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Carbon dioxide is constantly being removed from the atmosphere as part of the carbon cycle. If this did not happen, the world would heat up and become a lot like Venus. The mechanisms that take carbon out of the atmosphere are called "carbon sinks."

The forests of the world are a big carbon sink. Deforestation is reducing the size of this sink, allowing more carbon dioxide to remain in the atmosphere. On the other hand, recent technological developments allow humans to build a new kind of carbon sink through carbon capture and storage. 

Let's look at carbon sinks and how they work.

Photosynthesis

Photosynthesis removes carbon from the atmosphere. It is the process that enables plants to live and grow. Photosynthesis occurs in the leaves of plants and the cells of microscopic organisms that live on land and near the surface of seas and oceans. The process uses energy from solar radiation (sunlight) to convert carbon dioxide and water into carbon-based sugars such as glucose.

Carbon dioxide (CO₂) + Water (H₂O) + EnergyGlucose (C₆H₁₂O₆) + Oxygen (O₂)

The sugars are then converted into other molecules such as starch, fats, proteins, enzymes, and all of the other molecules that occur in living plants. Photosynthesis also releases oxygen into the atmosphere, which plants and animals need for respiration.

Photosynthesis accounts for about half of the carbon extracted from the atmosphere. Land plants take most of their carbon dioxide from the air around them. Photosynthesis by aquatic plants in lakes, seas, and oceans uses carbon dioxide dissolved in water.

The graph below shows an interesting aspect of how photosynthesis affects atmospheric CO₂ levels.

It is based on readings taken at the Mauna Loa Observatory in Hawaii. We see that the levels of CO₂ have risen steadily since 1958. But why the wiggles? The rises occur during the Northern Hemisphere autumn and winter, peaking just before spring. CO₂ concentrations decrease during spring and summer. This is because during the growing season, plants are absorbing more CO₂ than they are giving off through respiration. Deciduous trees grow leaves, and seasonal plants such as grasses come to life. During autumn and winter, grasses die and these trees become dormant and stop functioning as a CO₂ sink. Tropical plants absorb CO₂ throughout the year.

In the Southern Hemisphere the seasons are reversed. But since the Northern Hemisphere has a much greater land area, the net effect worldwide is that plants provide a greater CO₂ sink during spring and summer than during autumn and winter.

Diffusion into Oceans

Carbon dioxide is the gas that is usually used to make the “fizz” in soda drinks. It is most soluble at high pressures and low temperatures, which is why soda is normally served cold from cans or bottles. Some CO₂ will remain dissolved with the can or bottle open and at room temperature. This is also why colder oceans can absorb more CO₂ than warmer waters.

Molecules of carbon dioxide are continuously being exchanged between the atmosphere and water through a process called diffusion. Diffusion of carbon dioxide into the oceans accounts for nearly half of the carbon extracted from the atmosphere.

Carbonate Precipitation

Many forms of sea life extract carbon and oxygen from seawater and combine them with calcium to produce calcium carbonate (CaCO₃). This is used to produce shells and other hard body parts by a variety of organisms, such as coral, clams, oysters, and some microscopic plants and animals. When these organisms die, their shells and body parts sink to the seabed. Over very long periods of time, enormous numbers of dead organisms can create thick layers of carbonate-rich deposits on the ocean floor. Over millions of years, these deposits become buried by more and more carbonates and/or sand or mud, creating heat and pressure that physically and chemically changes them into sedimentary rocks such as limestone, marble, and chalk. These rocks may eventually be raised to become exposed land due to plate tectonics—the movement of continents and oceans around the globe.

Under the right conditions of temperature and CO₂ concentration, calcium carbonate may precipitate out of ocean water directly, without the intervention of marine organisms.

The sedimentary rocks of the Earth contain about 2,000 times more carbon than there is in all of the water, plants, and animals in today’s oceans. Marine organisms today continue to make skeletons and shells that, millions of years from now, might become hard rock.

Some carbon is fixed into rocks over millions of years through organic processes on land and under the sea. This creates fossil fuels such as peat and coal (from buried dead plants) and oil and gas (mostly from buried dead microorganisms). This represents only a tiny proportion of global carbon—20,000 times less than in sedimentary rocks.

Runoff

Some carbon is washed from the land into the oceans by water. Falling raindrops absorb some CO₂ from the atmosphere, creating very dilute carbonic acid. When this slightly acidic rain meets carbonate rocks such as limestone or chalk, it dissolves some of the rock, which is then carried by streams and rivers back to the ocean. This process causes the underground caves that are often found in limestone. Carbonate solution by rainwater also contributes toward the erosion of buildings and statues made from limestone and marble. Streams and rivers also carry particles of organic carbon from dead land plants and animals out into the oceans.

How Does CO₂ Enter the Atmosphere?

There are numerous sources adding CO₂ to the atmosphere. Find out more…