By Melanie Friedel, American Forests

Look at all that carbon sinking! Credit: Max Pixel

Don’t be fooled by the name; a carbon sink is not where we go to wash carbon. Actually, it’s something found in nature that holds or stores carbon — technically anything that absorbs more carbon that it releases.

Forests are great examples. In fact, U.S. forests alone store 14 percent of all annual carbon dioxide (CO2) emissions from the national economy. But how does it happen? You may know that trees survive by performing a process called photosynthesis, in which the tree actually consumes CO2. Being absorbed by trees is just one way that carbon moves through forests as part of the carbon cycle. This cycle is the process by which carbon travels from the atmosphere into the Earth and its organisms, and then travels back into the atmosphere.

During photosynthesis, trees and plants “sequester,” or absorb, carbon from the atmosphere in the form of CO2, using it as food. The chemical equation for photosynthesis is: 6 CO2 (the carbon they take in) + 6 H2O (the water they absorb) + sunlight = C6H12O6 (a sugar called glucose) + 6 O2 (the oxygen they release). The carbon from the CO2 becomes part of the plant and is stored as wood. Eventually, when the plant or tree dies, the carbon it has been storing is released into the atmosphere. This, however, is not the only route carbon can take back into the atmosphere.

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Credit: S. Luyssaert et-al.

Humans and other animals eat plants, thereby taking in the carbon that has become part of the plant. Then, we breathe. (And we can thank trees for that — not only do trees take up the carbon we don’t want, but they also provide us with the oxygen we need to survive!) Our breathing process is called cellular respiration, and it looks like this: C6H12O6 (the glucose from the plant) + 6 O2 (the oxygen we breathe) = 6 CO2 (the carbon we release) + 6 H2O (the water vapor we release when we breathe) + ATP (the energy that our cells use to keep working). During that process, we release back into the atmosphere the same carbon that the plant absorbed in the first place, thus continuing the cycle of the carbon.

So if plants and trees eventually contribute to the release of carbon, then how are they considered carbon sinks? Good question. Forests aren’t always carbon sinks; they can sometimes be a carbon source. When a forest releases more carbon than it absorbs, such as during a forest fire or when there are more dead than living trees, it is a carbon source. But in most other cases, forests absorb more than they release, making them carbon sinks.

We prefer forests to be carbon sinks, because too much CO2 in the atmosphere is bad for air quality and human health. Carbon dioxide is a greenhouse gas that traps heat in the lower levels of the atmosphere and contributes to climate change’s trend of globally increasing temperatures. CO2 is not just released by cellular respiration: The main source of CO2 emission is the combustion of fossil fuels by industry and cars. A lot of the carbon produced by these activities is just being introduced into the atmosphere for the first time, even though it will remain cycling through it forever. In 2007 alone, 8.5 billion tons of carbon were added to the carbon cycle by oil, coal, and gas combustion, but before then, it was all being stored underground, far away from the atmosphere where it now exists.

Fossil fuel emissions will require a concentrated effort to offset. Photo Credit: Martin Meissner

Planting trees and conserving forests is an important step towards reducing our carbon footprint, but it won’t do the job on its own. Carbon release from forests can occur at any time if triggered by deforestation, tree decay, forest fires or decomposition of other organic matter. Keeping in mind that carbon will not be stored in trees forever and that the overall carbon levels will keep increasing if emissions do, it’s crucial that we do our job of reducing our carbon emissions and our dependence on fossil fuels in addition to keeping our forest carbon sinks healthy and safe, so that they can do their job too.