What do we know about the roles played by various water bodies in the global carbon cycle?
After reviewing this subject with respect to the vast reaches of the planet's open oceans, del Giorgio and Duarte (2002) concluded that "we do not know whether the biota of the world's oceans is a net source or sink for carbon." Near the oceans' edges, however, we know via the review of Cebrian (2002) that biotic communities in tidal regions sequester large amounts of carbon, and that marshes, seagrasses and mangroves are the greatest organic carbon traps of all marine ecosystems.
Tidal marshes exhibit especially high rates of productivity. Along the southern coastal region of North America, their rates of net primary production average about 8000 g/m2/year (Mitsch and Gosselink, 1993). They also exhibit low rates of organic matter decomposition, because the anaerobic decomposers of these oxygen-depleted environments operate at slower rates than do their aerobic counterparts of terrestrial environments (Humphrey and Pluth, 1996; Amador and Jones, 1997). Thus, it can readily be appreciated that as seas rise and encroach upon the land, rates of carbon sequestration in coastal marshes rise right along with them.
How significant is this phenomenon? Choi et al. (2001) studied coastal marshes of the St. Marks National Wildlife Refuge in Wakulla County, Florida, USA, where they discovered that high-marsh soils contain 73% more organic carbon than nearby upland forest soils, that older middle-marsh soils contain 100% more carbon, and that still-older low-marsh soils contain 287% more carbon, indicative of the fact that as seawater encroaches upon the land, carbon sequestration at any submerged location grows ever larger.
How does it happen? "The increased accumulation of soil organic carbon," in the words of Choi et al., "is the result of reduced decomposition and increased primary production." In the specific wetland studied by them, for example, productivity increased from 243 g/m2/year in the high marsh to 595 g/m2/year in the middle marsh to 949 g/m2/year in the low marsh. In addition, other studies of marshes in the same general area have indicated they are four to five times more productive than the adjacent upland forests (Krucznski et al., 1978; Hsieh, 1996) and that their soils store fully ten times more organic carbon than do those of nearby forests (Coultas, 1996). Hence, as noted by Choi et al., "carbon is being sequestrated into soils as coastal wetland evolves from high marsh to low marsh" as sea level rises.
Additional evidence for this trend is provided by Hussein and Rabenhorst (2002), who studied submerged upland tidal-marsh soils on the eastern seaboard of the United States in the lower eastern portion of Chesapeake Bay, where they established a set of transects that led from upland areas across various marshes to the main streams that feed them. Along these transects, soil cores were extracted and brought back to the laboratory for numerous analyses, including age dating and carbon (C) and nitrogen (N) content determinations. Among other things, these measurements demonstrated that the accumulation of organic C and total N were significantly related, with r2 of 0.94 and an essentially constant C/N ratio of 20:1.
Hussein and Rabenhorst also found that over the 2000-year period they studied, the deposition of organic matter in the marshes generally kept pace with the sea-level rise, so that the top of the marsh-soil's organic matter horizon typically rose just as fast as the sea level, which led them to conclude that "the sequestration of total N (g/m2 over the entire thickness of the organic horizon) will increase with time, and that sea-level rise is the primary driving force." In addition, we note that the tight coupling of the marsh-soil's organic C and total N contents implies essentially the same thing about soil carbon sequestration: it is driven by sea-level rise and will increase with time ... but only, of course, if sea level continues to rise.
Hussein and Rabenhorst further determined that the mean rate of total N sequestration at two of the tidal marshes they studied was 1.47 ± 0.3 g/m2/year over the past two millennia, while over the last century and a half it was 4.2 ± 1.15 g/m2/year, or nearly three times greater. Even more impressive were their projections for the coming century. Based on predicted rates of sea level rise, they calculated a mean total N sequestration rate of 20.0 ± 7.9 g/m2/year, which is nearly five times greater than the past century's rate and more than thirteen times greater than the mean rate of the prior two thousand years. And, again, the tight coupling they observed between the organic C and total N contents of the marsh soils implies a similar amplification of marsh-soil carbon storage rate in the next century.
