The Carbon Cycle 1. The global carbon budget... 1 2. The oceanic carbon cycle... 7 3. The terrestrial carbon cycle and "missing sink"... 12 4. Fossil fuel emissions of CO 2... 15 1. The Global Carbon Budget Concerns about global climate change have arisen largely because of increasing concentrations of atmospheric CO 2, due fossil fuel burning and to deforestation, which releases CO 2 from oxidation of organic matter stored in trees and soils. In order to understand concerns over the changing composition of the atmosphere, and to project future concentrations of greenhouse gases, we will study the global cycles of carbon through the atmosphere, biosphere, soils, and oceans. Recent records of atmospheric CO 2 concentrations date back only to the late 1950's. Before 1960, it was not even known that atmospheric CO 2 concentrations were increasing with time. Since that time much scientific work has been done to determine the history of atmospheric CO 2. An especially fruitful approach has been to analyze CO 2 content of air trapped in polar ice cores. Ice core data The following figures show the records of CO 2 retrieved from polar ice cores, using different time intervals on the x-axis in a sequence leading up to the recent past. There have been peaks and dips in CO 2 concentrations over hundreds of thousands of years. This pattern repeats several times back to the oldest ice ever recovered, about 450,000 years before the present. However, the current peak, the one for which we are responsible, is the highest that has occurred on the planet in 450,000 years. This graph was made in 1995, present concentrations are just under 370 ppm. During the last ice age the atmospheric concentration of CO 2, inferred from ice cores, was about 200 ppm. At the end of the ice age, mean surface temperatures increased by 10 C while the atmospheric abundance of CO 2 increased by 80 ppm to 280 ppm. (1 ppm (part per million) corresponds to 2.1 billion tons (Gtons) of carbon in atmospheric CO 2.) Atmospheric CO 2 and global temperature track each other over geological time. During warm periods ("inter-glacials"), CO 2 was typically about 280 ppm, and values were much lower (~180 ppm) during cold periods of glacial advance. We cannot determine from the ice core record whether the CO 2 increases caused the temperature to increase at the end of the ice ages, or vice versa. However, we do know that CO 2 is a very efficient greenhouse gas, and most likely warming climate caused CO 2 to increase and vice versa, i.e. there was positive feedback. Page 1
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In the following graph, you will notice the rise in CO 2 concentrations over the past decades. You should also notice that the rate at which the concentration is rising is beginning to slow. That is, the graph appears to be leveling off a bit. The graph on the following page shows the concentrations of CO 2 at a number of stations throughout the world. The latitudes of the different stations are included in the index. Can you postulate as to why there is a "saw-toothed" appearance in the trend? Why are the "teeth" longer for some stations than for others? The saw tooth pattern superimposed on the long-term trend is due to the influence of photosynthesis and respiration, especially by terrestrial (land) plants. In the spring and summer when the surface vegetation is growing, photosynthesis uses up CO 2 from the atmosphere to make organic matter, and thus we see a decline in the concentration of atmospheric CO 2, reaching a minimum at the end of the summer. In fall and winter when leaves are decaying and vegetation and soils respire, CO 2 is released to the atmosphere and thus we see an increasing concentration, reaching a maximum in late winter. At sites very far removed from surface vegetation the amplitude of the saw tooth pattern is significantly reduced. For example, similar measurements from Antarctica show the gradual increase in the abundance of CO 2, but the season cycle due to photosynthesis and respiration is very small. Page 3
One would also expect the seasonal signal to be smaller in the Southern Hemisphere since most of the land mass on the Earth exists in the Northern Hemisphere. In fact, this is exactly what is observed. The seasonal change is largest at the highest northern latitudes, where most of the surface is covered by land and where the vegetation has the strongest seasonal growing pattern. Page 4
