Humans and the Carbon Cycle

Some people must think that humans are not part of nature according to two comments to my post: Carbon Dioxide and the Greenhouse Effect . The comments alleged: “Human carbon emissions are not a part of the natural carbon cycle.” and “We are now releasing huge amounts of fossil carbon too rapidly for natural processes to adjust.” Both claim that human carbon dioxide emissions upset “the balance of nature.” This belief reflects a misunderstanding of what “balance” really is. Nature is never really “in balance” or static, it is always seeking equilibrium between forces that upset the status quo.

This misunderstanding is reflected in one of the comments: “The natural carbon cycle involves the production/consumption of carbon. Humans do exhale – but energy production involves humans using historic carbon from earlier carbon cycles that are not contemporary. It isn’t part of a ‘natural’ carbon cycle.”

Tell me, how can nature distinguish between a carbon dioxide molecule produced by someone burning wood in a fireplace versus carbon dioxide resulting from burning wood in a forest fire? How can nature distinguish between a molecule of carbon dioxide produced by burning coal to generate electricity versus coal burning in a seam due to natural spontaneous combustion? Yes, that does happen. So much for “historic carbon.”

There are actually two carbon cycles. The geologic carbon cycle stores carbon in limestone, dolomite, petroleum, and coal deposits. Carbon dioxide from the atmosphere is used up during the weathering of silicate rocks, a process that speeds up with increasing temperature or increasing carbon dioxide, thereby forming a negative feedback or thermostat. It takes millions of years, usually, for this carbon to cycle back into the biosphere. Volcanoes recycle carbonate rocks and emit 200 million metric tons of carbon dioxide per year according to the U.S. Geological Survey. There are also carbon dioxide gas seeps. Carbon dioxide is also produced from metamorphism of carbonate rocks.

The biologic carbon cycle is exchange of carbon dioxide between the atmosphere, biosphere, and ocean as shown in the graphic below. The biologic process involves photosynthesis, respiration, ocean absorption, and biological use of carbonates to form shells and other structures. Human emissions are part of these natural cycles.


The relative amount of carbon in each “sink” is shown in the table below.


Notice that the amount of carbon stored as fossil fuel deposits is just one-tenth of that stored in the oceans, and the ocean store in continually in flux. The ocean is also the connection between the geologic carbon cycle and the biologic carbon cycle. As the amount of carbon dioxide in the atmosphere increases, ocean uptake also increases. The carbon dioxide is stored not only as dissolved gas, but also as carbonate ions which are sequestered by marine life and the production of limestone and dolomite deposits.

There is another complication. Some carbon is missing. When calculating the carbon flux, i.e., the emissions from known sources versus carbon sequestration by known sinks, there should be more carbon dioxide in the atmosphere than there is. So, either there is an unknown process taking up carbon dioxide or a known process is working faster than we thought (seeking equilibrium).

There is some observational evidence for that last process. We see that terrestrial plant life has increased its net primary productivity by growing more robustly and by making better use of nitrogen in the soil. (See here ) There are also new studies showing that small marine creatures, such as Thaliacea, are depositing more carbon into the geologic sink than previously realized.

Perhaps we still don’t know as much about the carbon cycle as we thought.

To put things in perspective, according to data from the Energy Information Administration, based on data derived from the IPCC, human carbon dioxide emissions represent about 3% of the total carbon dioxide flux, and 98.5% of that is reabsorbed in the biologic carbon cycle. (Source )

Slightly off subject but important: A new paper in Geophysical Research Abstracts (Vol. 13, EGU2011-4505-1, 2011) based on detailed spectrographic analysis of the atmosphere found that because the absorbance of water vapor overlaps the frequencies of long wave radiation that are absorbed by carbon dioxide and methane, the effective sensitivity of carbon dioxide and methane as greenhouse gases is only one-seven that claimed by the IPCC and used in climate models.

That makes our emissions from burning fossil fuels of even less concern.



  1. Very nice expanation of the carbon cycle.
    BUT what does it matter?
    There is more CO2 in the air than can be in use by the available energy photons, or, the Greenhouse effect is limited and controlled by the number of available proven every night when the number of photons decreases and the temperature goes DOWN in spite of Man adding more CO2.
    What this means is that when we add more CO2 it just adds more CO2, it does not add more warming. So reducing CO2 emissions does nothing for global warming.
    To reduce global warming Man must reduce the number of photons and since most of this energy comes from gravity and  planetary eccentricity, then Man can do nothing about the warming. (see Gravity causes Climate Change at
    STOP wasting money on trying.

  2. Human carbon dioxide emissions have no measurable effect on global temperature.

    Not even gonna qualify the statement, huh?  Just present it as generally-accepted-as-fact?  Thousands of climatologists are mistaken?  Maybe, just maybe. 

