Carbon sequestration is the process of capturing gaseous and atmospheric carbon dioxide and storing it in a solid or liquid form. Trees sequester carbon naturally by absorbing carbon dioxide and converting it into biomass. Technological carbon dioxide removal seeks to offset the human release of carbon dioxide and other greenhouse gas emissions, which come from the burning of fossil fuels such as coal and natural gas.

Carbon is the sixth element on the periodic table. It is a non-metal with atomic number six and a standard mass of roughly 12 atomic mass units. Carbon is the building block of organic matter—in fact, the scientific definition of the word “organic” implies the presence of carbon.

When climate scientists and environmentalists talk about carbon, they’re typically referring to carbon compounds such as carbon dioxide (CO2), carbon monoxide (CO), and methane (CH4). Methane and CO2 emissions play a critical role in global warming because they trap heat in Earth’s atmosphere rather than letting it escape into space. This produces a greenhouse effect wherein the Earth gets progressively warmer, affecting both local and global ecosystems.

By removing large amounts of carbon from the atmosphere—mainly in the form of CO2 and CH4—climate engineers may be able to reduce some effects of global warming. Soil carbon sequestration minimizes atmospheric CO2 concentration by channeling millions of tons of carbon gas into underground storage. Forests and grasslands accomplish this naturally.

Climate scientists have demonstrated that reducing the amount of carbon dioxide in the atmosphere can lead to a decrease in the average global temperature. The long-term effects of temperature decrease could mean fewer forest fires, fewer extreme storms, lower ocean levels, increases in the world’s forest area, increased soil fertility, and a rebound of habitats currently threatened by human activities.

Once carbon has been harnessed, it must be securely stored in a way that will not allow it to leak back into the atmosphere. There are several ways to do this, including:

• Biological sequestration: By far the simplest and most efficient way to capture more carbon is to plant more trees and preserve existing forests. Woody biomass can absorb tremendous amounts of carbon dioxide. By protecting forests from logging and clear-cutting for agricultural use—and by restoring damaged wetlands, grasslands, and forests—we can call upon natural processes and low-tech land management practices to sequester more carbon on a global scale.
• Geological sequestration: Another method is to capture CO2 from a source like a coal plant and compress it into fluid form and pump it into exhausted deposits of oil and natural gas. Depleted oil fields and natural gas fields contain the types of geologic formations that can handle long-term storage of carbon, as they can withstand the high pressure of compressed gas. However, depleted gas and oil fields will not provide enough geological carbon storage to reduce atmospheric CO2 concentration in a meaningful way.
• Ocean sequestration: Ocean storage of carbon involves injecting a CO2 hydrate along the seafloor, where the high-pressure environment will theoretically contain the carbon dioxide in liquid form. This process could be detrimental to sea life and risk further ocean acidification.