Processes of Carbon Flow in the Human Realm
Humans have exerted an enormous influence on the global carbon cycle, largely through deforestation and fossil fuel burning. In this section, we explore how these processes have led to changes in the dynamics of carbon in the atmosphere.
Fossil Fuel Burning
Another pathway for carbon to move from the sedimentary rock reservoir to the atmosphere is through the burning of fossil fuels by humans. Fossil fuels include petroleum, natural gas, and coal, all of which are produced by slow transformation of organic carbon deposited in sedimentary rocks — essentially the fossilized remains of marine and land plants. In general, this transformation takes many millions of years; most of the oil and gas we now extract from sedimentary rocks is on the order of 70-100 million years old. New fossil fuels take a very long time to form, and we are using them up much, much faster than they are being formed, meaning that if we keep using fossil fuels at the rate we are today, we will run out! This run out date depends on new discoveries and our ability to extract fuels more efficiently by processes like fracking, but we will be close to running out late this century.
These fossil fuels are primarily composed of carbon and hydrogen. For instance, methane, the main component of natural gas, has a chemical formula of CH4; petroleum is a more complex compound, but it, too, involves carbon and hydrogen (along with nitrogen, sulfur, and other impurities). The combustion of fossils fuels involves the use of oxygen and the release of carbon dioxide and water, as represented by the following description of burning natural gas:
CH4 + 2O2 => CO2 + 2H2O
Beginning with the onset of the industrial revolution at the end of the last century, humans have been burning increasing quantities of fossil fuels as our primary energy source.
As a consequence, the amount of CO2 emitted from this burning has undergone an exponential rise that follows the exponential rise in the human population. The magnitude of this flow is currently about 9 Gt C/yr. This number also includes the CO2 generated in the production of cement, where limestone is burned, liberating CO2.
As you can see in the graph above, this flow has changed considerably over time, as human population has increased and as our economies have become more industrialized with a big thirst for the energy provided from the combustion of fossil fuels. The model we will work within the lab activity for this module includes this history, beginning in 1880 and going up to 2010; beyond 2010 is the realm of future projections, which can be altered to explore the consequences of choices we might make or not make in the future. Part of the new energy economy that is key to our future is the use of so-called renewable energy sources, including wind, solar and geothermal energy, that emit little or no carbon. There are some interesting map view representations of this history of fossil fuel carbon emissions in the video below:
Video: Annual Fossil-Fuel CO2 Emissions (2:03) This video is not narrated.
If the video does not play, you can view it at YouTube: Annual Fossil-Fuel CO2 Emissions
Land-Use Changes - Forest Burning and Soil Disruption
The other form of human alteration of the global carbon cycle is through forest cutting and burning and the disruption of soils associated with agriculture. When deforestation occurs, most of the plant matter is either left to decompose on the ground, or it is burned, the latter being the more common occurrence. This process reduces the size (the mass) of the land biota reservoir, and the burning adds carbon to the atmosphere. Land-use changes other than deforestation can also add carbon to the atmosphere. Agriculture, for instance, involves tilling the soil, which leads to very rapid decomposition and oxidation of soil organic matter. This means that in terms of a system, we are talking about two separate flows here — one draining the land biota reservoir, the other draining the soil reservoir; both flows transfer carbon to the atmosphere. Current estimates place the total addition to the atmosphere from forest burning and soil disruption at around 2-3 Gt C/yr; estimates divide this into 70% to 50% forest burning, with soil disruption making up the remainder.
The actual history of this alteration to the natural carbon cycle is not well-constrained — not nearly as well known as the fossil fuel burning history — but we include a reasonable history that reflects patterns of land settlement and forest clearing.