    1. OK I will qualify the statement: So far, no one has presented any physical evidence to show that human carbon dioxide emissions have a significant effect on global temperatures.  But I think the statement stands, since there is also no physical evidence of measurable effect either, only theory and computer modeling. In my post Your Carbon Footprint Doesn’t Matter, I estimate that if we stopped all CO2 emissions in the U.S. is could make a difference of 0.003 degrees C. Hardly measurable.

  3. I asserted that nature sought balance.
    I answered the question involving your fireplace metaphor. There is no difference. Both involve contemporary carbon – not sequestered carbon.
    Mr. Pidwirny … here …

    … said …

    Human role in the carbon cycle
    Until recently, the flow of carbon stored in fossil fuels to the atmosphere was minuscule—nearly zero. The fossil fuel storage represented a “dead-end” for the carbon cycle. The Industrial Revolution increased the use of coal, oil, and natural gas. Burning fossil fuels completes the process of break-down back to carbon dioxide and water. In 2000, humans burned about 5.1 billion short tons (4.6 billion metric tons) of coal, 28.1 billion barrels of oil, and 89 trillion cubic feet of natural gas, which caused about 6.5 petagrams (6.5 billion metric tons) of carbon to flow from the fossil fuel storage pool to the atmosphere.
    The combustion of fossil fuels is not the only flow in the carbon cycle affected by economic activity. Prior to the expansion of human civilization, the amount of carbon stored in biota changed very slowly from year to year because the amount taken up through photosynthesis was nearly equal to the amount emitted through respiration and decomposition. But human activity has disturbed the biotic storage pool. Over the last several hundred years, humans have reduced the area covered byforests, a process known as deforestation. By reducing the number of trees through burning and/or chopping them down and allowing them to decay, deforestation reduces the amount of carbon stored in the biota. This carbon flows to the atmosphere. In the 1990’s, deforestation and other changes in land use caused 1-2 petagrams (1-2 billion metric tons) of carbon to flow from the biota to the atmosphere annually.
    The other important set of flows moves carbon from the atmosphere to the ocean and from the ocean to the atmosphere. For a long time, these two flows were approximately equal. This balance was created and maintained by the spontaneous flow of carbon from the storage of high concentration to the storage with the lower concentration. These movements created an equilibrium between the amount of carbon in the atmosphere and ocean.
    This equilibrium has been disrupted by the combustion of fossil fuels and deforestation. These two flows add carbon to the atmosphere, which causes the concentration of carbon to increase in the atmosphere relative to the ocean. The increased atmospheric concentration of carbon causes carbon to flow spontaneously from the atmosphere to the ocean. The size of this flow is limited by anegative feedback loop, termed the Revelle Factor, which slows the flow of carbon from the atmosphere to the ocean relative to the flow of carbon to the atmosphere. As carbon dioxide dissolves in the ocean, it reduces the ocean’s pH (makes it more acidic). The lower pH slows the rate at which carbon dioxide dissolves in the ocean. Currently, the flow of carbon from the atmosphere to the ocean is about 2 petagrams (2 billion metric tons) greater than the flow of carbon from the ocean to the atmosphere.
    … and more. He sees no issue with the assertion that human use of non-contemporary or historically sequestered carbon effects the nature of the climate/atmosphere.
    And I reassert that a biological system that is continuously seeking balance is being perturbed by the introduction of historic carbon from previous ‘carbon cycles’ resulting in rebalancing. A rebalancing that is effecting many things from mean temps to ocean ph.
    There was one issue mentioned in Mr. Pidwirny’s article that I hadnt mentioned. That being deforestation. Just another example of man’s activities tangibly modifying the biota.

  4. As I said in the first paragraph, nature is never really in balance, but always seeking equilibrium.

  5. We have only been taking temperature measurements with any degree of accuracy or any controls on how measurements are taken for a very short time.  Theories of temperature variations and estimations of such are gross extrapolations of available facts.  All dating techniques outside reliable and verifiable recorded historical data are dependent on completely theoretical assumptions on conditions at the time which would produce the materials being used to make the dating conclusions (often done using completely circular arguments).  Scientifically there is no reason other than these circular arguments to make conclusions that dinosaurs existed millions of years ago, or 50,000 years ago.  All this being said; why do we require some sort of fear of disaster to improve the quality and reliability of our lives?  There is no reason we should not be taking maximum advantage of wind and solar energy wherever we can.  All other forms should be conserved for use when wind or solar cannot be used.  In Arizona, certainly, wind or solar or both are always sufficiently available to provide all energy needs, but we take almost no advantage of them.  Even if the U.S. operated completely isolated from the rest of the world it would be quite capable of producing far more than all the food, clothing, housing, transportation, energy by wind-solar-water power for everyone in the country.  We need to stop arguing about “should” we do these things, and determine how to make it happen. 

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