Response to Comments on "Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change (original) (raw)

Two decades of ocean CO 2 sink and variability

Tellus B, 2003

Atmospheric CO 2 has increased at a nearly identical average rate of 3.3 and 3.2 Pg C yr −1 for the decades of the 1980s and the 1990s, in spite of a large increase in fossil fuel emissions from 5.4 to 6.3 Pg C yr −1 . Thus, the sum of the ocean and land CO 2 sinks was 1 Pg C yr −1 larger in the 1990s than in to the 1980s. Here we quantify the ocean and land sinks for these two decades using recent atmospheric inversions and ocean models. The ocean and land sinks are estimated to be, respectively, 0.3 (0.1 to 0.6) and 0.7 (0.4 to 0.9) Pg C yr −1 larger in the 1990s than in the 1980s. When variability less than 5 yr is removed, all estimates show a global oceanic sink more or less steadily increasing with time, and a large anomaly in the land sink during [1990][1991][1992][1993][1994]. For year-to-year variability, all estimates show 1/3 to 1/2 less variability in the ocean than on land, but the amplitude and phase of the oceanic variability remain poorly determined. A mean oceanic sink of 1.9 Pg C yr −1 for the 1990s based on O 2 observations corrected for ocean outgassing is supported by these estimates, but an uncertainty on the mean value of the order of ±0.7 Pg C yr −1 remains. The difference between the two decades appears to be more robust than the absolute value of either of the two decades.

The oceanic sink for anthropogenic CO 2 from 1994 to 2007

Science, 2019

The state of ocean CO 2 uptake The ocean is an important sink for anthropogenic CO 2 and has absorbed roughly 30% of our emissions between the beginning of the industrial revolution and the mid-1990s. This effect is an important moderator of climate change, but can we count on it to remain as strong in the future? Gruber et al. calculated the ocean uptake of anthropogenic CO 2 for the interval from 1994 to 2007, which continued as expected. They also observed clear regional deviations from this pattern, suggesting that there is no guarantee that uptake will remain as robust with time. Science , this issue p. 1193

Coastal Southern Ocean: A strong anthropogenic CO 2 sink

Geophysical Research Letters, 2008

1] Large-scale estimates of the Southern Ocean CO 2 sink do not adequately resolve the fluxes associated with Antarctic continental shelves. Using a mechanistic threedimensional biogeochemical model of the Ross Sea, we show that Antarctic shelf waters are a strong sink for CO 2 due to high biological productivity, intense winds, high ventilation rates, and extensive winter sea ice cover. Net primary production (NPP) in these waters is $0.055 Pg C yr À1 . Some of this carbon sinks to depth, driving an influx of CO 2 of 20-50 g C m À2 yr À1 . Although currently unaccounted for, the total atmospheric CO 2 sink on the Ross Sea continental shelf of 0.013 Pg C yr À1 is equivalent to 27% of the most recent estimate of the CO 2 sink for the entire Southern Ocean. Given these results, these and other highly productive waters around the Antarctic continent need to be included in future budgets of anthropogenic CO 2 .

Oceanic sources, sinks, and transport of atmospheric CO 2

Global Biogeochemical Cycles, 2009

1] We synthesize estimates of the contemporary net air-sea CO 2 flux on the basis of an inversion of interior ocean carbon observations using a suite of 10 ocean general circulation models and compare them to estimates based on a new climatology of the air-sea difference of the partial pressure of CO 2 (pCO 2 ) . These two independent flux estimates reveal a consistent description of the regional distribution of annual mean sources and sinks of atmospheric CO 2 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 À1 . This distribution is characterized by outgassing in the tropics, uptake in midlatitudes, and comparatively small fluxes in the high latitudes. Both estimates point toward a small ($ À0.3 Pg C a À1 ) contemporary CO 2 sink in the Southern Ocean (south of 44°S), a result of the near cancellation between a substantial outgassing of natural CO 2 and a strong uptake of anthropogenic CO 2 . 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 pCO 2 -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 CO 2 is estimated to be À1.7 ± 0.4 Pg C a À1 (inversion) and À1.4 ± 0.7 Pg C a À1 (pCO 2 -climatology), respectively, consisting of an outgassing flux of river-derived carbon of $+0.5 Pg C a À1 , and an uptake flux of anthropogenic carbon of À2.2 ± 0.3 Pg C a À1 (inversion) and À1.9 ± 0.7 Pg C a À1 (pCO 2 -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 À0.2 and À0.3 Pg C a À1 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 CO 2 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.

