Taro Takahashi

Observational Contrains on the Global Atmospheric Co2 Budget

Observed atmospheric concentrations of CO2 and data on the partial pressures of CO2 in surface ocean waters are combined to identify globally significant sources and sinks of CO2. The atmospheric data are compared with boundary layer concentrations calculated with the transport fields generated by a general circulation model (GCM) for specified source-sink distributions. In the model the observed north-south atmospheric concentration gradient can be maintained only if sinks for CO2 are greater in the Northern than in the Southern Hemisphere. The observed differences between the partial pressure of CO2 in the surface waters of the Northern Hemisphere and the atmosphere are too small for the oceans to be the major sink of fossil fuel CO2. Therefore, a large amount of the CO2 is apparently absorbed on the continents by terrestrial ecosystems.

Global sea–air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects

Based on about 940,000 measurements of surface-water pCO2 obtained since the International Geophysical Year of 1956–59, the climatological, monthly distribution of pCO2 in the global surface waters representing mean non-El Nino conditions has been obtained with a spatial resolution of 4°×5° for a reference year 1995. The monthly and annual net sea–air CO2 flux has been computed using the NCEP/NCAR 41-year mean monthly wind speeds. An annual net uptake flux of CO2 by the global oceans has been estimated to be 2.2 (+22% or ?19%) Pg C yr?1 using the (wind speed)2 dependence of the CO2 gas transfer velocity of Wanninkhof (J. Geophys. Res. 97 (1992) 7373). The errors associated with the wind-speed variation have been estimated using one standard deviation (about±2 m s?1) from the mean monthly wind speed observed over each 4°×5° pixel area of the global oceans. The new global uptake flux obtained with the Wanninkhof (wind speed)2 dependence is compared with those obtained previously using a smaller number of measurements, about 250,000 and 550,000, respectively, and are found to be consistent within±0.2 Pg C yr?1. This estimate for the global ocean uptake flux is consistent with the values of 2.0±0.6 Pg C yr?1 estimated on the basis of the observed changes in the atmospheric CO2 and oxygen concentrations during the 1990s (Nature 381 (1996) 218; Science 287 (2000) 2467). However, if the (wind speed)3 dependence of Wanninkhof and McGillis (Res. Lett. 26 (1999) 1889) is used instead, the annual ocean uptake as well as the sensitivity to wind-speed variability is increased by about 70%. A zone between 40° and 60° latitudes in both the northern and southern hemispheres is found to be a major sink for atmospheric CO2. In these areas, poleward-flowing warm waters meet and mix with the cold subpolar waters rich in nutrients. The pCO2 in the surface water is decreased by the cooling effect on warm waters and by the biological drawdown of pCO2 in subpolar waters. High wind speeds over these low pCO2 waters increase the CO2 uptake rate by the ocean waters. The pCO2 in surface waters of the global oceans varies seasonally over a wide range of about 60% above and below the current atmospheric pCO2 level of about 360 ?atm. A global map showing the seasonal amplitude of surface-water pCO2 is presented. The effect of biological utilization of CO2 is differentiated from that of seasonal temperature changes using seasonal temperature data. The seasonal amplitude of surface-water pCO2 in high-latitude waters located poleward of about 40° latitude and in the equatorial zone is dominated by the biology effect, whereas that in the temperate gyre regions is dominated by the temperature effect. These effects are about 6 months out of phase. Accordingly, along the boundaries between these two regimes, they tend to cancel each other, forming a zone of small pCO2 amplitude. In the oligotrophic waters of the northern and southern temperate gyres, the biology effect is about 35 ?atm on average. This is consistent with the biological export flux estimated by Laws et al. (Glob. Biogeochem. Cycles 14 (2000) 1231). Small areas such as the northwestern Arabian Sea and the eastern equatorial Pacific, where seasonal upwelling occurs, exhibit intense seasonal changes in pCO2 due to the biological drawdown of CO2.

