carbon cycle
In this article, we will discuss the element carbon and the ways in which it moves around our planet. Carbon is the forth most abundant element in the universe and the basic building block for all life. Carbon atoms are everywhere - the stone we walk on, the CO2 in the air, the backbone of polymers, and most importantly, in all life. Without carbon, there would be no life - no you, no me.
For the purposes of this discussion and most practical purposes, I’m assuming that mass cannot be created or destroyed. Since essentially nothing besides energy is entering or leaving the earth, the combined amount of carbon on the earth is constant. It’s like a bank account. If you don’t deposit or withdraw any money, then your balance remains the same. The same is true of carbon on the earth. If no carbon enters or leaves or is created or destroyed, then the total amount is constant. This called the law of Conservation of Mass. Imagine a log of wood inside of a large metal box filled with air. Weight the entire box and its contents. Now seal the box and cut the log up until it is just a pile of sawdust. Weight the box. Now burn the sawdust inside. Weight the box again. The weight is the same as before. Mass is not being created or destroyed when you cut the log up or burn it. It simply changes form. If you were to uniquely label each atom in the original log, you would still be able to find every atom after the log was burned. The atoms would be arranged differently but they would all be somewhere in the box. For example, a carbon atom from the wood’s cellulose would now be a carbon atom in a molecule of CO2.
[Click the image above to see how atoms move around during chemical reactions but note that they are not created or destroyed. They can all be accounted for. Image courtesy of chemistryland]
carbon problem
So if the number of carbon atoms on the planet is constant no matter what we do, then why is there suddenly a “carbon problem?” The problem is that we are converting a lot of carbon from one form to another. When we burn fossil fuels we don’t create carbon, we just convert it from a hydrocarbon, which was safely buried under the earth’s surface, to CO2 in the atmosphere which is a greenhouse gas (a gas that basically traps heat in the earth’s atmosphere).
Plants, on the other hand, take CO2 from the air and make sugars by a process called photosynthesis. The process converts light energy from the sun into chemical energy, which the plant can use later. If this plant is used as sugar for ethanol production or is decomposed by an animal or bacteria, then these carbon atoms that it took out of the air will be released as CO2 again. See how this is a cycle? Plants are a sort of carbon converter, meaning that they have the ability to remove carbon from the air and store it for a little while. If the plant (or more commonly little phytoplankton that live on the surface of the ocean) die and are preserved in the right way, then they can be transformed over millions of years into a fossil fuel like crude oil. During the last century or two, humans have been finding and burning a lot of fossil fuels and releasing the stored carbon in order to use the stored energy to do work. We are releasing all that carbon that the original plant removed from the atmosphere millions of years ago. The graph below shows how CO2 levels have increased over the last 200 years since humans started burning a lot of fossil fuels.
[Note how quickly humans have changed things. Volcanic activity accounts for the peaks on the far left. Image courtesy of NASA]
The problem that our world is currently facing is that too much of the earth’s carbon is being converted to the form of CO2 in the atmosphere. The process of a plant or organism removing CO2 from the air by photosynthesis and then dying and becoming a fossil fuel takes millions of years (thus fossil fuels are non-renewable). But to dig up that fossil fuel and combust it hardly takes any time at all, especially at the pace we are going at. So we’re basically draining a non-rechargeable battery and dumping CO2 into the atmosphere several times faster than it could ever be pulled back out. Eventually we’ll be out of this free energy and there will be a lot more CO2 in the atmosphere than we are used to. The carbon cycle is a delicate balance that can swing too far in one direction or the other, potentially eliminating natural life on the planet.
The picture below is a simple diagram of the carbon cycle. The black labels and numbers shows the amount of carbon stored in various parts of the planet. The deep oceans, for example, are the largest carbon sink, storing 38,100 gigatons of carbon. The purple arrows and numbers show how the carbon is moving around our planet. Humans release about 5.5 gigatons of carbon every year from fossil fuels alone. That’s about 40,300,000,000,000 pounds of CO2 a year, nearly enough gas to fill 1.8 Billion Goodyear blimps or cover the surface of the earth in CO2 filled ping-pong balls. As CO2 concentrations in the atmosphere get higher, the average temperature of the earth rises. As the earth’s temperature rises, the oceans become warmer. Cold water can store more dissolved CO2 than warmer water so the oceans release even more CO2 into the atmosphere as they warm up. [Have you ever opened a warm coke before and it made a huge mess all over yourself? That’s because the CO2 that makes the coke fizzy was less soluble in the coke (mostly sugar water) than if the coke had been cold]
[Units are gigatons of carbon. Image courtesy of NASA]
who cares?
Who cares? You do – you just may not fully realize it yet. Currently, we’re releasing far more CO2 into the atmosphere than is being removed. 1.8 Billion Goodyear blimps filled with anything is a lot. The CO2 in the atmosphere will soon reach high enough levels to do serious permanent damage to the earth and its inhabitant (that includes humans). You wouldn’t give your child or grandchild a warm coke and let them deal with the sticky mess. So why would you give them a messy planet to clean up?