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Chapter 10
In the last several hundred years, man has affected the carbon cycle through burning of fossil fuels and clearing of forests. Both these activities can be viewed as fluxes of carbon to the atmosphere, the former from sedimentary organic carbon, the latter from the terrestrial biosphere. The carbon flux from fossil fuel burning increased significantly over the 20 th^ century and is presently around 7 Gt per year, a reasonably well-known value, and is growing; the deforestation flux is uncertain, but 2 Gt per year is a commonly cited figure. This has resulted in a roughly 0.5% per year annual increase in the concentration of CO 2 in the atmosphere (Figure 10.30), as determined by a global system of monitoring stations, the first of which were installed by C. D. Keeling in the late 1950’s at Mauna Loa and the South Pole. This is equivalent to an average increase in the mass of atmospheric CO 2 reservoir of about 3 Gt/year since 1960. This increase in atmospheric CO 2 is only about 60% of the fossil fuel flux and
49% of the total estimated anthropogenic car- bon flux. Thus roughly 3 to 5 Gt of carbon are “missing” in the sense they are going into some reservoir other than the atmosphere, pre- sumably the ocean or terrestrial biosphere. Both sources of the anthropogenic carbon flux, biospheric carbon and sedimentary or- ganic carbon, have highly negative δ^13 C (the isotopic composition of fossil fuel burned has varied over time from δ^13 C ≈ -24‰ in 1850 to δ^13 C ≈ -27.3‰ in 1980 as coal has been partly replaced by oil and gas). Thus we might expect to see a decrease in the δ^13 C of atmospheric CO (^2) with time. This is indeed observed. First, Fig- ure 10.31 shows the δ^13 C of atmospheric CO (^2) measured at Mauna Loa over the period 1994-
- There is a clear decrease in δ^13 C over time. Superimposed on the temporal decrease are seasonal variations reflecting uptake a light carbon (making atmospheric carbon heavier) in the spring as photosynthesis increases and re- lease of light carbon the fall as respiration be-
Figure 10.32. Variation is δ^13 C in an ice core from Sipple Station, Antarctica (open squares; Friedli et al., 1986) and direct atmospheric samples from the South Pole (crosses; Keeling et al., 1989). After Friedli et al. (1986).
Figure 10.31. Smoothed monthly variation in carbon isotopic compo- sition of atmospheric CO 2 measured at Mauna Loa, Hawaii between 1994 and 2009. Data from the NOAA-ESRL Global Monitoring Sys- tem.
Chapter 10
comes dominant over photosynthesis. On a longer time scale, measurements of δ^13 C in tree rings and ice cores, compliment the direct measurements and show that δ^13 C of atmospheric CO 2 has declined by about 1.5‰ since 1800 (e.g., Figure 10.32). This is significantly greater (up to a factor of 2 greater) than that expected from burning of fossil fuel alone, which is one line of evidence that there is has been a sig- nificant destruction of the terrestrial biosphere over the last 200 years. To what degree the “missing” CO 2 (i.e., that fraction of CO 2 produced by burning fossil fuel and ter- restrial biosphere destruction that has not accumulated in the atmosphere) has been taken up by the oceans or by terrestrial reservoirs remains a debated question. Accurate predictions of future increases in atmospheric CO 2 require an answer, because storage of carbon in these two reservoirs is quite differ- ent. Once stored in the oceans, most carbon is unlikely to re-enter the atmosphere soon. However, in- creases in the terrestrial biomass or detritus and soil carbon may be unique, short-lived phenomena and, furthermore, may be susceptible to continued human intervention and climate change. Several teams of investigators have attempted use to δ^13 C changes in the atmosphere and ocean to de- termine what has happened to the balance of the anthropogenic carbon. Unfortunately, the uncer- tainties involved are such that several of these teams have arrived at somewhat different conclusions. The small concentration gradient between hemispheres (as indicated by the similar CO 2 concentrations at Mauna Loa and the South Pole) requires that much of the anthropogenic CO 2 be taken up in the northern hemisphere. Based on global isotopic measurements of δ^13 C in the atmosphere, Keeling et al. (1989) concluded that the uptake by the oceans was 2.2 Gt/year in 1980. In their model, the hemi- spheric gradient is explained by a large northern hemisphere oceanic sink (the North Atlantic?). Quay et al. (1992) concluded based on measurement of the depth-integrated change of δ^13 C in the oceans from 1970 to 1990 that the oceanic uptake rate was about 2.1 Gt/year. Tans et al. (1993) used the isotopic disequilibrium between the atmosphere and surface ocean to estimate an oceanic uptake rate of less than 1 Gt/year. By comparing seasonal and latitudinal variations in atmospheric δ^13 C, Ciais et al. (1995) concluded that the terrestrial biosphere north of 30°N took up 3.6 Gt/yr in 1992-1993, while the global ocean took up