Bronte Tilbrook

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.

Oceanic Uptake of Fossil Fuel CO2: Carbon-13 Evidence

The δ13C value of the dissolved inorganic carbon in the surface waters of the Pacific Ocean has decreased by about 0.4 per mil between 1970 and 1990. This decrease has resulted from the uptake of atmospheric CO2 derived from fossil fuel combustion and deforestation. The net amounts of CO2 taken up by the oceans and released from the biosphere between 1970 and 1990 have been determined from the changes in three measured values: the concentration of atmospheric CO2, the δ13C of atmospheric CO2 and the δ13C value of dissolved inorganic carbon in the ocean. The calculated average net oceanic CO2 uptake is 2.1 gigatons of carbon per year. This amount implies that the ocean is the dominant net sink for anthropogenically produced CO2 and that there has been no significant net CO2 released from the biosphere during the last 20 years.

The Southern Ocean Biological Response to Aeolian Iron Deposition

Biogeochemical rate processes in the Southern Ocean have an important impact on the global environment. Here, we summarize an extensive set of published and new data that establishes the pattern of gross primary production and net community production over large areas of the Southern Ocean. We compare these rates with model estimates of dissolved iron that is added to surface waters by aerosols. This comparison shows that net community production, which is comparable to export production, is proportional to modeled input of soluble iron in aerosols. Our results strengthen the evidence that the addition of aerosol iron fertilizes export production in the Southern Ocean. The data also show that aerosol iron input particularly enhances gross primary production over the large area of the Southern Ocean downwind of dry continental areas.

High CO2 enhances the competitive strength of seaweeds over corals

Space competition between corals and seaweeds is an important ecological process underlying coral-reef dynamics. Processes promoting seaweed growth and survival, such as herbivore overfishing and eutrophication, can lead to local reef degradation. Here, we present the case that increasing concentrations of atmospheric CO2 may be an additional process driving a shift from corals to seaweeds on reefs. Coral (Acropora intermedia) mortality in contact with a common coral-reef seaweed (Lobophora papenfussii) increased two- to threefold between background CO2 (400 ppm) and highest level projected for late 21st century (1140 ppm). The strong interaction between CO2 and seaweeds on coral mortality was most likely attributable to a chemical competitive mechanism, as control corals with algal mimics showed no mortality. Our results suggest that coral (Acropora) reefs may become increasingly susceptible to seaweed proliferation under ocean acidification, and processes regulating algal abundance (e.g. herbivory) will play an increasingly important role in maintaining coral abundance.

The reinvigoration of the Southern Ocean carbon sink

Uptake uptick Has global warming slowed the uptake of atmospheric CO2 by the Southern Ocean? Landschützer et al. say no (see the Perspective by Fletcher). Previous work suggested that the strength of the Southern Ocean carbon sink fell during the 1990s. This raised concerns that such a decline would exacerbate the rise of atmospheric CO2 and thereby increase global surface air temperatures and ocean acidity. The newer data show that the Southern Ocean carbon sink strengthened again over the past decade, which illustrates the dynamic nature of the process and alleviates some of the anxiety about its earlier weakening trend. Science, this issue p. 1221; see also p. 1165 Carbon uptake by the Southern Ocean has increased again after its slowdown in the 1990s. [Also see Perspective by Fletcher] Several studies have suggested that the carbon sink in the Southern Ocean—the ocean’s strongest region for the uptake of anthropogenic CO2 —has weakened in recent decades. We demonstrated, on the basis of multidecadal analyses of surface ocean CO2 observations, that this weakening trend stopped around 2002, and by 2012, the Southern Ocean had regained its expected strength based on the growth of atmospheric CO2. All three Southern Ocean sectors have contributed to this reinvigoration of the carbon sink, yet differences in the processes between sectors exist, related to a tendency toward a zonally more asymmetric atmospheric circulation. The large decadal variations in the Southern Ocean carbon sink suggest a rather dynamic ocean carbon cycle that varies more in time than previously recognized.

