## Lesson

Introduction

Carbon is a very important element. You could not live without carbon. If something you eat has protein or fats, then it contains carbon. When your body breaks down that food to produce energy, you breathe out carbon dioxide. Carbon is also a very important element on Earth. Carbon is provided by the environment, moves through organisms and then returns to the environment again. When all this happens in balance, the ecosystem remains in balance too.

Carbon atoms frequently move from place to place, change their chemical partners, and change their physical state. Most of this happens within a relatively short amount of time but some processes take millions of years. Scientists define two carbon cycles: the long term and short term carbon cycle. The short term carbon cycle is caused by biochemical processes. The long carbon cycle is less known and has to do with long-term geochemical processes. Both cycles will be discussed in the next paragraphs.

Short term carbon cycle

The short term carbon cycle begins with carbon dioxide (a gas composed of one carbon and two oxygen atoms) and the process of photosynthesis. Each day, plants absorb carbon dioxide from the air. Through photosynthesis, carbon dioxide plus water and energy from sunlight is transformed into food with oxygen given off as a waste product. Chemists write equations for different types of chemical reactions. The equation for photosynthesis looks like this (equation 1) :

The amazing transformation that has happened here is changing energy from sunlight into chemical energy that plants and animals can use as food (figure 7). Through the food chain, this carbon moves into all other living things.

We can use the energy stored in plants in other ways too. Scientists are interested in biomass energy for things such as fuel for your car (figure 8). Biomass can be found all over the world and there is an endless supply since it can keep growing. Things such as corn stalks that are leftover from harvesting, and forest brush that may cause a fire hazard, can be converted into fuels. These biomass fuels burn cleaner than gas or oil does, so it is also better for the environment.

Long Term Carbon Cycle

As described above, an individual carbon atom could cycle very quickly if the plant takes in carbon dioxide to make food and then is eaten by an animal, which in turn breathes out carbon dioxide. Carbon might also be stored as chemical energy in the cells of the plant or the animal. If this happens, the carbon will stay stored as part of the organic material that makes up the plant or animal until it dies. Some of the time, when a plant or animal dies, it decomposes and the carbon is released back into the environment. Other times, the organic material of the organism is buried and transformed over millions of years into coal, oil, or natural gas. When this happens, it can take millions of years before the carbon becomes available again.

Carbon Sinks and Carbon Sources

We can think of different areas of the ecosystem that use and give back carbon as carbon sources and carbon 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.

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 simple hydrocarbon looks like this (equation 2):

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. 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.

Importance of 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 a gas that can act like a greenhouse (figure 9). In our atmosphere, it helps hold some of the sun’s energy within the earth’s biosphere and keeps our planet at a liveable temperature. Increasing the level of carbon dioxide in our atmosphere causes our planet to hold more of the sun’s energy within the earth’s biosphere. This appears to be increasing the average temperature of the earth. (People refer to this as Global Warming.) This may also be altering the earth’s climates in unexpected ways. (People refer to that as Global Climate Change.)

Vocabulary

Carbon sink: an area of an ecosystem that has absorbed more carbon dioxide than it has produced.

Carbon source: an area of an ecosystem that emits more carbon dioxide than it absorbs.

Greenhouse gas: gases like carbon dioxide that absorb and hold heat from the earth's infrared radiation.

Global warming: warming of the Earth brought about by adding additional greenhouse gases to the atmosphere.

Photosynthesis: the process using carbon dioxide, water, and energy from sunlight by which plants and algae produce their own food. Although algae are plants as well, it’s part in the carbon cycle is so big,