Seasonal variation of CO2 and nutrients in the high‐latitude surface oceans: A comparative study

Seasonal data for pCO2 and the concentrations of CO2 and nutrients in high-latitude surface oceans obtained by the Lamont-Doherty CO2 group and Marine Research Institute, Reykjavik, are presented and analyzed. The seasonal progression and relationships between these properties are described, and their inter-ocean variation is compared. Spring phytoplankton blooms in the surface water of the North Atlantic Ocean and Iceland Sea caused a precipitous reduction of surface water pCO2 and the concentrations of CO2 and nutrients within two weeks, and proceeded until the nutrient salts were exhausted. This type of seasonal behavior is limited to the high-latitude (north of approximately 40°N) North Atlantic Ocean and adjoining seas. In contrast, seasonal changes in CO2 and nutrients were more gradual in the North Pacific and the nutrients were only partially consumed in the surface waters of the subarctic North Pacific Ocean and Southern Ocean. The magnitude of seasonal changes in nutrient concentrations in the North Pacific and Southern Oceans was similar to that observed in the North Atlantic and adjoining seas. In the subpolar and polar waters of the North and South Atlantic and North Pacific Oceans, pCO2 and the concentrations Of CO2 and nutrients were much higher during winter than summer. During winter, the high latitude areas of the North Atlantic, North Pacific, and Weddell Sea were sources for atmospheric CO2; during summer, they became CO2 sinks. This is attributed to the upwelling of deep waters rich in CO2 and nutrients during winter, and the intense photosynthesis occurring in strongly stratified upper layers during summer. On the other hand, subtropical waters were a CO2 source in summer and a sink in winter. Since these waters were depleted of nutrients and could only sustain low levels of primary production, the seasonal variation of pCO2 in subtropical waters and the CO2 sink/source condition were governed primarily by temperature. An intense CO2 sink zone was found along the confluence of the subtropical and subpolar waters (or the subtropical convergence). Its formation is attributed to the combined effects of cooling in subtropical waters and photosynthetic drawdown of CO2 in subpolar waters.

Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef

The concentration of CO2 in the atmosphere is projected to reach twice the preindustrial level by the middle of the 21st century. This increase will reduce the concentration of CO32− of the surface ocean by 30% relative to the preindustrial level and will reduce the calcium carbonate saturation state of the surface ocean by an equal percentage. Using the large 2650 m3 coral reef mesocosm at the BIOSPHERE-2 facility near Tucson, Arizona, we investigated the effect of the projected changes in seawater carbonate chemistry on the calcification of coral reef organisms at the community scale. Our experimental design was to obtain a long (3.8 years) time series of the net calcification of the complete system and all relevant physical and chemical variables (temperature, salinity, light, nutrients, Ca2+,pCO2, TCO2, and total alkalinity). Periodic additions of NaHCO3, Na2CO3, and/or CaCl2 were made to change the calcium carbonate saturation state of the water. We found that there were consistent and reproducible changes in the rate of calcification in response to our manipulations of the saturation state. We show that the net community calcification rate responds to manipulations in the concentrations of both Ca2+ and CO32− and that the rate is well described as a linear function of the ion concentration product, [Ca2+]0.69[CO32−]. This suggests that saturation state or a closely related quantity is a primary environmental factor that influences calcification on coral reefs at the ecosystem level. We compare the sensitivity of calcification to short-term (days) and long-term (months to years) changes in saturation state and found that the response was not significantly different. This indicates that coral reef organisms do not seem to be able to acclimate to changing saturation state. The predicted decrease in coral reef calcification between the years 1880 and 2065 A.D. based on our long-term results is 40%. Previous small-scale, short-term organismal studies predicted a calcification reduction of 14-30%. This much longer, community-scale study suggests that the impact on coral reefs may be greater than previously suspected. In the next century coral reefs will be less able to cope with rising sea level and other anthropogenic stresses.

Fate of fossil fuel carbon dioxide and the global carbon budget

The fate of fossil fuel carbon dioxide released into the atmosphere depends on the exchange rates of carbon between the atmosphere and three major carbon reservoirs, namely, the oceans, shallow-water sediments, and the terrestrial biosphere. Various assumptions and models used to estimate the global carbon budget for the last 20 years are reviewed and evaluated. Several versions of recent atmosphere-ocean models appear to give reliable and mutually consistent estimates for carbon dioxide uptake by the oceans. On the other hand, there is no compelling evidence which establishes that the terrestrial biomass has decreased at a rate comparable to that of fossil fuel combustion over the last two decades, as has been recently claimed.

