Sulfate jrom fossil fuel burning
Organic carbon Biomass burning
Land use (albedo)
Aerosol indirect J- effect
High Medium Medium Low
Very Very Very low low low
Very Very Very Very low low low low
Fig. 10.2 The impact of various human-related climate drivers on the energy budget of the Earth, in Watts per square meter, relative to the year 1750. Replotted from IPCC (2001).
1750 (Fig. 10.1) is clearly the result of deforestation, perhaps in the New World (the "pioneer effect"). Today, visible deforestation is mostly to be found in the tropics, and accounts for about 2 Gton C/year release of CO2 to the atmosphere.
The other main anthropogenic CO2 source is of course the combustion of fossil fuels discussed in the last chapter. Fossil fuel combustion releases 5 Gton C/year, rising s
exponentially, driven by population growth and economic growth, rising in spite of increases in energy and carbon fuel efficiency.
Combining fossil fuel combustion and tropical deforestation, mankind is releasing carbon to the atmosphere at a rate of about 7Gton C/year. The atmospheric CO2 inventory is rising at a rate of about 3 Gton C/year. Where is the other 4 Gton C/year? There are two main natural sinks for CO2 that are operating today; one is the oceans and the other is the terrestrial biosphere.
Carbon uptake by the terrestrial biosphere on land, the terrestrial carbon sink, is difficult to measure. In comparison with the ocean, the distribution of carbon on land is very spotty. Recall from Chapter 7 that most of the carbon on land is in the soil, rather than in the trees where we could see it. In soils, the amount of carbon depends on the recent history of the land: fires, agriculture, erosion, and so on. It is difficult to know precisely how much carbon there is on land because the measurements are so variable; you would have to make a lot of measurements in order to average out all the noise of natural variations. As a result of this, it would be possible to increase the amount of carbon on land, a little bit here or there, in a way that would be entirely invisible to direct measurements. The land is playing two roles in the carbon budget story, one as a visible deforestation source and another as a potential invisible carbon uptake sink.
One way to estimate the invisible terrestrial uptake is by putting together the rest of the carbon budget and assign the terrestrial biosphere whatever is left over. The chemical properties of seawater vary more smoothly than they do on land, so our estimates of ocean uptake are better than they are for the land. Another is to measure CO2 concentrations in the atmosphere, in the winds as they blow across the land, to see if CO2 is going into or coming out of a given forest. This doesn't sound easy, but it can be done. There is a network of "CO2 observatories" around the globe, where precise CO2 concentration measurements are made daily, for uses such as this.
There are several reasons why the land may be willing to take up CO2 as the atmospheric CO2 concentration rises. One is that with warming, there will be a longer growing season. This has been observed in many climate and botanical records. With warming, some tundra areas become amenable to conquest by forests. Rising CO2 in the atmosphere may also directly encourage plants to grow faster by a process known as CO2 fertilization. Plants run their photosynthetic machinery inside waxy walls on the surfaces of leaves. Gases are exchanged with the outside atmosphere through adjustable vents called stomata. When the leaf needs CO2 for photosynthesis, the stomata open. The cost of opening stomata, though, is loss of water. So if CO2 concentrations were higher in the outside atmosphere, plants could get the CO2 they need without opening their stomata as much or as often. They could therefore be stingier with their water. There is no doubt that this is a real effect; CO2 concentrations in greenhouses are typically higher than in the outside atmosphere, one of the ways that greenhouses are good for plants. However, in the real world, plant growth is very often limited by something else other than water stress, such as fertilizers like nitrogen or phosphorus. Scientists do CO2 fertilization experiments in natural settings by pumping CO2 continuously into the air. When the wind changes, they adjust the location of the CO2 vent, so that the target grove is always downwind from a CO2 source. These experiments go on for years!
What they tend to find is an initial growth spurt from CO2 fertilization, followed by a leveling off at something like the initial rates.
There is another process that might affect CO2 storage on land, which is temperature sensitivity to respiration, the process that converts soil organic carbon back into CO2. Soil respiration really gets going as it gets warmer. Think of a ham sandwich, half of which is safely stowed in the refrigerator while the other half sits on a plate in the Sun. Which half will stay tasty longer? For this reason, there is very little organic matter in tropical soils, while high latitudes may host peat deposits that contain prodigious carbon deposits for thousands of years. Warming and melting and decomposition of high-latitude permafrost may contribute CO2 to the atmosphere.