These observations illustrate one of the many important ways in which earth's biosphere tends to counter increases in planetary temperature. The negative feedback effect is initiated by the photosynthetic removal of CO2 from the atmosphere by tidal-marsh vegetation and magnified by the enhanced sequestration of the biologically-captured carbon in submerged organic soils, which process is driven by rising sea levels that are sustained by the rising temperature. This interim effect of enhanced organic matter sequestration either lowers the air's CO2 content, stabilizes it, or slows its rate of rise, which leads to a reduction in the atmosphere's greenhouse effect that either reverses, stops or slows the temperature rise.
Interestingly, when the air's CO2 content is in a rising mode, the power of this negative feedback phenomenon is even greater; for the meticulous and voluminous work of Dr. B.G. Drake and a host of collaborators on the very same Chesapeake Bay wetlands has abundantly demonstrated that elevated atmospheric CO2 concentrations significantly stimulate the productivity of the marsh plants that grow there (Drake et al., 1989; Curtis et al., 1990; Arp et al., 1991; Long and Drake, 1991; Drake, 1992; Jacob et al., 1995; Drake et al., 1996; Dakora and Drake, 2000).
Moving from marine to freshwater environments, Einsele et al. (2001) have determined that freshwater lake basins, which cover only 0.8% of the area covered by the world's oceans, bury an amount of carbon that annually "reaches more than one fourth of the annual atmospheric carbon burial in the modern oceans."
How is this amazing feat accomplished? Primarily, say Einsele et al., "by the rapid accumulation of lacustrine sediments and a very high preservation factor (on average 50 times higher than that in the oceans)." In addition, smaller lakes exhibit much greater accumulation rates of atmospheric-derived organic carbon than do larger lakes (about eight times greater); and there is an enormous number of small lakes in the world. Also, man-made reservoirs are more effective yet, exhibiting atmospheric-derived organic carbon burial rates that are more than an order of magnitude (approximately 12.5 times) greater than those of small lakes. Hence, as Einsele et al. note, the planet's man-made reservoirs bury more organic carbon each year than do all natural lake basins combined.
How do these lake and reservoir sequestration rates compare with anthropogenic emission rates? According to Einsele et al., present-day carbon emissions due to the burning of fossil fuels amount to about 5.5 x 109 tons C per year, while lakes and reservoirs remove about 0.3 x 109 tons C per year, which is 5.5% of what we put into the atmosphere annually as a consequence of our utilization of coal, gas and oil. Hence, as Einsele et al. note, "the contribution of lakes and artificial reservoirs in counteracting man-made CO2 emissions should not be neglected," or, as might have been done in the past, discounted as effective ameliorative measures.
But what about the future? How are these numbers likely to change if, as most climate models predict, earth's air temperature rises and its hydrologic cycle intensifies? Einsele et al.'s findings suggest that lake and reservoir organic carbon burial rates will only grow larger under these conditions; for they say that "carbon burial rates in lakes commonly increase with change to wetter and warmer climate," due to increasing lake size, higher rates of carbonate precipitation, more stratified lake water and, hence, more oxygen-deficient bottom water, which retards decomposition.
What about the increasing number of people on the planet? How might humanity alter the picture? For one thing, more people would be expected to construct more reservoirs; and the authors note that this is indeed the case, as the total area of reservoirs throughout the world is "steadily increasing." In addition, as people and lakes inevitably come into closer contact with each other, both the production and preservation of organic carbon in lake basins rise, often by a factor of three to four. Agriculture and other types of land use, for example, tend to promote soil erosion, which leads to an increase in the mass accumulation rate of lake sediments and a consequent higher organic carbon burial rate and preservation potential. Simultaneously, say Einsele et al., "enhanced riverine nutrient supply can augment primary organic production in the lakes," as has been observed "in many lake basins close to densely populated and industrialized regions."
Clearly, these several negative feedback processes have the potential to increase the carbon sequestering prowess of lakes and reservoirs several-fold as time progresses; and a several-fold increase in their current ability to remove 5.5% of the atmospheric carbon that results from the burning of fossil fuels is truly a force to be reckoned with.