These data show that the terrestrial biosphere plays an important role in the global carbon budget. The seasonal cycle is much bigger than the year-to-year increase. Annually, anthropogenic (human) activity releases about 7.6 G tons C/yr (1 G ton = 1 billion tons = 1.0 x 10 12 kg of Carbon) to the atmosphere. However, we only see an increase of about 3 G tons C/year (~1.5 ppm/yr) in the atmosphere (see Table 1 in Section 4, Emissions of CO 2 ). Thus the biosphere and the oceans must take up almost half of the CO 2 released into the atmosphere by combustion and deforestation yearly. To determine the fate of anthropogenic carbon in the future we must understand how carbon is stored globally. Will the oceans and atmosphere continue remove half of what we burn? There is no assurance that this might hold true for all time. Carbon Reservoirs In the pre-industrial atmosphere the abundance of carbon was about 615 G tons (about 280 ppm of CO 2 ). Today it is about 776 G tons. As mentioned above, photosynthesis and respiration in the biosphere play an important role in the carbon cycle. Annually, the biosphere and soils are responsible for a flux of carbon out of and into the atmosphere of about 62 G tons. We assume that the system is in steady-state (i.e. the fluxes out of and into each reservoir are equal). If, for example, the flux into the biosphere were greater than the flux out of it, over the last few thousand years, all the carbon in the atmosphere would have been accumulating in the biosphere. Obviously this is not the case. In the diagram on the next page we present a simplified picture of the various global reservoirs of carbon. The budgets in the reservoirs are estimated for the pre-industrial atmosphere. Photosynthesis and respiration by organisms in the surface of the oceans are also responsible for a flux of about 60 G tons of carbon per year out of and into the atmosphere. The carbon absorbed by the photosynthetic organisms is passed up the food chain in the oceans, and as organisms die, the organic matter falls to the deep oceans. Along the way it may be consumed by other organisms, or decay, releasing CO 2. Eventually some of this organic matter may make it to the ocean floor and deposited in the sediments. This flux of organic carbon and calcium carbonate (skeletons of animals) out of the ocean reservoir, about 0.23 G ton/yr, is very small relative to the fluxes into and out of other reservoirs. But as the diagram shows, the sediments are a significant reservoir of carbon because the residence time in sediments is millions of years. Sediments may either be transported to the surface through crustal up-lift or they may be subducted down into the mantle. Once in the mantle the carbon can be returned to the surface as CO 2 either through volcanos or hot springs. On average, a carbon atom resides in the sediments for 391 million years. In comparison, the residence time of a carbon atom in the ocean is only about 614 years. Therefore, if plate tectonics were to cease, in a geologically short period of time, all the carbon on earth would become locked up in the sediments. This includes the carbon in our bodies - what would be the prospect for life on Earth? The residence time for a given reservoir is the average time that an atom of carbon resides in that reservoir. Mathematically, it is given by the total amount of carbon in the reservoir divided by the flux out of the reservoir (or the flux into it, since we are assuming steady- state the flux in must balance the flux out). We know that the system is no longer in balance since each year human activity adds an extra 7.6 G tons of carbon to the atmosphere, extracted from the Page 5
deep sediments. The question which we need to answer is how will the carbon from this new source becomes partitioned among the reservoirs that we care about, especially the atmosphere. If in time most of goes into the biosphere or the ocean then we have no reason to be concerned about build-up of CO 2 and possible associated global warming. Currently we think about half of the CO 2 released to the atmosphere by humans remains in the atmosphere. But how sensitive are the biospheric and oceanic sinks to climatic perturbations? What effect would global warming have on the ability of the oceans to continue their current rate of carbon uptake? Would increased atmospheric CO 2 cause the biosphere to Page 6
