Water is the universal example for diagraming state changes; however it is far from the only molecule capable of doing this. As it turns out, water and hydrocarbons share more than a love for covalent bonding. Carbon is also capable of phase changes, and cycles through our planet and atmosphere just like water does.
The water cycle is elegantly simple. The sun heats water in oceans, lakes, and rivers, causing it to evaporate and form clouds. These clouds eventually release this water as precipitation that waters the earth. Runoff from this process makes its way back into oceans, lakes, and rivers, and the whole process begins again.
With carbon, things are a little more complicated, but the general idea is the same. Lets go through the carbon cycle, looking at each step in more detail:
Step 1: The Atmosphere
The earth's atmosphere contains two main types of carbon molecules: carbon dioxide and methane.
Carbon dioxide comes from humans, along with other land animals, who breathe in oxygen and breathe out carbon dioxide, releasing large quantities of the gas into the atmosphere. Wildfires, volcanic activity, microorganisms participating in fermentation, and the natural decay of organic material (examples include fallen tree leaves and dead animals) also release carbon dioxide into the atmosphere.
As the simplest form of carbon, it is no surprise that methane is released into the atmosphere. A large source of methane comes from an unexpected place: cows.
How now brown cow. Why so much methane?
Cows (and sheep) survive on a diet of plants, and their gastrointestinal tracts contain bacteria that release methane as a by-product of digestion. This means that any sort of "digestive release" enacted by cows, including burping and defecating, results in the release of methane. As silly as it sounds, cow farts play a significant role in the carbon cycle.
Wetlands are another source of natural methane release, mainly through the decay of soils and plants.
An interesting, although extremely minor, source of methane release is the methane reserves that are trapped under permafrost. As global temperatures continue to rise and the permafrost recedes, the release of this stored methane becomes more prevalent.
Step 2: The Earth's Surface
All life on earth is formed from carbon. Taken together, adding up all the plants and animals (including over 7 billion humans) results in a lot of carbon. Living organisms, including plants, are really just borrowing our carbon from the universe. We ingest carbon in the food that we eat, and release it at the end of our digestion process. When organisms pass away, the carbon in their bodies decays and returns to the soil.
Step 3: Within the Earth
Carbon found on the earth's surface is mostly in the organic form. Carbon can also be stored in inorganic forms, such as the calcium carbonate found in most rock formations. While this carbon is extremely slow to release into the atmosphere, it is still a vital part of the carbon cycle.
Calcium Carbonate is found in rock formations. Image from here.
Fossil Fuels, the most basic of all hydrocarbons, store a massive amount of the earth's carbon. They are named because they are the result of thousands of years of organic decay…these fuels are literally fossils. Left to nature these reserves would also be slow to release carbon, however in today's world fossil fuels are drilled and removed and converted to methane at a high rate.
Drilling for fossil fuels. Image from here.
Step 4: The Oceans
The molecule most associated with oceans is NaCl. Surprisingly, the oceans are also a large reservoir of carbon. Carbon dioxide from the atmosphere is sucked up by the ocean and dissolved into the water, some of which goes on to be stored as bicarbonate. Plants living in the oceans use some of this carbon in photosynthesis and store it as organic mass. Animals use dissolved carbon, in the form of calcium carbonate, to create their protective shells.
Crabby over carbon.
Honorary Mention: Human Activity
While it is not a natural part of the carbon cycle, human activity is certainly influential in the flow of carbon. Human activity releases enormous amounts of carbon into the atmosphere, putting a stress on the atmospheric step of the carbon cycle. Systems in stress react using Le Chatelier's principle, meaning a stress of increased atmospheric carbon leads to an increase in the ocean absorbing more atmospheric carbon than it normally would.
The less flattering side of carbon.
Urban expansion disrupts the carbon cycle through deforestation, effectively removing trees and plants that store organic carbon and increasing the release of carbon into the atmosphere in one blow.