only 1.82 Gt/yr in these years. They concluded that there was a net flux of 1. Gt/yr from the tropical terrestrial biosphere (30°S to 30°N) to the atmosphere in these years, presuma- bly because of deforestation. Heimann and Maier-Reimer (1996) also used the rate of δ^13 C change in the ocean to estimate an oceanic uptake rate of 2.1±0.9 Gt/yr. They also pointed out the importance of the riverine carbon flux to the ocean, which previous workers had neglected. Thus most estimates of ocean uptake are around 2 Gt/yr but there is substantial uncertainty surrounding this number. An ocean uptake of 2 Gt per year leaves at least additional up to to 3 Gt per year, more than the de- forestation flux, that is apparently being taken up by the terrestrial biosphere. Ciais et al. (1995) con- cluded most of this occurs in northern hemisphere temperate and polar regions. This also consistent with the hemispheric gradient in atmospheric CO 2. Since most of the fossil fuel burning occurs in the northern hemisphere, we would expect the concentration of CO 2 to be slightly higher at Mauna Loa than at the South Pole. This is indeed the case; however, the hemispheric gradient in less than that pre- dicted by most models of atmospheric CO 2 transport, indicating much of the missing CO 2 must be taken up in the northern hemisphere. It would appear then that expansion of the northern hemisphere terrestrial biosphere at least bal- ances, and likely exceeds, deforestation, which now occurs mainly in the tropics. There are several pos- sible explanations for this. These are as follows.
- As agriculture became more efficient in the 20 th^ century, land cleared for agriculture in Europe and North America in previous centuries has been abandoned and is returning to forest.
- Average global temperature has increased by over 0.5°C over the last century, perhaps as a result of rising atmospheric CO 2 concentrations. This temperature increase may be producing an expansion of boreal forests.
- Pollution, particularly by nitrates emitted when fossil fuel is burned, may be fertilizing and en- hancing growth of the biosphere.
Chapter 10 Spring 2011
CO 2 in each interglacial interval reaches about the same 280-300 ppm level as in the recent, pre- industrial ancient past. Interglacial CO 2 levels appear to be lower in the 450,000-750,000 year interval. During cold intervals, atmospheric CO 2 decreases by 100 ppm more than 35% to 180 ppm, or about 100 ppm than during interglacial periods. In the Antarctic ice records, air temperatures appear to rise sev- eral hundred years before CO 2 rises when glacial epochs end (i.e., at “terminations”), implying that climate change is somehow forcing CO 2. It is clear that CO 2 is following climate at acting as a positive feedback. But what is the mechanism by which atmospheric CO 2 concentrations change in glacial cycles? Changes in seawater temperature (CO 2 is more soluble in water at lower temperature), changes in ocean volume, changes the terrestrial bio- sphere with climate and with changing sea level, high latitude peat deposits and soil carbon, the effi- ciency of the oceanic biological pump, and the vertical circulation, or “ventilation”, of the oceans have all been suspected as being part of the feedback system. Research over the last 30 years, including car- bon isotope studies, suggests that changes in ocean circulation linked to climate-related changes in atmospheric circulation may be the most important of these effects. Even from the early data, it was apparent that CO 2 had risen quite rapidly at the end of the last glaciation. The rapid changes in both atmospheric CO 2 and the larger difference in δ^13 C between ocean and atmosphere during glacial periods suggested to Broecker (1982) that the ocean must somehow be involved, since it is a much larger carbon reservoir and exchanges relatively quickly with the atmosphere. He noted that one obvious mechanism, changing the solubility of CO 2 in the ocean due to changing temperature (solubility of CO 2 increases with decreasing temperature), would produce only about a 20 ppm de- crease in atmospheric CO 2 during glacial times, and about half this would be offset by decreasing volume of the oceans. Broecker suggested the changes in atmos- pheric CO 2 resulted from chang-
Figure 10.3 4. a. Map showing the position of the strongest Westerlies today and at the Last Glacial Maximum in relation to the Antarctic Circumpolar Current (ACC). The Westerlies are pushed to these limits by the positive feedback between atmospheric temperature and CO. The threshold, which ex- tends into the Indian and Atlantic, is the northernmost zone where the Westerlies can induce strong upwelling. b. Sche- matic diagram of simplified deep circulation of the ocean. Re- spired CO 2 accumulates during glacial periods in the deep southern domain because the southern circulation was inac- tive or very weak. The box labeled ‘‘DP/ACC’’ depicts the ocean’s main upwelling zone along the southern flank of the ACC in the latitude band of Drake Passage. After Toggweiler et al. (2006).
