natural gas. When this happens, it can take millions of years before the carbon becomes
available again.
Another way that carbon is stored for long periods of time happens when carbon is used by
ocean organisms. Many ocean creatures use calcium carbonate (CaCO 3 ) to make their shells
or to make the reef material where coral animals live. When algae die, their organic material
becomes part of the ocean sediments, which may stay at the bottom of the ocean for many,
many years. Over millions of years, those same ocean sediments can be forced down into
the mantle when oceanic crust is consumed in deep ocean trenches. As the ocean sediments
melt and form magma, carbon dioxide is eventually released when volcanoes erupt.
Carbon Sinks and Carbon Sources
We can think of different areas of the ecosystem that use and give back carbon as car-
bon sourcesandcarbon sinks. Carbon sources are places where carbon enters into the
environment and is available to be used by organisms. One source of available carbon in
the environment happens when an animal breathes out carbon dioxide. So carbon dioxide
added to our atmosphere through the process of respiration is a carbon source. Carbon sinks
are places where carbon is stored because more carbon dioxide is absorbed than is emitted.
Healthy living forests and our oceans act as carbon sinks.
In the natural situation, the amount of carbon dioxide in the atmosphere is very low. This
meansthatwecanquicklychangetheamountofcarbondioxideinouratmosphere. Scientists
can use data from air bubbles trapped in the ice of glaciers to determine what the natural
level of carbon dioxide was before the Industrial Revolution, when humans began to use
lots of fossil fuels. Measurements of the different gases in the air bubbles tell us that the
natural level of carbon dioxide was about 280 parts per million. Today the amount of carbon
dioxide in our atmosphere is 388 parts per million and that amount continues to rise every
year. Scientists have been making measurements in the middle of the Pacific Ocean, far from
any large land areas for fifty years. The graph (Figure18.16) of this data shows that the
amount of carbon dioxide has been steadily increasing every year.
Human Actions Impact the Carbon Cycle
Humans have changed the natural balance of the carbon cycle because we use coal, oil, and
natural gas to supply our energy demands. Remember that in the natural cycle, the carbon
that makes up coal, oil, and natural gas would be stored for millions of years. When we burn
coal, oil, or natural gas, we release the stored carbon in the process of combustion. That
means that combustion of fossil fuels is also a carbon source.
The equation for combustion of propane, which is a simplehydrocarbonlooks like this
(Figure18.17):