|1-4: En Español||1-4: Em Português|
Eugene S. Takle
The earth's atmosphere, as discussed in unit 1-2, is mostly
nitrogen and oxygen. These two constituents alone account for 99.03% of
all atmospheric "dry air" gases below 100 km (water vapor accounts for 0-4%
depending on latitude, altitude and weather conditions). Concentrations of
trace gases are given in Table 1. Water vapor, carbon dioxide, and ozone
are, to different degrees, variable in concentration, but all others are
One of the first indications that humans have altered the composition of the global atmosphere comes from the measurements of atmospheric carbon dioxide. Figure 1 shows that from 1973 to 1985, carbon dioxide concentrations, expressed in parts per million by volume, in the earth's atmosphere have increased from 320 to 350. More recent measurements now show values in excess of 360 parts per million. Figure 1 shows that the upward trend is independent of location and occurs in both the Northern and Southern Hemispheres. The main differences in the curves are the different amplitudes of the annual cycle. Careful examination shows that the concentration reaches a peak in April or May in the Northern Hemisphere and a minimum in July, corresponding to the absorption by plants during the growing season and the net release back to the atmosphere due to decaying vegetation outside the growing season. A look at the global vegetation maps for January and July will allow you to explain the different amplitudes at different locations on the earth. However, regardless of location, there is compelling evidence that the trend in concentrations is upward.
If we examine proxy data, we can get very good estimates of the carbon dioxide levels over longer periods of time. At the time of the industrial revolution in the late 1700s, the amount of carbon dioxide in the earth's atmosphere was about 270 parts per million. The record showed that concentrations grew slowly until the 20th century but have grown very rapidly since then, particularly in the last 50 years (figure 2).
We now have the capability of looking at an even longer record by examining ice cores taken from a 2-km deep hole drilled in the Antarctic ice sheet. A team of scientists from France and the former Soviet Union analyzed tiny bubbles of air trapped in this core at various levels below the present ice surface. Deeper layers correspond to times in the more distant past. These bubbles can be analyzed for the relative abundance of carbon dioxide to estimate atmospheric carbon dioxide levels at times extending back 160,000 years. Concentrations of isotopes of oxygen trapped in these bubbles are also measured, and their ratio, being temperature dependent, gives an estimate of the surface temperature on the Antarctic continent at the time the ice was formed. Figure 3 shows that carbon dioxide concentrations have gone from about 180 parts per million up to about 300 over the last 160,000 years. And you can see from our previous diagram also plotted here, that present concentrations exceed 350 parts per million, higher than it's been in the last 160,000 years. Figure 3a extends the CO2 and temperature back 400,000 years. Note the correspondence between CO2 and temperature over this period
Our discussion of magnitudes of carbon dioxide emissions on a global scale requires a system of units that we're not accustomed to using for everyday purposes. Figure 4 defines the prefixes, such as kilo- and mega- that we normally use, and other to terms that may be unfamiliar. You may find it useful to return to this table to refresh your memory on the meaning of tera-, peta-, or exa-. I have given a couple of conversions to help conceptualize the magnitudes of these numbers. One gigaton is the equivalent of one petagram, and one gigaton of water is the equivalent of about one cubic kilometer.
Trace gas concentrations will be given in parts per million by volume (ppmv), parts per billion by volume (ppbv), or parts per trillion by volume (pptv). One ppmv means one molecule out of a million. Occasionally, concentrations are reported in parts per million by mass, but such usage will be avoided in this course to retain uniformity.
We know the reason carbon dioxide is increasing in the earth's atmosphere. Deforestation is one of these causes, although not the main cause. Figure 5 shows the current annual release to the atmosphere of teragrams of carbon by several different countries due to deforestation. Brazil contributes the most, followed by Indonesia, Columbia, and other mostly tropical countries. It is important to recognize that developed countries, like the US and European nations do not appear on the chart of current deforestation. These countries have, in fact, reduced their forest area, but did so over the last few hundred years.
Another source of carbon dioxide in the earth's atmosphere is the emission from cement plants (figure 6). Carbonaceous material used for making cement releases significant amounts of carbon dioxide in creating the final product. This source of carbon dioxide will come up again in our discussion of sustainable development, since cement for roads, bridges, buildings, power and manufacturing plants is a key ingredient for economic development. Of particular interest are countries such as China and India that have such enormous potential for development.