increase or decrease its uptake of carbon? These are the questions we need to answer to address the issue of global warming. Major points of this section: The atmospheric concentration of CO 2 has risen over the past century from 280 ppm to 370 ppm. The current rate of increase is ~1.8 ppm per year, corresponding to a CO 2 accumulation rate in the atmosphere of 4.0 G tons C/yr. Note: 1 ppm CO 2 = 2.1 Gtons Carbon The rise in CO 2 is due to fossil fuel combustion (presently about 6.0 G tons C/yr) and to deforestation in the tropics (presently 1.6 G tons C/yr)). Thus the total rate of CO 2 emission to the atmosphere is 7.6 G tons C/yr. Comparison of the total rate of CO 2 emission to the atmosphere (7.6 Gtons C/yr) to the actual accumulation rate of CO 2 in the atmosphere (4.0 Gtons C/yr) implies that 3.6 Gtons C/yr (or almost half of the CO 2 emitted) is being removed from the atmosphere. The two processes removing CO 2 from the atmosphere are uptake by the biosphere (photosynthesis) and dissolution in the oceans. We call these processes "sinks" for CO 2. Both are of comparable importance. We presented a figure showing the global cycling of carbon between its geochemical reservoirs for the natural ("pre-industrial") atmosphere. The carbon amounts in each reservoir, and the fluxes of carbon between reservoirs, are estimated with variable reliability. The total amount of CO 2 in the atmosphere was 615 Gtons in 1800 (280 ppm, the pre-industrial value). The rate of transfer of CO 2 to and from the oceans is 60 Gtons/yr, to and from the terrestrial biosphere, 62 Gtons/yr. The RESIDENCE TIME of carbon in any of its reservoirs is defined as the average time that an atom of carbon resides in that reservoir. It is often estiamted from knowledge of the components of the reservoir, e.g. the lifetime of a tree. Mathematically, the residence time is calculated as the total amount of carbon in the reservoir divided by the flux out of the reservoir. From the figure above you can see that the atmospheric residence time of CO 2 is 615/(60+62) = 5 years. CO 2 is thus rapidly transferred to terrestrial vegetation and to the oceans.and back again! It is imprtant to distinguish between the size (Gtons of carbon) of a reservoir and the flux (Gtons of carbon/yr) through the reservoir. 2. The role of the oceans in the budget of carbon dioxide The largest fraction of all the carbon on Earth is stored in the oceans, apart from deep sediments and rocks that don't enter into the carbon cycle on time scales of interest to us. Uptake of CO 2 occurs at the surface of the ocean where there is contact between the atmosphere and the ocean. CO 2 taken up in surface waters can then be transported to other oceanic reservoirs, such the intermediate ocean and the deep ocean. These large reservoirs, which are not in contact with the atmosphere, account for the long residence time of carbon in the ocean. The uptake of CO 2 occurs through both biological and chemical processes. Organisms at the surface of the ocean take up CO 2 during photosynthesis. The organic carbon stored in these organisms is then transferred up the food chain. CO 2 can also be taken up by the ocean through Page 7
the dissolution of atmospheric CO 2 into the surface waters. The capacity for this chemical process to take up CO 2 is determined by the ph of the ocean, and by the transport rates of water. Page 8
625 G tons Atmosphere 1 55 56 6 5 796 Gton warm surface water 17 46 Gton cold surface water 30 8 22 <1 3 1 9744 Gtons Intermediate Ocean 162 205 26280 Gtons Deep Ocean 40 0.23 90,000,000 Gtons Sediments Carbon Cycle of the Ocean (after McElroy, 2000) Reservoir sizes are in Gtons C, transfer fluxes in Gtons C/ yr. ----- Biological transport by sinking fecal pellets in the ocean. This hypothetical schematic of the carbon cycle describes CO 2 before the recent increases in atmospheric concentrations (280 ppm in the atmosphere) and assumes that all reservoirs are in steady state and undisturbed by man or by climate change. Page 9
Ocean Circulation The figure above shows a schematic picture of the ocean, with the CO 2 amounts in each reservoir. We have distinguished the warm and the cold surface ocean because only in cold regions does water become sufficiently dense in winter to sink to the deep. Most of the deep water in the ocean arrived there by cooling and sinking from the surface in polar regions. Thus the ocean circulation looks something the atmospheric Hadley circulation in reverse: negative buoyancy is created at the poles, cold dense water sinks, circulates to the tropics, and is eventually forced up to the surface and warmed. Cold water can hold more CO 2 than warm water, and thus this circulation removes CO 2 from the atmosphere and circulates it through the deep ocean, a process that takes about 600 years. This circulation may be traced out on the diagram. Acidic and basic solutions Water constantly dissociates into H + and OH - ions. The ions in turn recombine rapidly to form H 2 O again: H + + OH - <=> H 2 O. Eq (I) Reactions in both directions proceed rapidly, leading to a balance in