Urban Ecology

Where does all the carbon go?

by Diane Boudreau

Carbon is an element with many forms. Those different forms flow between the biosphere, the atmosphere, and the oceans. This global flow is called the carbon cycle.

Scientists study how carbon is exchanged between these elements. They also look at where carbon is stored for long periods of time. Because the amounts of carbon involved are so huge, scientists use the term gigaton as a unit of measure. One gigaton is equal to 1 billion tons of carbon!

The largest stores of carbon lie underground. The carbon is part of fossil fuels like oil and coal, and in sedimentary rock deposits. Other huge amounts are found at the bottom of the oceans. About 44,000 gigatons of carbon is trapped in these stores. When humans burn fossil fuels and clear land they release huge amounts of carbon into the atmosphere. These activities release about 6 gigatons of carbon each year.

The atmosphere holds about 750 gigatons of carbon in the form of carbon dioxide. Scientists say that the amount of CO2 is the atmosphere is on the rise. Current levels are 25 percent higher than they were before the Industrial Revolution began in the late 1700s.

Plants absorb carbon dioxide from the atmosphere during photosynthesis. That is the process plants use to turn sunlight into food. Plants also release CO2 back into the air through respiration.

About 800 gigatons of carbon is dissolved in the surface layers of the world’s oceans. Marine plants use that dissolved CO2.

Plants and animals also store carbon in their bodies. About half the weight of a mature tree is carbon. Scientists believe that 550 gigatons of carbon exist in living plant and animal matter. Another 1,300 gigatons of carbon is trapped in dead leaves, twigs, branches, other ground litter, and soils.

These numbers are estimates, of course. Scientists only have a general understanding of the carbon cycle. They have not totally accounted for the rates of change between the atmosphere, land, and ocean.

Scientists still can’t account for about 20 percent of the CO2 released each year. That is between 1 and 2 gigatons. Not a small amount! Scientists are still working to discover where this disappearing carbon goes.