The major cause, by far, for the rise in concentrations of carbon dioxide in the earth's atmosphere is the burning of fossil fuels. Figure 7 shows the fossil fuel emissions in gigatons per year as a function of time. Emissions before the 1860s were well below 1 gigaton per year and rose quite slowly until the middle of the present century. Since the late 1940's and early 1950's, this curve has risen dramatically to present emission levels of about 6 gigatons per year.
A break-down of the kinds of fossil fuels contributing to this rise shows that coal use has increased steadily over the last 100 years and is increasing quite significantly at present. Use of oil for heating, manufacturing and automobiles started later but also is increasing at a rapid rate. Use of all types of fossil fuels is increasing, and this trend will likely continue well into the future (figure 8).
To understand the human impact of increasing atmospheric carbon dioxide, we first must consider the natural reservoirs and fluxes of carbon in the earth/atmosphere/ocean system. We must include inventories of the carbon stored in terrestrial plants and animals, carbon in the ocean, carbon in the atmosphere, and reserves of fossil carbon that represent terrestrial carbon taken out of the biosphere at times in the distant past. Each of these reservoirs takes up and releases carbon at different rates that must be estimated to put anthropogenic emissions into perspective.
The accompanying sketch (figure 9) shows these reservoirs, with amounts given in petagrams of carbon and fluxes in units of petagrams of carbon per year. So, for instance, the atmosphere contains about 740 units of carbon. Terrestrial biology, including all of the plants from phytoplankton to giant sequoia trees and animals from mice to elephants, accounts for about 550 units. Even these crude numbers allow us to make simple evaluations. For instance, if humans carried deforestation to the ultimate, in other words, if we incinerated the whole terrestrial biosphere and put this carbon in the atmosphere, we would approximately double the amount of carbon dioxide in the earth's atmosphere.
Soil contains molecular carbon, carbon of organisms living in the soil, and detritus (broken parts of dead plants, corn stalks, and tree leaves, etc.) in total amount of about 1200 units. This suggests that the soil contains about twice as much carbon as the terrestrial biology. The ocean presents a more complicated problem for evaluating carbon stores and fluxes. The deep ocean has about 34,000 units, and the surface ocean has about 600 units in what is considered the warm ocean and 300 units in the cold ocean. It should be noted that the deep ocean has a very large reservoir of carbon, but it's essentially stored there permanently, since the cycling time from the deep ocean is on the order of hundreds to thousands of years.
Near the ocean surface, turbulent motions promote the uptake of atmospheric carbon dioxide by the ocean through the formation of weak carbonic acid. Approximately 22 units per year go from the cold ocean into the atmosphere, and about 35 units come back, making the cold ocean a net sink for carbon dioxide from the earth's atmosphere. The warm ocean, by contrast, is a net source because it's emitting 80 units and taking in only 70, giving a net flux outward of about 10 units.
Terrestrial (land) biology consumes about 110 units per year, mostly in support of terrestrial plant growth. About 50 units of that goes back into the atmosphere in the decay process, and about 60 units goes into the soil. The soil, it turn, releases about 60 units back to the atmosphere, resulting in a balance of fluxes in and out of the natural terrestrial biological system.
If we now consider the effects of anthropogenic changes to this natural cycle, we must focus on the two largest contributions, namely burning of fossil fuels and deforestation. Fossil fuels represent terrestrial carbon that has been taken out of the rapidly changing part of the carbon cycle and stored more or less permanently below the earth's surface where natural processes cannot release it back to the atmosphere. Fossil fuels account for about 5 to 6 units of emission into the atmosphere, and the burning of standing carbon in the form of old growth tropical forests contributes about 1 to 2 units.
The carbon cycle includes other carbon-containing compounds in addition to carbon dioxide. For instance, methane molecules, which have one carbon atom and 4 hydrogen atoms, may be produced rather than carbon dioxide in the decay process, particularly in moist soils, marshes, and boreal tundra. Methane also is produced by ruminant animals, such as cattle and sheep. These animals eat large amount of grain or forage, which goes into the first of 4 stomachs known as the rumen. The digestion process produces significant amounts of methane that are released to the atmosphere. Carbon monoxide also is released to the atmosphere from the burning of fossil fuels and from decay of plant material. Volcanoes are natural sources of both carbon dioxide and carbon monoxide.
