I.  Global Carbon pools

Most Carbon is stored in sedimentary rocks in the crust as limestone (CaCO3) or dolomite (CaMg(CO3)2), which are in large part created and deposited by living organisms (corals, clams, algae).  Additional carbon pools in the crust are present in coal, oil, and natural gas deposits.

Oceans are the second largest pool, where Carbon is stored as Carbonic Acid, which forms when Carbon Dioxide combines with water (CO2 + H2O = H2CO3).  This molecule is soluable in water, creating a very weak acid, and is responsible for the fact that rainwater has always been slightly acidic.

The atmosphere currently has the smallest global carbon pool, where carbon is stored as Carbon Dioxide (CO2), Carbon Monoxide (CO), or Methane (natural gas; CH4).

The biosphere is also an important pool, where carbon is stored as organic compounds in organisms (forest trees are particularly important), the soils, and as peat deposits (very common in cold environments).

 Cycling times in these pools varies:
Pool                             Cycling time
Biological pool              less than 1000 years
Crust                            100 million years
 

 Well, duh, they make up the backbone of ALL organic molecules.

What's more interesting is that carbon exists on earth in organic (sugars, proteins, fats) and inorganic (carbon dioxide, methane) forms, and that life-forms are able to move carbon between these forms.

Creation of organic from inorganic carbon:

Is done by photosynthesizing organisms (plants, algae, seaweeds) through process of photosynthesis.  In this, organism absorbs sunlight (roughly 1/2 of total spectrum which reaches the surface), and converts this light energy to chemical energy (by creating ATP from ADP).  This energy is then used to make sugar by combining  inorganic carbon dioxide with inorganic water:

12H2O + 6CO2 + energy = C6H12O6 + 6O2  + 6H2O

Once captured as a simple organic compound, this carbon can then be used to create all other organic molecules.

Creation of inorganic carbon from organic carbon:

This process occurs through respiration or combustion, in which organic molecules are combined with oxygen to create carbon dioxide and water:
 

C6H12O6 + 6O2  =    6CO2 + 6H20 + energy
When this reaction occurs in our bodies, it is called respiration.

When this reaction occurs abiotically (through burning of organic matter) it is called combustion.

Long-term carbon storage from the atmosphere and biosphere to the crust happens in water, where carbonic acid dissolves.  If a Calcium ion comes in contact with this compound, it will adhere to it, creating Calcium Carbonate, or limestone (CaCO3).  As this compound is insoluable, it percipitates out of the water, moving the carbon into the crustal pool.
 

Since the discovery of fire, humans have been able to generate energy for our use through the burning of organic matter.  Until the mid-1700's most of the material burned was recently alive (wood, peat, grass, cow dung).  At this time, we learned how to harness the energy released in fire through engines of various types.  This discovery powered the industrial revolution, and our current life style.

However, there is not enough living biomass on the planet to meet these energy needs.  So, we began using carbon fuels (oil, coal, natural gas) which are stored in the crust.  In this way, we have been taking from the crustal pool and giving to the atmospheric pool.

This can be seen in the yearly trend in global atmospheric carbon dioxide levels.  During each year, atmospheric carbon dioxide levels peak in Northern Hemisphere winter, and are lowest in Northern Hemisphere summer.  However, in each year, the yealy maxima and minima are higher than the year before it.

Why is this important?  As we found out earlier, carbon dioxide makes the atmosphere opaque to infrared light energy.  As such, more carbon dioxide in the atmosphere will allow for more heat to be trapped.  This will likely cause the global air temperature to increase.

One main factor which will dictate how bad this will become is the world's oceans.  If the extra carbon can be removed by the oceans through production of more limestone, then atmospheric carbon dioxide levels may not stay permanently high.  In fact, this may already be occurring, as not all of the carbon emitted by humans can be accounted for by the measured increase in atmospheric carbon dioxide levels.