We learned in Chapter 4 that the amount of carbon dioxide now in the atmosphere
is but a tiny fraction of the total that has come out of volcanoes over the
ages We remarked there that the process of removal of carbon dioxide from the
atmosphere over time has been dissolution in ocean water followed by
precipitation of carbonate and organic sediments onto the ocean floors. To
this fact we now add that all of the carbon in living things on land and in the
ocean has also come from the carbon dioxide in the atmosphere. In this chapter
we will trace the paths of carbon as it passes from the atmosphere to these
other reservoirs. We will see that the reverse paths are also open and on
these paths carbon passes from the reservoirs back into the atmosphere as
carbon dioxide as part of a carbon cycle. One
objective of this chapter is to make it clear that the human activities of land
clearing and of fossil fuel burning constitute a major disturbance to the
natural carbon cycle.
Figure 6.1. Concentration of carbon dioxide in parts per million (ppm) as a function of time at Mauna Loa, Hawaii.
To trace out the carbon cycle we must keep track of carbon atoms rather than carbon dioxide molecules. We do this by means of weight. The standard unit is 10[12] kg of carbon atoms. Our unit of measurement of carbon dioxide concentration in the atmosphere has been the ppm (see Fig. 6.1). It turns out that 2.1 x 10[12] kg of carbon atoms are required to yield a concentration of one ppm carbon dioxide throughout the global atmosphere. Thus the statement that the present concentration of carbon dioxide in the atmosphere is 357 ppm is the same as saying that the atmosphere presently contains 750 x 10[12] kg of carbon in the form of carbon dioxide. Let us refer back to the lower panel of Fig. 5.2 and agree that the "natural" level of carbon dioxide concentration prior to the era of extensive land clearning and industrial development that began around 1800 was 285 ppm. This concentration is equivalent to 600 x 10[12] kg of carbon in the form of carbon dioxide. Since we are adopting 10[12] kg as the unit, we will henceforth omit this factor and, for example, refer to the number just mentioned as 600 units of carbon.
The current rate at which the carbon dioxide concentration in the atmosphere is increasing is slightly less than 1.5 ppm annually (see Fig. 6.1). This is equivalent to 3 units of carbon. The result of a detailed inventory of global fossil fuel consumption is shown in Fig. 6.2. The present rate of emission of carbon dioxide into the atmosphere from this source is 5 units per year. As we will see later, it is estimated that deforestation (land clearing) is the source of a further 2 units per year. With these numbers in front of us, we can see that less than half of the carbon emitted into the atmosphere through human activities is accumulating there. In this chapter we will address the question of what constitutes the "sink" that is absorbing the 4 units of carbon annually.
Figure 6.2. Global annual emissions of carbon dioxide from the burning of fossil fuels expressed in 10[12] kg of carbon per year.
How carbon is exchanged between the atmosphere and the land portion of the
biosphere is illustrated in Fig. 6.3. The cycle begins with the intake of
carbon dioxide by plants. By means of photosynthesis, this carbon dioxide and
water are used to create organic carbon molecules of one sort or another, here
represented generically by the symbol (CH2O[):
CO2 + H20 -----> (CH20) + 02]
Figure 6.3. A schematic of the carbon exchange between the atmosphere and the biosphere.
Carbon passes through this cycle at quite a rapid rate. We show the numbers in
Fig. 6.4. This is the natural (that is, pre-industrial) carbon cycle. The
figure appears very complicated at first, largely because it also contains the
marine portion of the carbon cycle, which we will examine in the next section.
For now we focus only on the extreme left hand side of the figure. There we
see that plants on land globally assimilate atmospheric carbon dioxide at the
rate of 100 units of carbon annually. Plant respiration returns 50 units
annually, with the result that annual net primary production is 50 units. The
arrow pointing vertically downward into the soil reservoir represents both
litter fall and animal consumption. All of this is respired, as represented by
the arrow labelled with 50 units that is directed vertically from the soil to
the atmosphere. What we have just said frames an important conclusion: the
land portion of the natural carbon cycle is closed, that is, it constitutes
neither a source nor a sink of carbon.
Figure 6.4. The geochemical cycle of carbon before the industrial era. The amounts in the reservoirs are in units of 10[12] kg and the transfers between reservoirs are in units of 10[12] kg per year.