Uptake of fossil fuel CO2 by the oceans is called the ocean carbon sink. The ocean sink depends on ocean circulation, and on the chemical forms that dissolved CO2 takes in seawater. The ocean covers 70% of the Earth's surface, and the length and width of the ocean are huge compared with its depth, which averages about 4 km. The deep ocean is so close to the surface, and yet it is so very far away. The way the ocean circulates, the deep ocean is very cold, and the only place where surface waters are cold enough to mix with the deep ocean is in high latitudes. The ocean surface is huge, but the deep ocean, which is the largest water type in the ocean, only sees the atmosphere through a very small area of sea surface.
The densest water at the sea surface is in the Antarctic and in the North Atlantic because it is cold there. Surface waters from these locations sink to the deep ocean, filling up the entire deep ocean like a bucket with cold polar water that is only a few degrees warmer than freezing. The cold deep ocean fills up until cold polar waters underlie the warm surface waters in lower latitudes. The warmer waters mix with the cooler, eroding the cold water and making room for more of the coldest water to continue filling the deep sea. As new cold water flows from the high-latitude surface ocean into the abyss, it carries with it atmospheric gases like anthropogenic CO2, a process known as ocean ventilation. It takes centuries for the waters of the deep ocean to travel through this cycle. For this reason, the timescale for getting anthropogenic CO2 into the deep ocean is centuries.
The shallower ocean has other water masses and circulation modes that are wondrous to learn about and study if one is of a mind to. The zone of the ocean separating the warm from the cold is called the thermocline. Thermocline waters may be exposed to the atmosphere in winter, when the sea surface waters are cold. Once a parcel of thermocline water becomes isolated from the sea surface, it follows a trajectory determined by its density and by the rotation of the Earth. Thermocline waters ventilate to the atmosphere on a timescale of decades.
The surface ocean water mass is not as large as the deep sea or the thermocline, but it is a respectable carbon reservoir of its own. Turbulence generated by the wind acts to mix the surface ocean down to a typical depth of 100 m. For most gases, this surface ocean layer 100 m thick would equilibrate with the atmosphere in about a month.
CO2 differs from other gases, however, in that it has chemical equilibrium reactions with water and hydrogen ions. Hydrogen ions are very reactive. If a solution has a high concentration of hydrogen ions, we call it acidic. A strongly acidic solution, such as battery acid for example, can burn your skin or clothes by chemical reaction with hydrogen ions. The acidity of a solution is described by a number called the pH of the solution, which can be calculated as pH = -log«, [H+]
The log10 is the base-10 logarithm, meaning that if x = 10y, then log10y = x. The hydrogen ion concentration is denoted by the square brackets, and is expressed in units of moles of H+ per liter of solution. A mole is simply a set number of atoms or molecules called the Avogadro's number and is equal to 6.023 ■ 1023. The hydrogen ion concentration in seawater usually ranges from 10-7 to 10-83 mol of H+ per liter. The pH of seawater therefore ranges from 7 to 8.3. Note that the more acidic the solution, the lower the pH of the solution. Ads for shampoo claim "low pH" as though pH were some toxic ingredient. I guess it sounded better than calling the shampoo "strongly acidic".
When CO2 dissolves in water, it reacts with water to form carbonic acid, H2CO3.
Carbonic acid loses a hydrogen ion (that's what acids do, in general; release hydrogen ions) to form bicarbonate ion, HCO-
A second hydrogen ion can be released to form carbonate ion, CO=
The concentrations of carbonic acid, bicarbonate, and carbonate ions control the acidity of the ocean, just as they control the acidity of our blood and cell plasma.
These chemical reactions are fast enough that the distribution of chemical species will always be in their lowest energy distribution. Many chemical reactions that we will encounter will not be in equilibrium, so we should enjoy this equilibrium system while we have it. Equilibrium reactions can be easily and very precisely predicted. In a qualitative way, we can use an idea known as le Chatelier's principle to take a stab at the behavior of an equilibrium system. Le Chatelier's principle states that an addition or removal of a chemical on one side of the chemical equilibrium will cause the reaction to run in the opposite direction to compensate for the change. Take some of something out, the equilibrium will put some of the something back. Add more something, the equilibrium will remove some of the something.
Le Chatelier's principle is treacherous for students of the carbon system in seawa-ter, though, because we have an innate human tendency, I have found, to ignore the hydrogen ions. They are such tiny things, after all. There are far fewer hydrogen ions in seawater than there are of the dissolved carbon species. What this means, however, is that a small change in the concentrations of the carbonate species might make a huge change in the hydrogen ion concentration. The safest assumption to make is that the carbon species have to get along together without counting on sloughing off too many hydrogen ions at all. Hydrogen ion is such a tiny slush fund, it might as well not exist. We can combine reactions (10.1-10.3) into asingle reaction, in such a way that we don't allow any production or consumption of hydrogen ions:
To this reaction we can apply le Chatelier principle with impunity. If we were to add CO2 to this system, the equilibrium would compensate somewhat by shifting to the right, consuming some of the CO2 by reacting it with CO=.
