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.