The Carbon Cycle Explained
by L.J. Martin
Most people have heard that we should all cut down on our carbon emissions. This means we should release less carbon dioxide (CO2) into the atmosphere. However, this is an overly simplistic definition. The problem is not CO2 in itself, but CO2 from the burning of fossil fuels.
All living organisms, apart from some specialized bacteria, are always releasing CO2 into the environment. Many people are confused about this, and think that all CO2 emissions are bad. This is not true, as CO2 emissions from living organisms are perfectly natural and harmless, while CO2 emissions from fossil fuels may be said to be bad. To understand this distinction, you must understand The Carbon Cycle.
The carbon cycle refers to the path of carbon atoms around the biosphere, which includes the atmosphere and all living things. There are an infinite number of paths around the carbon cycle. The carbon always starts off and ends up in the atmosphere, which is the carbon cycle's reservoir. There are many possible routes round the cycle. The first step is photosynthesis, where CO2 is absorbed by plants or algae and turned into glucose. The process by which living things burn carbon and release CO2 is called respiration. The CO2 could also be released by physical burning, for example in a forest fire.
Diagram of the Carbon Cycle
There are an infinite number of paths around the carbon cycle. The carbon could be absorbed by a plant, burned by that plant as fuel and returned to the atmosphere within minutes or hours. Alternatively, it could be stored within a plant for many months in fruits or grains, then eaten by an animal, which burns the carbon for fuel and returns the CO2 into the atmosphere. A plant or animal could die and be broken down by bacteria, which burn the carbon for energy and return it to the atmosphere as CO2. Plants could also be burned by humans for energy, returning the carbon to the atmosphere as CO2. Once the carbon has completed the cycle, it will start over again, and be endlessly recycled.
It is important to understand that all CO2 released by living things has come from the atmosphere relatively recently, generally within the past year or two. An exception is in the decay or burning of trees, which have held onto their carbon for decades, or a couple of centuries at most. This is still recent, in relative terms. The amount of CO2 in the atmosphere decreases when a plant photosynthesizes, then increases back to its initial level when that carbon is burned and returned back into the atmosphere. In other words, the release of CO2 from living or recently living organisms has no overall effect on atmospheric CO2 levels, and is said to be carbon neutral. When natural materials are burned for energy, they are called biofuels. As these are within the carbon cycle, and burning them simply carries on the cycle, they are carbon neutral. Biofuels, including fuels such as biodiesel, bioethanol and wood, can never cause an overall increase in atmospheric CO2, as the carbon in biofuels recently came from the atmosphere.
This is in stark contrast to fossil fuels. They were also part of the carbon cycle, but instead of the carbon being released back into the atmosphere, it was locked up beneath the earth's surface for tens of millions of years. The level of CO2 in the atmosphere decreased when this fossil carbon was locked up. When these fossil fuels are burned, the atmospheric CO2 levels increase. There is not a short term balance of CO2 reductions and increases as there is in the normal carbon cycle. Fossil fuels, such as coal, natural gas and oil, increase the overall level of carbon in the biosphere, and the atmosphere in particular.
To sum up, carbon is endlessly recycled through the biosphere in a process called the carbon cycle. It exists in carbon dioxide gas, and in organic molecules such as fats, oils, proteins and alcohols. CO2 breathed out by animals, or created from burning plant materials, is balanced by the CO2 consumed by plants at the start of the carbon cycle, and is considered carbon neutral. Burning fossil fuels adds extra carbon to the cycle, as fossil carbon has been locked away outside the cycle for millions of years. This is why fossil fuels are considered bad by many people, while carbon neutral biofuels are considered good.