As was previously pointed out, the ocean is both a major source and sink of carbon for the atmosphere. Inorganic carbon in the form of dissolved carbon dioxide and carbonates accounts for about 37,000 gigatons. Dissolved organic carbon contributes an additional 1,000 gigatons. Particulate carbon such as from live organisms or dead plants and animals or fragments add about 30 units. We should remember that there are large uncertainties in these estimates because of the wide variations over the planetary oceans.
Oceans regulate carbon in 3 different ways: by physical processes, chemical processes, and biological processes. Physical processes include the movement of carbon by ocean circulation from one location to another. Meteorologists and oceanographers call this process advection. Differences in temperature and salt content (salinity), in addition to the driving force of the wind and rotation of the earth, lead to bulk transport of carbon within and between major ocean basins. Another physical process is the diffusive mixing of water from one vertical level to another. Carbon dioxide dissolved in surface water is in equilibrium with CO2 in the atmosphere because of efficient mixing in the ocean surface water.
Chemical processes transform carbon among different molecular forms. Biological processes include the production and decomposition of organic matter, which are confined to the upper layer of the ocean where photosynthesis can operate. If this biological material remains near the surface, it will continue to cycle with the atmosphere. Some carbon, such as in the form of phytoplankton that thrive in the surface water of the ocean, are eaten by small fish and eventually larger fish or animals that ultimately die, leaving skeletons or carbonate shells that sink to the ocean floor. This process, sometimes referred to as "biological pumping", takes carbon from the rapidly changing part of the cycle near the ocean's surface to the deep ocean where it may be stored for thousands of years. The deep ocean is richer in dissolved inorganic carbon, and the surface water has a predominance of organic carbon.
The increase of ultraviolet radiation, which we will discuss in connection with ozone depletion, also has implications for ocean biology. Many of the simple organisms in the ocean surface are very vulnerable to ultraviolet radiation, and increases in ultraviolet radiation due to ozone depletion could significantly affect simple organisms at the bottom of the food chain that thrive in the ocean surface water. This presents a linkage between the ozone depletion problem and the build-up of atmospheric CO2: decreased ozone could contribute to increased ultraviolet light, which decreases ocean plant life, which decreases CO2 consumption by the ocean, which allows for increased rates of CO2 build-up in the atmosphere.
Sunlight penetrating the ocean surface is depleted as it passes downward, creating what is called the euphotic zone where sunlight is sufficiently intense to promote photosynthesis (figure 10). In the region below the euphotic zone, the net photosynthetic rate is negative due to lack of solar energy, resulting in very little biological activity below a certain level. The ingredients in addition to sunlight that are needed for biological production are nutrients, particularly nitrogen and phosphates. Figure 11 shows a typical nutrient deficiency in the surface layer due to consumption by micro-organisms. Deeper layers, where photosynthesis is suppressed due to lack of light, tend to have elevated levels of nutrients. If a mechanism were available to bring deep, nutrient-rich water in the euphotic zone, phytoplankton and algae would flourish, as would the marine life that live on these tiny organisms. Large-scale ocean circulation patterns in certain geographical regions and vertical motions in the ocean near continents circulate nutrient-rich water to provide the necessary nourishment for the euphotic zone.
The accompanying satellite picture (figure 12) shows ocean biological activity in the vicinity of Antarctica during the Southern Hemisphere spring. The color coding indicates the level of biological activity, going from a magenta, which represents essentially no biological activity, to blue, yellow, green, and finally red, which represents the highest observed concentration of phytoplankton. The arrival of sunshine to the arctic region in the spring and early summer leads to a rapid "bloom" of phytoplankton in this region. Notice that the regions of highest biological activity are along continental coastlines and around the Antarctic Continent. Equatorial ocean areas far from continents, on the other hand, are virtual biological deserts by comparison. Certainly sunlight is not lacking in these areas, so we must conclude that lack of nutrients prevents these regions from becoming biologically productive. Upwelling near continents creates rich biological regions, but closer examination reveals that not all coastal areas are equally productive. Again, we can conclude that differences in upwelling of nutrient-rich water must be the cause of these differences in ocean biology.
