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What causes ice ages?

 
 
Reply Sun 4 Mar, 2007 12:04 pm
The graph goes back 450k years. Warm periods (like the present) are fairly brief in the scheme of things.

http://img355.imageshack.us/img355/6058/carbondioxidekz6.jpg

(This question is not political. I would like to understand what is causing these natural cycles (pre-industrialization). Thanks.)


What causes the temperature to spike up so rapidly (relatively speaking)?

And once it gets warm, what happens to make it drop off so quickly again?
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stuh505
 
  1  
Reply Sun 4 Mar, 2007 12:30 pm
Well the Earth's axis rotates very slowly over 30,000 years or so (in the opposite direction of the Earth's rotation). If it's more off-axis then it will recieve less consistent direct sunlight allowing the edges to cool more thus lowering overall average temperature. BTW this precession of the equinoxes as it is called also changes the signs of the zodiac, but they never update the zodiac sign based on the year so your signs are actually wrong now Smile
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littlek
 
  1  
Reply Sun 4 Mar, 2007 12:46 pm
30,000 year cycles don't seem to correlate to Ros' graph......
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stuh505
 
  1  
Reply Sun 4 Mar, 2007 01:08 pm
Yeah hmm 100k

Well, what is obvious from that graph is that temp is probably going to jump up to 10+ pretty soon because CO2 just had a huge spike and thats going to absorb a lot of radiation!
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patiodog
 
  1  
Reply Sun 4 Mar, 2007 01:21 pm
That's assuming previous carbon dioxide fluctuations caused the temperature shifts and not vice versa. Just a though -- not sure what a mechanism might be either way.

I've been wondering why nobody ever seems to bring up the fact that we're in a warm blip in the middle of an otherwise very cold epoch in the earth's history in the global warming debate. At risk of being glib, maybe forestalling the next temperature plunge will be good for yooman beans.
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patiodog
 
  1  
Reply Sun 4 Mar, 2007 01:25 pm
I do remember reading -- I don't remember the source but I do remember that it was a lay text, so many grains of salt here -- that models suggest that all it would take is one cold summer (where snow doesn't melt at subarctic latitudes) in the northern hemisphere to send us right back into full-blown ice age. All to do with the reflective properties of snow and the failure to retain heat.

I don't know if there are actually any good models to suggest this, or what the relationship with atmospheric CO2 would be.

I do wonder if the cycle is tied in anyway to absorption of CO2 into the oceans (which is nowhere near saturated so can buffer CO2 concentrations, theoretically).
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rosborne979
 
  1  
Reply Sun 4 Mar, 2007 02:20 pm
stuh505 wrote:
Well the Earth's axis rotates very slowly over 30,000 years or so (in the opposite direction of the Earth's rotation). If it's more off-axis then it will recieve less consistent direct sunlight allowing the edges to cool more thus lowering overall average temperature. BTW this precession of the equinoxes as it is called also changes the signs of the zodiac, but they never update the zodiac sign based on the year so your signs are actually wrong now Smile


Yes, some of these are called Milankovitch cycles, and I'm sure stellar motion has some input to the system.

But I'm more interested in why the spikes are so sharp. It seems like if orbital cycles were all there was to it, the spikes wouldn't be so sharp.

For the most part the earth seems to equalize to a lower temperature state. But then every 75k years or so, it starts warming up rapidly, hits a high point and then crashes down again.
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rosborne979
 
  1  
Reply Sun 4 Mar, 2007 02:30 pm
patiodog wrote:
That's assuming previous carbon dioxide fluctuations caused the temperature shifts and not vice versa. Just a though -- not sure what a mechanism might be either way.


It isn't clear from the graphs (and I don't know either) which comes first, changes in CO2 or Temperature, but it's clear that they track together.

patiodog wrote:
I've been wondering why nobody ever seems to bring up the fact that we're in a warm blip in the middle of an otherwise very cold epoch in the earth's history in the global warming debate.


I don't think most people know the climate spikes like this. Most people (who even know about climate cycles) probably know ice ages come and go, but think they are relatively balanced. It's hard for people who only know the world they are born into, to imagine that our present conditions are anything other than 'normal'.

patiodog wrote:
At risk of being glib, maybe forestalling the next temperature plunge will be good for yooman beans.


Judging from the graph, temperature increases don't forestall plunges, they precipitate them.

And that's another of my questions... what event(s) precipitate the plunge?
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patiodog
 
  1  
Reply Sun 4 Mar, 2007 02:32 pm
Here's a bit suggesting its a combination of ideal conditions vis a vis solar output, the wobble of the earth's axis, and the precessional cycle of the earths eccentric orbit (which, it says, takes about 100K years).

http://culter.colorado.edu/~saelias/glacier.html




Geological sources of carbon dioxide?
http://www.pbs.org/wgbh/nova/ice/chill.html
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rosborne979
 
  1  
Reply Sun 4 Mar, 2007 02:34 pm
patiodog wrote:
I do wonder if the cycle is tied in anyway to absorption of CO2 into the oceans (which is nowhere near saturated so can buffer CO2 concentrations, theoretically).