McCarty and Ritchie (2002) have recently expanded on the agricultural soil erosion aspect of this phenomenon. Working on a small watershed in Maryland, they meticulously measured soil carbon contents at 25-meter grid-points across the area's agricultural fields, as well as within the riparian ecosystem associated with a first-order stream that meanders through it. On exposed farmland hilltops, they found soil C contents to average about 1%; but in the riparian ecosystem bordering the stream, they measured soil C contents on the order of fully 20%.
Although much of this higher-elevation to lower-elevation soil C content increase was the result of C redistribution (and, thus, represented no net carbon gain for the watershed), McCarty and Ritchie noticed several things that indicate that this erosion-transport-storage process is responsible for much more than mere C redistribution.
First, they report that yearly-recurring growth and decay processes in the watershed's agricultural erosion areas "promote formation of new organic C in zones of soil loss," as predicted by Stallard (1998) and observed by Harden et al. (1999). This phenomenon results in the continual generation of new C on the erosion-prone agricultural fields and thereby provides a steady source of C for subsequent transfer to storage sites along the stream.
Second, the researchers note that carbon may be stored for much longer periods of time in the anoxic soils of the riparian ecosystem than in the aerobic soils of the watershed's farmland. On agricultural soils operated under so-called best management practices, for example, storage rates of about 0.2 tons C per hectare per year are typically observed, while rates of 1.6 to 2.2 tons C per hectare per year -- a full order of magnitude larger -- were found to prevail in the wetland soils the scientists studied.
Third, McCarty and Ritchie note that wetlands often acquire extra phosphorous from the sediments they receive from agricultural lands, and that they likewise acquire extra nitrogen via that which infiltrates the ground as a consequence of excess fertilizer applications. Both of these nutrient inputs help to boost the net primary production of the riparian ecosystem to a much higher level than what it otherwise would be able to sustain (Aerts et al., 1999; Bedford et al., 1999); and, hence, they thereby increase its active participation in the carbon sequestration process.
As a result of these three phenomena, McCarty and Ritchie say that rates of carbon sequestration in the riparian zone of the watershed "are much higher than rates that have occurred over the pre-modern history of the wetland." In fact, they find that the carbon sequestration capacity of wetlands is anywhere from four to seven times greater than what was previously believed.
It is also interesting to note in this regard that much soil carbon travels all the way to the world's oceans via river transport. Based on a 48-year record derived from an average of 17 alkalinity measurements per year, for example, Raymond and Cole (2003) recently demonstrated that the export of carbonate and bicarbonate alkalinity from the USA's Mississippi River to the Gulf of Mexico has increased by approximately 60% since 1953. "This increased export," in their words, "is in part the result of increased flow resulting from higher rainfall in the Mississippi basin," which has led to a 40% increase in annual Mississippi River discharge to the Gulf of Mexico over the same time period. The remainder of the increased alkalinity export, however, must be due to increased rates of chemical weathering of soil minerals; and our calculations indicate that the CO2-induced stimulation of basin-wide agro- and natural ecosystem productivity due to the increase in the air's CO2 content over this period, together with its associated increase in plant and microbial production of CO2 and organic acids in soils, is just sufficient to account for the implied increase in soil mineral weathering inherent in Raymond and Cole's observed alkalinity export trend (see our Editorial of 16 July 2003).
Interestingly, dying ecosystems will also lead to increased carbon export to the sea via river transport, as has been demonstrated by Schilman et al. (2001), who developed a 3600-year history of carbon isotope changes on the continental slope of the southeast sector of the Mediterranean Sea that exhibited a gradual trend of decreasing 13C in the remains of both planktonic and benthic foraminifera. They also determined that the most likely cause of this long-term decline was the introduction of increasing amounts of dissolved carbon that were supplied to the Mediterranean Sea by the Nile River, as the once-lush biota of the verdant mid-Holocene Sahara slowly disintegrated as a result of global cooling that led to decreased rainfall over that vast region. In the case of the Mississippi River drainage basin, however, we know that the cause of enhanced carbon export to the sea was just the opposite, as the remote sensing study of Zhou et al. (2001) revealed there has been an 8.4% increase in the productivity of North America's plant life over just the last two decades of the 20th century.
