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Apr. 27th, 2009 02:12 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
etumukutenyakAn Online Guide to Carbon Cycles and Climate Change
CO2 in the Oceans
What exactly happens to the carbon dioxide introduced by burning wood, coal and hydrocarbons? This question is easy to ask but actually difficult to answer. At some point, some fifty years ago, it was thought that the buildup of carbon dioxide in the atmosphere would be rather slow because there is 50 times more carbon dioxide in the ocean than in the air. Scientists expected that the ocean and the atmosphere would share the newly introduced carbon dioxide, and that by far the greater proportion would be accommodated by the ocean, with its much larger capacity. Although this is not an unreasonable expectation, the fact to keep in mind is that carbon dioxide is first introduced into the atmosphere and from there it has to be passed along to the ocean. Carbon dioxide can only enter the ocean at its surface since the deep ocean is not immediately accessible. To fully reach the deep ocean takes a molecule of carbon dioxide about a thousand years. On the other hand, the uppermost ocean layer, called the “mixed layer,” can be accessed on a 10-year time-scale.
Carbon Cycle
These facts were first established by measuring the radiocarbon content of the ocean, from surface waters to great depths, by Hans Suess (1909-1989) at UCSD. Radiocarbon (14C) is made in the atmosphere, from bombardment of nitrogen atoms with cosmic rays. This radiocarbon reacts to make up a certain percentage of the carbon dioxide in our atmosphere. Half of a given number of radiocarbon atoms decay back to nitrogen within 6000 years. Radiocarbon in the ocean can only come from the atmosphere. Since it constantly disappears by decay, the amount of radiocarbon within the ocean depends on how fast the carbon dioxide from the atmosphere penetrates into the ocean and replenishes the store of radiocarbon there (See Glossary to learn more about Hans Suess and Radiocarbon.).
The fact that much of the ocean is not readily available to take up the additional carbon dioxide introduced through human activities raised a warning flag in the scientific community. However, another factor of equal importance emerged when considering how carbon dioxide enters the ocean. Seawater is somewhat reluctant to take on more carbon dioxide than it already has. The reason is that carbon dioxide reacts with water to make carbonic acid. In turn, the presence of this acid tends to discourage acceptance of additional carbon dioxide. This back-pressure effect has the consequence that the ability of the water to hold carbon dioxide increases by only about 1% for each 10% increase of the gas in the atmosphere. This ratio is widely known as the “Revelle buffer factor,” after Dr. Roger Revelle (1909-1991) who drew attention to this complication in the mid-1950.
There is one way to decrease this carbon dioxide back-pressure and that is to change the ocean’s chemistry by dissolving calcium carbonate. Carbonate is plentiful in the shells of marine organisms, including coral reef materials. This has led a number of scientists to raise the yet unanswered question of whether the continued addition of carbon dioxide to the atmosphere (which adds acid to the upper ocean) will eventually result in damage to coral reefs.
A graph of Dr. Keeling’s now famous curve of increasing CO2 concentration. The measurements are made at a station on top of Mauna Loa in Hawaii. Note carefully the magnitude of the increase from 1958 until present. We’ll discuss the seasonal variations (the squiggles) in Chapter 4. To link to the scientific article by Dr. Keeling and Dr. Whorf go to : Carbon Dioxide Information Analysis Center
The rise of carbon dioxide gas in our atmosphere has been measured continuously since 1958 and follows an oscillating line known as the "Keeling Curve," named after Dr. Charles David Keeling, professor at Scripps Institution of Oceanography. A renowned expert on the way carbon cycles itself on our planet, Keeling was the first to measure carbon dioxide in the atmosphere. He demonstrated its annual fluctuations (the little squiggles in the curve) and was the first to report that global atmospheric concentrations of carbon dioxide were rising. The concentration of carbon dioxide is given in units of parts per million by volume, also abbreviated ppmv. (For the more scientifically inclined: ppmv is the same as what chemists call the “mixing ratio” of a mixed gas, in this case the ratio of carbon dioxide molecules with all other air molecules, because equal volumes of gas at equal pressure hold equal numbers of molecules). Before the industrial era, circa 1800, atmospheric CO2 concentration was between 275 and 280 ppmv for several thousand of years (that is, between 275 and 280 molecules of CO2 for every one million molecules in the air); this we know from the composition of ancient air trapped in polar ice. Carbon dioxide has risen continuously since then, and the average value when Dr. Keeling took his measurements in 1958 was near 315 ppmv. By the year 2000 it has risen to about 367 ppmv (that is 367 molecules of CO2 for every one million molecules in the air). Thus, it is higher than pre-industrial values by one third of the pre-industrial era.
