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Carbon is critical to sustain a huge range of Earth’s functions. It is abundant in the atmosphere (air), biosphere (living and dead organisms), hydrosphere (oceans, rivers, and lakes), and lithosphere (soli and rocks). These act as storage areas or reservoirs of carbon. Processes such as erosion, evaporation, photosynthesis, respiration, and decomposition constantly move carbon between these reservoirs. Carbon enters, is stored, and leaves the different spheres of the Earth through different methods, and in different quantities. The carbon cycle is the term used to describe the ways in which carbon moves between them.
The flow of carbon is now strongly influenced by human activity. The most important human impact on the carbon cycle is the burning of fossil fuels, which releases carbon dioxide (CO2) into the atmosphere.
Under natural conditions the release of carbon from fossil fuels occurs slowly, as they are subducted into the mantle, and CO2 is released through volcanic activity. However, humans are heavily reliant on fossil fuels, and extract it from the lithosphere in great quantities. Fossil fuels, which include petroleum, natural gas, and coal, are used in nearly every aspect of the global economy. Automobiles are the most visible example, but more carbon dioxide is actually produced by coal and natural gas plants which produce electricity for both industrial and residential use. Industrial agriculture also runs on fossil fuel energy. All artificial fertilizers are synthesized by a process which burns fossil fuels – usually natural gas. Burning coal, oil, natural gas and other fossil fuels removes the carbon from them and emits it as CO2 into the atmosphere.
Another important human impact lies in changing the Earth’s land covers such as clearing forests or abandoning agricultural areas – which can release or take up atmospheric CO2.
Large amounts of carbon are stored in living plants. Therefore, land use changes, especially the clearance of forests (which are very densely inhabited by plants, and therefore contain a large amount of carbon), can influence the carbon cycle in two ways. Firstly, the removal of vegetation eliminates plants which would otherwise be capturing carbon from the atmosphere through photosynthesis, which is the process by which plants and some bacteria use the energy of sunlight to build carbohydrates out of carbon dioxide. Secondly, as dense forests are replaced by crops/pasture land/built environments, there is usually a net decrease in the carbon store, as smaller plants (and worse, concrete) store far less carbon than large trees. Deforestation also allows much more soil to be eroded, and carbon stored in the soil is rapidly taken into rivers. Though some areas have been set aside as wildlife preserves, far more are vulnerable to burning and clear-cutting for the purposes of timber harvest and the clearing of agricultural land.
Because of the cyclical nature of the carbon cycle, the impacts humans cause can lead to a number of amplifications and feedbacks. The changes in the carbon cycle impact each reservoir.
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 main concern about increasing carbon dioxide levels comes from the fact that carbon dioxide is a greenhouse gas. Carbon dioxide, methane, and halocarbons are greenhouse gases that absorb a wide range of energy—including infrared energy (heat) emitted by the Earth—and then re-emit it. The re-emitted energy travels out in all directions, but some returns to Earth, where it heats the surface. This is known as the greenhouse effect. The UN’s International Panel on Climate Change, believes that humans are upsetting the carbon cycle enough to drastically change the global climate, with potentially huge consequences for biodiversity, agriculture, weather, and the overall health of every ecosystem on the planet.
Carbon dioxide molecules provide the initial greenhouse heating needed to maintain water vapor concentrations. When carbon dioxide concentrations drop, Earth cools, some water vapor falls out of the atmosphere, and the greenhouse warming caused by water vapor drops. Likewise, when carbon dioxide concentrations rise, air temperatures go up, and more water vapor evaporates into the atmosphere—which then amplifies greenhouse heating. Carbon dioxide is the gas that sets the temperature. Carbon dioxide controls the amount of water vapor in the atmosphere and thus the size of the greenhouse effect. 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.
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.
Ocean acidification affects marine organisms in two ways. First, carbonic acid reacts with carbonate ions in the water to form bicarbonate. However, those same carbonate ions are what shell-building animals like coral need to create calcium carbonate shells. With less carbonate available, the animals need to expend more energy to build their shells. As a result, the shells end up being thinner and more fragile.
Second, the more acidic water is, the better it dissolves calcium carbonate. In the long run, this reaction will allow the ocean to soak up excess carbon dioxide because more acidic water will dissolve more rock, release more carbonate ions, and increase the ocean’s capacity to absorb carbon dioxide. In the meantime, though, more acidic water will dissolve the carbonate shells of marine organisms, making them pitted and weak.
Warmer oceans—a product of the greenhouse effect—could also decrease the abundance of phytoplankton, which grow better in cool, nutrient-rich waters. This could limit the ocean’s ability to take carbon from the atmosphere through the fast carbon cycle.
Plants on land have taken up approximately 25 percent of the carbon dioxide that humans have put into the atmosphere. The amount of carbon that plants take up varies greatly from year to year, but in general, the world’s plants have increased the amount of carbon dioxide they absorb since 1960. Only some of this increase occurred as a direct result of fossil fuel emissions.
With more atmospheric carbon dioxide available to convert to plant matter in photosynthesis, plants were able to grow more. This increased growth is referred to as carbon fertilization. There is a limit to how much carbon plants can take out of the atmosphere, and that limit varies from region to region. So far, it appears that carbon dioxide fertilization increases plant growth until the plant reaches a limit in the amount of water or nitrogen available.
The biggest changes in the land carbon cycle are likely to come because of climate change. 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.
Dry, water-stressed plants are also more susceptible to fire and insects when growing seasons become longer. In the far north, where an increase in temperature has the greatest impact, the forests have already started to burn more, releasing carbon from the plants and the soil into the atmosphere. Tropical forests may also be extremely susceptible to drying. With less water, tropical trees slow their growth and take up less carbon, or die and release their stored carbon to the atmosphere.
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.
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