The carbon cycle-Carbon's winding journey
How is it changing? Does it matter anyways?
Anyone who had even the slightest investigation into current environmental problems may be familiar with the term “carbon cycle”. The term is commonly discussed together with some sort of “change” and “harm” to the environment. Now, before we get all bewildered with the overwhelming information on the internet to how this changing of the carbon cycle will affect the human civilization, lets slow ourselves down and instead take a look at the carbon cycle itself, first.
The carbon cycle is essentially the exchanging of carbon between different natural carbon reservoirs on Earth: carbon starts in the atmosphere as carbon dioxide, then precipitates in the form of acid rain, and somehow end up as rocks—that is a taste of what the carbon cycle is like. It is all about the element of carbon getting tossed around from one part of the Earth to another.
However, the carbon cycle is a complex process--a winding journey for carbon atoms--that is not limited to the three stages we mentioned in the last paragraph. In fact, the carbon cycle itself contains two functioning, separate cycles: the slow carbon cycle and the fast carbon cycle. The carbon atoms in both cycles start their journey together in the atmosphere as carbon dioxide, and ends up in the atmosphere together, again, when both of their cycles are finished. The atmosphere is like the junction of the slow and fast carbon cycles.
You may be wondering: what is the point of having a carbon cycle and force the carbon element “move around” on our planet? Isn’t it good enough to have all our carbon stored in one place, like, the atmosphere? Well, that is the exact problem. We are talking about an enormous amount of carbon here that we are trying to get distributed to different reservoirs on Earth. If we keep them all in the atmosphere as carbon dioxide(which is a greenhouse gas), then the Earth is going to BURN in the immense heat greenhouse gases trap in(to know why this happens, look at our course on “Greenhouse gases-The chosen ones”). On the other hand, the Earth is going to freeze if we store all the carbon as calcium carbonate in rocks, as no more carbon(carbon dioxide) would be in the atmosphere and trap heat for us. The best scenario would be to have the exact amount of carbon—that would not cause the Earth to burn nor cause it to freeze—in the atmosphere as carbon dioxide. What about the rest of the carbon? They would have to go to the other carbon reservoirs we got here on Earth.
Slow carbon cycle
The Slow carbon cycle starts with the carbon—now existing as carbon dioxide—in the atmosphere. When carbon dioxide in the atmosphere meets with the water vapor in the atmosphere, they react. They form a liquid called the carbonic acid, otherwise known as acid rain. When these acid rain(containing the element of carbon from carbon dioxide) fall, there is a chance that they will fall onto a rock and eventually dissolve the rock in the continuous bombardment of more acid rain drops. This process of acid rain dissolving rocks is called Chemical Weathering. From the dissolving of the rocks, bio carbonate compounds are released. Together with the acid rain(contains carbon), these bio carbonate compounds(also contains carbon) flow into rivers, and end up in oceans.
In the ocean, the bio carbonate compounds are absorbed by shell-building marine animals, and combined with other chemicals(in the body of the shell-building animal) to form a calcium carbonate shell(contains carbon). When the animal die, these shells fall together with their body to the ocean floor. Over time, these shells are “squeezed” with other sediments and eventually form limestones(contains carbon) under the extreme pressure from the constantly piling of more sediments above. Now the carbon is in the limestones, trapped at the bottom of the ocean. When two tectonic plates collide, one of the plates will be pressed down, and the other plate lifted up. The limestones close to the brink of the plate that is pushed down will be submerged, along with the plate, into the lava that is below the Earth’s crust. These limestones will melt under the extreme heat and pressure, and form silicate minerals(DO NOT contain carbon) and carbon dioxide(contains carbon). The carbon dioxide will stay below the Earth’s crust for a while, until it is released into the atmosphere again through an erupting volcano.
Fast carbon cycle
If the limestones were on the plate that was pushed up during the collision, then they would have to wait for another instance when carbonic acid rains down and dissolves them so that they could get back to the ocean.
The fast carbon cycle also starts with carbon dioxide in the atmosphere. These carbon dioxide will be absorbed by plants, then processed together with solar energy and water to form sugar(contains carbon) and oxygen, in the process of photosynthesis.
Now there are several ways that this carbon in the sugar in the plant will go back to the atmosphere. Firstly, plants may breakdown this sugar(containing carbon) to get energy used in plant growth. Secondly, animals may eat plants, then breakdown the sugar stored in the plants to get energy for their own growth. Thirdly, plants(containing the sugar) may die and get decomposed by bacteria. Lastly, plants(containing the sugar) may get burned by fire. All of these four approaches of how carbon dioxide would be returned to the atmosphere includes a chemical reaction: sugar and oxygen combining to release carbon dioxide, water, and energy. The moment carbon dioxide appears as a product in the reaction, it either gets out of the organism’s body(in cases 1 and 2) and then enters the atmosphere, or it enters the atmosphere directly(in cases 3 and 4).
Above introduced are the two components of the carbon cycle: the slow cycle, which the carbon will take 100-200 million years to complete(as the carbon has to pass through all those stages), and the fast cycle, which takes the carbon varying lengths of time—depending on the life span of the organisms it goes through—to complete.
The carbon cycle seems to be functioning alright. But artificial activity easily changed the delicate system of the Carbon cycle.