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

Global Biogeochemical Cycles, 2008

1] We investigate the multidecadal and decadal trends in the flux of CO 2 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 CO 2 concentrations so that we can separately analyze trends in the natural and anthropogenic CO 2 fluxes. We find that the Southern Ocean (<35°S) CO 2 sink has weakened by 0.1 Pg C a À1 from 1979-2004, relative to the expected sink from rising atmospheric CO 2 and fixed physical climate. Although the magnitude of this trend is in agreement with prior studies , 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 CO 2 , 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 CO 2 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., , these mechanistic results are useful for projecting the future behavior of the Southern Ocean carbon sink.

Oceanic sources and sinks of atmospheric CO2

2006

We present new estimates of the contemporary net air-sea CO 2 fluxes based on the inversion of interior ocean carbon observations using a suite of 10 Ocean General Circulation Models. The comparison of these inverse estimates with those based on a greatly expanded climatology of the air-sea difference of the partial pressure of CO 2 reveal a consistent description of the regional distribution of annual mean sources and sinks of atmospheric CO 2 . This distribution is characterized by outgassing in the tropics, uptake in mid-latitudes, and comparatively small fluxes in the high-latitudes. In particular, both estimates point toward a substantially smaller present CO 2 sink in the Southern Ocean than previous estimates . The inversion permits us to attribute this small sink to a near cancellation between a substantial outgassing of natural CO 2 and a strong uptake of anthropogenic CO 2 . Globally, the inverse estimate of the uptake of anthropogenic CO 2 by the ocean amounts to 2.20 ± 0.25 Pg C yr −1 for a nominal year of 1995, in excellent agreement with most global estimates. Interpreted in the context of the global anthropogenic CO 2 budget for the 1990s, this oceanic uptake rate constrains the balance of the terrestrial biosphere for this decade to a net sink of 9 ± 5 Pg C.

Interannual variability of the oceanic sink of CO2from 1979 through 1997

Global Biogeochemical Cycles, 2000

We have estimated the interannual variability in the oceanic sink of CO2 with a three-dimensional global-scale model which includes ocean circulation and simple biogeochemistry. The model was forced from 1979 to 1997 by a combination of daily to weekly data from the European Centre for Medium-Range Weather Forecast and the National Centers for Environmental PredictionSNational Center for Atmospheric Research reanalysis as well as European Remote Sensing satellite observations. For this period, the ocean sink of CO2 is estimated to vary between 1.4 and 2.2 Pg C yr-1, as a result of annually averaged interannual variability of 4.0.4 Pg C yr-1 that fluctuates about a mean of 1.8 Pg C yr-•. Our interannual variability roughly agrees in amplitude with previous ocean-based estimates but is 2 to 4 times less than estimates based on atmospheric observations. About 70% of the global variance in our modeled flux of CO• originated in the equatorial Pacific. In that region, our modeled variability in the flux of CO• generally agreed with that observed to 4-0.1 Pg C yr-•. The predominance of the equatorial Pacific for interannual variability is caused by three factors: (1) interannual variability associated with E1 Nifio events occurs in phase over the entire basin, whereas elsewhere positive and negative anomalies partly cancel each other out (e.g., for events such as Antarctic Circumpolar Wave and the North Atlantic Oscillation); (2) dynamic processes dominate in the equatorial Pacific, whereas dynamic, thermodynamic, and biological processes partly cancel one another at higher latitudes; and (3) our model underestimates the variability in ocean dynamics and biology at high latitudes. 1. Introduction Interannual variability in atmospheric CO2 provides clues which can be exploited to help unravel the relative roles of the ocean and terrestrial biosphere in absorbing and releasing atmospheric CO2. Although changes in atmospheric CO2 are well documented [Keeling et al., 1989; Conway et al., 1994; Tans et al., 1996], interannual changes in sea-air flux of CO2 are poorly constrained. Figure i illustrates the present controversy. Relatively small interannual variability in the sea-air flux of CO2 (+0.1 to +0.5 Pg C yr-1) is deduced from measurements of oceanic partial pressure of CO2