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1 Carbon II2 http://www.globalwarmingart.com/wiki/Image:Phanerozoic_Carbon_Dioxide.png Direct determination of past carbon dioxide levels relies primarily on the interpretation of carbon isotopic ratios in fossilized soils or shells and through measure of stomatal density in fossil plants.3 Sources and sinks of CO2 during the industrial period. Current anthropogenic emissions of CO2 are primarily the result of the combustion of fossil fuels. This figure (Figure 3.3 IPCC) illustrates both the fossil fuel emissions of carbon (PgC/yr = Gton /yr). The red and blue lines are estimates of the increase in atmospheric CO2 from annual and monthly mean measurements of CO2 from a group of ground stations. In addition to the fossil fuel emissions, it is estimated that land use change (deforestation, etc.) contributes and additional 10-30% more carbon. It is clear from this figure that only a fraction (~50%) of the emitted carbon remains in the atmosphere. The rest of the carbon must have been taken up by the land and ocean. The uptake is quite variable. Arrows mark the periods of El Nino. Oxygen Measurements of atmospheric oxygen can be used to constrain the relative role of land and ocean processes in sequestering the 'missing' carbon. Keeling (UCSD-Scripps) obtained the first oxygen measurements with sufficient precision to see the decrease due to fossil fuel combustion. Because the [O2]/[Air] is 209,000 ppm, only a minute change occurs. The uptake of CO2 by the ocean and terrestrial biosphere have different effects on the concentration of O2 in the atmosphere. Dissolution of CO2 in the ocean does not alter O2 while net terrestrial uptake (photosynthesis - respiration and other oxidation processes, including fire) increases O2 in a known stochiometric ratio. The different influences of these processes can be used to partition the total CO2 uptake into land and ocean components, as shown in this figure from IPCC, 2001.4 The “fossil fuel burning” arrow denotes the effect of the combustion of fossil fuels based on the relatively well known O2:CO2 stoichiometric of the different fuels. Uptake by land and ocean is constrained by the known O2:CO2 stoichiometric ratio of these processes, defining the slopes of the respective arrows. A small correction is made for differential outgassing of O2 and N2 with the increased temperature of the ocean as estimated by Levitus et al. (2000). During the 1990s, both ocean and terrestrial sinks were essentially equally important for removing CO2. An important question for future CO2 is will these sinks continue as CO2 increases further. Physical uptake of carbon in the surface ocean mixed layer. From last lecture we learned that the equilibrium exchange of carbon between the atmosphere and ocean is driven by: CO2 + CO32- + H2O ⇔ 2 HCO3-. CO2 increases in the atmosphere drive increases in DIC (uptake of CO2). The capacity of surface waters to take up anthropogenic CO2, however, is decreasing as CO2 levels increase. As atmospheric CO2 increases, the dissolved CO2 content of surface seawater increases at a similar rate, but most of the added CO2 ends up as HCO3- and the CO32- content decreases. This restricts further uptake, so that the overall ability of surface sea water to take up CO2 decreases at higher atmospheric CO2 levels. The effect is large. For a 100 ppm increase in atmospheric CO2 above today’s level (i.e., from 370 to 470 ppm) the DIC concentration increase of surface sea water is already about 40% smaller than would have been caused by a similar 100 ppm increase relative to pre-industrial levels (i.e., from 280 to 380 ppm). The contemporary DIC increase is about 60% greater than would result if atmospheric CO2 were to increase from 750 to 850 ppm. The uptake capacity for CO2 also varies significantly due to additional factors, most importantly seawater temperature (the equilibrium pCO2 in seawater increases by about 10 to 20 ppm per °C), salinity and alkalinity (the latter being a measurable quantity approximately equal to [HCO3-] + 2 x [CO32-]). Alkalinity is influenced primarily by the cycle of CaCO3 formation (in shells and corals) and dissolution.5 On longer time scales (decade to century), atmospheric CO2 is strongly influenced by mixing of deep ocean water to the surface. Although deep waters is generally supersaturated with respect to the even the modern atmosphere, mixing of these water masses to the surface tends to draw down atmospheric CO2 in the end as the high nutrients stimulate formation of organic carbon. Several coupled atmosphere-ocean models have shown, however, that global warming is accompanied by an increase in vertical stratification within the ocean and therefore reduced vertical mixing. On its own, this effect would tend to reduce the ocean CO2 uptake. However, changes in stratification may also drive changes in the natural carbon cycle. The magnitude and even the sign of changes in the natural cycle are much more difficult to predict because of the complexity of ocean biological processes (as discussed last lecture). The uptake (and release) of carbon from the land. Inverse models have been used to estimate the geographical variability in the sources and sinks of carbon. In these models, a 'best fit' to the atmospheric CO2 record is determined from estimates of the sources with variation in the size and location of the sinks. A model of the winds (and in some models the ocean) is used to transport the CO2 and estimates of fossil fuel CO2 uptake in three broad bands are produced. The results from eight such models are shown in this figure from the IPCC, 2001 report. Positive numbers denote fluxes to the atmosphere; negative numbers denote uptake from the atmosphere. The ocean-atmosphere fluxes represent the natural carbon cycle; the land-atmosphere fluxes may be considered as estimates of the uptake of anthropogenic CO2 by the land. The sum of land-atmosphere and ocean-atmosphere fluxes is shown because it is somewhat better constrained by observations than the separate fluxes, especially for the 1980s when the measurement network was less extensive than it is today.6 For the oceans, carbon is suggested to be taken up at high latitude and degassed at low latitude (the natural carbon cycle driven by thermal effects in the ocean. In the tropics, most models suggest a net flux of carbon from the land to the atmosphere due to biomass burning. In the northern hemisphere a large land sink is


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CALTECH ESE 148A - Carbon II

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