Sink For Anthropogenic CO2 Research Articles

Exploring continental margin carbon fluxes on a global scale

How quickly anthropogenic CO2 is being absorbed by the ocean remains one of the most critical and yet unanswered questions asked by climate researchers. Since the ocean is a major sink of anthropogenic CO2,an accurate estimate of the present oceanic uptake rate of anthropogenic CO2 is essential for reliable prediction of future atmospheric CO2. This number is difficult to pin down because the net airto‐sea CO2 flux is a small difference (about 2 Pg C yr−1, 1 Pg = 1015 g) between two huge fluxes—namely, the uptake and the release of CO2 at sea surface (each about 90 Pg C yr−1).

Simulations of storage of anthropogenic carbon dioxide in the North Pacific using an ocean general circulation model

There is a large uncertainty of how much anthropogenic CO2 has been and will be taken up by the ocean. The North Pacific is normally considered a small sink of anthropogenic CO2. Recently, some researchers have proposed that the North Pacific may take up more anthropogenic CO2 than thought previously. Here we explore this issue with a basin-wide OGCM of the North Pacific. The sensitivities of ocean circulation and the redistribution of dissolved anthropogenic CO2 in the North Pacific to the values of some mixing parameters are examined. The increase of isopycnal diffusivity generally leads to improvement of distributions of water masses. Larger isopycnal diffusivity produces larger CO2 uptake in the subpolar region but smaller CO2 uptake in the tropical region. Increasing thickness diffusivity reduces CO2 uptake in both the subpolar and subtropical regions, and also reduces the inventory of CO2 in the western subtropical region. Both smaller isopycnal and thickness diffusivities result in a large net transport of CO2 from the North Pacific to the South Pacific. Simulated results show that the North Pacific has taken up about 23 GtC of excess carbon dioxide released by human activities between 1800 and 1997. The averaged uptake rate in the North Pacific during 1990–1997 is 0.40 GtC/year. Our model estimates the largest air–sea fluxes along the western boundary around 42°N, 150°E and in the equatorial Pacific. Our simulated inventories slightly overestimate data-based estimates in the eastern North Pacific, but exhibit less penetration of anthropogenic carbon dioxide in the western North Pacific.

Biogeochemical responses of the carbon cycle to natural and human perturbations; past, present, and future

In the past three centuries, human perturbations of the environ- ment have affected the biogeochemical behavior of the global carbon cycle and that of the other three nutrient elements closely coupled to carbon: nitrogen, phosphorus, and sulfur. The partitioning of anthropogenic CO2 among its various sinks in the past, for the present, and for projections into the near future is controlled by the interactions of these four elemental cycles within the major environmental domains of the land, atmosphere, coastal oceanic zone, and open ocean. We analyze the past, present, and future behavior of the global carbon cycle using the Terrestrial-Ocean-aTmosphere Ecosystem Model (TOTEM), a unique process-based model of the four global coupled biogeochemical cycles of carbon, nitrogen, phosphorus, and sulfur. We find that during the past 300 yrs, anthropogenic CO2 was mainly stored in the atmosphere and in the open ocean. Human activities on land caused an enhanced loss of mass from the terrestrial organic matter reservoirs (phytomass and humus) mainly through deforestation and consequently increased humus remineralization, erosion, and transport to the coastal margins by rivers and runoff. Photosynthetic uptake by the terrestrial phytomass was enhanced owing to fertilization by increasing atmospheric CO2 concentrations and supported by nutrients remineralized from organic matter. TOTEM results indicate that through most of the past 300 yrs, the loss of C from deforestation and other land-use activities was greater than the gain from the enhanced photosynthetic uptake. During the decade of the 1980s, the terrestrial organic reservoirs were in rough carbon balance. Organic and carbonate carbon accumulating in coastal marine sediments is a small but significant sink for anthropogenic CO2. Increasing inputs of terrestrial organic matter and its subse- quent oxidation in the coastal margin (increasing heterotrophy) were significant sources of CO2 in coastal waters in the 20th century. However, the coastal ocean did not evolve into a greater net source of CO2 to the atmosphere during this period because of the opposing pressure from rising atmospheric CO2. Since pre-industrial time (since 1700), the net flux of CO2 from the coastal waters has decreased by 40 percent, from 0.20 Gt C/yr to 0.12 Gt C/yr. TOTEM analyses of atmospheric CO2 concentrations for the 21st century were based on the fossil-fuel emission projections of IPCC (''business as usual'' scenario) and of the more restrictive UN 1997 Kyoto Protocol. By the mid-21st century, the projected atmospheric CO2 concentrations range from about 550 ppmv (TOTEM, based on IPCC projected emissions) to 510 ppmv (IPCC projection) and to 460 ppmv (TOTEM, based on the Kyoto Protocol reduced emissions). The difference of about 40 ppmv between the IPCC and TOTEM estimates by the year 2050 reflects the different mechanisms within the C-N-P-S cycles on land that are built into our model. The effects of the reduced emissions prescribed by the Kyoto Protocol begin to show in the atmospheric CO2 concentrations by the mid-21st century, when our model projects a rise to 460 (year 2050) and 490 ppmv (2075), relative to about 360 ppmv in 1995. However, these projected increases assume no major changes in the present biogeochemical feedback mechanisms within the system of the coupled C-N-P-S cycles, no global changes in the kind and distribution of ecosystems in response to the rising CO2 and possibly temperature, and no changes in the mechanisms of CO2 exchange between the atmosphere and the ocean, such as could be induced by changes in the intensity of oceanic thermoha- line circulation.