Impacts of ocean acidification in naturally variable coral reef flat ecosystems

Ocean acidification leads to changes in marine carbonate chemistry that are predicted to cause a decline in future coral reef calcification. Several laboratory and mesocosm experiments have described calcification responses of species and communities to increasing CO 2. The few in situ studies on natural coral reefs that have been carried out to date have shown a direct relationship between aragonite saturation state ( arag) and net community calcification (G net). However, these studies have been performed over a limited range of arag values, where extrapolation outside the observational range is required to predict future changes in coral reef calcification. We measured extreme diurnal variability in carbonate chemistry within a reef flat in the southern Great Barrier Reef, Australia. arag varied between 1.1 and 6.5, thus exceeding the magnitude of change expected this century in open ocean subtropical/tropical waters. The observed variability comes about through biological activity on the reef, where changes to the carbonate chemistry are enhanced at low tide when reef flat waters are isolated from open ocean water. We define a relationship between net community calcification and arag, using our in situ measurements. We find net community calcification to be linearly related to arag, while temperature and nutrients had no significant effect on G net. Using our relationship between G net and arag, we predict that net community calcification will decline by 55% of its preindustrial value by the end of the century. It is not known at this stage whether exposure to large variability in carbonate chemistry will make reef flat organisms more or less vulnerable to the non-calcifying physiological effects of increasing ocean CO 2 and future laboratory studies will need to incorporate this natural variability to address this question.

The observed evolution of oceanic pCO2 and its drivers over the last two decades

[1] We use a database of more than 4.4 million observations of ocean pCO2 to investigate oceanic pCO2 growth rates. We use pCO2 measurements, with corresponding sea surface temperature and salinity measurements, to reconstruct alkalinity and dissolved inorganic carbon to understand what is driving these growth rates in different ocean regions. If the oceanic pCO2 growth rate is faster (slower) than the atmospheric CO2 growth rate, the region can be interpreted as having a decreasing (increasing) atmospheric CO2 uptake. Only the Western subpolar and subtropical North Pacific, and the Southern Ocean are found to have sufficient spatial and temporal observations to calculate the growth rates of oceanic pCO2 in different seasons. Based on these regions, we find the strength of the ocean carbon sink has declined over the last two decades due to a combination of regional drivers (physical and biological). In the subpolar North Pacific reduced atmospheric CO2 uptake in the summer is associated with changes in the biological production, while in the subtropical North Pacific enhanced uptake in winter is associated with enhanced biological production. In the Indian and Pacific sectors of the Southern Ocean a reduced winter atmospheric CO2 uptake is associated with a positive SAM response. Conversely in the more stratified Atlantic Ocean sector enhanced summer uptake is associated with increased biological production and reduced vertical supply. We are not able to separate climate variability and change as the calculated growth rates are at the limit of detection and are associated with large uncertainties. Ongoing sustained observations of global oceanic pCO2 and its drivers, including dissolved inorganic carbon and alkalinity, are key to detecting and understanding how the ocean carbon sink will evolve in future and what processes are driving this change.

