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PAST AND FUTURE ATMOSPHERIC CONCENTRATIONS OF CARBON DIOXIDE
Pages 186-265

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From page 186...
... In reviewing the material it is clear that all controversy in this area has not been resolved. There are three principal goals that we seek in our evaluation of the carbon cycle among atmosphere, oceans, and biota.
From page 187...
... , where A is the increase in the carbon content of the atmosphere over any period, F is the release of carbon to the atmosphere from combustion of fossil fuels in the same period, S is the net transfer to the oceans in the same period, and B is the absorption or release of carbon by the biota in the same period, then the terms S and B are the least well determined. The "airborne fraction" is not directly determined from either oceanic or biosphere experiments or models.
From page 188...
... of methane hydrates in continental slope sediments suggests, that climate change may bring about surprising changes in fluxes of carbon. 3.2 CARBON DIOXIDE AND THE OCEANS Peter G
From page 189...
... The deep ocean waters have a salinity of about 34.9 parts per thousand and a temperature of about 2°C. Most of the deep waters of the world's oceans are formed in wintertime in the Norwegian and Greenland Seas and in the Weddell Sea.
From page 190...
... Reid and Lynn (l97l) have elegantly shown that the salinity maximum of the North Atlantic deep waters can be traced on their trajectory around the globe.
From page 191...
... In the North Atlantic Ocean this horizon occurs at great depth, approximately 5000 m. Progressing along the path of the deep circulation, oxygen and pH are lowered with increasing (X>2 levels, and progressively more calcium carbonate is dissolved.
From page 193...
... l93 CT LU o to *
From page 194...
... . The alkalinity of ocean surface waters is quite well correlated with salinity; at a salinity of 35°/oo it is approximately 2300 equivalents/kg.
From page 195...
... The resistance to change increases, the ocean absorbs proportionately less CO2, and the airborne fraction rises. This is a complex system, sensitive to the alkalinity/total CO2 ratio, and hence pH.
From page 196...
... Increasing CO2 levels raise the buffer factor and diminish the oceans tendency to absorb CO2. (From Takahashi et al., l980b.)
From page 197...
... Typical dissolved organic matter concentrations in ocean water are l mg of C/kg (83 y mol/kg)
From page 198...
... In this calculation, the dissolution of deep calcium carbonate was found to have little immediate effect on rising atmospheric CO2 levels, since the affected seawater would be sequestered in the deep ocean. As this water is brought into contact with the atmosphere, it will slowly draw down the atmospheric CO2
From page 199...
... In the future it appears inevitable that fossil fuel C02-indueed dissolution of calcium carbonate will take place in the ocean. The most sensitive site appears to be in the deep North Atlantic Ocean where waters enriched in industrial CO2 begin their deep ocean tour (Broecker and Takahashi, l977)
From page 200...
... has presented ocean CO2 data based on gas extraction and manometric measurement accurate to +0.5 ymol/kg. The current rate of increase of the total CO2 concentration in ocean surface waters is calculated to be approximately l pmol/kg/yr.
From page 201...
... 20l d id E 11)
From page 202...
... to the TTO-North Atlantic expedition (l98l)
From page 203...
... The calculations of gas exchange rates appear to rest on sound principles. The characteristic exchange time for CO2 is about l0 times longer than for the nonreactive gases (N2 and 02, for example)
From page 204...
... The results shown in Figure 3.6 represent a vertical slice through the western basin of the North Atlantic. They plainly reveal the formation of new deep waters; however, such a representation understates the great horizontal extent of the oceans.
From page 205...
... Tritium was principally injected into the atmosphere of the northern hemisphere by nuclear bomb tests in l962. The figure illustrates the labeling of the ocean by the invasion of a passive tracer in a l9-year period.
From page 206...
... First, there is no time series of ocean CO2 measurements of high accuracy that would match the Mauna Loa and other atmospheric records. Early oceanic CO2 measurements quite simply lack accuracy and precision and rest on an unsatisfactory thermodynamic basis.
From page 207...
