The Carbon Cycle

No discussion about the Earth is complete without taking a look at the topic of Global Warming. It seems to be of such great concern, that only this past spring has a national council of pastors convened to raise awareness of the increased amount of greenhouse gases and the observed rise in global temperatures with a result potentially disastrous rise in sea levels. Indeed, my aging Uncle Bill is truly concerned that the entire state of Florida might find itself underwater if predictions come to pass. To understand global warming, you must first understand how carbon is recycled and what greenhouses gases are.

The concentration of carbon in living matter (18%) is almost 100 times greater than its concentration in the earth (0.19%). So living things continually extract carbon from their nonliving environment and return it to that nonliving environment throughout the natural process of life and death. For life to continue, this carbon must be recycled. It is the recycling of carbon on a global scale that makes life on this planet possible. At present, there is not another place in the Universe where carbon is recycled because Earth is the only place where geological process that are responsible for this recycling happen.

Carbon exists in the nonliving environment as: a) carbon dioxide (CO2) in the atmosphere and dissolved in water (forming HCO3-), b) carbonate rocks (limestone and coral = CaCO3), c) deposits of coal, petroleum, and natural gas derived from once-living things, and d) dead organic matter, e.g., humus in the soil

Carbon enters the biotic world through the action of autotrophs: primarily photoautotrophs, like plants and algae, and to a small extent, chemoautotrophic bacteria. Since this lesson is focused for Astronomy students more than AP Biology students, a few definitions here might be helpful. Photoautotrophs are organisms that take energy from the Sun and carbon from atmospheric carbon dioxide fixed carbon dioxide into glucose. Chemoautotrophs are bacteria that get their energy from chemical reactions and their carbon from atmospheric carbon dioxide. Humans, and all other multicellular animals are chemoheterotrophs ... meaning we get ourenergy from chemical reactions and out carbon from consuming organic foods.

Carbon returns to the atmosphere and water in large quantities at plate tectonic spreading centers and to a somewhat lesser extent when volcanoes erupt. the interplay of carbon and the living world finds carbon returning to the atmosphere from: 1) respiration (as CO2), 2) burning, and 3) decay (producing CO2 if oxygen is present, methane (CH4) if it is not.

From a geologic standpoint, carbon also leaves the botic world when it is removed from the atmosphere by dissolving in water and forming carbonic acid

CO2 + H2O --> H2CO3 (carbonic acid)

Carbonic acid is used to weather rocks, yielding bicarbonate ions, other ions, and clays

H2CO3 + H2O + silicate minerals -> HCO3- + cations (Ca++, Fe++, Na+, etc.) + clays

Calcium carbonate is precipitated from calcium and bicarbonate ions in seawater by marine organisms like coral

Ca++ + 2HCO3- -> CaCO3 + CO2 + H2O and thus, the carbon is now stored on the seafloor in layers of limestone. Thinking this through, every time it rains some of the Earth's supply of carbon dioxide is lost to a seemingly permanent place in rocks. Were it not for seafloor spreading and volcanism, the Earthh would have depleted its entire available supply of carbon dioxide a long time ago ... trapping it forever in rock formations.

Fortunately, some of this carbon is returned to the atmosphere via metamorphism of limestone at depth in subduction zones or in orogenic belts

CaCO3 + SiO2 -> CO2 + CaSiO3 followed by outgassing at the Ring of Fire.

It is precisely this global recycling of carbon dioxide from the activity of Earth's plate tectonics that returns carbon dioxide to the atmosphere so that plants can convert it into glucose and release oxygen as a by-product of photosynthesis. WITHOUT THE GLOBAL RECYCLING OF CARBON DIOXIDE, LIFE HERE ON EARTH WOULD SURELY END, AND WITHOUT ACTIVE PLATE TECTONICS, A MECHANISM TO RETURN CARBON THAT HAS BEEN TRAPPED AS CARBONATE IN THE ROCK, LIFE WOULD BE IMPOSSIBLE. EARTH IS THE ONLY PLANET OR MOON AMONGTHE 153 PRESENTLY NAMED MAJOR SOLAR SYSTEM BODIES THAT POSSESSES ACTIVE TECTONIC PLATES.