aqueous (water) solutions where [H + ] [OH - ] = 10-14 (moles/liter) 2 DEFINITION:An acidic solution has [H + ] > [OH - ]. (NOTE:[ ] denotes concentration.) A basic solution has [H + ] < [OH - ]. From Eq. (I), [H + ] > 10-7 moles/liter in an acid, and [H + ] < 10-7 moles/liter is a base, and vice versa for [OH - ]. DEFINITION: ph = - log 10 [H + ] is convenient to describe [H + ], which varies by many orders of magnitude in solutions that we encounter every day. Examples: orange juice, vinegar, and Coca Cola are very acidic (ph = 2-3), soap and seawater are basic (ph 8-9), and cleaning solution for contact lenses is carefully kept neutral (ph = 7). Chemical and biological transformations of CO 2 in the oceans I. Chemical transformations The first step in dissolving CO 2 in the oceans is the simple uptake of gaseous CO 2 : CO 2 (g) CO 2 (aq). Eq (II) Carbon dioxide (aqueous) reacts with liquid water in a reversible reaction (i.e., goes in both directions): CO 2 (aq) + H 2 O (liquid) H 2 CO 3 (aq) Eq (III) (aqueous carbon dioxide + liquid water carbonic acid) Carbonic acid is a weak acid, which means that it that can liberate a hydrogen ion in a basic solution. This process produces a bicarbonate ion HCO - 3, and it also goes both ways: H 2 CO 3 (aq) HCO 3 - (aq) + H + (aq). Eq (IV) Page 10
In the present discussion we are considering the steps that take place when CO 2 is added to the atmosphere, which drives the reaction CO 2 (aq) + H 2 O (liquid) HCO 3 - (aq) + H + (aq). The bicarbonate HCO 3 - can also can liberate a hydrogen ion if the solution is sufficiently basic, producing a carbonate ion CO 3-2 : HCO 3 - (aq) CO 3-2 (aq) + H + (aq). Eq (V) Thus bicarbonate can act as either an acid, liberating H +, or a base (take up H + ). The carbonate ion is strictly a base, taking up H + (reverse direction of Eq. V). The activity of CO 3-2 as a base may also be look at as a reaction of CO 3-2 with water to release OH -, since H + and OH - must remain in balance (Eq. I),. CO 3-2 + H 2 O HCO 3 - + OH -. Eq (V') Seawater is basic, i.e. it has an excess of OH - ions over H + ions, by a factor greater than 10 4. Almost all dissolved CO 2 is in the form of HCO - 3 with about 10% as CO -2 3. Almost all of the H + ions liberated in these reactions will be removed by the vast excess of OH - (the ph changes only slightly) that are generated by Eq. V'. So the reactions of H 2 CO 3 and HCO - 3 that release H + are followed immediately by H + + OH - H 2 O We can summarize this series of reactions as CO 2 + OH - - HCO 3 Adding CO 2 to seawater neutralizes an OH - ion, and the process that re-supplies OH - is the reaction of water with carbonate ion, CO 3-2 + H 2 O HCO 3 - + OH - The net change in concentrations of "inorganic carbon" (CO 2, HCO 3 -, CO 3-2 ) due to adding CO 2 to seawater is therefore CO 2 + CO 3 2-2 HCO 3 -. Dissolution of CO 2 in seawater: how much CO 2 is removed by the oceans today? If we take equal volumes of air and seawater at an average temperature, say 18 C, add 15 molecules of CO 2 to the air, and wait for the process of dissolution to go to completion, approximately 14 will end up in the seawater and only one will remain in the air. This is one of the main processes that removes anthropogenic CO 2 from the atmosphere. It is reversible: the amount of CO 2 removed is limited by the quantity of seawater that comes in contact with the air, and changes in the ocean temperature, which makes CO 2 less soluble in seawater, can cause CO 2 to be emitted from the ocean. The effective depth of the atmosphere is the scale height, H (7 km), and of the oceans, 4 km. If we wait a very long time, all of the ocean water eventually comes into contact with the atmosphere (about 500 years for one "stirring"), so after a very long time (1000-1500 years) only 7-10% of fossil fuel CO 2 will remain in the atmosphere. This is the ultimate limit on the amount Page 11
of CO 2 that can be removed from the atmosphere by dissolution in the oceans on time scales of hundreds of years. Slower processes (10 4-10 6 years) involving sediments can eventually take up more CO 2, but these can be of little practical interest. Current estimates suggest that 2 Gtons C/yr of fossil fuel CO 2 dissolves in the oceans. Why is this so much smaller than the 90% that can be taken up by reaction of CO 2 with CO 3-2? Unfortunately, only a very small portion of the ocean is in intimate contact with the atmosphere in a year (the "mixed layer")--about 150 m (3%), so only about 0.15*15/7 =.30 (30%) of added CO 2 is removed annually from the atmosphere by this process. Therefore, of the 5-7 Gtons of carbon humans add to the