What Do Forest Carbon "Sequestration" and "Storage" Mean?

In conversations about forests and carbon, the term "sequestration" is often used—sometimes inappropriately. Trees both sequester and store carbon as different activities. To help improve discussions about atmospheric carbon, fossil fuels and the value of carbon offsets in a market context, this article will explain the difference between carbon sequestration and carbon storage.

Sequestration vs. Storage

Carbon Sequestration

Sequestration is the act of capturing a substance; in this case, it is capturing carbon molecules from the air. Carbon sequestration is the process of removing carbon dioxide–one of the leading greenhouse gases—from the atmosphere, thereby reducing the total concentration in the atmosphere. Living trees sequester carbon through photosynthesis. Using light, leaves create sugar by capturing carbon dioxide molecules and combining them with water molecules. Oxygen is released into the atmosphere as a byproduct of this process. The sequestration occurring in forests is part of the carbon cycle that helps regulate the climate. Increasing carbon sequestration in trees helps remove additional carbon released through the use of fossil fuels. Trees that grow rapidly (e.g., young trees) are more preferred in a carbon market context, because they take up more carbon in a shorter timespan. Trees are also ideal for carbon sequestration because of their large size and the practice of forest management is well established which can help enhance sequestration services by strategically increasing primary production or tree growth.

Carbon Storage

Storage is the act of retaining carbon in a solid form which keeps it out of the atmosphere for a long time (50 years or more). Carbon storage is a key but often overlooked function of trees and forests. One of the standards for selling carbon is "permanence" or the promise that the carbon will be locked away for a very long time. All life on earth is made of carbon, but that carbon is released when organisms die. Organisms that decompose quickly after death, thereby releasing CO₂, are poor at storing carbon for long periods. Trees are good at holding onto carbon for long periods, even after the tree stops growing. Roughly 50% of the dry mass of a tree is carbon, much of which is stored in the wood fibers. Wood molecules, like lignin, are so difficult to decompose that only a handful of fungi can break them down. Wood products, like furniture and building material, are protected from rot and have the potential to extend the storage of carbon. The difficulty in breaking down woody material is also why there is so much coal in Pennsylvania. Coal is made from ancient forests and plants that didn’t fully decompose because they were covered by dirt, water, and rocks. After a few millions of years of heat and pressure, the dead trees and plants were transformed into coal.