... The carbon dioxide concentration in the North Atlantic has been measured in parallel with the tritium section in Figure 3.7 (Brewer, l983)
From page 208...
... (From Brewer, l983.) takes place in the northern hemisphere.
From page 209...
... 90° S 40° S 20° S 0 20 N LATlTUDE (deg) 40° N 90° N FIGURE 3.9 Atmospheric CO2 levels along a Pacific Ocean transect, normalized to zero at the South Pole.
From page 210...
... However, this is likely a lower limit since oceanic deep waters are formed in areas of marked negative disequilibrium with the atmosphere. As our ocean data base grows, the one-dimensional models will become increasingly inadequate, and incorporation of the C02 and tracer data into new models will be required.
From page 211...
... We do not know when and where calcium carbonate dissolution will occur. We do not know how future warming will affect ocean circulation and whether we will detect this warming in the CO2 signal.
From page 212...
... . The Global Carbon Cycle.
From page 213...
... . The Global Carbon Cycle: What we know and could know from atmospheric, biospheric and oceanic observations.
From page 214...
... . The carbon dioxide content in the surface waters of the Pacific Ocean.
From page 215...
... . Geographical, seasonal, and secular variations of the partial pressure of CO2 in surface waters of the North Atlantic Ocean: The results of the North Atlantic TTO Program.
From page 216...
... . It is about 5 ppm at Mauna Loa and l ppm at the South Pole (Keeling et al., l976a,b)
From page 217...
... of terrestrial biomass have suggested additionally that the average standing crop of organic matter per unit area is lower than estimated by Whittaker and Likens (l973)
From page 218...
... : ( NEP « GP - (RA + %) , where NEP is the net ecosystem production, the net flux of carbon into or from an ecosystem; GP is the gross production, total photosynthesis of the ecosystem; RA is the respiration of the autotrophs, the green plants; Rg is the respiration of the heterotrophs, including all animals and organisms of decay; and RA + Rg is the total .
From page 219...
... in which gross photosynthesis is equaled by total respiration. Any change in the relationship between gross production and either segment of the total respiration shifts the ratio and causes a positive or negative net ecosystem production.
From page 220...
... O M ir O O i 330 8 J310 11/76 M M FIGURE 3.l2 (a) The course of total respiration and gross photosynthesis of an oak-pine forest in central Long Island, New York.
From page 221...
... . This new curve follows closely the pattern of oscillation observed in the CO2 concentration at Mauna Loa and elsewhere, although the amplitude observed around the world differs greatly from that calculated for the forest of central Long Island shown in Figure 3.l2.
From page 222...
... forests still dominate the biotic segment of the global carbon cycle. Plants in natural forests live in conditions of extreme competition for light, water, nutrients, space, and, probably, C02 during daylight.
From page 223...
... introduced a factor into their analysis that allowed an expansion of the biotic pool of carbon as a function of the increase in CO2 in air. It was assumed that this so-called "B-factor" was the only important biotic consideration in the global carbon cycle.
From page 224...
... The use of the 6-factor was a pragmatic solution to a complex and puzzling issue that arose from attempts to analyze the global carbon cycle through a simple model. Its use should now be replaced by separate analyses of the effects of (a)
From page 225...
... , the 0.5°C warming observed may have increased total respiration of terrestrial ecosystems by l0-l5%. Such a change would appear as a reduction in net ecosystem production; it might, of course, simply offset an increase in gross photosynthesis.
From page 226...
... 226 TABLE 3.3 Estimates of Annual Net Carbon Flux between Terrestrial Ecosystems and the Atmosphere in or about l980*
From page 227...
... 227 carbon storage in forests, locally or globally. Such a model, constructed around the central principle of ecological succession, has been developed and used; results are reported below.
From page 228...
... v o oo in crt mr^ro vom ifl r- in^ „ •» r- m 0j ^J UH a o 3 ooo ooo oo o o oo 41 a o J3 *
From page 229...
... The results are not consistent with current estimates of other segments of the global carbon cycle. The global carbon balance can be expressed as A » F - S + B, where A is the increase in the carbon content of the atmosphere over any period, F is the release of carbon to the atmosphere from combustion of fossil fuels in the same period, S is the net transfer to the oceans in the same period, and B is the absorption or release of carbon by the biota in the same period.