The reservoir contents are also given in Fig. 6.4. It can be seen there that life on land (which consists mostly of plants) contains about as much carbon as the atmosphere, and the soil contains a little over twice as much carbon as the atmosphere. Given that the atmosphere contains 600 units of carbon and that 100 units are exhanged annually with life on land, we see that the residence time of carbon dioxide in the atmosphere is 6 years for this process. ( It is actually 3.4 years when the exchange rate of 74 units annually with the mixed layer of the ocean is considered). This residence time is so brief as to bring us to the point of view that atmospheric carbon dioxide is quite a dynamic atmospheric gas that is intimately connected with life. The whole carbon dioxide component of the atmosphere literally passes through life on land every 6 years. On the other hand, this residence time of 6 years is very long compared to the residence time of 12 days that we deduced for atmospheric water vapor. This is the reason why atmospheric carbon dioxide is "well mixed" (concentration in ppm the same everywhere in the atmosphere) while water vapor varies from almost zero up to 4000 ppm depending on time and place.
The first stage of the marine portion of the carbon cycle is the exchange of 74 units of carbon annually between the atmosphere and the mixed layer of the ocean (Fig. 6.4). This global exchange occurs because carbon dioxide dissolves more readily in cold water than in warm water. Thus, for the most part, the exchange from the atmosphere to the ocean takes place in high latitudes and the reverse exchange takes place in lower latitudes.
Unlike other constitutents of the atmosphere such as molecular nitrogen and molecular oxygen, carbon dioxide does not simply dissolve into ocean water. Instead, it dissolves and undergoes the following reaction:
C02 + H20 -----> H[+] + HC0[-]3
The resulting components here are a hydrogen ion (H[+], which is a hydrogen atom less its electron) and a bicarbonate ion (HC0[-]3[,] which has carried away the hydrogen atom's electron). Because of this reaction, almost all of the carbon dioxide dissolved in ocean water is in the form of bicarbonate ions. A little bit stays as dissolved carbon dioxide (that is, C02) and a little bit of the HC0[-]3 further dissociates into H[+] and C0[=]3 (the carbonate ion). As chemical oceanographers do, we will refer to the sum total of carbon in these three forms as if it were all in the form of CO2; that is, we simply call it dissolved carbon dioxide.
Just as atmospheric carbon dioxide is the source of all carbon for life on land, so also dissolved carbon dioxide is the source of all carbon for life in the ocean. The primary producers (photosynthesizing plants) in the ocean are the phytoplankton. The photosynthesizing reaction (the same as on land) takes place in the mixed layer, where there is light. The ocean herbivores are the oxygen breathing zooplankton, which graze on the phytoplankton. Higher on the food chain are the fish and other consumers. As shown in Fig. 6.4, the reservoir of carbon contained in the biota of the ocean is 3 units.
It can be seen in Fig. 6.4 that the exchange of dissolved carbon dioxide in the mixed layer with life is not a closed cycle. Rather, there is a net export of 4 units of carbon annually from the biota of the mixed layer to the deep ocean. This is known as the biological pump. Further inspection of Fig. 6.4 reveals that the downward export of carbon by the biological pump is accompanied by a separate export of 33 units. This downward flow of 33 units and the upward flow of 37 units that balances it and the units from the biological pump is achieved by the deep ocean circulation that was diagrammed in Fig. 4.6. We will later examine this larger transport of carbon. First we turn to an explanation of how the biological pump works.
When any of the organisms in the mixed layer die, nearly all of their mass decomposes (is respired) in the mixed layer. The photosynthesis reaction then runs in reverse, and oxygen is consumed and carbon dioxide is released into the water. The remaining small fraction of organic carbon manages to sink out of the mixed layer and into the deep ocean. This constitutes for the mixed layer a net loss of carbon and a net gain of dissolved oxygen. Nearly all of this mass of dead organisms drifting down from the mixed layer subsequently decomposes in the deep ocean. In other words, nearly all of the carbon raining down from the mixed layer to the deep ocean ends up as dissolved carbon dioxide. However, a little bit of this mass manages to fall to the bottom and is buried in the sediments. This is the source of the organic carbon stored in sediments. This burial of carbon constitutes for the whole ocean a net loss of carbon.
