Archive for April, 2008

Earth’s Carbon Cycle

April 14, 2008

The first image portrays the Earth’s carbon cycle in the absence of life and the biosphere.  Atmospheric carbon levels are quite high, lacking the feedbacks that a living biosphere provides.

In the geological carbon cycle, carbon moves between rocks and minerals, seawater, and the atmosphere. Carbon dioxide in the atmosphere reacts with some minerals to form the mineral calcium carbonate (limestone). This mineral is then dissolved by rainwater and carried to the oceans. Once there, it can precipitate out of the ocean water, forming layers of sediment on the sea floor. As the Earth’s plates move, through the processes of plate tectonics, these sediments are subducted underneath the continents. Under the great heat and pressure far below the Earth’s surface, the limestone melts and reacts with other minerals, releasing carbon dioxide. The carbon dioxide is then re-emitted into the atmosphere through volcanic eruptions. (Illustration by Robert Simmon, NASA GSFC)

The balance between weathering, subduction, and volcanism controls atmospheric carbon dioxide concentrations over time periods of hundreds of millions of years. The oldest geologic sediments suggest that, before life evolved, the concentration of atmospheric carbon dioxide may have been one-hundred times that of the present… Source

The next image looks at the much richer carbon cycle that includes many of the contributions of the biospheres of land and sea.  We are just beginning to quantify the movement of carbon between the various biospheric compartments, the oceans, the lithosphere, and the atmosphere.  At least a third of human-generated CO2 ends up sequestered by either the biosphere of land and sea, or by going into solution in the oceans.

Local land biosphere carbon sequestration may involve incorporation into growing plants such as trees, or incorporating into micro-organisms, fungi, and animals in the soil.

This graphic looks at global turnover of carbon from various compartments.  As you can see, human contributions to atmospheric carbon are quite low compared to natural contributions.  As long as the biosphere is allowed to grow, it is capable of sequestering the larger part of human emissions of CO2.

This image looks at the photosynthetic activity of the Earth seen from space, during the time period of 18-25 Dec, 2000.  Observe that the northern hemisphere displays very low photosynthetic activity compared to the southern hemisphere, at that time of year.  You would expect to see a mirror image of photosynthesis north to south when viewing the time period of 18-25 June.

CO2 concentration in the atmosphere is roughly 380 parts per million, or 0.000380 of the atmosphere, or 0.038 per cent of the atmosphere.   Since modern measurements of CO2 have begun, CO2 concentrations have steadily increased–although the rate of increase may have temporarily slowed, as measured at Mauna Loa.

These images are meant as a preliminary to a future more detailed look at the carbon cycle, and possible biological feedbacks in the carbon cycle.  Significant sequestering of CO2 can be carried out through a number of human interventions, including biochar agriculture, wiser forestry practises, and other ways of making carbon work for us as the valuable resource that it is.