Chapter 10 Spring 2011
ing biological productivity in the oceans, in other words, the effectiveness of the biological pump. He suggested that as sea level rose, phosphorus was removed by bio- logical processes from the ocean and de- posited on continental shelves. Because the water column is short above continental shelves, there is less opportunity for falling organic matter to be recycled before being incorporated in the sediment. He supposed that phosphorus is the limiting nutrient in the oceans; lowering its concentration would de- crease marine biological productivity and thereby allow the concentration of CO 2 in the atmosphere to rise. Boyle (1988) proposed a different mechanism for changing atmospheric CO 2 , but one that nevertheless involved the oceans. In his model, the primary driving factor is a redistribution of nutrients and metabolic CO 2 in the ocean so that they are concentrated in deep rather than in intermediate waters as in the present ocean. As the concentrations of CO 2 in the bottom water increase, pH drops and calcium carbonate sediment on the deep ocean floor dissolves. This, in turn, increases ocean alkalinity, allowing it to dissolve more CO 2 from the atmosphere. Following on this idea, Toggweiler (1999) and Toggweiler et al., (2006) suggests that this redistribution of CO 2 reflects differences in ocean circulation in the Southern Ocean as well as the North Atlantic. Toggweiler (1999) suggested that reduced ventilation of deep water reduced atmospheric CO 2 by 21 ppm, that an additional 36 ppm reduction occurs due to the consequent carbonate dissolution and reduction in ocean alkalinity, and that cold water temperatures further reduced atmospheric CO 2 by 23 ppm (CO 2 is more soluble in water at lower temperatures). Toggweiler et al. (2006) suggested specifically the ocean circu- lation changes result from a climate-driven migration of the westerly winds in the Southern Ocean. In the present, interglacial climate, these most intense Westerlies are located south of the Antarctic polar front. As a result of a phenomenon called Ekman transport, these winds drive water away from Antarc- tica, and as a result, water rises, or “upwells” from depth (Figure 10.3 4 a). This upwelling allows CO 2 built-up by respiration in the deep ocean to vent to the atmosphere, keeping atmospheric CO 2 concen- trations high. As climate cools in glacial periods, these winds migrate northward where they produce considerable less upwelling. The result is that water in the “Southern Circuit” (Figure 10.3 4 b) circulates slowly during glacial periods, allowing respired CO 2 to accumulate. Many of the mechanisms by which climate could drive changes in CO 2 involve biology, i.e., photo- synthesis and respiration. Because of the large fractionation of carbon isotopes associated with photo- synthesis, these mechanisms predict changes in δ^13 C in the oceans and atmosphere. For example, look- ing at Figure 10.2 9 , we can see that since the terrestrial biosphere has lower δ^13 C than the atmosphere, storage of carbon in the biosphere should raise atmospheric δ^13 C. On the other hand, since the oceans
Figure 10.35. Comparison of modern and glacial period in δ^13 C in shells of the benthic forams of the Cibicidoides genus recovered from cores from the South Atlantic. After Hodell et al. (2003).