The burial of organic carbon also constitutes for the whole ocean a net gain of
oxygen. To see this we return to the equation for photosynthesis that was
written at the beginning of this section. For every molecule of organic
material, represented by (CH2O), that is buried, an oxygen molecule remains
behind in the ocean. Burial of organic material is thus accompanied by an
increase in dissolved oxygen in the ocean. Since the amount of dissolved
oxygen in the ocean is in equilibrium with (that is, proportional to) the
amount of oxygen in the atmosphere, some of the increase in dissolved oxygen in
the ocean gets shared with the atmosphere. This is the process alluded to in
Chapter 5 that is responsible for all of the oxygen that is found in the
atmosphere. The buildup of atmospheric oxygen that occurred much earlier in
the history of the earth has now ceased. This is because the present rate of
increase of oxygen in the atmosphere is balanced by a loss due to the
decomposition of organic material that was buried a long time ago on the ocean
floor and has since been consolidated into rock and raised up as dry land by
geological processes.
Figure 6.5. A three-dimensional perspective of the global ocean circulation. Continents are shaded in black.
The transfer of organic carbon out of the mixed layer as just described is one component of the biological pump. There is another component. Both the phytoplankton and the zooplankton are prolific builders of tiny skeletal shells. These are made of calcium carbonate (CaCO3). The reaction by which the shells are constructed is (see also Section 5.2):
The ocean contains plenty of calcium ions
(Ca[++], the two plus signs reflect the fact that two electrons have left the calcium atom) and the organisms use these together with twice as many bicarbonate ions to construct CaC03.
Upon the death of the organism, and even after decomposition of its organic body, the shell survives to sink into the deep ocean. About 80% of them disintegrate there, which restores carbon dioxide to the ocean water. The remaining 20% make it all the way to the bottom and are buried there, and this constitutes for the whole ocean a net loss of carbon. This is the other source for the carbonate sediments reservoir.
We now need to return to the circulation of the ocean described in Section 4.2 in order to understand the other exchange of carbon between the mixed layer and the deep ocean. For convenience, we repeat here as Fig. 6.5 the schematic of the thermohaline circulation previously shown as Fig. 4.6. As we discussed in Section 4.2, we can view the deep ocean circulation as starting from the sinking of cold, salty water into the deep ocean in the North Atlantic. This water moves very slowly as a river in the direction shown, mixing laterally with deep-ocean water at its sides, until it eventually comes to the surface in the Indian and Pacific Oceans. As this deep water flows, there is a steady rain into it from above of dead organic matter and calcium carbonate. This is the output of the biological pump, as we have just described. The material is decaying and disintegrating as it falls, putting into the deep ocean most of the dissolved carbon dioxide that was taken out of the mixed layer. As the deep water makes its way eastward, it accumulates more and more of this. As a result, the deep ocean in the Pacific contains more dissolved carbon dioxide than does the deep ocean in the Atlantic. Eventually, the dissolved carbon dioxide returns to the mixed layer when the water upwells.
Combining the results of the deliberations of this section we can see in Fig. 6.4 that 33 units of carbon are carried down into the deep ocean in the North Atlantic Ocean (and, although not shown, also near Antarctica, see Section 4.2) while 37 units are upwelled primarily in the Indian and Pacific Oceans. The sum of the contributions of carbon contained in the biological pump and sinking zones are balanced by the carbon contained in the upwelling zones.
Figure 6.6 shows how dissolved carbon dioxide is distributed vertically in the ocean. The particular example shown here is for the North Pacific, but it can be considered for our purposes here as representative of any location in the global ocean. The feature of this schematic that we wish to call attention to is that there is about 12% less dissolved carbon dioxide in the mixed layer (top 100 m) of the ocean compared to that in the deep ocean. This is a consequence of the presence of life in the mixed layer.
Figure 6.6. Concentration of dissolved carbon dioxide in percent of the total volume of ocean water as a function of depth (in meters).