atmosphere each year, about 30% dissolves in the ocean. There is another factor that reduces oceanic uptake of CO 2. The capacity of the ocean to take up CO 2 will diminish slowly with time, as the ph of the ocean declines due to uptake of CO 2. The ocean becomes (slightly) acidified and the dissolution reactions slow down. This effect is especially important in the surface waters that are exposed to the highest CO 2 concentrations. Uptake of CO 2 by chemical dissolution is limited by the rate for exchange between deep ocean water and surface water, and by acidification of seawater as CO 2 is added. II. Biological processes The "biological pump" Biological processes can help to transfer carbon from the surface waters of the ocean to the deep ocean, helping to remove fossil fuel CO 2 from the atmosphere. CO 2 is removed by growing plants via photosynthesis: CO 2 + H 2 O + sunlight + nutrients "CH 2 O" Most of the growing plants in the ocean are small organisms, plankton that float near the surface so that they can remain in waters where sunlight is available for photosynthesis. When animals (small ones like Daphnia, or large ones like whales) eat and excrete these organisms, they are packaged into fecal pellets large enough to sink into the deep. This provides a "rain of organic matter" that extracts organic carbon from the near-surface environment, where exchange with the atmosphere is rapid, to the deep ocean, where carbon is stored for centuries. Eventually almost all of it is oxidized to CO 2, but it can't return to the atmosphere until the deep water exchanges with surface water, a process that takes hundreds of years. A small fraction is not oxidized, but is incorporated into organic-rich sediments. A *very* small portion of that may be converted into the oil deposits used as fossil fuel. Nutrient limitation on the "biological pump" The rate at which the biological pump can operate is limited by the supply of nutrients (N, P, Fe, essentially fertilizer) in the surface waters. These nutrients have to come from the deep waters, and when you bring them up, you bring up the CO 2 that descended in past centuries! Deep waters always contain more CO 2 than would be there in equilibrium with the atmosphere, and can serve to release additional CO 2 to the atmosphere. Page 12
In this figure, we represent organic matter as [C:N:P] to emphasize that marine organic material has (approximately) a mean composition with molar ratios C:N:P=106:16:1. Organic matter that descends by gravitational settling (sedimentation) is mostly oxidized in the deep water, releasing CO 2, NO 3 - (nitrate) and PO 4 3- (phosphate) in the same molar ratios. Growth of marine plants, followed by grazing and sedimentation, effectively strips the upper ocean of nutrients. Nutrients are supplied by upwelling of deep water to the surface, and also from land as dust or run-off in rivers. When deep water is brought to the surface, CO 2 comes along with the nutrients. This process keeps high concentrations of CO 2 in the deep, but there is little effect on removing anthropogenic CO 2 from the atmosphere. However when nutrients are added from the land, the effect can be a net removal of CO 2 from the atmosphere. Recently attention has focussed on iron as a limiting nutrient for the "biological pump" of CO 2. Iron is a very common element, and it is essential for plants to make chlorophyll, but iron is very insoluble in seawater, and thus it is in short supply in most of the ocean. Experiments in which iron has been added to the sea have shown increased growth of phytoplankton, and a shift in the type of organisms present to favor larger plankton. Some scientists have argued that we should consider adding iron to seawater to stimulate growth of phytoplankton and remove CO 2 from the atmosphere. Others think this is a "risky scheme" and voice concerns about the impact of shifts in the species of plankton, or the possibility that carbon removed in this way may return to the atmosphere in a brief time by upwelling from deeper waters. Main points of Ocean Processes The oceans have a great capacity to take up CO 2, due to both chemical and biological processes: Chemical processes involve reaction of atmospheric CO 2 (a weak acid) with basic ions (OH - ) in seawater. The ocean becomes slightly acidified, and the over-all reaction may be represented by CO 2 + CO 3-2 2 HCO 3 -. The rate of removal of fossil fuel CO 2 is limited by the rate for exchange of cold deep ocean water with surface water. Biological processes involving sinking of organic matter from the surface. The rate for removal of fossil fuel CO 2 is limited by the rate of