The Thermohaline Conveyor Belt Shutdown

The IPCCs Third Assessment Report in 2001 provided a comprehensive review of the thermohaline conveyor system in both hemispheres. By analyzing a series of models with varying global warming scenarios they predicted and showed a general weakening of the system.

Simulated water-volume transport change of the Atlantic "conveyor belt" (Atlantic overturning)

 in a range of global warming scenarios computed by different climate research centres.

Observations appear to agree with this. As a reminder, this is what the deep water oceanic circulation looks like:

The Thermohaline Conveyor Belt

What Bryden, Longworth and Cunningham found was that from 1957 to 2004 some more tropical regions around the Bahamas and Florida Straits showed little to no evidence of changes above or beyond the annual variation in Gulf Stream transport. But this wasn’t the case in the North Atlantic where there has been a tendency for a weakening and shallowing of water flow; a 50% decrease in the southward deep ocean flow (3000-5000m depth), but mid-ocean circulation increasing 50% southward. 

Southward water flow is different in the Norwegian-Greenland Sea, where deep water formation appears to have ceased as a direct result of increased melt from nearby ice sheets. More freshwater has lowered the density gradient between the Gulf Stream and the local waters in the area, making it harder for the current to sink. Such changes have altered the structure of deep water circulation. 

Climate models have predicted that this weakening of the Gulf Stream will cool northwestern Europe by 4°C. For perspective, the Gulf Stream is thought to artificially increase European temperatures by up to 10°C. Because of this, some people believe that should the Gulf Stream completely shut down, then it may be enough to push Europe, and subsequently the rest of the world into the another glacial, through rapid glaciation of the continent. 

The IPCC rejects this, by first proposing that the Gulf Stream will only weaken and not shut off completely. However, the UK has now seen a 30% fall in the quantity of warm water it receives. Temperatures are thought to remain artificially high due to radiative effects of atmospheric greenhouse gases. There is a danger that the warming in the North Atlantic may trigger the destabilisation of 2.5Gt of methane trapped in methane clathrates, which as we have seen earlier, is a highly potent greenhouse gas able to rapidly spread throughout the global systems (geologically speaking of course). This is only a fraction (0.2% at most) of the methane required to cause the PETM (2000Gt). Knowledge of the global extent of methane clathrate destabilisation is still largely unknown but likely to be significantly higher than 2.5Gt. So, understanding the PETM is now more important than ever!