Reservoirs of Carbon The amount of carbon in the atmosphere is surprisingly small. What keeps it at a low level? Why is carbon dioxide a trace gas (about 367 ppmv) rather than making up most of the atmosphere, as is the case for the sibling planets of Earth, Venus and Mars? To tackle these questions, we first need a little background.
Sizes of reservoirs are given in mass units. For example, the atmospheric reservoir of carbon (mostly in the form of carbon dioxide) is about 750 GtC (Gigatonnes of carbon – see the glossary of scientific units for further clarification). The ocean is near 40,000 GtC; the biosphere is near 610 GtC; and, depending on how it is defined, soil is almost 1600 GtC. We can immediately see that the ocean is extremely important in the study of atmospheric carbon dioxide since it is so large a reservoir and is in intimate contact with the air.
Also, when considering there is about 5000 GtC in the form of fossil fuels ready to be burned, we immediately realize that the atmosphere could be easily overwhelmed by all the carbon available for industrial use. Also we realize that planting trees, while a good thing, could not solve the problem of carbon emissions for long. While the biosphere (mostly trees) has roughly the same mass as the atmosphere, doubling the mass of trees would help out with about 10 percent of the potential problem. Doubling the mass of trees, of course, would be a major undertaking in itself, especially when considering that deforestation is occurring at a rapid pace in the tropics.
An important point in this scheme of reservoirs and fluxes is that they differ greatly in size and in their ability to respond to changes, a property called “reactivity.” Large reservoirs with small fluxes in and out (called “input” and “output”) are not very reactive. Small reservoirs with relatively large fluxes in and out are very reactive - as far as carbon is concerned, the atmosphere is such a Reservoir. Fortunately, the atmosphere is closely coupled to the ocean, a large Reservoir that can offset this problem and stabilize the atmosphere. Unfortunately, the atmosphere's dependency on the ocean Reservoir has a drawback: if the ocean reacts to climate change by giving off a small proportion of its carbon dioxide, the atmosphere, with its low concentrations of carbon dioxide, greatly amplifies the effect. In other words, what seems a small adjustment for the ocean results in a big change in the atmosphere.
CO2 in the Oceans
What exactly happens to the carbon dioxide introduced by burning wood, coal and hydrocarbons? This question is easy to ask but actually difficult to answer. At some point, some fifty years ago, it was thought that the buildup of carbon dioxide in the atmosphere would be rather slow because there is 50 times more carbon dioxide in the ocean than in the air. Scientists expected that the ocean and the atmosphere would share the newly introduced carbon dioxide, and that by far the greater proportion would be accommodated by the ocean, with its much larger capacity. Although this is not an unreasonable expectation, the fact to keep in mind is that carbon dioxide is first introduced into the atmosphere and from there it has to be passed along to the ocean. Carbon dioxide can only enter the ocean at its surface since the deep ocean is not immediately accessible. To fully reach the deep ocean takes a molecule of carbon dioxide about a thousand years. On the other hand, the uppermost ocean layer, called the “mixed layer,” can be accessed on a 10-year time-scale.
Carbon Cycle
These facts were first established by measuring the radiocarbon content of the ocean, from surface waters to great depths, by Hans Suess (1909-1989) at UCSD. Radiocarbon (14C) is made in the atmosphere, from bombardment of nitrogen atoms with cosmic rays. This radiocarbon reacts to make up a certain percentage of the carbon dioxide in our atmosphere. Half of a given number of radiocarbon atoms decay back to nitrogen within 6000 years. Radiocarbon in the ocean can only come from the atmosphere. Since it constantly disappears by decay, the amount of radiocarbon within the ocean depends on how fast the carbon dioxide from the atmosphere penetrates into the ocean and replenishes the store of radiocarbon there (See Glossary to learn more about Hans Suess and Radiocarbon.).
The fact that much of the ocean is not readily available to take up the additional carbon dioxide introduced through human activities raised a warning flag in the scientific community. However, another factor of equal importance emerged when considering how carbon dioxide enters the ocean. Seawater is somewhat reluctant to take on more carbon dioxide than it already has. The reason is that carbon dioxide reacts with water to make carbonic acid. In turn, the presence of this acid tends to discourage acceptance of additional carbon dioxide. This back-pressure effect has the consequence that the ability of the water to hold carbon dioxide increases by only about 1% for each 10% increase of the gas in the atmosphere. This ratio is widely known as the “Revelle buffer factor,” after Dr. Roger Revelle (1909-1991) who drew attention to this complication in the mid-1950.