supply of nutrients to surface waters. Some people think that anthropogenic nutrients (N, P in sewage, N and S in combustion exhaust gases, Fe in dust) could be stimulating uptake of CO 2 by the oceans today. The oceans currently take up about 1/3 of the CO 2 entering the atmosphere each year due to human use of fossil fuels and to clearance of forests for agriculture. The capacity of the oceans to take up CO 2 each year by the chemical process may be expected to decline over time as the ocean becomes more acidic (ph declines). The ultimate capacity of the ocean to absorb anthropogenic CO 2 is very great, leaving < 15% in the atmosphere, but the time scale for this process is many hundreds years. Oceanic uptake is reversible! Just as the oceans "buffer" atmospheric levels of CO 2 by storing > 90% of what is added, they will tend to keep the higher levels in place so that the perturbation persists for a very long time. Page 13
3. The carbon cycle and the fate of anthropogenic carbon dioxide. SOURCES Fossil fuel Forests (deforestation React organic (reduced) carbon with O 2 atmosphere SINKS Dissolution in the ocean Inorganic C, oxidation state = 0 Organic C, oxidation state = -4 Forests (reforestation The fate of anthropogenic carbon dioxide Anthropogenic carbon dioxide is emitted to the atmosphere from burning of fossil fuel and from clearing of forests, with minor sources from production of cement and from use of wood as fuel. The amount of carbon emitted per unit of usable energy is not the same for different fuels--highest for coal (by far the most abundant fuel), lowest for natural gas (the least abundant, according to present ideas). The more hydrogen in a fuel, the lower the oxidation state of carbon, the more energy is derived from oxidizing -H to water, as summarized in the following table. oxidation state stoichiomtery fuel 0 (CH2O)n wood, peat -1 to -2.5 (CH2±) coal, oil -2.5 to -3 C2H6 ; C4H10 LPG -4 CH4 natural gas The budget of anthropogenic carbon presents a mystery CO 2 budget for 1980-1990 (IPCC, 1995) sources Average 1980-1990 fossil fuel 5.3 Gt C/yr tropical deforestation 1-2 total input 6.3 7.3 sinks and accumulation Average 1980-1990 atmospheric increase 3.2. ocean uptake 2.1 "Missing Sink" 1 2 Note: 2.12 Gtons C = 1 ppm atmospheric CO 2 The sources of carbon dioxide from human activity in the last decade were: Burning of fossil fuel 5.3 Gtons of C per year, averaged over the last decade (from U.N. estimates, +/- 5% (?)) Tropical deforestation 1-2 Gtons of C per year (from U.N. estimates, +/- 50-100% (?)) It takes 2.12 Gtons of carbon to raise the concentration in the atmosphere by 1 ppm, so the amount released is sufficient to raise the atmospheric concentration by 3.5-4 ppm / yr. The observed rise is 1-2 ppm/yr, averaging about 1.5 ppm/yr over the last decade. Where we think the carbon is going: Stored in the atmosphere: 3.2 Gtons/yr (atmospheric data, +/-15%(?)) Stored in the oceans: 2.1 Gtons/yr (from ocean partial pressure of CO 2, from 14 CO 2, and from from 13 CO 2 ; +/-20%(?)) The "missing sink" is 1 2 Gtons/yr. This CO 2 is "missing" from our budget analysis. If the missing carbon is going into the deep ocean, that's good news: it is gone for a very long time (see Page 14
next lecture). If it's going into forests, the uptake will stop as forests mature, or may even reverse if more forests are degraded or converted to agriculture. We need to identify and understand the "missing sink" in order to be able to predict the future concentrations of CO 2 : will there be 400 ppm or 700 50 years from now? Analysis of atmospheric data to infer the nature of the "missing sink" A classic analysis of the carbon cycle takes observed variations of the CO 2 concentration in the atmosphere and tries to work backwards to infer the sources. For example, Tans, Conway and Takahashi (1989) analyzed the observed concentrations of CO 2 and examined the latitude distribution of sources and CO 2 concentrations. The CO 2 emitted to the atmosphere enters mostly in the northern hemisphere and in the tropics, very little in the southern hemisphere. On average, this is expected to give rise to a mean excess of CO 2 in the northern hemisphere. Some of the CO 2 is removed by dissolution in the oceans and by growth of plants; this removal is seasonallyvarying, peaking in late spring and early fall, and it is partially reversed (net input from vegetation) in the other seasons. The net effect is a rather complex pattern of CO 2 concentrations over time and latitude, superimposed on a long-term increase (recall the figure earlier in the chapter with the different CO 2 monitoring stations and their characteristic 'sawtooth' patterns). Since most fossil fuel is burned in the northern hemisphere, most models predict a large excess Page 15
