Anthropogenic CO2 in the Atlantic Ocean

The anthropogenic CO2 in the Atlantic Ocean is separated from the large natural variability of dissolved inorganic carbon using the method developed by Gruber et al. [1996]. Surface concentrations of anthropogenic CO2 are found to be highest in the tropical to subtropical regions and to decrease toward the high latitudes. They are very close to what is expected from thermodynamic considerations assuming that the surface ocean followed the atmospheric CO2 perturbation. Highest specific inventories (inventory per square meter) of anthropogenic CO2 occur in the subtropical convergence zones. Large differences exist between the North and South Atlantic high latitudes: In the North Atlantic, anthropogenic CO2 has already invaded deeply into the interior; north of 50°N it has even reached the bottom. By contrast, waters south of 50°S contain relatively little anthropogenic CO2, and hence specific inventories are very low. An anthropogenic CO2 inventory of about 22 ± 5 Gt C is estimated for the Atlantic north of the equator for 1982, and 18 ± 4 Gt C is estimated for the Atlantic south of the equator for 1989. The Princeton ocean biogeochemistry model predicts anthropogenic CO2 inventories of 20.0 Gt C (North Atlantic, 1982) and 17.7 Gt C (South Atlantic, 1989) for the same regions in good agreement with the observed inventories. Important differences exist on a more regional scale, associated with known deficiencies of the model

Long-term trends in surface ocean pH in the North Atlantic

Presently available direct pH measurements do not have a sufficient data density in space or time in order to determine long-term trends across wider geographic regions, limiting our ability to assess the magnitude and impacts of ocean acidification. We overcome this limitation by using the much more frequently measured fugacity of CO2 (fCO2), as synthesized in the SOCAT data product, from which we calculate pH using algorithms for alkalinity based on temperature and salinity. The estimated pH at 25 °C, i.e., pHsws25 °C has a calculation error of 0.0033 ± 0.0003, and evaluation using co-located pH observations yields a RMSE of 0.010 and a non-significant bias of 0.004. The estimated pHsws25 °C is rather sensitive to uncertainties and biases in fCO2, while uncertainties in alkalinity, temperature, and salinity matter much less. The high precision and low bias of the computed pH permit us to apply this method to data from the North Atlantic Subpolar Gyre, for which we find a statistically significant trend in surface ocean pHswsinsitu of − 0.0022 ± 0.0004 yr− 1 over the period 1981 to 2007. This long-term trend in pH is nearly entirely driven by the long-term trend in surface ocean fCO2, while the impact of temperature is negligible. This pH trend is very close to that expected based on the assumption of thermodynamic equilibrium of CO2 between the atmosphere and the surface ocean.publishedVersio

Decadal variations and trends of the global ocean carbon sink

We investigate the variations of the ocean CO2 sink during the past three decades using global surface ocean maps of the partial pressure of CO2 reconstructed from observations contained in the Surface Ocean CO2 Atlas Version 2. To create these maps, we used the neural network-based data-interpolation method of [Landschützer2014], but extended the work in time from 1998 through 2011 to the period from 1982 through 2011. Our results suggest strong decadal variations in the global ocean carbon sink around a long-term increase that corresponds roughly to that expected from the rise in atmospheric CO2. The sink is estimated to have weakened during the 1990s toward a minimum uptake of only -0.8 ± 0.5 Pg C yr − 1 in 2000, and thereafter to have strengthened considerably to rates of more than -2.0 ± 0.5 Pg C yr − 1. These decadal variations originate mostly from the extratropical oceans while the tropical regions contribute primarily to interannual variations. Changes in sea-surface temperature affecting the solubility of CO2 explain part of these variations, particularly at subtropical latitudes. But most of the higher latitude changes are attributed to modifications in the surface concentration of dissolved inorganic carbon and alkalinity, induced by decadal variations in atmospheric forcing, with patterns that are reminiscent of those of the Northern and Southern Annular Modes. These decadal variations lead to a substantially smaller cumulative anthropogenic CO2 uptake of the ocean over the 1982 through 2011 period (reduction of 7.5 ± 5.5 Pg C) relative to that derived by the Global Carbon Budget

Toward a mechanistic understanding of the decadal trends in the Southern Ocean carbon sink

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 22 (2008): GB3016, doi:10.1029/2007GB003139.We investigate the multidecadal and decadal trends in the flux of CO2 between the atmosphere and the Southern Ocean using output from hindcast simulations of an ocean circulation model with embedded biogeochemistry. The simulations are run with NCEP-1 forcing under both preindustrial and historical atmospheric CO2 concentrations so that we can separately analyze trends in the natural and anthropogenic CO2 fluxes. We find that the Southern Ocean (<35°S) CO2 sink has weakened by 0.1 Pg C a−1 from 1979–2004, relative to the expected sink from rising atmospheric CO2 and fixed physical climate. Although the magnitude of this trend is in agreement with prior studies (Le Quéré et al., 2007), its size may not be entirely robust because of uncertainties associated with the trend in the NCEP-1 atmospheric forcing. We attribute the weakening sink to an outgassing trend of natural CO2, driven by enhanced upwelling and equatorward transport of carbon-rich water, which are caused by a trend toward stronger and southward shifted winds over the Southern Ocean (associated with the positive trend in the Southern Annular Mode (SAM)). In contrast, the trend in the anthropogenic CO2 uptake is largely unaffected by the trend in the wind and ocean circulation. We regard this attribution of the trend as robust, and show that surface and interior ocean observations may help to solidify our findings. As coupled climate models consistently show a positive trend in the SAM in the coming century [e.g., Meehl et al., 2007], these mechanistic results are useful for projecting the future behavior of the Southern Ocean carbon sink.This work was supported by funding from various agencies. NSL was supported by NASA grant NNG05GP78H and the NOAA Climate and Global Change postdoctoral fellowship. NG was supported by NASA grant NNG04GH53G and by ETH Zurich. SCD was supported by NASA grant NNG05GG30G

