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The basic carbon cycle of life is: (1) the conversion of atmospheric carbon dioxide to carbohydrates by photosynthesis in plants; (2) the consumption and oxidation of these carbohydrates by animals and microorganisms to produce carbon dioxide and other products; and (3) the return of carbon dioxide to the atmosphere. On a global level, the total carbon cycle is more complex, and involves carbon stored in fossil fuels, soils, oceans, and rocks.
We can organize all the carbon on earth into five main pools, listed in order of the size of the pool:
Carbon moves back and forth among these various pools. Nearly all of the carbon on earth is locked up in the lithosphere as sedimentary rock deposits and fossil fuels. And about 99.999% of this carbon is fixed in place and essentially off the table as far as the carbon cycle is concerned. Only the amount stored as fossil fuels enters the carbon cycle, and only then through human activities.
A sizable percentage of the "free" carbon on Earth exists in the atmosphere. As the carbon cycle undergoes shifts and fluxes through the eons, the amount of carbon in the atmosphere tends to increase or decrease to buffer the changes. Currently, the atmospheric carbon pool is expanding by about 6.1 gigatons per year, and the fossil fuel carbon pool is shrinking by about 4 to 5 gigatons per year. This is one aspect of the carbon cycle that can be readily manipulated by human activity.
The ocean absorbs 2.5 gigatons of carbon more from the atmosphere than it gives off to the atmosphere. But that extra amount of carbon is utilized by marine biota and eventually gets incorporated into deep sea deposits and sediments. So the net level of carbon in the ocean remains roughly the same every year.
The soil organic matter pool is currently losing about 1 to 2 gigatons of carbon per year to the atmospheric pool. About 60 gigatons of carbon per year enters the soil organic carbon sink as decaying biomass remains in the soil. About 61 to 62 gigatons of carbon are lost from this pool as soil organic matter is oxidized by the atmosphere. This is the other main cycle that can be manipulated by human activity. Changes in land use patterns and agricultural practices can affect the amount of carbon released into the atmosphere from soil organic matter.
The biosphere represents a significant carbon pool on Earth. About 110 gigatons per year of carbon is absorbed by the atmosphere into plant life through the process of photosynthesis. Of that amount, about 60 gigatons of carbon is released into the atmosphere through respiration, decay, and gaseous waste elimination from living animal biomass, both on land and in the ocean. The other 50 tons is incorporated into soil organic carbon, part of which can be readily oxidized and part of which is relatively stable for many years.
Before the industrial revolution, the main source of fluctuation in atmospheric carbon was from changes in biomass and soil organic carbon. Now, fossil fuel burning is the greatest factor in atmospheric carbon fluctuations.
The bottom line of all this is that the amount of carbon in the atmosphere is increasing by about 6.1 gigatons per year, mostly due to fossil fuel burning and land use changes that destroy soil organic carbon. This increase needs to stop, or at least slow down, since carbon dioxide in the atmosphere traps heat and becomes a greenhouse gas that can lead to global warming.
The atmospheric carbon balance sheet looks like this:
|Factor||Carbon flux into atmosphere
|Movement of C out of atmosphere
|Fossil Fuel Burning||4-5|
|Soil organic matter oxidation/erosion||61-62|
|Respiration from organisms in biosphere||50|
|Incorporation into biosphere through photosynthesis||(110)|
|Diffusion into oceans||(2.5)|
|Overall Annual Net Increase in Atmospheric Carbon||+4.5-6.5|
So how can humans manipulate the carbon cycle so that the atmospheric carbon pool stops expanding? Let's look at the two ends of the equation: where atmospheric carbon goes, and where it comes from. This will give us an idea of what changes can be made to reduce carbon buildup in the atmosphere.