Controls on the carbon isotopic composition of southern ocean phytoplankton

Carbon isotopic compositions of suspended organic matter and biomarker compounds were determined for 59 samples filtered from Southern Ocean surface waters in January 1994 along two north-south transects (WOCE SR3 from Tasmania to Antarctica, and across the Princess Elizabeth Trough (PET) east of Prydz Bay, Antarctica). Along the SR3 line, bulk organic matter show generally decreasing 13C contents southward, which are well correlated with increasing dissolved molecular carbon dioxide concentrations, CO2(aq). This relationship does not hold along the PET transect. Using concentrations and isotopic compositions of molecular compounds, we evaluate the relative roles of several factors affecting the δ13C of Southern Ocean suspended particulate organic matter. Along the WOCE SR3 transect, the concentration of CO2(aq) plays an important role. It is well described by a supply versus demand model for the extent of cellular CO2 utilization and its associated linear dependence of isotopic fractionation (EP) on the reciprocal of CO2(aq). An equally important factor appears to be changes in algal assemblages along the SR3 transect, with their contribution to isotopic fractionation also well described by the supply and demand model, when formulated to include the cell surface/volume control of supply. Changes in microalgal growth rates appear to have a minor effect on EP. Along the PET transect, algal assemblage changes and possibly changes in microalgal growth rates appear to strongly affect the carbon isotopic variations of suspended organic matter. These results can be used to improve the formulation of modern carbon cycle models that include phytoplankton carbon isotopic fractionation.

Australian dust storms in 2002–2003 and their impact on Southern Ocean biogeochemistry

During late 2002 and early 2003 southern Australia was in the grip of drought and experienced one of its most active dust storm seasons in the last 40 years with large dust plumes frequently advected over the adjacent Southern Ocean. We use meteorological records of dust activity, satellite ocean colour and aerosol optical depth data, and dust transport modeling to investigate the transport and deposition of mineral dust from Australia over adjacent ocean regions and to correlate it with biological response in phytoplankton standing stock as measured by chlorophyll-a concentration in five-degree latitude bands from 40-60°S. Seasonal maxima in mean surface chlorophyll-a of ~0.5 mg m-3 were not achieved until late Jan 2003 or during February in the more southerly bands, which when compared with a 9-year satellite mean climatology suggests the phenology of the bloom in 2002-03 was atypical. Contemporaneous field data on CO2 fugacity collected on transects between Tasmania and Antarctica show that significant atmospheric CO2 drawdown occurred as far south as 60°S during February 2003. Our results provide strong evidence for a large-scale natural dust fertilization event in the Australian sector of the Southern Ocean, and highlight the importance of dust-derived nutrients in the marine carbon cycle of the Southern Ocean.

Accumulation and uptake of anthropogenic CO2 in the Southern Ocean, south of Australia between 1968 and 1996

Increases in the anthropogenic CO2 inventory for Southern Ocean waters to the south of Australia were estimated by comparing measurements made 28 years apart in 1968 and 1996. For this period, the deepest penetration of anthropogenic CO2 is consistent with the depth of 28 year old water from CFC-11 age estimates. Significant accumulation of anthropogenic CO2 (13±10 μmol kg−1) was found in Antarctic Bottom Water (AABW) from 500–4500 m south of 58°S suggesting AABW to be an important water mass for the storage of anthropogenic CO2 in the deep ocean. Deep penetration of anthropogenic CO2 (<1900 m) was found in the sub-Antarctic zone (SAZ) between 48°S and 51°S while little accumulation (<400 m) was observed south of 53°S. A simple one dimensional box model was used to investigate the anthropogenic CO2 uptake potential in the SAZ based on observed CFC-11 concentrations and the calculated anthropogenic CO2 inventory. Formation and export of sub-Antarctic Mode Water (SAMW) was estimated by reproducing the observed present day SAMW CFC-11 concentration using time histories of atmospheric CFC-11 concentrations. The anthropogenic CO2 uptake for the SAZ (45°–50°S) ranged from 0.73 to 0.86 μmol kg yr−1. The calculated range of SAMW formation and export rates only increased the anthropogenic CO2 uptake potential for the SAZ by as much as 18% (<0.13 μmol kg−1 yr−1). Extrapolating our estimate to the circumpolar SAZ gives an anthropogenic CO2 uptake of 0.07–0.08 GTC yr−1. Even though SAMW formation does not considerably enhance anthropogenic CO2 uptake in the SAZ, it provides an important mechanism for transporting anthropogenic CO2 into the ocean interior. SAMW therefore may be important for anthropogenic CO2 uptake on a larger regional scale.