Oceanic sources, sinks, and transport of atmospheric CO2

We synthesize estimates of the contemporary net air-sea CO2 flux on the basis of an inversion of interior ocean carbon observations using a suite of 10 ocean general circulation models (Mikaloff Fletcher et al., 2006, 2007) and compare them to estimates based on a new climatology of the air-sea difference of the partial pressure of CO2 (pCO2) (Takahashi et al., 2008). These two independent flux estimates reveal a consistent description of the regional distribution of annual mean sources and sinks of atmospheric CO2 for the decade of the 1990s and the early 2000s with differences at the regional level of generally less than 0.1 Pg C a−1. This distribution is characterized by outgassing in the tropics, uptake in midlatitudes, and comparatively small fluxes in thehigh latitudes. Both estimates point toward a small (∼ −0.3 Pg C a−1) contemporary CO2 sink in the Southern Ocean (south of 44°S), a result of the near cancellation between a substantial outgassing of natural CO2 and a strong uptake of anthropogenic CO2. A notable exception in the generally good agreement between the two estimates exists within the Southern Ocean: the ocean inversion suggests a relatively uniform uptake, while the pCO2-based estimate suggests strong uptake in the region between 58°S and 44°S, and a source in the region south of 58°S. Globally and for a nominal period between 1995 and 2000, the contemporary net air-sea flux of CO2 is estimated to be −1.7 ± 0.4 Pg C a−1 (inversion) and −1.4 ± 0.7 Pg C a−1 (pCO2-climatology), respectively, consisting of an outgassing flux of river-derived carbon of ∼+0.5 Pg C a−1, and an uptake flux of anthropogenic carbon of −2.2 ± 0.3 Pg C a−1 (inversion) and −1.9 ± 0.7 Pg C a−1 (pCO2-climatology). The two flux estimates also imply a consistent description of the contemporary meridional transport of carbon with southward ocean transport throughout most of the Atlantic basin, and strong equatorward convergence in the Indo-Pacific basins. Both transport estimates suggest a small hemispheric asymmetry with a southward transport of between −0.2 and −0.3 Pg C a−1 across the equator. While the convergence of these two independent estimates is encouraging and suggests that it is now possible to provide relatively tight constraints for the net air-sea CO2 fluxes at the regional basis, both studies are limited by their lack of consideration of long-term changes in the ocean carbon cycle, such as the recent possible stalling in the expected growth of the Southern Ocean carbon sink.

Primary production at 47°N and 20°W in the North Atlantic Ocean: a comparison between the 14C incubation method and the mixed layer carbon budget

Abstract Primary production in the oceanshas been estimated mainly on the basis of in vitro incubation measurements. An implicit assumption is that the growth rate of phytoplankton observed in vitro represents that occurring in the freely circulating water of the euphotic zone. We have tested this assumption at 47°N-20°W in the eastern North Atlantic Ocean during the initial stages of a spring phytoplankton bloom. The daily primary production was measured by means of the 14C assimilation method, in which the incubation bottles were suspended in the ocean from dawn to dusk daily (about 14 h). The mean daily carbon assimilation rate in the photic zone and in the mixed layer was 107 ± 23 and 84 ± 18 mmol C m −2 day ∗−1 (where day ∗ = 14 daylight hours), respectively, during the 12-day period between 26 April and 7 May 1989. The mixed layer carbon assimilation data are found to be consistent with the in situ CO2 utilization rate of 82 ± 17 mmol C m − day ∗−1 estimated on the basis of the thickness of surface mixed layer, the CO2 concentration in it, and the air-sea CO2 flux. We conclude that primary production in the open ocean appears to be well represented by the in vitro measurements, if the samples are incubated under the in situ light and temperature conditions. The mean daily reduction rate of the total CO2 concentration observed in the mixed layer over the 12-day period is 2.3 μmol kg−1 day−1, about 75% of the rate, 3.1 μmol kg−1 day∗−1, expected from the rate of primary production. About 8.5% of this difference is explained by the atmospheric CO2 flux, and the remaining 16.5% may be attributed to the respiration and the influx of CO2-rich waters from the mixed layer.

Carbon dioxide sequestration in deep-sea basalt.

Developing a method for secure sequestration of anthropogenic carbon dioxide in geological formations is one of our most pressing global scientific problems. Injection into deep-sea basalt formations provides unique and significant advantages over other potential geological storage options, including (i) vast reservoir capacities sufficient to accommodate centuries-long U.S. production of fossil fuel CO2 at locations within pipeline distances to populated areas and CO2 sources along the U.S. west coast; (ii) sufficiently closed water-rock circulation pathways for the chemical reaction of CO2 with basalt to produce stable and nontoxic (Ca2+, Mg2+, Fe2+)CO3 infilling minerals, and (iii) significant risk reduction for post-injection leakage by geological, gravitational, and hydrate-trapping mechanisms. CO2 sequestration in established sediment-covered basalt aquifers on the Juan de Fuca plate offer promising locations to securely accommodate more than a century of future U.S. emissions, warranting energized scientific research, technological assessment, and economic evaluation to establish a viable pilot injection program in the future.

