Any idea why carbon dioxide levels naturally go up and down in a cycle?

In the graph here

http://www.seed.slb.com/en/scictr/watch/...

you can see that carbon dioxide would go up and down even without humans (though humans make it worse).

My question: why did carbon dioxide go up naturally in the past, and why are the rises so much steeper than the falls??

Any link which gives some precise theories, or your theory much appreciated...

Answer:
jj said:
"One of the Carbon isotopes mostly found in fossil fuels does not get utilzed by photosynthetic organisms as much as the other isotopes, so decay of the molecule, which takes more time is what is largely responsible for its reduction."

This is butchered a bit. Fossil fuels are higher in the most common isotope of carbon, C-12. This is true of most plant species, 99% of which use the C3 pathway that FAVORS C-12 uptake - the opposite of what jj stated.

Where some scientists have used isotopes to support the idea that excess CO2 originates from fossil fuel burning and not from ocean release is that they can point to an increase in the proportion of C-12 to C-13. Since the sea surface has a higher concentration of C-13 than the atmosphere normally has, scientists claim that this means the excess CO2 is not released (inorganically) from the oceans.

Whether this is true or not is debatable. The sea surface where CO2 is likely released is certainly higher in C-13 than the atmosphere, however the deep ocean, which stores almost 40 times the CO2 that surface waters do, like fossil fuels is much higher in C-12. But bt what mechanism could this deep ocean sink release the CO2 to the atmosphere.

Now, to answer your question, a common mechanism given for this CO2 increase that accompanies a temperature increase is the release of CO2 from warming water. The ability of a gas to remain dissolved in a liquid decreases as temperature increases (That's why thermal pollution of a lake will literally suffocate the wildlife living in it.) But this graph you linked - when you increase the temporal resolution - also shows a lag between the rise in temperature and the subsequent rise in CO2. So how can this lag be explained?

The lag is important to global warming scientists because their numbers show a disappearance of a lag. This, they claim, points to a departure from the normal cyclical lag and suggests an anthropogenic influence. Is the lag gone? Depends on who you listen to. The "lag" cycle was derived from ice core samples whereas the current portion is derived from direct measurement, so I guess that leaves the matter up for debate.

And as for that mechanism that could account for the lag, could account for a large increase of C-12 CO2, is the Thermohaline conveyor. The thermohaline conveyor is already intimately associated with global climate: it is responsible for el nino/la nina climate patterns, responsible for the temperate European climate, regulates polar ice, and is viewed as the culprit of the Little Ice Age that immediately preceded the warming period we are now in. These conveyor currents subduct large portions of the ocean, maintaining a thermal "memory" with varying cycles up to 1600 years before those waters reach the surface again.

Is this the smoking gun? I don't know. The truth is that no one knows to what extent the conveyor ties into the climate phenomenon we are witnessing. I believe it warrants further investigation.

http://homepage.mac.com/uriarte/carbon13...
http://thermohaline-circulation.zdnet.co...
http://www.natice.noaa.gov/icefree/navya...
http://www.pnas.org/cgi/content/full/97/...
Yes
The greater the temperature, the faster enzyme catalyzed reactions occur.
Which means the higher the temperature, the higher the production of CO2 by microorganisms, followed by a higher rate of remediation by photsynthetic organisms, after a short lag time.
But that chart still fails to explain why the levels are that high to begin with, and doesn't reflect the different isotopes of carbon that are emitted by different sources. One of the Carbon isotopes mostly found in fossil fuels C12 is most abundant in the atmosphere and climbing while C14 does not get utilized by photosynthetic organisms as much as the other isotopes, so decay of the molecule, which takes more time is what is largely responsible for its reduction.
It's mostly speculation, but one theory goes something like this:

As the Earth enters one of it's cycles where it is closer to the sun, the oceans warm, increasing the numbers of photosynthesizing algae and bacteria. These reproduce rapidly, and the CO2 levels climb sharply. In response to the increased food source, animals which consume the bacteria and algae increase their numbers, and so on up the food chain. But because of the slower rate of reproduction, CO2 levels decline much more slowly.

Climate is complicated, and this is no doubt oversimplified, but it's likely to be at least part of the story.
No theories needed, this is scientific fact. Note the time period, about 100,000 years.

The Earth and the Sun go through changes in position which cause solar radiation to get bigger or smaller, called Milankovic cycles, about 100,000 years long. When the cycles are in a warming phase they warm the oceans. The warmer water can't hold as much CO2 and it's released. The CO2 causes even more warming due to the greenhouse effect and the process spins out of control, fast, raising CO2 levels sharply. Eventually it does stop (covering the whole Earth with clouds may help), the solar radiation gets less, and the CO2 goes back into the ocean. But that's a much slower process.

How do we know the present warming isn't due to that? Lots of reasons. We measure solar radiation and it isn't increasing enough to cause the warming. The Milankovic cycles say we should be in a period of stable climate. And, when warming is due to the Sun, there's a lag of several hundred years between temperature increases and CO2 increases. This time CO2 is going up immediately, because it's actually the cause of the warming.

All that is part of the evidence which causes virtually all scientists agree the present warming is mostly caused by us.

Some good places for more information below. But this stuff is hard, it takes time to learn.

http://en.wikipedia.org/wiki/milankovitc...
http://www.globalwarmingart.com/wiki/ima...
http://environment.newscientist.com/chan...
http://profend.com/global-warming/...

and, more difficult:

http://www.aip.org/history/climate/cycle...
http://ipcc-wg1.ucar.edu/wg1/wg1-report.
summarized at:
http://www.ipcc.ch/spm2feb07.pdf...
http://www.ucsusa.org/global_warming/sci...
http://www.realclimate.org
"climate science from climate scientists"
CO2 dissolves easily in the ocean, but cold water is capable of holding more CO2 than warm water. Therefore, if the oceans are at their saturation point and the temperature rises just at bit, from some outside cause (in the case of ice ages, the outside cause is orbital forcing), then the warmer ocean emits some of its CO2 into the air. That newly-emitted CO2 increases the greenhouse effect, which warms the earth, which causes more CO2 to be emitted from the oceans, which increases the greenhouse effect, which warms the earth, and so on. This is called a "positive feedback loop," and the climate system is full of them.

Going the other direction -- from warm to cool -- also produces a positive feedback loop, but the effect is much slower because it takes more time for the ocean to absorb CO2 when cooling than to emit it when warming.
carbon is a lightest element period''got it
Rising temps cause CO2 levels to increase, and decreasing temps cause CO2 levels to decrease. A better graph would show clearly that there is an 800 year lag in CO2 compared to the temps.

Some people think that the lagging co2 causes an increase in temps, but they couldn't be any more wrong.
bacuse we produce more one time and less another
Two words - Orbital Inclination.

OI is related to Milankovitch Cycles, although not one of the MC's it is explained on this page about MC's http://en.wikipedia.org/wiki/milankovitc...

OI should be taken in context of what is often referred to as the '100,000 year problem' which is described here http://en.wikipedia.org/wiki/100%2c000_y...

Reading both pages will explain it better than I can given the limitations of answering here.
This documentary covers that issue.
oceans get cooler, oceans get warmer, do the sums.
hint; think plankton...

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