MadSci Network: Earth Sciences

Re: Why doesn't the earth's oxygen/carbon dioxide ever get out of balance?

Area: Earth Sciences
Posted By: Karen Culver-Rymsza, Grad student oceanography
Date: Mon Oct 20 17:50:07 1997
Area of science: Earth Sciences
ID: 876445291.Es
Dear Aussie,

Such a simple question....Not such a simple answer.  I have tried to 
give you a complete, though not rigorous answer below.  I urge you to 
consult references for more information on this very topical question.
There are many good books around on this subject including general texts.  
Much of the information presented here can be found in:

     The Global Carbon Cycle by Martin Heimann (1993) pub. Springer-Verlag 
     Atmospheric Carbon Dioxide and the Global Carbon Cycle edited by J.   
       Trabalka pub. US Dept of Energy

	1) The equilibrium between O2 and CO2 HAS changed a lot in the 
history of the Earth
	2) The balance is mediated in part by feedback mechanisms that keep 
it from getting too far out of whack
	3) The biological influence on CO2/O2 is not the only mechanism to 
moderate concentrations.
	4)   It is unlikely that CO2 or O2 will be diminished so much that 
either plants or animals would expire AS LONG AS there is not a 
perturbation to the system that pushes it beyond its ability to recover.

	The global carbon cycle illustrates the principle of feedback which 
continues today to keep the amount of CO2 in the atmosphere from getting 
too far out of kilter.  However, there have been very large variations in 
atmospheric CO2 and O2 since the Earth was formed.

	Let's look at a little history to start with.  The history of 
atmospheric CO2 goes back about 100 million years.  This is not because 
there was no prior history, but because the recording of that history in 
sediment and ice cores is not available to us.  At that time (10^8 yrs BP) 
the concentration of CO2 in the atmosphere was about (1000-5000 ppm) or 
approximately 5-15 times modern concentration.  That concentration steadily 
decreased to 200-300 ppm characteristic of the glacial-interglacial period. 
 CO2 fluctuated in concert with climate change associated with the advance 
and retreat of glaciers of the last few million years.  This fluctuation 
between glacial (about 200 ppm) and interglacial (about 270 ppm) periods 
was regular, with a frequency of about 10,000 years. The post-glacial 
period, since about 10.000 yrs ago, CO2 has been slightly higher, about 290 
ppm, and is considered natural as human impact would be minimal during most 
of that time.  

	Finally, we reach the last 100 yrs or so.  During that time CO2 
concentration has increased from about 280 ppm to about 340 ppm in 1985 and 
continues.  This increase was illustrated in the pioneering work by Keeling 
at Mauna Loa, a site used to monitor a variety of atmospheric parameters 
because it is little impacted by local anthropogenic contamination, that 
is, it is clean, natural air.  If you look at his data, you would see a 
graph that shows a wavy ascending curve.  The increase in average CO2 is 
that mentioned above.  The waves are the seasonal change in CO2 from winter 
to summer (about 10%)as photosynthetic activity  changes over the course of 
the Northern hemisphere annual cycle.  This is an example of the kind of 
change you ask about, but there is not enough activity to bring the CO2 
concentration to zero before winter sets in.  This fluctuation has shown 
some variation in the last decade or so.  The increase in amplitude of the 
seasonal variation has increased 1-2% per year.

	It is an oversimplification to think about CO2 and O2 as only 
cycling through plants and animals.  By that I mean there are other pools 
for carbon dioxide and oxygen besides living tissue and other processes 
that cycle these elements beside photosynthesis and respiration.  

	Only about 1-5% of the total carbon of the Earth has been released 
from rocks.  Of that about 80% has been coverted to limestome via marine 
carbonates (shells, reefs, etc), This leaves about 0.1% of the total carbon 
in sedimentary rocks to circulate between atmosphere, biosphere and oceans. 
 Of that most is in the oceans as a result of the dissolution of CO2 in 
water (about 50 times as much as in the atmosphere).  This is a large 
buffer of CO2 concentration because as more is put into the atmosphere, 
more is dissolved in the ocean water.  Even if we consider only organic 
pool, much of that is stored as well.  Thus we have fossil fuels, and peat 
bogs and leaf litter and tree trunks.  These cycle slowly and act as 
temporary reservoirs for excess CO2.  It remains to be seen if trees get 
woodier with increasing atmospheric CO2 concentration.

	Now we get to the part you are really interested in, and where 
feedback is more easily visualized.  Even though feedback regulation 
occurs at the scale of rocks (dissolution, sedimentation, etc) the feedback 
exhibited by living organisms is the crux of your question.   If we start 
with the simplest assumptions we can see how feedback would work.  Let's 
ignore bacteria, herbivory, food sources and so forth and consider only 
plants and animals connected only by the atmosphere.  What we have is: 
	Plants use CO2 and give off O2.  
	Animals use O2 and give off CO2.  
	In both cases they require what they take in and what they give off 
           is "toxic".  

	Imagine a world with little or no oxygem but lots of CO2.  In that 
world, plants would thrive and release more O2, and more and more.  This 
would continue until either they used up all the CO2 or were poisoned by 
all the O2 they gave off.  The same would happen in an oxygen rich world 
filled with only animals  as animals used up all the O2, they would give 
off more CO2, and when starved for O2 or poisoned by CO2, they would die. 
But if there were plants and animals in this world, CO2 released by animal 
respiration would be used for photosynthesis and photosynthesis would 
release more O2 for animals and on and on.  As these two opposing processes 
(respiration and photosynthesis) approach equilibrium, there is feedback.  
More CO2 fixation means more oxygen available for respiration and vice 
versa.  If we add back predation, herbivory, food sources, and other biotic 
interactions, the feedback is more finely controlled at the 
level of populations.  When we add back geochemical interactions such as 
CO2 dssolving in seawater, rocks eroding, and carbonate formation there are 
yet more means of moderating changes.

	Finally we get to some limitations on the fluctuations and the 
feedback.  If the growth of plants were determined only by the amount of 
CO2 in the atmosphere, we would have bigger problems than we have.  In most 
of the world, aquatic and terrestrial plants are limited by the 
availability of nutrients, not CO2.  So how far photosynthesis can shift 
the equilibrium is limited.  In the same way, animals are limited by the 
availability of food. There is a whole other level of control on 
populations due to these factors. 
	If we consider global effects, we can imagine more scenarios.  
Plants draw down CO2, an ice age comes, growth slows down, but decay 
processes continue to release CO2, and the level rebounds.  But what you 
must consider is that different processes proceed at different rates.  When 
these rates are overcome by a very large perturbation such as fossil fuel 
burning, then there can be changes seen in the atmospheric content of CO2

    I hope this helps.

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