Returning to the sea for one last item of interest with respect to the subject of carbon sequestration in water bodies, we note that Bates et al. (1998) have calculated, on the basis of real-world measurements, that "hurricanes and tropical storms in the latitudinal band 40°S to 40°N should contribute to the ocean-to-atmosphere flux of CO2 by between 0.042 and 0.509 Pg C year-1." One way to get a feel for the significance of these numbers is to compare them to estimates of the atmosphere-to-ocean -- or oppositely-directed -- net global carbon flux of 0.290 to 1.340 Pg C per year. Doing so, one finds that the presence of hurricanes provides an effective counterforce to something on the order of 14 to 38% of this important global impetus for oceanic carbon sequestration.
What do these findings imply about the future? Obviously, it depends upon how earth's climate may change and how that change will affect hurricane characteristics such as frequency and intensity. Taking the climate-alarmist path in this instance, Bates et al. say "there may be a modest 10-20% increase in tropical cyclone intensity" as a consequence of what is nearly universally predicted for the climate of the planet, i.e., global warming; and if such were to occur, the phenomenon they elucidated would, as they say, "increase the importance of the ocean-to-atmosphere CO2 flux of hurricanes." But if the opposite were to occur, we could validly expect that (1) the sea-to-air flux of CO2 would be reduced, (2) less CO2 would accumulate in the atmosphere, and (3) there would be a smaller CO2-induced impetus for warming than would otherwise exist, which chain of events completes an important negative feedback loop between the initial increase in temperature and the warming-driven decrease in the forcing that is characteristically claimed to be responsible for the initial rise in temperature, i.e., the accumulation of CO2 in the atmosphere.
Which of these two outcomes of future global warming -- more and stronger hurricanes, or fewer and weaker hurricanes -- is more likely to be true? There is really only one way to find out; and that is to discover how hurricane frequency and intensity have changed in the past in response to global warming.
With respect to hurricane frequency, there is little evidence of real-world hurricanes occurring more frequently during real-world periods of greater, as opposed to lesser, warmth. However, several scientific studies describe just the opposite behavior, i.e., reductions in hurricane frequencies in response to global warming: Boose et al. (2001), Easterling et al. (2000), Elsner and Bossak (2001), Elsner et al. (2000), Landsea et al. (1999), Liu and Fearn (2000), Liu et al. (2001), Muller and Stone (2001), Parisi and Lund (2000), Singh et al. (2000, 2001), and Wilson (1999).
With respect to hurricane intensity, much the same is true, although the data are more sketchy. Again, for example, there is little evidence of real-world hurricanes intensifying during real-world periods of greater, as opposed to lesser, warmth; but there are some indications of reductions in hurricane intensity during warmer periods: Cerveny and Balling (1998), Landsea et al. (1999), and Pielke and Landsea (1999).
In view of these real-world observations, we think it more likely than not that if the world were to warm in the future -- for whatever reason -- there would be a modest reduction in the frequency of occurrence of hurricanes and tropical storms, as well as a small decrease in their intensities, especially in the Atlantic basin, to which most of the cited empirical studies specifically apply. Consequently, these changes would tend to keep more carbon in the world's oceans and, therefore, reduce the rate at which CO2 is accumulating in the atmosphere, which would then complete the negative feedback loop and provide a natural brake upon the warming that was responsible for the changes in hurricane characteristics.
If it is possible to sum up this vast and diverse array of findings in any meaningful way, it is perhaps appropriate to note that (1) there are a number of important processes that are related to the sequestration of carbon in various types of water bodies, and that (2) these processes typically lead to negative feedback phenomena that tend to enhance the removal of carbon from the atmosphere in the face of an increasing CO2-induced impetus for warming.
References
Aerts, R., Verhoeven, J.T.A. and Whigham, D.F. 1999. Plant-mediated controls on nutrient cycling in temperate fens and bogs. Ecology 80: 2170-2181.
Amador, J.A. and Jones, R.D. 1997. Response of carbon mineralization to combined changes in soil moisture and carbon-phosphorus ratio in a low phosphorus histosol. Soil Science 162: 275-282.