Seawater has the capacity to absorb or release more CO2 than it would if CO2 had no pH chemistry because of the other carbon reservoirs HCO- and CO=. It is like sitting at a poker game with a rich uncle sitting behind you covering most of your losses and taking splits of your winnings. ThepH reactions ofH2CO3, HCO-, and CO= are called a buffer because any changes to the chemistry tend to be resisted or buffered by the chemical reactions. The CO2 concentration of seawater is buffered by its pH chemistry. Another way of saying this is that the seawater has greater capacity to hold carbon than it would if CO2 were not buffered.
The strength of the buffer is about a factor of 10, meaning that seawater has a capacity to hold 10 times as much CO2 as it would if there were no buffer chemistry. The factor of 10 comes from the fact that there is about 10 times more CO= than dissolved CO2 in surface ocean water. Carbonate ion is our anti-CO2, reacting with new CO2, hiding it away as bicarbonate; this is the action of the buffer.
As the CO2 concentration in the atmosphere increases, and CO2 invades the ocean, the concentration of carbonate ion goes down, according to the equilibrium chemical reaction (10.4). As the carbonate ion becomes depleted, so is its ability to buffer CO2. Ocean uptake of new CO2 would decrease as a result of this. The future of ocean uptake of fossil fuel CO2 may also be affected by changes in the circulation of the ocean. Surface warming is expected to be most intense in high latitudes because of the ice albedo feedback (Chapter 7). If the high latitudes warm, the overall circulation of the subsurface ocean may decrease. The circulation of the ocean may stagnate, slowing uptake ofCO2.
Biology in the ocean acts to decrease the CO2 concentration of surface waters by converting CO2 into organic carbon via photosynthesis (Chapter 8). Dead phytoplank-ton sink from surface waters exporting their carbon to the deep sea. This processes has been termed the biological pump. If all the life in the ocean were killed, that is, if the biological pump were stopped, then the CO2 concentration of the atmosphere would rise. If the biological pump were stimulated to work harder, it could decrease the CO2 concentration of the atmosphere. One proposal for dealing with global warming is to fertilize the ocean with iron. Iron concentrations are extremely low in remote parts of the ocean, far away from iron deposition from dust and iron bleeding from surface sediments. Supplying iron to the phytoplankton has been shown to stimulate phyto-plankton growth in the Southern Ocean around Antarctica for example. The problem is that it takes hundreds of years for the ocean and the atmosphere to negotiate what the atmospheric CO2 concentration should be; it's slow, recall, because the ocean circulation is so slow. Model studies have shown that fertilizing the Southern Ocean for hundreds of years might bring the CO2 concentration of the atmosphere down, but fertilizing for a few decades has very little impact.
Decreasing carbonate ion may also be detrimental to coral reef and other organisms that produce limestone, CaCO3, from calcium ion, Ca2+, and carbonate ion. It's like pouring vinegar on limestone steps; you will see bubbles as the acid of the vinegar converts the CaCO3 to CO2. Fossil fuel CO2 is itself an acid, and drives CaCO3 to dissolve. Note the counterintuitive reverse behavior; one might have thought that adding CO2 to the oceans would lead to an increase in the amount of carbon that winds up as CaCO3. The response is opposite this expectation because CO2 is an acid and CaCO3 reacts with acid. Fish and other aquatic organisms also react poorly to the acidity and higher CO2 concentrations resulting from fossil fuel CO2 release. This danger is called the acid ocean.
Uptake of CO2 into the oceans has been estimated by a number of different independent methods. These include measurements of the chemical concentrations throughout the ocean, and modeling the circulation and carbon cycle. Other chemicals serve as tracers for how the ocean circulates. These include radioactive elements produced naturally by cosmic rays, or in nuclear bomb tests in the 1960s, and industrial chemicals like chlorofluorocarbons. Another distinction between dissolution of CO2 in the ocean versus uptake of CO2 by photosynthesis on land is that photosynthesis releases oxygen, whereas dissolution in water does not. So you can measure the change in CO2 and O2 in the atmosphere to figure out what fraction of the missing CO2 is going into the ocean versus into the terrestrial biosphere by photosynthesis.