One more fact of life will enable us to see the reason why there is 12% less dissolved carbon dioxide in the mixed layer than in the deep ocean. The biochemistry of living organisms requires atomic nitrogen (N) and phosphorous (P). These two items are referred to as inorganic nutrients. They are very scarce in the ocean, as they are also on land. Phosphorous is simply a scarce element everywhere on earth that comes into the oceans by rivers, having been washed out of rocks by rain. The scarcity of atomic nitrogen (N) may seem strange in view of the fact that nitrogen is the dominant gas in the atmosphere and is abundant also as dissolved nitrogen in ocean water (see Section 4.1). But this nitrogen is molecular nitrogen (N2) and it is difficult for molecular nitrogen to be converted into atomic nitrogen. The breakdown of N2 requires certain kinds of bacteria. Some of these bacteria are found in the oceans while other bacteria on land create atomic nitrogen that eventually washes into the oceans in the form of nitrate (NO3[-]). In any case, it is the lack of these inorganic nutrients in the ocean that limits the amount of life present there.
Analysis of dead organic material falling out of the mixed layer has revealed that it contains atoms of carbon, nitrogen, and phosphorous in the ratio of 100 to 15 to 1. The relative abundance of these three atomic species in deep water just as it begins to upwell into the mixed layer in the Pacific is in the ratio 800 to 15 to 1. What happens when this newly upwelled water arrives in the mixed layer is that life uses up both inorganic nutrients completely. Since oceanic life contains carbon, nitrogen, and phosphorous in the ratio 100 to 15 to 1, we can conclude that the leftovers consist of 700 of every 800 carbon atoms that come with the upwelling water. That is, there is 12% less dissolved carbon dioxide in the mixed layer than there is in the deep ocean.
The above considerations also explain another fact of life in the ocean: life is more abundant in the Pacific Ocean than in the other oceans. This is because the water upwelling there, at the end of the river in the ocean circulation diagram, has had the maximum chance to sweep up the inorganic nutrients that come down from the mixed layer inside dead organisms, which subsequently decay in the river as it progresses.
This is the way the ocean works. If the circulation were to stop for some reason, then the rain of dead organisms would soon remove all the nitrogen and phosphorous from the mixed layer and put it in the deep ocean. Without the ocean circulation to recycle these nutrients back up to the mixed layer, life in the ocean would cease.
Now let's tie up a few loose ends before we leave the ocean. In Section 4.2, we described the catastrophic effects on the fisheries along the west coast of South America when El Niño conditions prevail. You now should be able to understand better why this region normally is so prolific and how the changes associated with El Niño affect the entire food chain in that region.
It is literally true that inorganic nutrients are so scarce and life is so prevalent that these nutrients are completely used up as soon as deep water enters the Pacific mixed layer. It remains to be clarified how there can be life in the mixed layer elsewhere in global ocean. What happens is that life is short for the phytoplankton and zooplankton that constitute most of the ocean biomass. Recall that when organisms in the mixed layer die, only a small fraction of the material sinks into the deep ocean. The remainder decays in the mixed layer and the nutrients are then available for use again. There is then an intense recycling of nutrients within the mixed layer itself. This recycling is shown in Fig. 6.4. Finally, since 40 units of carbon enter the oceanic biota reservoir every year, we can then say that the residence time of carbon in the oceanic biota category is about .1 year.
Meanwhile, this pool of nutrients, which came up in the Pacific upwelling zones, is drifting westward in the surface current that is returning water to the Atlantic (see Fig. 6.5). There are, as a general rule, less nutrients available in the mixed layer the farther west you go, because of the continual drain of nutrients in the rain of dead organisms into the deep ocean.
Let us collect together the content of all the carbon reservoirs shown in Fig. 6.4.
Oceanic biota 3 Land biota 610 Atmosphere 600 Mixed layer 1,000 Soil and detritus 1,560 Deep Ocean 37,950 Sediments 66,000,000
By far the largest reservoir of carbon is in the form of sediments. This reservoir includes not only ocean sediments but also rocks on land whose chemical constituents were deposited as ocean sediments. All of the huge amount of carbon that is present in this reservoir came originally out of volcanoes, into the atmosphere, then into the ocean, and finally into the sediments. Carbon atoms making this journey for the first time constitute what geologists call juvenile carbon. It turns out that a very small fraction of the carbon atoms found in the form of carbon dioxide in the atmosphere is juvenile carbon. What this means is that the earth-atmosphere system has developed a way to recycle carbon. So, the cycle through the atmosphere and ocean and into the sediments does not stop there. Geological processes bring the carbon in the sediments back into the atmosphere through volcanoes and through sedimentary and metamorphic rocks in mountain ranges exposed to the atmosphere. The complete cycle, involving as it does both geological processes and chemistry is referred to as the geochemical cycle of carbon.
