The Carbon Cycle: How Can We Reduce Carbon Dioxide in the Atmosphere?

Dec 11, 2023

Life as we know it would not exist without carbon, which is an essential component of complex molecules, including DNA, proteins and carbohydrates. Carbon is the fourth most abundant element in the universe, according to Earth Observatory at NASA. Most of Earth's carbon is stored in rocks. The rest is present in the atmosphere, the ocean, living things, the soil and fossil fuels. The carbon cycle — which includes fast and slow components — describes how carbon naturally moves through these different repositories.

Since the beginning of the Industrial Revolution about 150 years ago, humans have been releasing carbon into the atmosphere from fossil fuels, such as coal, oil and natural gas. The energy from these sources enabled the widespread availability of manufactured goods, automobiles, electronics and other innovations that make modern life possible. However, the excess carbon is now altering the climate and threatening the ecosystems that keep life in balance. Thus, reducing carbon emissions has become an urgent international goal.

The fast carbon cycle is largely the movement of carbon through life forms on Earth. Carbon dioxide (CO2) in the atmosphere is taken up by plants undergoing photosynthesis, which uses energy from the sun to combine CO2 with water (H2O) to produce sugar (usually C6H12O6) and oxygen (O2).

The carbons captured through this process can be incorporated into a seemingly endless variety of complex organic molecules as the plants grow and support ecosystems that include animals, bacteria and fungi. In a reverse of the original reaction, sugar molecules can be broken down to produce energy and CO2 during digestion, decay or fire. This requires the input of oxygen and water and allows CO2 to be released back into the environment.

Humans are most familiar with CO2 in the atmosphere, but CO2 is also present in the ocean. Microscopic marine algae — called phytoplankton — take up this CO2 and use energy from the sun to undergo photosynthesis. These algae form the base of the marine food chain and provide 50% of the oxygen in the atmosphere, according to National Geographic.

The fastest part of the slow carbon cycle is the ocean. At the surface of the ocean, carbon dioxide is exchanged with the atmosphere. As humans have released more CO2 into the atmosphere, the ocean has taken up more CO2.

Once in the ocean, carbon dioxide reacts with water molecules to release hydrogen, making the ocean more acidic. The hydrogen reacts with carbonate to produce bicarbonate ions. Carbonate is also used by shell-building organisms, including phytoplankton, corals, oysters and starfish, to build their calcium carbonate shells. More carbon dioxide in the atmosphere has led to less carbonate in the oceans and more fragile shells.

As marine organisms die, they sink to the seafloor. Over time, layers of shells and sediment become cemented together to form limestone. The carbon trapped in limestone can be stored for millions of years, as evidenced by the fossil shells commonly found in limestone. About 80% of carbon-containing rock is made this way. The remaining 20% comes from land-dwelling organisms that became embedded in mud to form shale, which is another type of sedimentary rock. If dead plants build up faster than they can decay, they may become fossil fuels.

Over geological time, these sedimentary rocks may be exposed to the atmosphere or volcanic activity, and their carbon may be returned to the atmosphere. Carbon dioxide in the atmosphere combines with water to form carbonic acid, which is a weak acid that falls as rain and dissolves exposed rocks through a process called chemical weathering. This releases ions — such as calcium needed to form calcium carbonate shells — and eventually leads to more carbon being deposited on the ocean floor. When volcanic activity causes sedimentary rocks to melt, they form fresh silicate minerals and release carbon dioxide into the atmosphere.

At present, volcanoes emit between 130 and 380 million metric tons of carbon dioxide per year. By burning fossil fuels, humans now emit about 36 billion metric tons of carbon dioxide per year, according to Statista. The amount of carbon dioxide in the atmosphere is now greater than any other time in the last 3.6 million years.

Carbon dioxide is a greenhouse gas. When present in the atmosphere, it absorbs heat emitted by Earth and reflects some of it back to Earth. Without greenhouse gases like water vapor, CO2 and methane, Earth would be completely frozen. However, too much carbon dioxide causes excess warming. Since 1880, CO2 concentrations in the atmosphere have risen from 280 parts per million to 387 parts per million, and average global temperatures have increased by 0.8 degrees Celsius (1.4 degrees Fahrenheit).

Reducing carbon emissions by using energy more efficiently and transitioning to clean, renewable energy whenever possible is essential to reversing this problem. It's also important to protect and restore natural carbon reservoirs, such as forests, permafrost and salt marshes. However, some industrial processes (e.g., iron smelting and making cement from limestone) will release carbon regardless of the energy source.

Therefore, we need ways to capture carbon dioxide from the atmosphere or from the point of emission and store it safely for thousands or millions of years.

According to Chemical and Engineering News, existing carbon capture technologies work but are prohibitively expensive in all but a few settings. Carbon capture is essentially a gas-separation problem, which becomes more energy intensive as CO2 becomes more dilute. The "low-hanging fruit" includes iron and steel plants, which emit streams with 15% to 80% CO2. Coal and natural gas power plants are more challenging, as they produce the largest share of gas emissions and emit gas streams with less than 15% CO2. Most challenging of all is ambient air, which has a CO2 concentration of around 0.041%.

Existing technologies include the use of filters, membranes or solvents that absorb CO2 while allowing other molecules to pass through. These processes often require heat to push gas through the filter to remove purified CO2 so it can be stored or to refresh the system so it can be reused. One promising option is to pair these energy requirements with clean energy sources or heat that would otherwise go to waste.

According to the environmental policy think tank Center for Climate and Energy Solutions, existing carbon capture technologies can capture more than 90% of CO2 emissions from power plants and industrial facilities. Geological formations being used or considered for CO2 storage include old oil and gas reservoirs, saline aquifers, basalt formations and shale basins.

Almost all the CO2 captured to date has been used in "enhanced oil recovery," where the CO2 is injected into declining oil fields to increase oil production. This would seem to negate the benefits of carbon capture and work against the goal of transitioning to renewable energy. However, oil and gas companies have been early adopters of carbon capture technology and have enormous budgets — both of which are essential for rapidly improving the technology to achieve net-zero carbon emissions.

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