The Climate Buffer

Because of the role of CO2 in climate, feedbacks in the carbon cycle act to maintain global temperatures within certain bounds so that the climate never gets too hot or too cold to support life on Earth. The process is a large-scale example of LeChatelier's Principle. This chemical principle states that if a reaction at equilibrium is perturbed by the addition or removal of a product or reactant, the reaction will adjust so as to attempt to bring that chemical species back to its original concentration. For example, as carbonic acid is removed from solution by weathering of rocks, the reaction will adjust by producing more carbonic acid. And since the dissolved CO2 is in equilibrium with atmospheric CO2, more CO2 is removed from the atmosphere to replace that removed from solution by weathering.

some examples:

If CO2 concentration increases in the atmosphere because of an increased rate of outgassing, global temperature will rise. Rising temperature and more dissolved CO2 will lead to increased weathering of crustal rocks as a result of faster reaction rates (temperature effect) and greater acidity. Enhanced weathering will use up the excess CO2 thereby cooling the climate.

If global temperature cools as a result of some astronomical forcing or tectonic/ocean circulation effect, the lower temperatures will result in lower rates of chemical weathering. Decreased weathering means less CO2 being drawn from the atmosphere by weathering reactions, leaving more CO2 in the atmosphere to increase temperatures.

If more rocks become available for rapid weathering as a result of mountain uplift the enhanced weathering will draw down atmospheric CO2 and decrease global temperatures. But the decreased temperatures will slow reaction rates, thereby using less CO2, thus allowing temperatures to moderate.

Presently, the uptake and return of CO2 are not in balance.

The carbon dioxide content of the atmosphere is gradually and steadily increasing. The graph below shows the CO2 concentration at the summit of Mauna Loa in Hawaii from 1958 through 1999. The values are in parts per million (ppm). The seasonal fluctuation is caused by the increased uptake of CO2 by plants in the summer.

The increase in CO2 probably began with the start of the industrial revolution. Samples of air trapped over the centuries in the glacial ice of Greenland show no change in CO2 content until 300 years ago. Since measurements of atmospheric CO2 began late in the nineteenth century, its concentration has risen over 20%. This increase is surely "anthropogenic"; that is, caused by human activities such as burning fossil fuels (coal, oil, natural gas) which returns to the atmosphere carbon that has been locked within the earth for millions of years. Additionally, South American nations have taken to clearing and burning of forests, especially in the tropics. In recent decades, large areas of the Amazon rain forest have been cleared for agriculture and cattle grazing.

Where is the missing carbon?

Curiously, the increase in atmospheric CO2 is only about one-half of what would have been expected from the amount of fossil fuel consumption and forest burning.

Where has the rest gone?

Research has shown that increased CO2 levels lead to increased net production by photoautotrophs. There is some evidence that the missing CO2 has been incorporated by
increased growth of forests, especially in North America and increased amounts of phytoplankton in the oceans. This was mentioned earlier in the Climate Buffer section.

The Greenhouse Effect and Global Warming

Despite these "sinks" for our greatly increased CO2 production, the concentration of atmospheric CO2 continues to rise? Should we be worried?

Carbon dioxide is transparent to light but rather opaque to heat rays. Therefore, CO2 in the atmosphere retards the radiation of heat from the earth back into space — the "greenhouse effect".

Has the increase in carbon dioxide led to global warming?

Average temperatures do seem to have increased slightly (~0.6°C) in the last century.

