The equation shows that when propane burns, it uses oxygen and the result is carbon dioxide
and water. So each time we burn a fossil fuel, we increase the amount of carbon dioxide
in the atmosphere. Another way that carbon dioxide is being added to our atmosphere is
through the cutting down of trees, called deforestation(Figure18.18). Trees are very
large plants, which naturally use carbon dioxide while they are alive. When we cut down
trees, we lose their ability to absorb carbon dioxide and we also add the carbon that was
stored in the tree into the environment. Healthy living forests act as a carbon sink, but when
we cut them down, they are a carbon source.
Figure 18.18: This forest in Mexico has been cut down and burned to clear forested land for
agriculture. ( 21 )
Coal, oil, and natural gas as well as calcium carbonate rocks and ocean sediments are long
term carbon sinks for the natural cycling of carbon. When humans extract and use these
resources, combustion makes them into carbon sources.
Why Do We Need to Know About the Carbon Cycle?
You may wonder why scientists study the carbon cycle or why we would be concerned about
such small amounts of carbon dioxide in our atmosphere. Carbon dioxide is agreenhouse
gas(Figure18.19). Different gases in our atmosphere absorb infrared energy, the longer
wavelengths of the Sun’s reflected rays. These gases hold onto heat energy that would
otherwise radiate out into space. As the heat is held in our atmosphere, it warms the Earth.
This is just like what happens in a greenhouse. The glass that makes up the greenhouse
holds in heat that would otherwise radiate out.
Whenouratmosphereholdsontomoreheatthanitwouldinthenaturalsituation,itproduces