There is one way to decrease this carbon dioxide back-pressure and that is to change the ocean’s chemistry by dissolving calcium carbonate. Carbonate is plentiful in the shells of marine organisms, including coral reef materials. This has led a number of scientists to raise the yet unanswered question of whether the continued addition of carbon dioxide to the atmosphere (which adds acid to the upper ocean) will eventually result in damage to coral reefs.
A graph of Dr. Keeling’s now famous curve of increasing CO2 concentration. The measurements are made at a station on top of Mauna Loa in Hawaii. Note carefully the magnitude of the increase from 1958 until present. We’ll discuss the seasonal variations (the squiggles) in Chapter 4. To link to the scientific article by Dr. Keeling and Dr. Whorf go to : Carbon Dioxide Information Analysis Center
The rise of carbon dioxide gas in our atmosphere has been measured continuously since 1958 and follows an oscillating line known as the "Keeling Curve," named after Dr. Charles David Keeling, professor at Scripps Institution of Oceanography. A renowned expert on the way carbon cycles itself on our planet, Keeling was the first to measure carbon dioxide in the atmosphere. He demonstrated its annual fluctuations (the little squiggles in the curve) and was the first to report that global atmospheric concentrations of carbon dioxide were rising. The concentration of carbon dioxide is given in units of parts per million by volume, also abbreviated ppmv. (For the more scientifically inclined: ppmv is the same as what chemists call the “mixing ratio” of a mixed gas, in this case the ratio of carbon dioxide molecules with all other air molecules, because equal volumes of gas at equal pressure hold equal numbers of molecules). Before the industrial era, circa 1800, atmospheric CO2 concentration was between 275 and 280 ppmv for several thousand of years (that is, between 275 and 280 molecules of CO2 for every one million molecules in the air); this we know from the composition of ancient air trapped in polar ice. Carbon dioxide has risen continuously since then, and the average value when Dr. Keeling took his measurements in 1958 was near 315 ppmv. By the year 2000 it has risen to about 367 ppmv (that is 367 molecules of CO2 for every one million molecules in the air). Thus, it is higher than pre-industrial values by one third of the pre-industrial era.
Reservoirs of Carbon The amount of carbon in the atmosphere is surprisingly small. What keeps it at a low level? Why is carbon dioxide a trace gas (about 367 ppmv) rather than making up most of the atmosphere, as is the case for the sibling planets of Earth, Venus and Mars? To tackle these questions, we first need a little background.
Sizes of reservoirs are given in mass units. For example, the atmospheric reservoir of carbon (mostly in the form of carbon dioxide) is about 750 GtC (Gigatonnes of carbon – see the glossary of scientific units for further clarification). The ocean is near 40,000 GtC; the biosphere is near 610 GtC; and, depending on how it is defined, soil is almost 1600 GtC. We can immediately see that the ocean is extremely important in the study of atmospheric carbon dioxide since it is so large a reservoir and is in intimate contact with the air.
Also, when considering there is about 5000 GtC in the form of fossil fuels ready to be burned, we immediately realize that the atmosphere could be easily overwhelmed by all the carbon available for industrial use. Also we realize that planting trees, while a good thing, could not solve the problem of carbon emissions for long. While the biosphere (mostly trees) has roughly the same mass as the atmosphere, doubling the mass of trees would help out with about 10 percent of the potential problem. Doubling the mass of trees, of course, would be a major undertaking in itself, especially when considering that deforestation is occurring at a rapid pace in the tropics.
An important point in this scheme of reservoirs and fluxes is that they differ greatly in size and in their ability to respond to changes, a property called “reactivity.” Large reservoirs with small fluxes in and out (called “input” and “output”) are not very reactive. Small reservoirs with relatively large fluxes in and out are very reactive - as far as carbon is concerned, the atmosphere is such a Reservoir. Fortunately, the atmosphere is closely coupled to the ocean, a large Reservoir that can offset this problem and stabilize the atmosphere. Unfortunately, the atmosphere's dependency on the ocean Reservoir has a drawback: if the ocean reacts to climate change by giving off a small proportion of its carbon dioxide, the atmosphere, with its low concentrations of carbon dioxide, greatly amplifies the effect. In other words, what seems a small adjustment for the ocean results in a big change in the atmosphere.