From page 230...
... As we have seen, most direct analyses of recent changes in the biotic reservoirs of carbon, when they include releases of carbon from decay of organic residues from the plants, organic matter in soils, and the decay of wood and forest products removed from the site, show a net release of carbon from destruction of forests. The release is estimated currently here on the basis of extensive experience as between l.8 and 4.7 Gt of C per year (Houghton et al., l983; Woodwell et al., l983a)
From page 231...
... A warm winter in the northern hemisphere, for example, would increase the total respiration without affecting the photosynthetic withdrawal during summer appreciably; the excess CO2 would appear as an increased late-winter peak in the Mauna Loa record. That CO2 would be mixed into the rest of the atmosphere over the ensuing weeks and would have little or no effect on the late summer minimum.
From page 232...
... . The airborne fraction, calculated solely on the basis of the.Mauna Loa data and estimates of combustion of fossil fuels was 0.55 for the period l959-l978, according to Bacastow and Keeling (l98l)
From page 233...
... on releases from combustion of fossil fuels. Time Period Biotic Release Fossil Release Total Release Atmospheric Increase Airborne Fraction e!
From page 234...
... The most powerful evidence for the importance of the biota in affecting the C02 content of the atmosphere is the annual oscillation in the C02 concentration observed at Mauna Loa and in virtually every other annual record of atmospheric CO2. The extent of the biotic influence and the factors that govern it remain uncertain.
From page 235...
... Any effect on total photosynthesis would be largely through an increase in the length of the growing season. An increase in the growing season with impact on photosynthesis comparable with that of temperature on respiration would not be expected from a l°C increase in temperature.
From page 236...
... . Atmospheric carbon dioxide and radiocarbon in the natural carbon cycle.
From page 237...
... In The Role of Terrestrial Vegetation in the Global Carbon Cycle: Measurement by Remote Sensing, G
From page 238...
... . Atmospheric carbon dioxide variations at Mauna Loa Observatory, Hawaii.
From page 239...
... In The Role of Terrestrial Vegetation in the Global Carbon Cycle: Measurement by Remote Sensing.
From page 240...
... . The Role of Terrestrial Vegetation in the Global Carbon Cycle: Measurement by Remote Sensing.
From page 241...
... . Report of the Woods Hole Conference on the Biotic Contributions to the Global Carbon Cycle at the Ecosystems Center.
From page 242...
... Stations were established at the South Pole and at ll,l50 feet aside Mauna Loa in Hawaii. The latter, the better record, is reproduced in Figure 3.l4.
From page 243...
... 3.4.2 Changes in Atmospheric CO? Growth Rate with Time and Space There appear to be two kinds of changes in the year-to-year growth rate at Mauna Loa (Figure 3.l4)
From page 245...
... ,'»1• -1 -- i i i i i i i i i i i * i i i i i i i 1958 1960 1962 1964 1966 1968 1970 1972 197419 YEAR FIGURE 3.l6 Time history of residuals of monthly carbon dioxide concentrations after removing the long-term upward trend and seasonal variability at Mauna Loa and the South Pole (lower section)
From page 246...
... An analysis using a two-dimensional transport model (vertical and north-south directions) suggests that the lag of changing concentrations among stations (Mauna Loa, the South Pole and Australia)
From page 247...
... This airborne fraction is determined from the ratio of the C02 increase in the atmosphere (as found from the average annual increases at Mauna Loa and the South Pole) to the amount added to the air from all sources that one wishes to assume.
From page 248...
... Weather Ship P at 50.0° N and (b) Mauna Loa Observatory at l9.5° N
From page 249...
... contends that the seasonal cycle in 13C measurements are consistent with land plants being the primary source of the annual cycle of CO2 concentration for northern hemisphere stations. At the South Pole the isotopic data suggest an oceanic source for the cause of the much smaller annual cycle.
From page 250...
