Terry M. Bella
The carbon cycle can appear as daunting, but ultimately it can be compartmentalized and efficiently delivered through classroom instruction quite easily. If one uses the approach of first addressing where the carbon is and then discusses how carbon moves from store to store it is readily understood by students. Students can pull from their prior knowledge of the water cycle, content that is learned in middle school, and apply their understanding of cycles and conservation of matter to the carbon cycle. The important facet of teaching about the carbon cycle is working with students to help them understand that the cycle has been going on for millennia but that humans have been impacting the cycle in a substantial way for the last 50 years. With the background knowledge covered about where the carbon is and what natural processes are cycling carbon students can then begin to work with anthropogenic processes that move carbon. The important learning objective resides in students making connections between natural cycle processes and possible implications of human manipulation of the cycle through activities that upset the balance of carbon flow.
Human activity affects the carbon cycle in a few key areas, all of which fall within the realm of carbon release into the atmosphere. We release carbon by burning forests, burning fossil fuels, and production of cement. It is important to note that burning of forests is considered carbon neutral because the carbon sequestered in plants is considered short term storage. Moving carbon from terrestrial ecosystems through burning and from the Earth’s crust by burning fossil fuels are likely prior knowledge for students, but cement making is not something that will have been covered in school previously. Secondly, we have to address the consequences of these activities and how they are compounding the problem of carbon release with acid rain production possible climate change.
Whether it is combustion of wood, oil, coal, or natural gas, carbon is liberated to release energy and carbon dioxide is formed. A combustion reaction, appropriate for a freshmen science class, is as follows:
Carbon Fuel Source (Oil) + Oxygen (O
2
) + Fire -> Water (H
2
O) + Carbon Dioxide (CO
2
) + Heat Energy
Carbon Fuel Source (Natural Gas: CH
4
) + Oxygen (O
2
) + Fire -> Water (H
2
O) + Carbon Dioxide (CO
2
) + Heat Energy
The carbon fuel source may be a fossil fuel or wood, but the end result is the same in complete combustion, carbon dioxide production. Take note though that combustion of natural gas, methane, produces half the carbon dioxide of coal or oil combustion per unit of energy yielded. The carbon dioxide is a gas and once in the atmosphere it will take part in the natural carbon cycle again. The question is whether or not natural processes can accommodate the excess of carbon dioxide in the air. It seems that the answer to that question is “no”. Atmospheric carbon dioxide has been rising year after year. Readings of carbon dioxide taken since the 1960’s show that carbon dioxide levels have risen from 320ppm to almost 400ppm. There is evidence in the oldest Antarctic ice cores that carbon dioxide levels have not been this high in 800,000 years. We do not currently have a completely accurate way of predicting what the implications will be. Students need not be overwhelmed with theories about what may happen but should rather develop an understanding about how the natural cycle is being disturbed. Human activity moves carbon from stores to the atmosphere faster than the cycle can move that carbon back into stores.
Global cement production is the other key human activity that releases carbon dioxide. Limestone and other calcium carbonate rich rocks are used to produce cement through a process that makes lime (CaO). Though a significant amount of carbon dioxide is released in the energy intensive process of cement making by burning fossil fuels, the formation of lime also releases carbon dioxide.
According the Carbon Dioxide Information Analysis Center of Oak Ridge National Laboratory in Tennessee, human caused release of carbon for 2011 was 9.449GT. The release of carbon is accounted for as follows: 1.7GT from burning of gas hydrocarbons; 3.317GT from burning of liquid hydrocarbons; 3.997GT from burning of solid hydrocarbons; 0.491GT from cement production; and 0.063GT from gas flaring. Gas flaring is essentially the burning of combustible gases, by petroleum production plants or other industrial plants, that are deemed unusable by-products or waste products.