of CO 2 in the north. The figures show that north/south gradient for CO 2 varies greatly over the course of the year, and changes from year to year also. On average the northern hemisphere has 2-3 ppm more CO 2 than the southern hemisphere, and this is about half of what most computer models predict if there is no additional sink in the northern hemisphere. The conclusion from this study was that the "missing sink" lies in the northern hemisphere, probably in the re-growing forests of North America and Scandinavia This is an elegant analysis, but possibly incorrect for several reasons: We don't know how much CO 2 the ocean might transfer from one hemisphere to the other (recall our graph of CO 2 in the ocean, showing the gas moving from cold to warm waters it could also move from north to south.. Some scientists studying ocean circulation believe that the ocean waters cool in the North Atlantic, circulate through the cold interior of the ocean, and warm up in the southern hemisphere, releasing CO 2. These authors claim that, in the absence of fossil fuel inputs, there would be less CO 2 in the northern hemisphere than the southern hemisphere. Anthropogenic CO 2 just fills in that gradient, in their view. Thus they don't accept the interpretation of the atmospheric data. A relatively small change in the gradient for CO 2 inferred from measurements could have a large effect on the conclusions. Due to the large seasonal variation, and the fact that measurements of atmospheric CO 2 are all made at the surface, there is some additional doubt about the interpretation of the model results. For example, the stations near the surface may be more strongly affected by seasonal uptake of CO 2 but the models may not correctly simulate this. What can we do to find out the identity of the "missing sink"? Main points of this section The turnover time for atmospheric CO 2, due to inputs and outputs of CO 2 from vegetation, is short, less than 10 years. However, net fluxes to/from vegetation must be small; inputs and outputs must balance almost exactly, since uptake occurs mostly to short-lived materials (leaves) that decay within a year. Burning of fossil fuels leads to release of CO 2 from large, very old stores of organic material. This leads to long-term increases in concentrations of CO 2 in the atmosphere and oceans; some may be converted to organic material and stored in forests and in the deep ocean. Scientists are presently not able to account fully for the fate of CO 2 in the atmosphere. There is evidence to support the idea that a significant fraction of CO 2 released from burning of fossil fuels is presently being taken up and stored by terrestrial vegetation. Modest net growth (or depletion) of organic carbon reservoirs, in soils or forests, might occur over the long term, affecting atmospheric levels of CO 2 by up to 100 ppm +/-. It is important to understand the role of terrestrial vegetation: is there is net growth, and if so, why? There is concern that, if uptake by vegetation is large, it may stop, or even reverse, over periods of decades (perhaps uptake might be sustained a little longer if forests are deliberately grown), leading to an acceleration of CO 2 increases in the atmosphere. Page 16
4. Emission rates of CO 2 from fossil fuels and other sources What is the history of CO 2 emissions? How do these emission rates compare to natural fluxes of CO 2? In the following section we examine data pncarbon sources and sinks in nature, and try to understand the issues raised by attempts to restrain the growth rates of atmospheric CO 2. Historical rates of CO 2 emissions Global emissions of CO 2 are increasing rapidly; the rate of increase for atmospheric CO 2 is actually slowing. It is important to understand this paradox, and to determine if rapid acceleration of CO 2 growth rates in the atmosphere is likely in the future. Page 17
US emissions, total and per capita Page 18
German emissions, total and per capita Japanese emissions, total and per capita Chinese emissions, total and per capita Former USSR Emissions, total and per capita Page 19
The contributions of various countries have changed radically in the past 50 years After World War II, the US emrged as the dominant economy in the world, accounting for nearly 45% of global fossil fuel use. The fraction is about 20% today and declining rapidly as China, Japan and other far-eastern countries grow rapidly. Brazil, India and other large countries are also increasing their shares. Page 20