ocean carbon sinks

This is complex thinking but indicates how easy it is to be rite about global warming and to think the answers are easy. The absorption of carbon by oceans is an important feature of the carbon chain and this is an important article by Gruber in Nature 11th March 2009
Recent studies have suggested that the carbon sink in the North Atlantic f Ocean has decreased in recent decades, possibly as a result of long-term climate change. Writing in Global Biogeochemical Cycles, Thomas et al. propose that this interpretation needs to be viewed with ” great caution. From their modelling work, they argue that the observed trends instead reflect fluctuations over decades and are a response to climate variability in the North Atlantic region.
The oceanic sink, which has removed about 30% of the global anthropogenic emissions over the past 250 years or so, might be stalling. Some of the best observational evidence comes from the North Atlantic Ocean, where long-term measurements of the surface ocean’s partial pressure of CO2 (PC02) indicate that its carbon uptake from the atmosphere has decreased in recent decades, perhaps owing to climate change. Thomas et al. challenge this interpretation.
The North Atlantic is the largest ocean sink for atmospheric CO2 in the Northern Hemisphere, with half of the flux in the North Atlantic being driven by the uptake of man made CO2 . The detection of long-term changes in this sink is challenging, however, because the sink varies substantially from year to year. That variation is largely associated with the North Atlantic Oscillation (NAO), which is the dominant mode of climate variability in this region.
The NAO is a large-scale seesaw in atmospheric mass between a subtropical high pressure system, typically near the Azores, and a subpolar low near Iceland. A positive phase of the NAO – that is, a stronger pressure gradient between these two systems – is associated with more and stronger winter storms crossing the North Atlantic on a more northerly route, causing major anomalies in sea surface temperature, currents and convective activity throughout the North Atlantic
In a positive phase, a stronger Azores high and stronger Icelandic low produce more and stronger winter storms on a more northerly track. As a consequence, the subtropical gyre extends northwards, and the North Atlantic Current accelerates, transporting increased amounts of warm, saline waters with low carbon concentrations northeastwards, This causes an intensified sink (minus sign) in the eastern subpolar North Atlantic, because these waters have the potential to take up a large amount of CO, from the atmosphere when they are cooled along their northward journey. At the same time, the Labrador Current intensifies, bringing fresher, colder waters with high carbon concentrations from the
Arctic into the subpolar gyre, creating a diminished sink (plus sign) near the coast of Canada. In the subtropical gyre, warm conditions and reduced convective activity also lead to reduced carbon uptakes.
Grubwer 2009 Fickle trends in the ocean Nature vol 458 pp 155-6

Martin Eastwood
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