Decadal variability of the air‐sea CO2 fluxes in the equatorial Pacific Ocean

[1]xa0In order to determine the interannual and decadal changes in the air-sea carbon fluxes of the equatorial Pacific, we developed seasonal and interannual relationships between the fugacity of CO2 (fCO2) and sea surface temperature (SST) from shipboard data that were applied to high-resolution temperature fields deduced from satellite data to obtain high-resolution large-scale estimates of the regional fluxes. The data were gathered on board research ships from November 1981 through June 2004 between 95°W and 165°E. The distribution of fCO2sw during five El Nino periods and four La Nina periods were documented. Observations made during the warm boreal winter-spring season and during the cooler boreal summer-fall season of each year enabled us to examine the interannual and seasonal variability of the fCO2sw-SST relationships. A linear fit through all of the data sets yields an inverse correlation between SST and fCO2sw, with both interannual and seasonal differences in slope. On average, the surface water fCO2 in the equatorial region has been increasing at a rate similar to the atmospheric CO2 increase. In addition, there appears to be a slight increase (∼27%) in the outgassing flux of CO2 after the 1997–1998 Pacific Decadal Oscillation (PDO) regime shift. Most of this flux increase is due to increase in wind speeds after the spring of 1998, although increases in fCO2sw after 1998 are also important. These increases are coincident with the recent rebound of the shallow water meridional overturning circulation in the tropical and subtropical Pacific after the regime shift.

Biogeochemical regimes, net community production and carbon export in the Ross Sea, Antarctica

Abstract The net community production (NCP) of the Ross Sea, from the early austral spring (mid-October) to the austral summer (mid-February), has been estimated from the seasonal drawdown of CO 2 concentrations integrated over the top 100xa0m of the water column. The deficits in nutrients and CO 2 indicate three distinct biogeochemical regimes. The regime in the southwestern Ross Sea (Region I) had relatively shallow mixed layers and was dominated by diatom growth, as evidenced by a silicate (Def(Si)) to NCP removal ratio (0.11±0.04) that was similar to the silicate-to-carbon ratio found in diatoms growing in temperate regions. High NCP values (4.9–8.7xa0molxa0m −2 ) and low ratios of surplus total organic carbon (Surp(TOC)) to NCP (from 0.27 to 0.67) show high organic carbon export out of the upper 100xa0m of the water column. The second regime (Region II), located in the center of the southern Ross Sea polynya, also had high NCPs (4.4–10.8xa0molxa0m −2 ) but the mixed layers were deeper. The average Def(Si)/NCP ratio was 0.04±0.02, much lower than the southwestern sector and consistent with the observed growth of the haptophyte Phaeocystis antarctica . An increase in Def(Si) and the Def(Si)/NCP ratio during re-occupations of selected stations indicate the presence and persistence of diatom growth late into the summer. The third regime (Region III), in the northeastern Ross Sea, had shallow mixed layers and a wider range of Def(Si)/NCP ratios (0.10–0.31), indicating variations in silicate-to-carbon uptake by diatoms. The low NCPs (1.2–4.2xa0molxa0Cxa0m −2 ) that distinguished this area also may be due to micro-nutrient deficiencies in addition to prolonged ice coverage. NCP over the continental shelf of the Ross Sea (441,000xa0km 2 , defined by the 1000-m isopleth) is estimated to be 25±10xa0Tg of carbon per year, with a mean rate of 4.8±1.9xa0molxa0m −2 xa0yr. By mid-February, the productivity peak of the 1997 growing season had passed, and 19±7% of NCP remained in the upper 100xa0m as DOC, which presumably would not be exported but remineralized prior to the next growing season. During the same time period, 16±25% of NCP had already been removed from the upper 100xa0m as sinking biogenic particles, leaving 65% present in the POC fraction to be exported later or remineralized. High export of organic carbon (>50% of NCP) was shown in both diatom- and Phaeocystis -dominated regimes.