Global air-sea flux of CO ₂ : An estimate based on measurements of sea–air pCO ₂  difference

Abstract

Approximately 250,000 measurements made for the pCO 2 difference between surface water and the marine atmosphere, ⌬pCO 2 , have been assembled for the global oceans. Observations made in the equatorial Pacific during El Nino events have been excluded from the data set. These observations are mapped on the global 4°؋ 5°grid for a single virtual calendar year (chosen arbitrarily to be 1990) representing a non-El Nino year. Monthly global distributions of ⌬pCO 2 have been constructed using an interpolation method based on a lateral advection-diffusion transport equation. The net f lux of CO 2 across the sea surface has been computed using ⌬pCO 2 distributions and CO 2 gas transfer coefficients across sea surface. The annual net uptake f lux of CO 2 by the global oceans thus estimated ranges from 0.60 to 1.34 GtC⅐yr ؊1 depending on different formulations used for wind speed dependence on the gas transfer coefficient. These estimates are subject to an error of up to 75% resulting from the numerical interpolation method used to estimate the distribution of ⌬pCO 2 over the global oceans. Temperate and polar oceans of the both hemispheres are the major sinks for atmospheric CO 2 , whereas the equatorial oceans are the major sources for CO 2 . The Atlantic Ocean is the most important CO 2 sink, providing about 60% of the global ocean uptake, while the Pacific Ocean is neutral because of its equatorial source f lux being balanced by the sink f lux of the temperate oceans. The Indian and Southern Oceans take up about 20% each.Measurements of the atmospheric CO 2 concentration indicate that it has been increasing at a rate about 50% of that which is expected from all industrial CO 2 emissions. The oceans have been considered to be a major sink for CO 2 . Hence the improved knowledge of the net transport flux across the air-sea interface is important for understanding the fate of this important greenhouse gas emitted into the earth's atmosphere (1-5).A number of different approaches has been used for estimating the role of the oceans as a CO 2 sink, yielding a wide range of estimates for the CO 2 uptake flux (3). Most commonly used are ocean-atmosphere CO 2 cycle models. In these models, ocean circulation is modeled using various schemes ranging from one-dimensional box-diffusion models (4, 5) to threedimensional ocean general circulation models (6-8), and biological processes are assumed to be invariant with time and are not explicitly described. These ''perturbation'' models yield an oceanic uptake of about 2 Gt-C⅐yr Ϫ1 (ϭ 2 ϫ 10 15 g of carbon⅐yr Ϫ1 ), which corresponds to about 35% of the current industrial CO 2 emission rate of about 6 Gt-C⅐yr Ϫ1 . On the basis of temporal changes of the 13 C͞ 12 C ratio in atmospheric and oceanic CO 2 , the annual oceanic uptake of atmospheric CO 2 has been estimated to be 1.6 Ϯ 0.9 Gt-C⅐yr Ϫ1 (2, 9). Tans et al.(1) combined the meridional gradient of atmospheric CO 2 concentration and the net CO 2 flux over the northern oceans to constrain the CO 2 budget and concluded that th...

The Atlantic Ocean at the Last Glacial Maximum: 2. Reconstructing the Current Systems with a Global Ocean Model

“…For the sea surface, this treatment was similar to the horizontal interpolation scheme by Takahashi et al (1997), but it allowed for a vertical exchange with the deep ocean. Accordingly, the resulting sea-surface distribution in Experiment M shows a general agreement between model and data (Figure 16).…”

“…Thus there were no surface fluxes over the vast areas of the ocean that were void of data. As far as the surface ocean is concerned, the model acted like an interpolation or extrapolation method using advective and diffusive fluxes (Takahashi et al 1997). The δ 18 O w tracer became even more artificial in the glacial experiments, but still served the purpose to highlight the effects of changes in advection and diffusion.…”

Multiple constraints on regional CO2 flux variations over land and oceans

“…Land fluxes are assigned prior monthly values (balanced annually and with no IAV) and prior geographic patterns within each region from the SIB-2 global biosphere model [Denning et al, 1996]. Ocean fluxes are assigned prior values and patterns (no IAV) from a global synthesis of air-sea-flux measurements [Takahashi et al, 1997]. Prior errors on net fluxes are set to ±1/ ffiffiffiffiffi 12 p GtC month À1 (1 GtC annually), identically over land and ocean regions.…”

The role of the oceans in climate

“…In the future, increasing water temperatures will also tend to reduce the capacity of the oceans to take up CO 2 . Current estimates (Takahashi et al, 1997) suggest that the major oceanographic sinks for CO 2 are in the North Atlantic, particularly the Norwegian Sea, and in the Southern Ocean. However, the limited nature of regular measurements of CO 2 concentrations throughout most of the world oceans, coupled with uncertainties over the exchange rate, means that there is currently an uncertainty of about a factor of two in calculations of the air-sea exchange of CO 2 , and particularly that of anthropogenic CO 2 .…”