Arp, W.J. and Drake, B.G. 1991. Increased photosynthetic capacity of Scirpus olneyi after 4 years of exposure to elevated CO2. Plant, Cell and Environment 14: 1003-1006.
Bates, N.R., Knap, A.H. and Michaels, A.F. 1998. Contribution of hurricanes to local and global estimates of air-sea exchange of CO2. Nature 395: 58-61.
Bedford, B.L., Walbridge, M.R. and Aldous, A. 1999. Patterns in nutrient availability and plant diversity of temperate North American wetlands. Ecology 80: 2151-2169.
Boose, E.R., Chamberlin, K.E. and Foster, D.R. 2001. Landscape and regional impacts of hurricanes in New England. Ecological Monographs 71: 27-48.
Cebrian, J. 2002. Variability and control of carbon consumption, export, and accumulation in marine communities. Limnology and Oceanography 47: 11-22.
Cerveny, R.S. and Balling Jr., R.C. 1998. Weekly cycles of air pollutants, precipitation and tropical cyclones in the coastal NW Atlantic region. Nature 394: 561-563.
Choi, Y., Wang, Y., Hsieh, Y.-P. and Robinson, L. 2001. Vegetation succession and carbon sequestration in a coastal wetland in northwest Florida: Evidence from carbon isotopes. Global Biogeochemical Cycles 15: 311-319.
Coultas, C.L. 1996. Soils of the intertidal marshes of Florida's Gulf Coast. In Coultas, C.L. and Hsieh, Y.-P. (Eds.), Ecology and Management of Tidal Marshes, St. Lucie Press, Delray, FL, pp. 53-75.
Curtis, P.S., Balduman, L.M., Drake, B.G. and Whigham, D.F. 1990. Elevated atmospheric CO2 effects on belowground processes in C3 and C4 estuarine marsh communities. Ecology 71: 2001-2006.
Dakora, F.D. and Drake, B.G. 2000. Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland. Plant, Cell and Environment 23: 943-953.
del Giorgio, P.A. and Duarte, C.M. 2002. Respiration in the open ocean. Nature 420: 379-384.
Drake, B.G. 1992. A field-study of the effects of elevated CO2 on ecosystem processes in a Chesapeake Bay wetland. Australian Journal of Botany 40: 579-595.
Drake, B.G., Leadley, P.W., Arp, W.J., Nassiry, D. and Curtis, P.S. 1989. An open top chamber for field studies of elevated atmospheric CO2 concentration on saltmarsh vegetation. Functional Ecology 3: 363-371.
Drake, B.G., Muehe, M.S., Peresta, G., GonzalezMeler, M.A. and Matamala, R. 1996. Acclimation of photosynthesis, respiration and ecosystem carbon flux of a wetland on Chesapeake Bay, Maryland to elevated atmospheric CO2 concentration. Plant and Soil 187: 111-118.
Easterling, D.R., Evans, J.L., Groisman, P.Ya., Karl, T.R., Kunkel, K.E. and Ambenje, P. 2000. Observed variability and trends in extreme climate events: A brief review. Bulletin of the American Meteorological Society 81: 417-425.
Einsele, G, Yan, J. and Hinderer, M. 2001. Atmospheric carbon burial in modern lake basins and its significance for the global carbon budget. Global and Planetary Change 30: 167-195.
Elsner, J.B. and Bossak, B.H. 2001. Secular changes to the ENSO-U.S. hurricane relationship. Geophysical Research Letters 28: 4123-4126.
Elsner, J.B., Liu, K.-b. and Kocher, B. 2000. Spatial variations in major U.S. hurricane activity: Statistics and a physical mechanism. Journal of Climate 13: 2293-2305.
Harden, J.W., Sharpe, J.M., Parton, W.J., Ojima, D.S., Fries, T.L., Huntington, T.G. and Dabney, S.M. 1999. Dynamic replacement and loss of soil carbon on eroding cropland. Global Biogeochemical Cycles 13: 885-901.
