cooling. Evaporation removes only molecules of pure water, resulting in an increase in the salinity of the seawater left behind, and thus an increase in the density of the water mass. In the Norwegian Sea evaporative cooling is predominant, and the sinking water mass, the North Atlantic Deep Water (NADW), fills the basin and spills southwards through crevasses in the submarine sills that connect Greenland, Iceland and Great Britain. It then flows very slowly into the deep abyssal plains of the Atlantic, always in a southerly direction. Flow from the Arctic Ocean Basin into the Pacific, however, is blocked by the narrow shallows of the Bering Strait."
If the cold water masses don't sink, extremely cold water stays at the ocean surface and thus the polar cold is not shunted into the deep ocean. So instead it would cool the atmosphere, particularly in the Northern Hemisphere and predominantly in North America and Europe.
There are far more variables to weather and climate than just CO2. There’s just no way it can all be attributed to one factor.
CO2 is only the third strongest greenhouse gas to begin with.
There’s no way the ocean’s circulation is going to stop.
Even if all that cold water got dammed up at the poles, doesn’t necessarily mean that it would cool the continents south of it. The Hadley cells don’t permit much interchange of atmosphere between the polar regions and the temperate regions so you’re not going to have much cooling through the atmosphere. And if the oceans circulation stops, unlikely as it seems, then there’s no cold water being drawn out to cool the continents.
The alleged drop in temperatures of the poles is not going to cool the whole planet. All it would do is increase the temperature gradient some between the equator and the poles.
Global warming is not going to cause global cooling. SOME parts, might temporarily get colder than others but that happens now anyway and that’s called weather.
Good, thanks.
"Climate science maverick James Lovelock believes catastrophe is inevitable, carbon offsetting is a joke and ethical living a scam. So what would he do? "
Eat, drink, and be merry for tomorrow we die?
Wouldn't it be easier to move inland than try to cross the widening English Channel to an island country that is flooding?
It certainly isn't! Rememeber, I was responding to your comment on "Increasing concentrations of atmospheric CO2 cause warming." Which they do. But that is only one factor in climate.
CO2 is only the third strongest greenhouse gas to begin with.
That fact doesn't change the truth of the statement that increasing atmospheric CO2 concentrations cause warming.
There’s no way the ocean’s circulation is going to stop.
It doesn't. The Younger Dryas mechanism is that deepwater formation in the North Atlantic is stopped. Ocean circulation goes into a different "mode" until deepwater formation resumes. For the Younger Dryas, it took about 1000 years.
The alleged drop in temperatures of the poles is not going to cool the whole planet. All it would do is increase the temperature gradient some between the equator and the poles.
Good point. The YD was most pronounced in the Northern Hemisphere, specifically North America and Europe. While there are YD signals elsewhere, it apparently didn't have a major global influence. Antarctic ice cores don't show nearly the big temperature drop that the Greenland cores show.
Global warming is not going to cause global cooling. SOME parts, might temporarily get colder than others but that happens now anyway and that’s called weather.
Climate is average weather. Regional climate is more variable than global climate, obviously.
Climate is average weather. Regional climate is more variable than global climate, obviously.
You're preaching to the choir.
Yup. A comet/asteroid impact or a super-volcano are more likely to kill us before 'global-warming/climate-change caused by humans.'
As I said, data support an explanatory framework. The influence of atmospheric CO2 concentrations on planetary temperatures is an explanatory framework. To demonstrate, I provided a reference of paleoclimatic data analyses over 420 million years indicating that doubling of atmospheric CO2 concentration induces, at a minimum, a 1.5 degree C global temperature increase. (Maximum 6.2 deg C, median 2.8 deg C). That is one example of how data and data analyses support an explanatory framework.
A wide variety of data types and analyses support the explanatory framework that higher atmospheric CO2 concentrations induce warmer global temperatures*. The alternative -- that they don't -- is not as strongly supported, by a long shot.
* The basis for this framework is the direct physical observation that the CO2 molecule absorbs longwave (infrared) radiation.
What you or anybody "think" (i.e. opinion) doesn't matter in science. What matters in science is the explanatory framework that is best supported by the data.
I have yet to meet a meteorologist who accepts the current global warming scenario.
When the data can be shown to be reliable, that might give it more credibility, but the evidence is so flimsy that the time frame so inadequate, that building a whole theory on it is ludicrous.
If you want to buy into the whole global warming thing, go ahead and buy your carbon offsets. Just don’t preach it as fact cause it ain’t.
Meteorologists may or may not know a lot about climate; it isn't necessary to be a climate expert to be a meteorologist. To be a meteorologist you need to know meteorology.
When the data can be shown to be reliable, that might give it more credibility, but the evidence is so flimsy that the time frame so inadequate, that building a whole theory on it is ludicrous.
There is a lot of data, of many different types. Your blanket characterization of it as "flimsy" is opinion.
Since we've now deviated considerably from the original point, I will agree to a conclusion of our discussion at this point.
OK :)
read later
From late 535 AD to 536, written records from across the world suggest a huge climate catastrophe. Dubbed the year "without a summer", the sun was completely dimmed and shadows were invisible even at noon. The cause of of the "worst year to be alive in history" has been long uncertain.
Was it a comet? An asteroid? A volcano? Archaeologist David Keys reveals the latter is to blame for the Dark Ages of famine and plague that shaped the world order of today. Episode 1 of 2.536 AD: The Year That The Sun Disappeared | Catastrophe | Chronicle
March 9, 2022 | Chronicle - Medieval History Documentaries
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