There’s one story that the climate science community likes to scare the world’s population with even more than The Methane Time Bomb and that is saturation of the upper ocean layers with CO2 leading to ocean acidification and the extinction of all carbonate based ecosystems. The worry is that on our static planet removal of CO2 from the upper ocean layers by the deeper ocean layers takes place on “a very long time scale” of hundreds of years.
This concept is totally at odds with my own perceptions of wild oceans and ocean currents churning seawater about on a daily basis. From my geochemistry background I have recollection of the ocean mixing time being roughly 1000 years. And this got me thinking about the Gulf Stream, the gigantic ocean conveyer system that carries water from the Indian Ocean, round Cape of Good Hope, northwards along the full length of the Atlantic until it eventually sinks in the North Atlantic somewhere between northern Norway and Greenland.
The Gulf Stream transports nearly four billion cubic feet of water per second, an amount greater than that carried by all of the world’s rivers combined.
That is one BIG number. The calculation below the fold suggests that the Gulf Stream sequesters the equivalent of the surface waters (333m layer) of the whole Atlantic Ocean once every decade. Map image from Met Office.
4^9 ft3 per second
= 2.4^11 ft3 per minute
= 1.44^13 ft3 per hour
= 3.456^14 ft3 per day
= 1.262^17 ft3 per year
= 3.6^15 m3 per year
= 3.6^6 km3 per year
I’m going to make the assumption that the annual flow of the Gulf Stream at the surface is matched by a similar reverse flow in the depths and therefore the flow rate equals the sequestration rate. Hence the Gulf Stream sequesters 3.6 million cubic kilometres of surface water every year. Is that still a BIG number? The upper ocean layer is normally taken to be the top 300 m or so, for simplicities sake I’m going to assume the top 333.3 m. 3.6^6 km3 spread out in a layer 333.3 m deep would cover an area of 3*3.6^6 = 10.8^6 square kms. The area of the Atlantic Ocean is 106.4^6 square kilometres. Hence The Gulf Stream would take 106.4/10.8 = 9.85 years to sequester the surface layer (333.3 m deep) of the Atlantic Ocean.
The total volume of seawater on Earth is 1.37^9 km3. Divide this by the annual sequestration of The Gulf Stream (3.6^6 km3) and you get 380 years for The Gulf Stream to cycle a volume of water equivalent to all the world’s oceans. Of course some of the water in the Gulf Stream may simply be going round and round but there must surely also be significant mixing along the edges and on the surface and this zeroes in on the 1000 year ocean mixing time quoted at the start. And of course The Gulf Stream is only one of many systems churning seawater on a daily basis.
These numbers surprised me, check my sums and let me know if I made a mistake.