The concept of reservoirs and residence time is central to understanding this ponderous cycle. Let's begin by examining the interaction of the atmospheric and oceanic components of the cycle with the aid of Fig. 6.4. The atmospheric reservoir contains about 600 units of carbon. Carbon dioxide dissolves into the surface of the ocean at a rate of about 74 units per year and is returned at the same rate. Considering only this gaseous exchange process, the residence time for carbon in the atmospheric reservoir would be 8.1 years.
The mixed layer of the ocean contains 1000 units of carbon. It exchanges carbon with the atmosphere at a rate of 74 units per year. Considering only this exchange process, the residence time for carbon in the mixed layer is 14 years. The residence time of carbon in the deep ocean is about 1000 years (as we have already seen in Section 4.2). The exchange here is between the deep ocean and the mixed layer. This residence time is very long compared to those characterizing the exchange between atmosphere and mixed layer (8 to 14 years, depending on which reservoir we focus). Because of this, the deep ocean sees the atmosphere and mixed layer as one and the same reservoir. These two reservoirs are able to adjust to one another much faster than the deep ocean is able to adjust to them.
So far we have reviewed two of the smaller (and faster) cycles of the carbon cycle: the exchange of carbon between the atmosphere and the mixed layer of the ocean and the exchange between this two-member combined reservoir and the deep-ocean reservoir. We go now to the big cycle, where the export of carbon is from the deep ocean and the sediments. Reference to Fig. 6.4 shows that the rate at which carbon passes from the deep ocean to the sediments is 0.2 units per year. Since the deep ocean contains 37,950 units of carbon, the residence time characterizing this process is 190,000 years. This is very much longer than the 1000 years characterizing the exchange between the deep ocean and the combined reservoir of atmosphere and mixed layer. So, as regards the process of loss of carbon to sediments, the atmosphere and the entire ocean respond as one reservoir.
We have just considered the flux of carbon out of the combined reservoir consisting of the atmosphere and ocean. Now, let's consider the flux of carbon into this combined reservoir as shown in Fig. 6.4. There is a flux of 0.8 unit of carbon brought into the ocean by rivers. This is equal to the flux out of the ocean into sediments together with a flux from the ocean into the atmosphere of 0.6 unit. The process which puts this carbon into rivers is the chemical weathering of rock by rain. What greatly assists rain to do this is that it is naturally acid by virtue of the fact that it contains dissolved atmospheric carbon dioxide. Chemical weathering therefore uses atmospheric carbon dioxide. In other words, part of the carbon that enters the ocean via rivers in the geochemical cycle comes directly from the atmosphere. The 0.6 units that is shown returning to the atmosphere is liberated in the formation of calcium carbonate by living organisms in the ocean, as described earlier. The remaining 0.2 units of carbon is destined for the sediments, and we now have to invoke the geological mechanism of plate tectonics to move ocean bottom sediments onto land areas as sedimentary or metamorphic rock, thereby completing the geochemical cycle of carbon.
The Current Numbers
We now return to the original question posed at the beginning of the chapter. What has happened to the carbon emitted into the atmosphere by the burning of fossil fuels and deforestation? Figure 6.7 shows a schematic similar to that shown earlier (Fig. 6.4) except that it represents the carbon cycle at the present time. Notice first near the left margin that two new sources of carbon for the atmosphere have been included: 5 units of carbon are added each year by fossil fuel burning and an additional 2 units are added by deforestation. Notice also that the atmospheric reservoir has increased by 150 units to 750 units during the industrial era while the deep ocean reservoir has increased by 170 units to 38,120 units.
There is considerable uncertainty about the amounts of carbon in each reservoir and the rates of exchange between them. Be that as it may, the schematic presented here (Fig. 6.7) accounts for all of the carbon introduced as a result of fossil fuel burning and deforestation during the industrial era. This has officially been estimated to be 320 units of carbon and this, according to Fig. 6.7, resides in the atmosphere (150 units) and in the deep ocean (170 units). What happens to the 7 units of carbon that are now being added each year as a result of the burning of fossil fuels and deforestation is that 3 units remain in the atmosphere; 2 units are fixed by plant life, possibly making their way into the soil; and the remaining 2 units are dissolved into the mixed layer and pass into the deep ocean via the deep ocean circulation.