Cold water dissolves more CO2. But I don't know how much, or what the thresholds are. A Coke in the fridge will stay fizzy longer than a warm one.
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patiodog
 
  1  
Reply Sun 4 Mar, 2007 02:36 pm
Quote:
Judging from the graph, temperature increases don't forestall plunges, they precipitate them.

And that's another of my questions... what event(s) precipitate the plunge?


Hmmmm.... Looks to me like there's a steady decrease in temperature prior to the spikes, and then the earth attempts to go back to "baseline" (i.e., really cold). If it was the other way around -- if temps were gradually climbing before the spike, which was then followed by a plunge, I'd say that it looked like the increases precipitate the plunge, but as it is it looks like theres a steady downward trend that is interrupted by big bumps in temperature.
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farmerman
 
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Reply Sun 4 Mar, 2007 02:40 pm
The Croll-Milankovitch cycles are a combined group of short term cyclical variations. These result from orbit elliptical eccentricities of period nmaxes of 100 and 400 K; Obliquity , with a period of about 41K years ; and axial precession which have periodicities of 19 and 23 K years. These were important "heat engines " during the Miocene through Pkleistocene ( see Clark,PO, RS Webb, and LD Kegwein (1999) Mechanisms of global climate change AM GEophys Union Monograph 112. (This is IMHO, the best explanation of the effects that the earths axes, plus it discusses all the effect that climatic convection cells that have grown around the drifting continents, have had on the surface water temperatures. On top of this the atmospheric Carbon burden has been beaten to death by ALley (at Penn State) and , new work by Holbrook (2005) is showing that upticking in the increase in deep oceanic diffusivity is affecting the "turning down" of oceanic streams like the Japanese current and the Gulf STream. Its been postualted that , once the Arctic ice melts, the diffusivity (rate of 3 dimensional spreading of cold water) goes nuts and that Ice Ages can, if the Milankovitch cycle is favorable, cause a very rapid onset of an Ice Age.

This isnt my area so Im just repeating things from my copy of Clark.. The Sangamon Interglacial warm period, had seen the dispersion of tropical plants as far north as Nova Scotia.
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rosborne979
 
  1  
Reply Sun 4 Mar, 2007 02:42 pm
patiodog wrote:
Here's a bit suggesting its a combination of ideal conditions vis a vis solar output, the wobble of the earth's axis, and the precessional cycle of the earths eccentric orbit (which, it says, takes about 100K years).

http://culter.colorado.edu/~saelias/glacier.html

Geological sources of carbon dioxide?
http://www.pbs.org/wgbh/nova/ice/chill.html


I agree. I've googled these things as well.

Most of the things I've read all seem like contributing factors to change, but most of them seem to be slow and steady processes rather than things which might cause spikes.

A periodic combination of otherwise unrelated inputs might overlap to cause a spike, but then I wouldn't expect them to be particularly regular as in the graph.

I can kind of see how the rise in temperature can suddenlty crash. The earth can probably only get so hot before some type of regulating system (perhaps ocean currents) stops and we get a backlash. But it's harder to see what causes the cold periods to suddenly have such a sharp rise. Especially since the cold periods seem to be more stable (last longer) than the hot ones.
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xingu
 
  1  
Reply Sun 4 Mar, 2007 02:43 pm
bm
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Pauligirl
 
  1  
Reply Sun 4 Mar, 2007 03:39 pm
Quote:
New ice age theory: Sun's 'dimmer switch'
Submitted by Vidura Panditaratne on Wed, 2007-01-24 21:06.Sci | Non-geographical | News
Sun has a dimmer switch
Ice ages are not caused by planet Earth's orbital variations as once thought, but by the dimmer switch inside the sun that causes its brightness to rise and fall on timescales of around 100,000 years which is exactly the same period as between ice ages on Earth, according to a radical new theory proposed by renowned astrophysicist Robert Ehrlich of George Mason University.
Ehrlich modelled the effect of temperature fluctuations in the sun's interior and showed that while the temperature of the sun's core is held constant by the opposing pressures of gravity and nuclear fusion, slight variations are possible.
His research builds upon the work of Attila Grandpierre and Gábor Ágoston who calculated that magnetic fields in the sun's core could produce small instabilities in the solar plasma inducing localised oscillations in temperature.
In an article appearing in the journal New Scientist, Ehrlich describes how some of these oscillations reinforce one another and become long lasting temperature variations, with the sun's core temperature to oscillating around its average temperature of 13.6 million kelvin in cycles lasting either 100,000 or 41,000 years.
According to the scientist random interactions within the sun's magnetic field could flip the fluctuations between the two cycles which correspond to the Earth's ice ages.
Over the past million years, ice ages have occurred roughly every 100,000 years and before that roughly every 41,000 years.
The currently accepted theories attribute the ice ages to subtle changes in Earth's orbit, known as the Milankovitch cycles, one of which describes the way Earth's orbit gradually changes shape from a circle to a slight ellipse and back again roughly every 100,000 years.
This should, in theory, alter the amount of solar radiation that Earth receives which in turn trigger the ice ages, but a hole in this theory has been its inability to explain why the ice ages changed frequency a million years ago.
http://pressesc.com/01169672696_
Related Publications:
http://xxx.lanl.gov/abs/astro-ph/0701117- Robert Ehrlich.