Some evidence:

Careful monitoring of both ocean and land temperatures.
Many glaciers and ice sheets are receding.
Woody shrubs are now growing in areas of northern Alaska that 50 years ago were barren tundra.
Many angiosperms in temperate climates are flowering earlier in the spring than they used to.
Many species of birds and butterflies are moving north and breeding earlier in the spring.
Will continued increase in carbon dioxide lead to more global warming and, if so, how much?

At this point, the answer depends on what assumptions you plug into your computer models. But as the different models have been improved, they seem to be converging on a consensus: a doubling of the CO2 concentration (expected by the end of this century) will cause the earth to warm somewhere in the range of 2.5–3.5°C.

While we worry about possible global warming from the additional CO2 we put into the atmosphere by burning fossil fuels, if there was no CO2 in the atmosphere the global climate would be significantly cooler. While I clearly agree with the research in the science literature that the global climate is warming, there is ample evidence of repeated episodes of warming and cooling to ice ages in the past. It is my contention that more is at play with the current warming trend than merely human factors and cows passing gas in the fields. As an astronomer, I believe scientists must take into account the vagaries of the Sunspot Cycle, potential slight changes in the Earth's orbit (Milankovitch Theory), and unusual changes in plate tectonic actitivy as well as mankind's contribution. More information and other links can be found at the USGS Carbon Cycle page.

GLOBAL WARMING PAGE

I have prepared a special section on the topic of Global Warming ... it is still under construction, but it is a central topic in any course on Earth Science. Click on the title above to learn more details.

Other Greenhouse Gases

Although their levels in the atmosphere are much lower than that of CO2, methane (CH4) and chlorofluorocarbons (CFCs) are also potent greenhouse gases. Methane ("marsh gas") is released by natural processes (e.g. from decay occurring in swamps), but human activities may now account for over one-half of the total. Some examples of human activites that raise global methane is the growing of rice in paddies and the burning of forests. On a somewhat humorous note, raising cattle (fermentation in their rumens produces methane that is expelled) collectively adds an estimated 100 million tons a year to the atmosphere. But estimates can be wrong. In 1990, the U.S. Environmental Protection Agency estimated that rice paddies were also adding about 100 million tons a year; accurate measurements later showed that this estimate was too high. And to add to the uncertainty, the discovery that plants naturally release methane to the atmosphere was reported in 2006. This previously-unrecognized source may account for 10–30% of the total.

So while the burning of the tropical rain forest adds to the atmospheric methane budget by incomplete combustion during burning and release from the GI tract of the cattle that are later placed on the cleared land, some of this may be offset by the reduction in the natural production by the trees removed from the forest. The methane concentration in the air is presently some 1.8 parts per million (ppm) and is growing at a rate of 1% per year. Although this concentration is far less than that of CO2, methane is 30 times as potent a greenhouse gas and so may now be responsible for 15–20% of the predicted global warming.

The marked warming of the earth that occurred at the end of the Paleocene epoch is thought to have been caused by the release of large amounts of methane from the sea floor. Today, geologists are discovering "fire ice" which is frozen methane crystals in deep ocean sediments. Their existence may promise a new resource for an energy-hungry world, but the substance is exceedingly volatile. Perhaps large quantities of this form of methane erupted at the end of the Paleocene!

Chlorofluorocarbons (CFCs) are synthetic gases in which the hydrogen atoms of methane are replaced by atoms of fluorine and chlorine (e.g., CHF2Cl, CFCl3, CF2Cl2).These gases are noninflammable, nontoxic, and very stable. They are widely used in industry as refrigerants (e.g., in refrigerators and air conditioners),
solvents, propellants in aerosol cans (now banned in some countries), and in the manufacture of plastic foams. They escape to the air from all of these uses (e.g., from leaky and discarded refrigeration units). Their chemical inertness, which makes CFCs so desirable for industry, also makes them a threat to the atmosphere. Once in the atmosphere, it may take 60–100 years for them to decompose and disappear. In the meantime, they may contribute to as much as 25% of the greenhouse effect. But perhaps even more worrisome is the threat they pose to the ozone shield. At the 1997 Nobel Conference at Gustavus College in St Peter, MN I heard a lecture from Sherry Rowland about CFCs. He stated that a single CFC molecule can degrade 300,000 molecules of Ozone. To learn more of this threat, read below.