There are two ways in which artificial activity caused the carbon cycle to be unbalanced(with too much carbon in the atmosphere). Deforestation is the first way. Deforestation is essentially killing trees, or in other words, removing reservoirs of carbon. As the trees are uprooted, the soil—that stores carbon released from the decomposed remnants of organisms—near the tree’s roots will be exposed to the air, causing the carbon dioxide previously trapped inside the soil to get released. Then, crops are cultivated in the original position of the trees. Despite the fact that both are plants, crops store way less carbon than trees do. There is also possibility that the land will be left barren for buildings to take place on, which is a worse situation as no other plants, not even crops, will be storing part of the carbon that the trees originally stored. Whatever happens to the land after the trees were removed—whether it is filled with crops or simply left barren—less carbon will be stored in the area than before, which means more carbon released into the atmosphere. The second way human activity can increase atmospheric carbon is by burning fossil fuels—usually for energy—releasing carbon dioxide into the atmosphere along with the smoke created.
Now, there are consequences to the having too much carbon in the carbon cycle being at the atmosphere. Despite how the plants and oceans on Earth will gradually take up 80% of the excess carbon added to the atmosphere, 20% of added carbon will remain in the atmosphere. These extra carbon in the atmosphere as carbon dioxide would cause Earth to warm up, as the atmosphere now traps more infrared radiation(heat) inside. To see the exact reason why extra carbon dioxide in the atmosphere warms the Earth up, visit our course “Greenhouse Gases-The chosen ones”.
The warming up of the Earth is a devastating consequence to the imbalance of the carbon cycle, but it’s not the sole consequence. The ocean and the atmosphere comes in contact, and carbon dioxide in the atmosphere can dissolve(react with water) in the ocean to form carbonic acid. Now that there is more CO2 in the atmosphere, more carbonic acid would end up in oceans. The ocean will be “taking up” the extra carbon in the atmosphere, as mentioned in the last paragraph. But this sudden increase of acid concentration in the ocean will cause ocean acidification. The large amount of carbonic acid in the ocean will react with carbonate ions to form bio carbonate, which leaves shell-building marine animals less carbonate ions for the building of their shells. The shells of these animals then evolve to become thinner and more fragile. Also, the ocean water, which is becoming acidic, can easily dissolve the carbonate shells of the shell-building animals. Both the acidifying of the water and the thinning of the carbonate shells cause shell-building animals to be more vulnerable in the ocean environment.
A third consequence to the addition of more carbon to the atmosphere would be the increase in water concentration in the atmosphere, which also results in global temperature rises. When lots of carbon dioxide—a greenhouse gas—has been added to the atmosphere, the Earth would warm up. This higher temperature will evaporate more water from rivers, lakes, and the ocean then before, causing the concentration of water vapor in the atmosphere to increase. Unfortunately, water vapor is also a greenhouse gas, meaning that an increase in its concentration also warms the Earth up. Things now go in a loop: the increased concentration of water vapor in the atmosphere will raise the temperature of the Earth, causing more water to evaporate, leading to an even higher concentration of water vapor in the atmosphere, which evaporates more water, and so on. The Earth would heat up continuously in this positive feed back loop, which originated with an increase of carbon in the atmosphere that caused the initial heating up of the Earth.
On land, things are going in a slightly different direction. With the increase of carbon in the atmosphere, there will be more carbon dioxide in the air for plants to take in, and do photosynthesis with. More photosynthesis means that more sugar, containing the energy necessary for plant growth, will be produced. Thus, more photosynthesis will lead to more plant growth, and the increased number of plants would absorb more carbon dioxide from the atmosphere, which leads to more plant growth, and so on. However, this cycle of more plant growth and more carbon absorption does not go on infinitely. To do photosynthesis, a plant needs all three components of sunshine, water, and carbon dioxide. To grow healthily, a plant will need way more components than the carbon dioxide in the air. If any other component necessary for a plant’s growth is limited, such as water and nitrogen, then the plant still would not grow even when there is abundant carbon dioxide, and there won’t be more plants that could absorb more carbon dioxide and keep the cycle going.
The three consequences of an unbalanced carbon cycle are listed above. In this course we learned what the carbon cycle is, what causes an unbalanced carbon cycle, and what would happen with an increased carbon concentration in the atmosphere. Human should act, we should cut the activities mentioned in this course that causes an unbalanced carbon cycle. However, not everyone has the ability to do so, as we all rely on deforestation and the combustion of fossil fuels to maintain our life standards. The task to eliminate these activities that add carbon to the atmosphere should be handled by businesses and governments. As civilians, our responsibility is to feel and to perceive the consequences of the change in the carbon cycle. We should look for signs of a more moist atmosphere, reports of shell-building marine organisms becoming more vulnerable, and any abnormal changes in the number of trees around us. We should look for these signs that indicate changes in the carbon cycle, and be aware of the fact there are still 20% of additional carbon currently swaying in the atmosphere and warming the Earth up. If we can not act, at least we should see and admit the imbalance of the carbon cycle.
Citation:
Riebeek, Holli. “The Carbon Cycle.” NASA, NASA, 16 June 2011, https://www.earthobservatory.nasa.gov/features/CarbonCycle.