I leave it to those who know this stuff as to plausibility.
P
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patiodog
 
  1  
Reply Sun 4 Mar, 2007 03:42 pm
Sounds like the electricity in my house...
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stuh505
 
  1  
Reply Sun 4 Mar, 2007 03:53 pm
patiodog wrote:
That's assuming previous carbon dioxide fluctuations caused the temperature shifts and not vice versa. Just a though -- not sure what a mechanism might be either way.


Well it's a fact that CO2 strongly absorbs solar radiation. Dont need a graph to tell you that.
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rosborne979
 
  1  
Reply Sun 4 Mar, 2007 04:26 pm
Mechanisms of abrupt climate change

"Why do glacial periods end abruptly?

Notice the saw-tooth shape of the Vostok record, with abrupt warmings followed by more gradual coolings. Warming at the end of glacial periods tends to happen more abruptly than the increase in solar insolation. There are several feedbacks that might be responsible for this. One is the ice-albedo feedback. Ice has a higher albedo (or reflectivity) than vegetation, soil, or water. Therefore, when ice is present, less solar radiation is absorbed by the surface, temperatures decrease, and the ice will persist. Once ice begins to melt and uncover land or water, more solar radiation will be absorbed by the surface, raising temperatures and causing even more ice to melt.

This effect might be greatest over the oceans because sea ice can melt much more quickly than large continental ice sheets. A possible feedback also exists with atmospheric CO2. The amount of atmospheric CO2 decreased during glacial periods, in part because more CO2 was stored in the ocean due to changes in either ocean mixing or biological activity, although vegetative feedbacks and the trapping of methane may also have played roles. This decreased the atmosphere's greenhouse effect and helped to maintain low temperatures. Once CO2 began to rise at the end of the glacial periods, however, the atmosphere's greenhouse effect increased and contributed to further warming."

Thermohaline Circulation

"Why did the thermohaline circulation change abruptly?

The Younger Dryas occurred during the transition from the last glacial period into the present interglacial (the Holocene). During this time, the continental ice sheets were rapidly melting. A pulse of this meltwater flowing into the North Atlantic (Figure 8) reduced the salinity and density of the surface ocean, causing a reduction in the rate of deepwater formation. As deepwater formation slowed, less warm water flowed north from the tropics and the North Atlantic became colder. Eventually, the meltwater flux slowed and other changes occurred, causing deepwater formation to increase. See Model of Abrupt Change in the Thermohaline."
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coluber2001
 
  1  
Reply Tue 6 Mar, 2007 01:59 pm
There is a difference in the terminology with respect to the ice ages and glaciation periods. Some scientists including the late Stephen J. Gould claimed that we were still in an ice age in an interglacial period. The ice age has been going on for over 3 million years and began with the formation of the Artic ice cap and the ice sheets of Greenland etc. One good explanation for the formation of the ice cap and sheets was the positioning of the continents around the Artic Ocean, slowing the currents through the Artic Ocean and allowing snow to accumulate on the northern part of the continents. Once the snow accumulated more sunlight was reflected creating a chain of cooling.

Apparantly, during this ice age, glaciations have occured on the average of every 90, 000 years. Since of lowest point of the last glaciation happened 18,000 years ago, the next warm peak shouldn't occur until about 26,000 years from now. Still, the ice sheets would survive. If the Artic ice cap and the ice sheets melt, it would be the first time since over 3,000,000 years ago.

The Milankovitch Curve is very complex and contains three factors, including two wobbles of the Earth's axis and a variation of the distance of our planet from the Sun. When these three factors coincide there are either peaks or dips in the cycle.
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farmerman
 
  1  
Reply Tue 6 Mar, 2007 05:19 pm
Pauli girl
Quote:
but a hole in this theory has been its inability to explain why the ice ages changed frequency a million years ago.
This was a good basis for not getting all excited about croll-Milankovitch. I wonder how this will be resolved? Its amazing how "dueling hypotheses" will appear and , within cometime in the future, may even co-exist.

Alley's made a prediction based on C-M cycles and , lets wait and see.
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