Carbon Exchange Dynamics and Mineral Weathering in a Temperate Forested Watershed (Northern Michigan): Links Between Forest Ecosystems and Groundwaters

Our research team is determining the fate of organic carbon produced in temperate forests, which may constitute a major sink for anthropogenic CO2. Importantly, studies of carbon allocation in forests under enhanced and ambient CO2 growth conditions show that above and below ground carbon storage, as well as root and microbial respiration, all increase at elevated PCO2 (e.g. Zak et al., 1993). A portion of this additional organic carbon is rapidly recycled via respiration to the atmosphere. However, an unknown amount of organic carbon may be transported from the soil zone to the regional groundwater system for longer term storage. Past investigations have demonstrated important links between carbon transport and carbonate mineral weathering under open and closed system conditions (e.g. Rightmire and Hanshaw, 1973, Reardon et al., 1979). As an extension of these classic studies, we are investigating how increased carbon fixation and processing in forest stands and soils may affect the overall carbon budget in groundwaters and the rate of carbon processing at the watershed scale.

Multiple timescales for neutralization of fossil fuel CO2

The long term abiological sinks for anthropogenic CO2 will be dissolution in the oceans and chemical neutralization by reaction with carbonates and basic igneous rocks. We use a detailed ocean/sediment carbon cycle model to simulate the response of the carbonate cycle in the ocean to a range of anthropogenic CO2 release scenarios. CaCO3 will play only a secondary role in buffering the CO2 concentration of the atmosphere because CaCO3 reaction uptake capacity and kinetics are limited by the dynamics of the ocean carbon cycle. Dissolution into ocean water sequesters 70–80% of the CO2 release on a time scale of several hundred years. Chemical neutralization of CO2 by reaction with CaCO3 on the sea floor accounts for another 9–15% decrease in the atmospheric concentration on a time scale of 5.5–6.8 kyr. Reaction with CaCO3 on land accounts for another 3–8%, with a time scale of 8.2 kyr. The final equilibrium with CaCO3 leaves 7.5–8% of the CO2 release remaining in the atmosphere. The carbonate chemistry of the oceans in contact with CaCO3 will act to buffer atmospheric CO2 at this higher concentration until the entire fossil fuel CO2 release is consumed by weathering of basic igneous rocks on a time scale of 200 kyr.

Spatial and Temporal Patterns in Terrestrial Carbon Storage Due to Deposition of Fossil Fuel Nitrogen

Fertilization of the biosphere by nitrogen deposition represents an important connection between atmospheric chemistry and the global carbon cycle. We describe a modeled estimate of terrestrial carbon storage arising from deposition of nitrogen derived from fossil fuels that accounts for spatial distributions in deposition and vegetation types, turnover of plant and soil carbon pools, and the cumulative effects of deposition. Vegetation type has a pronounced effect on C uptake; the combination of high C : N ratios and long lifetimes in wood may create a significant sink in forests, but much of the nitrogen falls on cultivated areas and grasslands, where there is limited capacity for long‐term carbon storage. We estimate 1990 net carbon uptake due to deposition of fossil‐fuel N to be between 0.3 and 1.3 Pg C/yr [1 Pg = 1015 g], depending on the fraction of C allocated to wood, with a best estimate of 0.44‐0.74 Pg/yr. Cumulative C storage since 1845 is estimated to be about 25% of the proposed terrestrial sink for anthropogenic CO2. Continued exposure to high N deposition, however, will decrease the extent of N limitation in terrestrial ecosystems, thereby limiting the persistence of any N‐derived carbon sink.