... The isotopic studies in tree rings and from current air samples offer potential to elucidate further the carbon cycle and the contribution of the nonfossil fuel C02. For example, following the fate with time of the nuclear weapons test l4CO2 in the atmosphere can continue to provide new information on the atmospheric residence time of the fossil fuel C02.
From page 251...
... . The global carbon cycle: what we know and could know from atmospheric, biospheric, and oceanic observations.
From page 252...
... A small part may represent release of methane from methane hydrates in continental slope sediments as the ocean responds to atmospheric warming. MacDonald (l982a)
From page 253...
... But with a rise in ocean-bottom temperatures, the uppermost layers of sediments would also become warmer and methane hydrates would become unstable in the upper limit of their depth range, that is, about 300 m in the Arctic and about 600 m at low latitudes. 3.5.2 Formation of Methane Clathrate in Continental Slope Sediments The quantity of clathrates that will be released from sediments under the seafloor as a result of ocean warming depends on the distribution of clathrates with depth and on their total abundance in the sediments.
From page 254...
... . Below water depths of 300 to 600 m, depending on bottom-water temperature, methane in excess of the quantity that can be dissolved in the interstitial water will be converted to methane hydrate as soon as it is formed.
From page 255...
... is the partial pressure of methane, which is equal to hydrostatic pressure when the water is saturated with methane. The water depths below which methane hydrate in the uppermost layers of marine bottom sediments will be stable at different temperatures can be calculated from this equation, with the simplifying assumption that
From page 256...
... _ _ 289 l 3l9 2 35l 3 388 4 429 5 475 6 528 7 588 8 650 9 724 l0 807 3.5.3 Effect of Carbon Dioxide-Induced Warming on Continental Slope Clathrates With carbon dioxide-induced warming of the atmosphere, ocean surface temperatures will rise by a nearly equal amount, and heat will be carried downward by advection and eddy diffusion into the subsurface water layers. For a doubling of atmospheric CO2 and the expected increase in other "greenhouse gases," with an assumed sensitivity of global average temperature of 3°C for a CO2 doubling, the temperature increase in different latitudes at the water depths below which methane hydrate is stable at present can be estimated (see this volume, Chapter 8, Section 8.3)
From page 257...
... Provided the relation between the concentration required for bubble formation and hydrostatic pressure is approximately linear, the required concentration at about 500 m will be 44 mmol. With our assumed concentration of 200 mmol kg"l of interstitial water, close to 80% of the methane released from clathrate should escape from the mud in bubbles and should rise rapidly to the sea surface before it can be oxidized in the water.
From page 259...
... Bell (l982) assumed that a CO2-induced warming of the Arctic Ocean would release methane hydrates during l00 years from the top 40 m of the bottom sediments over half the area between water depths of 280 and 370 m.
From page 260...
... . This shows the locations in the continental slopes of the ocean floor and in the Black and Caspian Seas, in which the existence of methane clathrates has been inferred from high gas contents in cores of the Deep Sea Drilling Project or in which their presence is suspected from acoustic reflections from a layer below the sediment surface that parallels the ocean-bottom topography (Bryan, l974; Stoll et al., l97l)
From page 261...
... . The Long-Term Impacts of Increasing Atmospheric Carbon Dioxide Levels.
From page 262...
... In some instances, other observations, not involved in model development, may also be used to validate the model. Few, if any, models of the carbon cycle would likely be published unless there were some agreement with the Mauna Loa C02 record.
From page 263...
... 3.6.2 Comparison of Parameters within a Single Model As part of the analysis of their carbon cycle model, Enting and Pearman (l982) have undertaken a sensitivity study.
From page 264...
... (%) -- Rate of exchange between air and sea 2x and 0.5x standard rate of exchange 2 0.3 Rate of exchange between mixed layer of the ocean and the deep ocean 2x and 0.5x standard rate of exchange 70 9 Both of above taken together 74 l0 Biospheric uptake due to enhanced atmospheric CO2 No uptake and a standard value of 0.266 229 29 Buffer factor Constant (l0)
From page 265...
... . The effects of carbon cycle model error in calculating future atmospheric carbon dioxide levels.


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