INTRODUCTION

The carbon cycle is one of several global geochemical cycles that determine conditions on Earth. It is an essential component of the climate system because the amount of carbon dioxide in the atmosphere – the primary driver of current global warming – is basically controlled by the flow of carbon between it and the ocean and biosphere. Furthermore, by maintaining a long-term balance in the amounts of carbon in the atmosphere, ocean, and solid Earth, the carbon cycle has acted as a long-term global thermostat, maintaining temperate surface conditions on Earth throughout nearly its entire history and providing a hospitable environment for life to begin and evolve.

However, recent scientific evidence indicates that humans have upset the carbon cycle, leaving more carbon dioxide and other greenhouse gases in the atmosphere. Burning fossil fuels such as oil and natural gas, burning biomass such as ethanol and wood, creating cement and releasing other chemicals into the atmosphere is upsetting the natural flow of carbon. For example, when you burn plant material such as a log the burning releases carbon(in the form of carbon dioxide) stored in the log as carbon dioxide. This process of burning biomass causes an overall flow of carbon from the biosphere into the atmosphere. Burning fossil fuels like gasoline, diesel and coal also releases carbon into the atmosphere. Every time you drive your gas car you are burning carbon that was stored underground for millions of years! Ever since the Industrial Revolution, man has been, knowingly or unknowingly, tampering with the carbon cycle.

Carbon dioxide is one of the primary greenhouse gases in the Earth’s atmosphere. Greenhouse gases in the atmosphere act almost like a blanket between the earth and frigid space. At lower levels, greenhouse gases trap some heat in our atmosphere while letting other heat radiate out into space. However, as the layer of greenhouse gases around our planet grows thicker more heat is trapped in our atmosphere, less radiates away and the earth slowly heats up. Therefore, with the ever increasing levels of carbon dioxide in the atmosphere, it is of paramount importance for humans to understand the carbon cycle, how they are tampering with it, and the potential damage it is causing to the environment.

EXPLANATION

Since the industrial revolution, human activity has modified the carbon cycle by changing its component’s functions and directly adding carbon to the atmosphere.

Burning of Fossil Fuels

The largest and most direct human influence on the carbon cycle is through direct emissions from burning fossil fuels, which transfers carbon from the geosphere into the atmosphere.

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

The graph below shows how the concentration of carbon dioxide in the atmosphere has increased over time. These measurements were made at Mauna Loa in Hawaii, where the atmopshere is well mixed as air blows across the Pacific. Measurements have been made at other sites around the world too, but the longest record we have is from Hawaii. Dr. Charles Keeling started making the monthly measurements (grey line) in 1958. The graph is known as the Keeling curve in his honor. The annual fluctuation in carbon dioxide is caused by seasonal change in the amount of carbon dioxide taken up by plants on land in the Northern Hemisphere. (NOAA data, graphed by the Global Warming Art Project)

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.

Effects of changing the Carbon Cycle

All of this extra carbon needs to go somewhere. So far, land plants and the ocean have taken up about 55 percent of the extra carbon people have put into the atmosphere while about 45 percent has stayed in the atmosphere. Eventually, the land and oceans will take up most of the extra carbon dioxide, but as much as 20 percent may remain in the atmosphere for many thousands of years.

The changes in the carbon cycle impact each reservoir. Excess carbon in the atmosphere warms the planet and helps plants on land grow more. Excess carbon in the ocean makes the water more acidic, putting marine life in danger.

1. Atmosphere: Carbon dioxide, methane, and halo-carbons are greenhouse gases that absorb a wide range of energy—including infrared energy (heat) emitted by the Earth—and then re-emit it. Rising carbon dioxide concentrations are already causing the planet to heat up. At the same time that greenhouse gases have been increasing, average global temperatures have risen 0.8 degrees Celsius (1.4 degrees Fahrenheit) since 1880. This rise in temperature isn’t all the warming we will see based on current carbon dioxide concentrations. Greenhouse warming doesn’t happen right away because the ocean soaks up heat. This means that Earth’s temperature will increase at least another 0.6 degrees Celsius (1 degree Fahrenheit) because of carbon dioxide already in the atmosphere. The degree to which temperatures go up beyond that depends in part on how much more carbon humans release into the atmosphere in the future.
2. Ocean: About 30 percent of the carbon dioxide that people have put into the atmosphere has diffused into the ocean through the direct chemical exchange. Dissolving carbon dioxide in the ocean creates carbonic acid, which increases the acidity of the water. Or rather, a slightly alkaline ocean becomes a little less alkaline. Since 1750, the pH of the ocean’s surface has dropped by 0.1, a 30 percent change in acidity. This can hugely affect marine organisms and their ecosystem.
3. Land: Carbon dioxide increases temperatures, extending the growing season and increasing humidity. Both factors have led to some additional plant growth. However, warmer temperatures also stress plants. With a longer, warmer growing season, plants need more water to survive. Scientists are already seeing evidence that plants in the Northern Hemisphere slow their growth in the summer because of warm temperatures and water shortages. The warming caused by rising greenhouse gases may also “bake” the soil, accelerating the rate at which carbon seeps out in some places. This is of particular concern in the far north, where frozen soil—permafrost—is thawing. Permafrost contains rich deposits of carbon from plant matter that has accumulated for thousands of years because the cold slows decay. When the soil warms, the organic matter decays and carbon—in the form of methane and carbon dioxide—seeps into the atmosphere.

REFERENCES

1. The Changing Carbon Cycle (UCAR Center for Science Education)
   http://scied.ucar.edu/longcontent/changing-carbon-cycle
2. Global Biogeochemical Cycles and the Physical Climate System (UCAR Center for Science Education)
   http://www.ucar.edu/communications/gcip/m4bgchem/m4html.html
3. The Carbon Cycle : Feature Articles (NASA)
   http://earthobservatory.nasa.gov/Features/CarbonCycle/
4. Archer, David: The Global Carbon Cycle.
   Princeton University Press, 2010.
5. Williams, Richard G., and Mick Follows: Ocean Dynamics and The Carbon Cycle: Principles and Mechanisms
   Cambridge University Press, 2011.