This leads us to a new application of the concept of residence time. It can be considered loosely to be the time that a reservoir takes to come into equilibrium after it has been disturbed by changes in another portion of the system. Consider this year's 7 units of carbon dioxide input to the atmosphere. It will take roughly 11 years (1000 units divided by 92 units per year) for the mixed layer of the ocean to adjust to this new atmospheric carbon dioxide. Otherwise stated, the mixed layer of the ocean has already responded to all of the carbon dioxide added to the atmosphere before roughly 11 years ago.
Figure 6.7. Global carbon reservoirs and rates of exchange between them. The unit of measure for the reservoirs is 10[12]kg while the rates of exchange are in 10[12] kg per year.
Of great significance to the global warming problem is that the deep layer of the ocean has the much longer residence time of about 1000 years. It is only just beginning to respond to all of the carbon dioxide that has been added to the atmosphere since the industrial revolution began some 150 years ago. In effect, the deep ocean is responding to all of the carbon dioxide that has been added to the combined reservoir of the atmosphere and mixed layer since that time, and is taking it in at a rate of 2 units of carbon out of every 7 units placed into the atmosphere. Consistent with our figures, we can also postulate that the land portion of the carbon cycle is absorbing 2 of the 7 units of carbon. However, we are not sure over what time span this carbon stored on land may be released back into the atmosphere or even whether the land biota will continue to take the carbon out of the atmosphere at this rate.
Our schematic in Fig. 6.7 shows that 2 units of carbon are placed per year into the atmosphere as a consequence of deforestation. In a superficial way, deforestation is the most correctable aspect of human activity that would have a signficant effect on the concentration of carbon dioxide in the atmosphere. To correct the problem, it is simply necessary to grow trees at a rate faster than the trees are being cleared. Or is it really so simple?
The tropical rain forests are the most productive ones in terms of the rate at which carbon is removed from the atmosphere and fixed through photosynthesis. The forests in mid-latitudes are both not as large and not as fast in removing carbon. Grasslands, tundra, deserts, and concrete jungles are even less effective at removing carbon dioxide.
Most of the deforestation underway at the present time takes place in the tropical forests of South America, South-east Asia, and Africa. The latest published figures from satellite surveilance indicates that 150,000 km[2] of tropical rain forest in the Amazon Basin fell to deforestation in the decade 1978-88. This area is almost equal to that of the whole state of Utah. This deforestation takes place as a result of a complex mix of economics, politics, and historical forces within the developing countries of the tropics.
As we have alluded to in the previous section, there is considerable uncertainty regarding how much of the carbon placed into the atmosphere through human activities is removed through storage in stadning biomass or in the soils. There is tantalizing evidence from the small transport of carbon dioxide that is presently observed to be northward across the equator that there must be a large sink of carbon as a result of the land biota in the Northern Hemisphere. It has been assumed that this sink exists in Fig. 6.7.
There is another aspect of human activity that may be affecting the fixing of carbon in the biomass. As we mentioned in the previous section, life depends on the availablity of the inorganic nutrients nitrogen and phosphorous. We have altered the natural abundance of these nutrients with intensive applications of fertilizers. In the mid to later 1980's, roughly 70 x 10[9] kg of nitrogen were applied to fields, for the most part located in the mid-latitudes of the Northern Hemisphere. The long-term effect of these additional nutrients is unknown.
Attempts have been made to estimate how much additional forest acreage needs to be planted in order to arrest the projected increase in the concentration of carbon dioxide. To take in 1 unit of carbon annually would require a new forest to be planted in temperature latitudes over nearly the area of the entire United States. In addition, as forests mature, they reach an equilibrium state between photosynthesis on the one hand and respiration and decay on the other hand, and therefore cease to consume additional carbon dioxide. It is unlikely that planting trees will even temporarily reverse the effects of continued fossil fuel burning and deforestation.
