Is There a Future Tipping Point in the Atlantic Meridional Thermohaline Circulation (AMOC)?

by Emil Morhardt

The AMOC is the set of ocean currents that begins with cold seawater off Greenland sinking to the bottom and flowing south, being replaced by warmer water flowing at the surface past Florida from the south, transferring warmth from the tropics to the east cost of North America and the West Coast of Europe. The full round trip cycle from, and back to, Greenland takes a thousand years. Without it, both western Europe and eastern North America would cool significantly, with large numbers of potential adverse effects. We know from ice core record temperature data from both the Greenland Ice Sheet (GrIS) and from Antarctic ice sheets, that the AMOC has come to an abrupt halt many times, and has characteristically taken a millennium to recover. The comparative abruptness of these cessations has led to the fear that there may be some threshold that once crossed—a tipping point—that cessation is inevitable. This would be nice to avoid.

The cause of these cessations is usually attributed to a larger amount than usual of freshwater floating on the sea surface near Greenland; freshwater even when near freezing, will not sink through seawater, which is denser, and as we know from ice cubes, becomes much less dense when frozen so will float no matter how cold it gets. Once the water around Greenland stops sinking, the AMOC stops as well. The source of the freshwater in the past may have been melting of ice on the Canadian Shield as probably happened 12,000 years ago, increased precipitation as is likely to happen with global warming, or loss of ice from the GrIS as is happening now. But, according to a paper written collectively by the global scientific community studying the AMOC (Bakker 2016), water from the melting of the GrIS has not been well modelled in most global climate models, not probabilistically modelled past 2100, and not much modelled at all past 2300. This paper employs a physics-based AMOC emulator to estimate the probability of various levels of AMOC slowing including effects of GrIS melting and other effects of global warming.

The principal outputs are graphs of the predicted slowing of the AMOS under two CO2 production scenarios: RCP8.5 which is regarded by the IPCC as the business-as-usual scenario, and RCP4.5, which is the level of CO2 buildup projected to result in a less than 2°C global warming. The results from eight major global climate models are shown, as well as their averages with and without GrIS melting and with and without other projected climate effects. There is a wide spread of projected slowing across models, but the bottom line is that if all are considered together, they collectively predict slowing but not collapse of the AMOC at RCP4.5, but a high likelihood of complete collapse by around 2250 under RCP8.5.

It is impossible to tell from these two scenarios (RCP4.5 and RCP8.5) if somewhere in the middle there would be something like a tipping point, but what is clear is that it appears dangerous to continue producing CO2 at business-as-usual levels.

It is also clear that there is much more work to be done in examining the probability of AMOC collapse.

Bakker, P., Schmittner, A., Lenaerts, J., Abe‐Ouchi, A., Bi, D., Broeke, M., Chan, W.L., Hu, A., Beadling, R., Marsland, S., 2016. Fate of the Atlantic Meridional Overturning Circulation: Strong decline under continued warming and Greenland melting. Geophysical Research Letters DOI: 10.1002/2016GL070457

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