There are discrepancies between the different estimates of ocean and land carbon uptake, and uncertainties associated with each method, but in general they all point to about a 50 :50 split. The ocean gets about 2 Gton C/year of the anthropogenic CO2 and the terrestrial biosphere gets 2 Gton C/year. The terrestrial uptake of new CO2 is thought to be taking place in high northern latitudes, perhaps into the great forests of Canada and Siberia.
What about carbon uptake by the natural world in the future? It is certainly possible that the land carbon reservoir will change in the coming centuries, depending on how people decide to use the land surface, in addition to biological factors such as CO2 fertilization and increases in soil respiration. So far the amount of carbon released, about 300 Gton C, is smaller than the size of the terrestrial biosphere (500 Gton C), especially if we consider soil carbon (1500 Gton C). It is reasonable to think about the land surface as a potential major player in the carbon budget. However, if we combust all of the coal or methane clathrate deposits, the amount of CO2 we could release could be 5000 Gton C or more, several times larger than the carbon stored on land. It would be difficult to imagine the terrestrial biosphere saving the day in that case.
After hundreds of years, about 75% of the fossil fuel CO2 will dissolve in the oceans, while the remaining 25% remains in the atmosphere, awaiting slow chemical reactions with rocks that will ultimately consume it (Fig. 10.3). The CO2 invasion lowers the pH of the ocean and the concentration of carbonate ion. On timescales of thousands of years, the pH and carbonate ion concentration of the ocean are controlled by limestone,
5000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 Year AD
Fig. 10.3 Long-term fate of fossil fuel CO2. Reprinted from Archer JGR (2005).
CaCO3. Limestone on land dissolves, a chemical reaction we have already defined as weathering. Dissolved limestone flows to the ocean in rivers. Plankton re-form the solid limestone to make shells of CaCO3, some of which sink to the sea floor and are buried.
The effect of the CO2 invasion of the ocean will be to make it harder for little shells to be buried. In fact, if we ultimately release 1000 Gton C or so, there will be net dissolution of CaCO3 from the sea floor. Mankind will have reversed the net sedimentation of the ocean! Engineers in Chicago early in the last century reversed the flow direction of the Chicago River. That was impressive in its time, but it was nothing compared to this. The rate of CaCO3 weathering will exceed CaCO3 burial, and so dissolved CaCO3 will accumulate in the ocean. The fossil fuel CO2 acts as an acid, lowering the pH of the ocean, while the dissolved CaCO3 is a base, pushing ocean pH back up toward its natural value. Restoring the pH of the ocean will draw down the atmospheric CO2 somewhat. This process will take thousands of years.
On timescales of hundreds of thousands of years, the silicate weathering thermostat, defined and described in Chapter 7, will act to pull CO2 down the rest of the way toward the pre-anthropogenic value. The bottom line is that about 15-30% of the CO2 released by burning fossil fuel will still be in the atmosphere in 1000 years, and 7% will remain after 100,000 years. Truly, global warming is forever.
Of the 7 Gton C/year that mankind is releasing to the atmosphere today, 4 Gton C/year is going away as quickly as we release it. This leads to a simple but powerful conclusion: if we want atmospheric CO2 to stop going up, tomorrow, we have to reduce our CO2 emissions from 7 Gton C/year down to 4 Gton C/year, say a reduction of total carbon emission by 40%. Then the CO2 concentration in the atmosphere would stop rising, but remain at its current level of 365 ppm. This could continue until the terrestrial biosphere and the ocean equilibrated at this new level, "filled up" with the new higher CO2. No one has any idea how long it would take for the terrestrial biosphere to saturate or fill up, but the ocean would take several centuries. If emissions were stopped after that, the atmospheric CO2 concentration would remain at 365 ppm for thousands of years.
The aim of the Kyoto Protocol, the international agreement to reduce CO2 emissions discussed in Chapter 13, is to reduce emissions to about 6% below the 1990 levels, resulting in emissions that are still very close to 7 Gton C/year. CO2 emissions under business-as-usual are projected to grow, so the rather modest-sounding 6%-below-1990 target actually amounts to about 30% reductions from the projected 2010 rate. Still, this is just a drop in the bucket of what would be required to truly stabilize the CO2 concentration of the atmosphere. The Kyoto protocol by itself is not sufficient to end the problem of global warming; it can only be the first step, alas.
1. The ozone hole is not global warming. They are different issues.
3. CO2 has a long lifetime in the atmosphere. Stabilizing CO2 in the atmosphere at some "safe level" (whatever that is) will require major new energy initiatives.
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