The Ozone Problems

Ozone is a highly active form of oxygen (O3 rather than O2). Ozone is made when a electric spark passes through air, and this accounts for the characteristic odor give off by some electrical motors. Ozone presents two quite different biological problems: too much at low levels of the atmosphere (the troposphere); too little at high altitudes (the stratosphere).

Ozone in the Troposphere
Ozone is produced by the reaction of sunlight, oxygen, and automobile exhaust (which contains hydrocarbons and nitrogen oxides). Ozone is largely responsible for the discomfort associated with photochemical smog. This form of smog, long familiar to people in the Los Angeles basin, is now common wherever sunlight and stagnant air occur in urban areas (Mexico City is a dramatic example with ozone levels that often exceed 100 ppb and sometimes rise above 350 ppb). High levels of ozone during smog build-up can cause difficulty to people with respiratory ailments like emphysema and asthma. Ozone also damages plants and may be an important factor in the damage that is occurring to forests in Europe and North America.

Ozone in the Stratosphere

While we often have too much ozone around us, the concentration of ozone high in the stratosphere (which begins about 7 miles up - where airliners cruise) has declined over the past two decades. Satellite monitoring of the stratosphere, which began in 1978, has revealed a marked decline. The most serious decline occurs over Antarctica in spring (October) when a precipitous drop in ozone causes an ozone hole. The figure (courtesy of NASA) shows a map of the ozone hole measured over Antarctica on 5 October 1987 by a device carried on the Nimbus 7 satellite. The tips of South America (upper right quadrant ) and Africa (lower right) are drawn in, as is the outline of Australia and New Zealand (lower left). The Dobson unit is a measure of the number of molecules of ozone in a vertical column of the atmosphere. You can see that the concentration of ozone decreases in ever-smaller concentric circles with the lowest reading centered over the South Pole.

The Ozone Shield
The spreading of this ozone-depleted air may account for the more gradual and more protracted declines that are being seen at midlatitudes. From 1978-1990, average ozone levels declined 8% over Europe and about 5% over the United States. This is ominous because ozone shields the earth's surface from much of the ultraviolet radiation reaching the earth from the sun. Ultraviolet rays can cause skin cancer, cataracts, and may depress the immune system. The graph (from C. R. Roy, et. al., in Nature 347:235, 1990) shows measurements of the intensity of ultraviolet light and the concentration of ozone on several sunny days in Melbourne, Australia during December 1987 and January 1988. When ozone levels were low, ultraviolet light was more intense and vice versa. The drop in ozone, which lasted about a month, was probably caused by ozone-depleted air drifting in from the ozone hole over the South Pole. Most of the ultraviolet light that reaches the earth is ultraviolet "B" (UVB), which includes wavelengths from 290 to 320 nm.


Chlorofluorocarbons (CFCs)
Although some of the recent depletion of ozone in the stratosphere was probably due to natural causes (volcanic eruptions, fewer sunspots), some is most likely caused by manmade chlorofluorocarbons (CFCs). These gases escape from such sources as aerosol spray cans, leaky or discarded refrigeration units, and a variety of industrial processes. The U.S., Canada, and the Scandinavian countries stopped using CFCs in aerosol cans over a decade ago, but this and other uses of CFCs have continued to grow worldwide. However, a multi-nation agreement drawn up in 1987 established a schedule for reducing the use of these materials.

And, in fact, monitoring shows that the concentration of CFCs in the stratosphere has been decreasing since the mid-90s.

You are ready to study the water cycle or go to the Earth's atmosphere, weather, and recycling of biogeochemicals, you are asked to move to a study of the Seasons before going on to the Moon, or you can either move return to the Earth Introduction.


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