How about reforesting the tropics? Estimates have been made that an area equivalent in size to the Amazon Basin could be replanted throughout the tropics (based on areas that were forested in the past or ones that are used for crops now). Most of the land to be reforested has been degraded in terms of reduced amounts of phosphorous and other inorganic nutrients, so the rate at which carbon could be fixed would be reduced compared to present natural tropical forests. Because of this even the replanting of such a large area would not its growing tage remove more than an additional 1.5 units of carbon from the atmosphere per year.
Based on both of these sets of calculations we can conclude that the global warming problem will not be solved in its entirety by reforesting the earth.
Imagine for the moment that all of the countries of the world united to stop the emissions of carbon dioxide now. What would be the projected concentrations in carbon dioxide in the atmosphere over the next 100 years? One possible answer is shown in Fig. 6.8.
Figure 6.8. One estimate of the projected concentration of carbon dioxide in ppm if no further emissions are put into the atmosphere.
Notice that even if all of the emissions were shut off immediately, the concentration of carbon dioxide would remain higher than that found in 1970 for the next hundred years. This level is considerably higher than that found prior to the industrial revolution.
Why won't the carbon dioxide concentration return to pre-industrial levels if we switch off all deforestation and all fossil-fuel based industry? As we saw in the previous sub-section, the carbon injected into the atmosphere over the past 150 years has upset the pre-industrial equilibrium. The present and future situation is not in equilibrium: the excess carbon introduced during the industrial era is still being absorbed in the deep ocean and in the land-biosphere reservoir. Finally, even if the anthropogenic sources of carbon dioxide are turned off, the higher concentrations of carbon in the deep ocean will eventually surface and lead to continued elevated levels in the atmosphere.
International groups of scientists have spent considerable time evaluating likely scenarios of the future changes in carbon dioxide concentrations. In order to even limit the concentration of carbon dioxide to its present value, it has been estimated that the emission of carbon dioxide into the atmosphere would have to be cut by 60%. Such a cut is highly unlikely.
The Intergovernmental Panel of Climate Change has developed four scenarios to span the range of possible measures for reducing carbon dioixde emissions. They can be summarized as follows:
* Business as usual. Scenario A. We try to muddle through using present technology and continue to deforest the globe.
* Natural gas. Scenario B. Coal intensive industries are converted to natural gas and deforestation is reversed.
* Nuclear after 50 years. Scenario C. Renewable energy sources (wind power, solar energy, etc.) and nuclear energy are dominant after about 50 years from now.
* Nuclear now. Scenario D. The shift to renewable energy sources and nuclear energy begins at the turn of the century.
Figure 6.9. Estimates of the anthropogenic emissions (in units of 10[12] kg per year) of carbon into the atmosphere projected to the year 2100 as a function of four scenarios.
Figure 6.9 summarizes their estimates of how these four scenarios would affect the emissions of carbon dioxide into the atmosphere. Notice that none of these scenarios reduce the emission rate of carbon to zero. If nothing is done (Scenario A), carbon emissions will triple over the next hundred years while the modest (but immensely painful economically) measures of Scenario B might leave the carbon emission rate below 10 units of carbon per year.
Figure 6.10 Estimates of the concentration of carbon dioxide (in ppm) projected over the next hundred years as a function of four scenarios.
The same group of scientists have estimated what these four scenarios for carbon emissions would yield in terms of the concentration of carbon dioxide in the atmosphere as shown in Fig. 6.10. If nothing significant is done (Scenario A), the projected concentration of atmospheric CO2 will double (from the current 350 ppm to 700 ppm) in less than a hundred years. Only with the wrenching changes proposed in Scenario D would it be likely that the concentration of carbon dioxide would remain less than 450 ppm by the year 2100.
1. What are the processes by which carbon in the atmosphere cycles through the land portion of the biosphere? Through the mairine portion of the biosphere?
2. How is carbon dioxide removed from the atmosphere?
3. What is the biological pump? How does it work and what does it do?
4. How much carbon is resently being emitted into the atmosphere per year due to human activities? How much of this is presently accumulating in the atmosphere per year? Where is the difference (between emission into and accumulating in the atmosphere) going and by what processes does it get there?
5. What is the geochemical cycle of carbon in the earth-atmosphere system?
6. Why is the Pacific Ocean richer in carbon than the Atlantic?
7. Why wouldn't the concentration of carbon dioxide in the atmosphere return to pre-industrial levels if we stopped burning fossil fuels and deforestation for the next five years?