Hsieh, Y.E.P. 1996. Assessing aboveground net primary production of vascular plants in marshes. Estuaries 19: 82-85.
Humphrey, W.D. and Pluth, D.J. 1996. Net nitrogen mineralization in natural and drained fen peatlands in Alberta, Canada. Soil Science Society of America Journal 60: 932-940.
Hussein, A.H. and Rabenhorst, M.C. 2002. Modeling of nitrogen sequestration in coastal marsh soils. Soil Science Society of America Journal 66: 324-330.
Jacob, J., Greitner, C. and Drake, B.G. 1995. Acclimation of photosynthesis in relation to rubisco and nonstructural carbohydrate contents and in situ carboxylase activity in Scirpus olneyi grown at elevated CO2 in the field. Plant, Cell and Environment 18: 875-884.
Krucznski, W.I., Subrahmanyam, C.B. and Drake, S.H. 1978. Studies on the plant community of a north Florida salt marsh. Bulletin of Marine Science 28: 316-334.
Landsea, C.N., Pielke Jr., R.A., Mestas-Nuñez, A.M. and Knaff, J.A. 1999. Atlantic basin hurricanes: indices of climatic changes. Climatic Change 42: 89-129.
Liu, K.-b. and Fearn, M.L. 2000. Reconstruction of prehistoric landfall frequencies of catastrophic hurricanes in northwestern Florida from lake sediment records. Quaternary Research 54: 238-245.
Liu, K.-b., Shen, C. and Louie, K.-s. 2001. A 1,000-year history of typhoon landfalls in Guangdong, southern China, reconstructed from Chinese historical documentary records. Annals of the Association of American Geographers 91: 453-464.
Long, S.P. and Drake, B.G. 1991. Effect of the long-term elevation of CO2 concentration in the field on the quantum yield of photosynthesis of the C3 sedge, Scirpus olneyi. Plant Physiology 96: 221-226.
McCarty, G.W. and Ritchie, J.C. 2002. Impact of soil movement on carbon sequestration in agricultural ecosystems. Environmental Pollution 116: 423-430.
Mitsch, W.J. and Gosselink, J.G. 1993. Wetlands. Van Nostrand Reinhold, New York, NY.
Muller, R.A. and Stone, G.W. 2001. A climatology of tropical storm and hurricane strikes to enhance vulnerability prediction for the southeast U.S. coast. Journal of Coastal Research 17: 949-956.
Parisi, F. and Lund, R. 2000. Seasonality and return periods of landfalling Atlantic basin hurricanes. Australian & New Zealand Journal of Statistics 42: 271-282.
Pielke Jr., R.A. and Landsea, C.N. 1999. La Niña, El Niño, and Atlantic hurricane damages in the United States. Bulletin of the American Meteorological Society 80: 2027-2033.
Raymond, P.A. and Cole, J.J. 2003. Increase in the export of alkalinity from North America's largest river. Science 301: 88-91.
Schilman, B., Almogi-Labin, A., Bar-Matthews, M., Labeyrie, L., Paterne, M. and Luz B. 2001. Long- and short-term carbon fluctuations in the Eastern Mediterranean during the late Holocene. Geology 29: 1099-1102.
Singh, O.P., Ali Khan, T.M. and Rahman, S. 2000. Changes in the frequency of tropical cyclones over the North Indian Ocean. Meteorology and Atmospheric Physics 75: 11-20.
Singh, O.P., Ali Kahn, T.M. and Rahman, S. 2001. Has the frequency of intense tropical cyclones increased in the North Indian Ocean? Current Science 80: 575-580.
Stallard, R.F. 1998. Terrestrial sedimentation and the carbon cycle: Coupling weathering and erosion to carbon burial. Global Biogeochemical Cycles 12: 231-257.
Wilson, R.M. 1999. Statistical aspects of major (intense) hurricanes in the Atlantic basin during the past 49 hurricane seasons (1950-1998): Implications for the current season. Geophysical Research Letters 26: 2957-2960.
Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback, D., Shabanov, N.V. and Myneni, R.B. 2001. Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999. Journal of Geophysical Research 106: 20,069-20,083.