760 Gtons of Carbon Atoms in The Form Co2
There are 760 Gtons of carbon atoms in the form CO2 in the atmosphere. Every year, 60
Gtons of carbon are “fixed” into plants by the process photosynthesis.
1a. How many years would it take to remove all of the CO2 from the atmosphere if
photosynthesis were to continue to fix carbon dioxide at a rate of 60 Gtons yr-1?
1b. Plants also respire and, when they die, they decompose. This process also occurs at a rate
of 60 Gton yr-1. If photosynthesis were somehow to be shut off, how long would it take to
produce 760 Gtons of carbon in the form of CO2 in the atmosphere (i.e., now long would
it take to produce 760 Gtons, which would cause CO2 in the atmosphere to double)?
1c. In Homework 3 we determined that fossil fuel consumption and deforestation (mainly
“slash and burn” agricultural practices) produces 3.3 Gtons yr-1 of carbon. This represents
an additional source of CO2 to the atmosphere such that removal by photosynthesis is not
perfectly balanced by reemission of CO2 to the atmosphere by respiration and decay.
How long will it take to double the present amount of CO2 (i.e., add an additional 760
Gtons) if humans continue to produce this additional 3.3 Gtons yr-1?
II. There is an enormous amount of organic (“fixed”) carbon buried in sediments – about
10,000,000 Gtons (or 107 Gtons). Some of this is in the form of fossil fuels (mainly coal).
Plate tectonics slowly buries carbon in sediments at the bottom of the ocean at a rate of 0.05
2a. Assuming this rate of burial has remained constant, how long did it take to produce the
amount of organic carbon that is currently buried in sediments?
2b. It is believed that extractable fossil fuels represent about 0.2% of all of the organic carbon
buried in sediments, or about 20,000 Gtons. Humans are presently burning fossil fuels at
the rate of 7.6 Gton yr-1. At this rate, how long would it take to use up all of the
extractable fossil fuels buried in sediments?
2c. If all of these ‘fuels’ were to be burned and emitted to the atmosphere as CO2, how much
CO2 would there be in the atmosphere? (Don’t forget the current amount of carbon in the
atmosphere – 760 Gtons.)
2d. Convert the value in 2c above to ppm using the relationship 1 GtonC = 0.45 ppm CO2.
Recall that in the movie “Crude: The Incredible Journey of Oil”, we learned that if
atmospheric abundances of CO2 reach 1200-1400 parts per million the polar ice caps
will likely melt and deep water formation will cease, leading to an oceanic anoxic event.
Is the amount of CO2 you determined in Part 2c more or less than the amount necessary
to trigger an oceanic anoxic event?
III. Photosynthesis in the oceans takes up (“uptake”) 90 Gtons yr-1 of carbon from the
atmosphere and respiration and decay (“rerelease”) return 88 Gtons yr-1 of carbon back to the
3a. Assuming the imbalance between uptake and rerelease remains the same over long
periods of time, how long would it take to remove the excess 20,000 Gtons of organic
carbon in the deep ocean? Note – we call the process of transporting organic carbon from
the surface of the ocean to the deep ocean the biological pump.
3b. If the oceans become anoxic, release of carbon back to the atmosphere ceases (i.e., the 88
Gtons yr-1 of rerelease essentially drops to zero). If this were to occur, how long would it
take to remove the excess 20,000 Gton of CO2 from the atmosphere in return it to the
deep ocean (i.e. how long would it take to remove 20,000 Gtons of carbon from the
atmosphere)? We can consider this the shortest time possible to restore balance back to
the organic carbon cycle.