The Carbon Cycle

Reader Contribution by Richard Hilderman and Ph.D.
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Photo by Pixabay/Foundry

As we discussed in my first posting, carbon dioxide helps stabilize the planet’s temperature, but increasing the atmospheric concentration of carbon dioxide will increase the global temperature.  Over millions of years Earth has been able to regulate the level of atmospheric carbon dioxide through the carbon cycle.  The burning of fossil fuels, such as coal and oil, in our factories, power plants and cars has disrupted this balance so that higher levels of carbon dioxide accumulate in the atmosphere. To appreciate the impact our burning of fossil fuels is placing on the planet we need to understand how the Earth recycles carbon dioxide through the carbon cycle.

The carbon cycle refers to the flow of carbon between the atmosphere, rocks, oceans and biosphere (all of Earth’s life forms).  Each of these is part of a reservoir which contains all the carbon on the planet. The carbon cycle is composed of two reservoirs: a long-term and a short-term.  By circulating carbon through these interconnected reservoirs the planet regulates the level of atmospheric carbon dioxide.

The long-term reservoir contains about 99.9 percent of the total carbon which is found mainly in rocks and fossil fuels and takes up to millions of years to recycle carbon dioxide.  In the long-term reservoir atmospheric carbon dioxide reacts with water and minerals in rocks to form calcium bicarbonate which enters rivers and ends up in the ocean where it becomes shells of various marine organisms.  When these organisms die the shells accumulate on the ocean floor and are eventually transformed into rocks and petroleum.  Over millions of years this transformed material is buried at depths of thousands of feet and the heat and pressure melts the rocks and converts the carbonate back to carbon dioxide.  Some of these rocks become part of volcanoes and the carbonate is released as carbon dioxide via volcanic eruptions. 

Carbon dioxide is also removed from the atmosphere by plants and the burial of dead plant matter.  In swamps this material is transformed into coal and in river deltas the material is converted into carbonaceous shale.

In the short-term reservoir, carbon is stored in the atmosphere, oceans and biosphere with the ocean containing the largest amount of carbon.  It takes months to centuries to recycle carbon dioxide through the short-term reservoir.  The ocean is the primary regulator of atmospheric carbon dioxide in the short-term reservoir because atmospheric and ocean carbon dioxide are in chemical equilibrium.  If there is an increase in atmospheric carbon dioxide there is a corresponding increase in oceanic carbon dioxide and vice versa.  There are two main ways in the short-term reservoir that carbon dioxide is removed from the atmosphere and enters the ocean.

In the first mechanism, atmospheric carbon dioxide enters the ocean by the growth and death of plants, animals and microbes.  Secondly, atmospheric carbon dioxide is dissolved in the ocean which helps maintain a stable pH for life.

In the carbon cycle, carbon dioxide is removed from the atmosphere and converted to fossil fuels and rocks which are components of the long-term reservoir.  Eventually the carbon dioxide in the long-term reservoir enters the ocean and atmosphere and becomes part of the short-term reservoir through volcanic eruptions and melting of rocks.

The crux of our current dilemma is that the drilling of oil and the mining of coal and their use as an energy source has disrupted the natural balance between the long-term and short-term reservoirs. When we burn fossil fuels, over half of the carbon dioxide enters the ocean and the rest stays in the atmosphere1.  This disrupts the long term carbon balance because carbon dioxide can’t be transferred from the atmosphere back into the long-term reservoir fast enough.  It is estimated that it will take up to hundreds of thousands of years to remove this excess carbon dioxide from the atmosphere1.

What has been the effect of disrupting this natural balance?  Preindustrial revolution levels of carbon dioxide were 285 ppm. But, in 2006 the level was 385 ppm2and it is currently increasing by 2 ppm per year3!  The 2006 level of atmospheric carbon dioxide is substantially higher than any time in the past 800,000 years3!  Over the past 100 years, global temperature has increased 1.8o F4. In recent decades the increase in global temperature per decade is much faster than in earlier decades of the 20th Century.  In the past three decades the temperature has increased 1.4o F4!  The first eight months of 2010 matched 1998 as the hottest January to August period on record5!  Some people believe that the disruption of this natural carbon balance could lead to a “runaway greenhouse” in which so much carbon dioxide accumulates in the atmosphere as to create a global temperature unable to support life.

There are several natural feedback factors that will amplify a “runaway greenhouse” and increase the probability that the planet will reach a tipping point where the current gradual global temperature increase will be replaced by an abrupt and significant increase in global temperature.  My next posting will address some of the feedback factors.


  1. C. Hamilton, Requiem for a Species: Why We Resist the Truth About Climate Change,(2010)       pp 8-9. Earthscan
  2. D. Keeling and T. Wharf, Scripps Institute of Oceanography, La Jolla California, http://scrippsCO2.ucsd.eduAfter Climate Change, 1994. Intergovernmental Panel on Climate Change, Cambridge: Cambridge University Press.
  3. U. Siegenthaler, et  al,2005, Science, 310, 1313-1317.;  D. Luthi, et al, 2008, Nature, 453, 379-383.
  4. P. Brohan, J. Kennedy, I. Harris, S. Tett and P.Jones. “Uncertainty Estimates in Regional and Global Observed Temperature Changes:  A New Data Set from 1850,” Journal of Geophyscial Research 111, 2006, doi: 10.1029/2005jdoo6548.
  5. The National Oceanic and Atmospheric Administration
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