Bank-derived carbonate sediment transport and dissolution in the Hawaiian Archipelago

This investigation used two approaches to examine the flux of bank-derived carbonate particles and determine the potential influence of benthic carbonate particle dissolution on the carbon chemistry of the waters around the Hawaiian Archipelago. First, the particle flux near several representative carbonate banks in the Hawaiian Archipelago was measured and compared with the flux at a distal site (ALOHA) approximately 100 km north of Oahu, Hawaii. The results of four sediment trap deployments on three carbonate banks in the Hawaiian Archipelago demonstrate that the flux of bank-derived carbonate particles are consistently one to two orders of magnitude higher than the fluxes at the distal station. Furthermore, the mineralogy of the carbonate flux near the banks, which includes very soluble bank-derived aragonite and magnesian calcite particles, is distinctly different from that of the distal fluxes. Second, the chemistry of the waters at each bank station along the archipelago was characterized and compared with the chemistry of the distal waters to determine if differences in the particle flux were reflected in the carbon chemistry. Higher alkalinity and carbonate ion concentrations were observed around all of the banks studied. The saturation state of these waters suggests that the dissolution of some magnesian calcite and aragonite phases could explain the higher alkalinity values. Calculations suggest that the dissolution of benthically-derived aragonite and magnesian calcite may be an important component of the North Pacific alkalinity budget and a potential sink for anthropogenic CO2.

Vegetation effects on the isotope composition of oxygen in atmospheric CO2

THE 18O/16O ratio in atmospheric CO2 is a signal dominated by CO2 exchange with the terrestrial biosphere and it has considerable potential to resolve the current importance of the oceans and individual terrestrial biomes as net sinks for anthropogenic CO2. Fractionation of the oxygen isotopes of CO2 occurs in plants owing to differential diffusion of C18O16O and C16O2 and to isotope effects in oxygen exchange with chloroplast water. Kere we investigate the consequences of these effects for the global distribution of oxygen isotopes in CO2. We predict that 18O isotopic exchange fluxes, especially between the atmosphere and terrestrial biosphere, are large, with considerable spatial variation. Near 70° N, where precipitation (and soil water) is most depleted in 18O, photosynthesis and respiration both deplete the atmospheric CO2 of O. This provides an explanation for the depletion of 18O in atmospheric CO2 at high northern latitudes1.

Air–sea gas exchange in rough and stormy seas measured by a dual-tracer technique

THE transfer of dissolved atmospheric gases across the air/sea interface is a strong function of wind speed and state of the sea surface. The form of the dependence on wind velocities is needed to calculate oceanic sources and sinks of climatically significant gases, particularly carbon dioxide1 and dimethylsulphide2,3. Previous measurements at sea have used techniques that reveal this dependence only when averaged over weeks or months4,5, making it impossible to study the effects of high winds, which are generally episodic. Here we report first results from the application of a tracer technique which enables accurate measurements to be made over periods of 24–72 hours, including the first determination of gas exchange in storm conditions. Our results support the wind-speed dependence suggested by Liss and Merlivat6 on the basis of lake and wind-tunnel experiments, and are consistent with most measurements made by the radon-deficit technique. They suggest slower rates for CO2exchange than those obtained by 14C budgeting, and favour recent lower estimates1,7 of the global oceanic sink for anthropogenic CO2.

The ocean as a net heterotrophic system: Implications From the carbon biogeochemical cycle

The global ocean apparently consumes more organic carbon than it produces. The excess heterotrophy probably occurs in the nearshore zone. This nearshore heterotrophy has significant implications with respect to processes such as organic matter transport from the nearshore zone to the adjacent open ocean, nutrient limitation of primary production, and the role of the coastal zone as a short‐term sink for anthropogenic CO2.