In an article published September 29th 2006 in Global Biogeochemical Cycles the authors investigate whether the uptake of atmospheric CO2 is decreasing in the Nordic Seas. Not only do the results show that the uptake is decreasing, but they are also indicating that the changes are a consequence of the thermohaline circulation.
Due to the consumption of fossil fuels the concentration of CO2 in the atmosphere is increasing, and for ocean regions undersaturated with respect to the atmospheric CO2 concentration, one expects that the transfer (flux) of CO2 from the atmosphere to the ocean will increase with time. The North Atlantic Ocean is a classical example of such an area. However, the area has not quite lived up to expectations. Truls Johannessen, Abdirahman Omar and Are Olsen have earlier reported that the uptake of CO2 from the atmosphere has decreased in this region. The sea surface CO2 partial pressure (pCO2) has increased at a greater rate than the atmospheric so that the flux into the ocean has decreased. A number of causes have been suggested, for instance reduced biological activity, reduced convection and/or changes in the distribution of surface water masses. Scientists worry that the uptake of atmospheric CO2 will come to a halt in the North Atlantic. This may increase atmospheric CO2 growth rate, ultimately resulting in more rapid climate change.
The motivation for the study was therefore to investigate whether the uptake of atmospheric CO2 is decreasing in the Nordic Seas as well, and if that is the case, to elucidate the cause. To address these issues the authors compared inorganic ocean carbon data collected at the TTO cruise of 1981 with data collected by scientists from the Bjerknes Centre on cruises to the Iceland, Norwegian, and Greenland Seas in 2002 and 2003. The comparison technique accounted for changes in hydrography and biology and so only the changes due to uptake of anthropogenic CO2 was quantified. The comparison revealed substantial changes, in particular in the Atlantic inflow region between the Faeroes and Shetland and northwards along the Norwegian Atlantic Current. The surface ocean pCO2 in these regions has increased at a greater rate than the atmospheric pCO2. Further west on the other hand, across the Arctic front, the partial pressure has increased at a lower rate than the atmospheric. The decreasing uptake of atmospheric CO2 appears to be tied to Atlantic Waters.
The authors did also investigate whether the isotopic composition of the inorganic ocean carbon pool is changing. Fossil fuel CO2 is enriched with the light carbon isotope 12C so that the atmospheric ratio of this and the heavier isotope 13C, expressed in the parameter δ13C is declining (more negative). The changes are transferred to the ocean through isotopic flux, but this is a slow process and takes about ten years. The comparison of the Nordic Seas δ13C data revealed that the Atlantic water δ13C had decreased at the same rate as the atmospheric. This implies that the changes can not have arisen locally inside the Nordic Seas but must have been advected into the region with the Atlantic water since the residence time for this water in the region is too short to allow for establishment of isotopic equilibrium.
Anthropogenic carbon transport can also explain why the surface ocean pCO2 is increasing at a greater rate than the atmospheric. This was shown by Are Olsen and Leif Anderson in 2002. Because of the buffer effect the ability of seawater to absorb fossil fuel CO2 declines as temperature decreases, since cooling is accompanied by CO2 uptake. This implies that if warm waters (farther south) saturated with anthropogenic CO2 are transported northwards, the cooling and CO2 uptake will eventually cause the pCO2 of these waters to increase at a greater rate than the atmospheric.
The authors conclude that advection of waters saturated with anthropogenic CO2 is a very likely cause for the reduced uptake of CO2 from the atmosphere. Ultimately convection brings this water down to depth and so the atmosphere is shielded from the CO2. This process acts to reduce the atmospheric CO2 growth rate. It has also become clear that ocean uptake of anthropogenic CO2 is closely tied to the ocean circulation and so there a numerous possible feedbacks to atmospheric CO2 levels that should be taken into consideration when projecting future atmospheric CO2 levels.
Citation:
Olsen, A., et al. (2006), Magnitude and origin of the anthropogenic CO2 increase and 13C Suess effect in the Nordic seas since 1981, Global Biogeochem Cycles, 20, GB3027, doi:10.1029/2005GB002669.