The Global Carbon Cycle

It has taken Earth millions of years to achieve a perfect balance in the carbon cycle, but now humans have created an imbalance, resulting in an increase in the planet’s temperature and the acidity of the oceans. We have the necessary technology to reverse this process and it is our duty to help the carbon cycle regain its balance.

Text and photos: Javier Pinzón

Carbon is fundamental to the existence of all living beings because it is the basic component of life. It has an amazing ability to bind to other compounds to form essential elements, such as sugars, starches, fats, and proteins. In fact, carbon represents approximately half of the total dry mass of living beings. Earth’s marvelous system is also a huge reservoir of carbon, preserved in the atmosphere, the soil, the oceans, and the earth’s crust; the movement of carbon between these deposits is called flow.

But before we talk about the carbon cycle, let’s start from the beginning. Let’s go back 3.8 billion years, after a rain of meteorites and rocky objects fell from space, when the surface of the Earth cooled and solidified. The water vapor in the atmosphere condensed and fell like rain, creating the oceans. Since the sun was darker, our planet received less solar radiation, so the planet’s surface should have been well below the freezing point, too cold for life. However, evidence indicates the opposite: there was liquid water on Earth and the first forms of life began to appear just a few years later, 3.5 billion years ago.

How was this possible? Thanks, first of all, to greenhouse gases: volcanoes emitted CO2 as a by-product of heating that took place within the Earth’s crust. But instead of developing a runaway greenhouse effect, as happened on Venus, the Earth’s temperatures remained within a moderate range thanks to what we know as the “carbon cycle.” This is the process through which carbon is added to the atmosphere when it is needed and eliminated when there is an excess; it is stored in “carbon pools.” When sources of carbon are equal to the pools, we can say that the carbon cycle is in balance, meaning there is no change in the size of the pools over time. Maintaining a constant amount of CO2 in the atmosphere helps to maintain stable average temperatures on a global scale.

Studying this cycle is very complex because it includes all the plants, animals, and microbes on Earth. The carbon cycle encompasses each leaf that performs photosynthesis and every fallen tree; every ocean, lake, pond, and puddle; every bit of soil, sediment, and carbonate rock; every breath of fresh air; every volcanic eruption; and the bubbles that rise to the surface of a marsh, among many other factors. To understand it, scientists have focused on processes on a global scale, carbon deposits, and the most important flows between these deposits.

Carbon Deposits

Carbon deposits on Earth can be grouped into several categories, four of which are most significant to the overall carbon cycle.

The Earth’s Crust: The greatest amount of carbon is stored in sedimentary rocks, which were produced by the hardening of mud (which contains organic matter) during the geological age. The heat and pressure compressed the mud and carbon over millions of years, forming sedimentary rocks, such as slate. In special cases, when dead plant matter accumulates faster than it can be decomposed, layers of organic carbon are converted into oil, coal, or natural gas instead of sedimentary rock. Together, all the sedimentary rocks on Earth store 100,000,000 petagrams of carbon (PgC). Considering that one Pg is equal to a trillion kilograms, this is clearly a large mass of carbon. Another 4,000 PgC are stored in the Earth’s crust as hydrocarbons formed from living organisms over millions of years.

Oceans: In the oceans, 38,000 inorganic PgC is stored at great depths where it remains for long periods of time. There is another 1,000 PgC on the surface of the oceans, where it is rapidly exchanged with the atmosphere through physical processes.

Atmosphere: Around 750 PgC is found in the atmosphere, mostly in the form of CO2. Although the content is lower than in the oceans and the Earth’s crust, this carbon fulfills a very important role in maintaining the climate and the greenhouse effect. Due to its small concentration, the carbon in the atmosphere is highly sensitive to changes within the cycle. We are now observing the results of these changes: it is estimated that before the burning of fossil fuels and deforestation, the atmosphere contained around 560 PgC. This increase is one of the causes of global warming.

Terrestrial Ecosystems: The carbon here is found in the form of plants, animals, soil, and microorganisms. Unlike the Earth’s crust and the oceans, most of the carbon in terrestrial ecosystems exists in organic form, which means compounds produced by living beings, including leaves, wood, roots, decaying plant material, and the organic matter of soil. The total carbon stored in plants is approximately 560 PgC, with wood from trees being the largest container. There are an estimated 1,500 PgC in the world’s soil.

Carbon Flows

This process is the transfer of carbon from one group to another. Often a single carbon group can have several flows that add and eliminate carbon simultaneously.

Geological Processes: These processes represent an important control in the carbon cycle that occurs over hundreds of millions of years. This includes the formation of sedimentary rocks, the process of erosion, and volcanic eruptions. Ultimately, the carbon in living organisms becomes sedimentary rock that continues moving through the tectonic plates, and eventually to the depths of the Earth’s crust, where, thanks to volcanic eruptions, they will return to the atmosphere in the form of carbon. These flows, which occur very slowly, have allowed life on the planet to take place, because the carbon does not remain trapped in the rocks, but returns to the atmosphere.

Photosynthesis: During this process, plants use the sun’s energy to combine CO2 from the atmosphere with water from the soil. This results in the carbohydrates that nourish plants. This is how CO2 is transferred from the atmosphere and stored in plants. It is estimated that photosynthesis removes 120 PgC annually from the atmosphere, but because many plants, such as trees, have a very long lifespan, they can store carbon for long periods of time; trees store around 610 PgC.

Plant Respiration: Through the process of respiration, plants release 60 PgC annually into the atmosphere. This occurs when they use the carbohydrates created during photosynthesis to obtain energy.

Fallen Leaves: Living plants shed some of their leaves, roots, and branches each year. All plant parts are carbon, so the loss of these parts to the soil is a transfer of carbon from the plant to the soil through the process of decomposition.

Soil Respiration: The release of CO2 through respiration is not exclusive to plants. All organisms do this, including the microscopic organisms that live in the soil. When dead organic material decomposes (consumed by bacteria and fungi), CO2 is released into the atmosphere at an average rate of around 60 PgC annually on a global scale.

Ocean-Atmosphere Exchange: Inorganic carbon is absorbed and released on ocean surfaces and in the surrounding air. In this process CO2 reacts with water, forming carbonic acid (oceans are acidifying due to the large amount of CO2 they are absorbing), in which the anion is carbonate. The latter is very important for marine organisms such as corals and mollusks, which use it to make their shells. Carbon is also recycled through the photosynthesis, respiration, and decomposition of aquatic plants.

Combustion of Fossil Fuels and Change of Land Cover: These flows created by humans are the main source of imbalance in the system. The use of fossil fuels captured over millions of years in the Earth’s crust (such as coal, petroleum and natural gas, the byproduct of which is CO2) has disproportionately increased the flow of CO2 into the atmosphere by an average of 6 to 8 PgC per year. Likewise, deforestation has released around 1.5 PgC per year into the atmosphere, increasing the cycle’s imbalance.

It has taken millions of years to arrive at the perfect balance of the carbon cycle, which has allowed life on our planet to flourish. Now we have caused an imbalance, producing an increase in the temperature of the planet and the acidity of the oceans. It is our duty to help the planet get back in balance. We have the technology and the power to achieve this, but we need a greater willingness to change, not only by countries, but also by every individual on this planet. Recycling, using our cars less, reforesting, using clean energy… every step we take, no matter how small it may seem, multiplied by the millions of people living on our planet, would be like taking one giant step toward regaining that balance.