The reinvigoration of the Southern Ocean carbon sink

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

Decoupling marine export production from new production

We investigate the relationship between annually integrated new and export production for the central Californian marine upwelling system using an eddy-resolving coupled physical-ecosystem-biogeochemical model. We find that when averaged over the annual cycle lateral transport leads to a substantial spatial decoupling of export from new production, with a length-scale of decoupling on the order of 300 km. The decoupling is largely caused by mean horizontal fluxes induced by persistent meso- and submesoscale circulation structures and to a lesser degree by the mean lateral offshore transport induced by Ekman transport. This indicates that the concept of numerically equal new and export production has to be used with great care, particularly in dynamic oceanic environments

Constraining future terrestrial carbon cycle projections using observation‐based water and carbon flux estimates

The terrestrial biosphere is currently acting as a sink for about a third of the total anthropogenic CO2 emissions. However, the future fate of this sink in the coming decades is very uncertain, as current earth system models (ESMs) simulate diverging responses of the terrestrial carbon cycle to upcoming climate change. Here, we use observation-based constraints of water and carbon fluxes to reduce uncertainties in the projected terrestrial carbon cycle response derived from simulations of ESMs conducted as part of the 5th phase of the Coupled Model Intercomparison Project (CMIP5). We find in the ESMs a clear linear relationship between present-day evapotranspiration (ET) and gross primary productivity (GPP), as well as between these present-day fluxes and projected changes in GPP, thus providing an emergent constraint on projected GPP. Constraining the ESMs based on their ability to simulate present-day ET and GPP leads to a substantial decrease in the projected GPP and to a ca. 50% reduction in the associated model spread in GPP by the end of the century. Given the strong correlation between projected changes in GPP and in NBP in the ESMs, applying the constraints on net biome productivity (NBP) reduces the model spread in the projected land sink by more than 30% by 2100. Moreover, the projected decline in the land sink is at least doubled in the constrained ensembles and the probability that the terrestrial biosphere is turned into a net carbon source by the end of the century is strongly increased. This indicates that the decline in the future land carbon uptake might be stronger than previously thought, which would have important implications for the rate of increase in the atmospheric CO2 concentration and for future climate change

Single-Image based unsupervised joint segmentation and denoising

In this work, we develop an unsupervised method for the joint segmentation and denoising of a single image. To this end, we combine the advantages of a variational segmentation method with the power of a self-supervised, single-image based deep learning approach. One major strength of our method lies in the fact, that in contrast to data-driven methods, where huge amounts of labeled samples are necessary, our model can segment an image into multiple meaningful regions without any training database. Further, we introduce a novel energy functional in which denoising and segmentation are coupled in a way that both tasks benefit from each other. The limitations of existing single-image based variational segmentation methods, which are not capable of dealing with high noise or generic texture, are tackled by this specific combination with self-supervised image denoising. We propose a unified optimisation strategy and show that, especially for very noisy images available in microscopy, our proposed joint approach outperforms its sequential counterpart as well as alternative methods focused purely on denoising or segmentation. Another comparison is conducted with a supervised deep learning approach designed for the same application, highlighting the good performance of our approach

Trends and drivers in global surface ocean pH over the past 3 decades

We report global long-term trends in surface ocean pH using a new pH data set computed by combining fCO2 observations from the Surface Ocean CO2 Atlas (SOCAT) version 2 with surface alkalinity estimates based on temperature and salinity. Trends were determined over the periods 1981–2011 and 1991–2011 for a set of 17 biomes using a weighted linear least squares method. We observe significant decreases in surface ocean pH in ~70% of all biomes and a mean rate of decrease of 0.0018 ± 0.0004 yr−1 for 1991–2011. We are not able to calculate a global trend for 1981–2011 because too few biomes have enough data for this. In half the biomes, the rate of change is commensurate with the trends expected based on the assumption that the surface ocean pH change is only driven by the surface ocean CO2 chemistry remaining in a transient equilibrium with the increase in atmospheric CO2. In the remaining biomes, deviations from such equilibrium may reflect that the trend of surface ocean fCO2 is not equal to that of the atmosphere, most notably in the equatorial Pacific Ocean, or may reflect changes in the oceanic buffer (Revelle) factor. We conclude that well-planned and long-term sustained observational networks are key to reliably document the ongoing and future changes in ocean carbon chemistry due to anthropogenic forcing.publishedVersio