The next photograph of the Northern Hemisphere (figure 13), centered on the North Pole shows that the whole North Atlantic Ocean is a very biologically productive region. In the North Pacific Ocean, coastal regions between Alaska and Russia also show high levels of phytoplankton. Considering that these small organisms cover such a large area and consume CO2 , we must conclude that the polar oceans are tremendous sinks (removal mechanisms) for atmospheric carbon dioxide. In contrast to lower latitudes where lack of nutrients limits biological activity except near coasts, in the polar regions the supply of nutrients is persistent, but the lack of sunlight during winter periods shuts down the phytoplankton production beginning in autumn. However the large extent of the summer blooms means that a tremendous amount of carbon dioxide is used seasonally by these organisms. A closer look at coastal California in figure 14 shows that northern California experiences a rich phytoplankton bloom, which is less pronounced in the ocean off southern California. Ocean currents (to be discussed later in the course) produce upwelling preferentially in the northern part of the state, a situation that also is responsible for ocean temperatures off San Francisco being much colder than those off Los Angeles.
A limiting factor for the growth of phytoplankton and algae around Antarctica, even during the spring bloom, is insufficient amounts of iron. A proposal has been made that by fertilizing the Antarctic ocean with iron, the growth in ocean marine plants would sequester large additional amounts of carbon dioxide from the atmosphere and counteract the anthropogenic increases.
Terrestrial plants consume carbon dioxide during the daytime period and respire some of this CO2 back to the atmosphere at night. The accompanying sketch (figure 15) shows typical levels of CO2 within a vigorously growing plant canopy at different times of day and night. At night, plants tend to respire or give up carbon dioxide, raising the ambient level well above the global mean (which was about 330 ppm when this sketch was printed). During the day however, if the plants are not limited by lack of water or nutrients, photosynthesis may draw down the amount of CO2 to approximately 300 parts per million. The carbon taken up by the plants is converted to plant carbon and remains in this form until the decay process begins. For typical agricultural plants, the biomass is broken down during the dormant period and is converted to soil carbon and atmospheric carbon dioxide at rates that depend on temperature and moisture conditions.
Woody plants play a special role in the capture (sequestration) of carbon from the atmosphere. Trees lose their leaves (deciduous) and some or all needles (conifers) every year and return a portion of the season's carbon capture back to the atmosphere. The woody parts of the tree, however, may persist as plant carbon for several decades until the tree dies and decay begins. Trees, therefore, like annual plants, participate in the rapid part of the carbon cycle by cycling CO2 back to the atmosphere in 1-3 years, but also store some carbon that is not recycled to the atmosphere for 50 to 100 years. Tropical rain forests have very large amounts of carbon sequestered in the trunks of trees as a part of this longer term cycle. Deforestation interrupts this natural long term cycle and puts woody material back into the atmosphere before the natural decay process would recycle it into CO2 .
The "greenness index" of the planet, shown in figure 16 for July and August shows very rich terrestrial biological activity in the Northern Hemisphere continental areas, particularly in the boreal forests of northern Russia and Canada. At this time of year, the Southern Hemisphere, of course, is experiencing the biologically dormant winter period. Major desert regions, located approximately 30o north and south of the equator, show lack of vegetation in both hemisphere (both summer and winter). During the Northern Hemisphere winter, shown in figure 17, the boreal forest regions are seen to have shut down their biological production. In the Southern Hemisphere the smaller amount of land mass does not reveal the dramatic seasonal variation of the Northern Hemisphere. This suggests that global seasonal cycles in atmospheric carbon dioxide will be dominated by the seasonal cycle in the Northern Hemisphere.
Figure 18 shows biological production potential for the entire planet, including both ocean and land areas. An important conclusion to be drawn from this figure is that relatively large regions of the planet, including tropical oceans, great deserts on land, and Antarctica, have very low potential for biological production. Essentially all the biologically productive land that is not in forests or urban use is under human agricultural management, and most of the productive ocean areas are heavily fished by global fishing fleets from many countries. Increased food production to feed a growing population on the planet will require higher production on present agricultural land or more conversion of forest land to agricultural land, both of which present problems and challenges. This figure reminds us of the finiteness of our planet and the need for preservation and stewardship of its food-producing resources.