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.