The thermohaline circulation is part of a huge oceanic current system reliant on surface temperature and salinity gradients. Surface water pushed by poleward winds cools as it travels to higher latitudes, resulting in a gradual transition to sea ice. Consequently, surrounding waters become saltier due to salt left behind during sea ice formation. As the ocean salinity increases, the water becomes denser and ultimately sinks, pulling surface water into its place. It is this process of colder, saltier water sinking and dragging warmer water poleward that results in the formation of these global currents.

This system is hugely important for global temperature and weather regulation, as well as the upkeep of plankton stocks. The issue we're facing pertains to global temperature increase, where glacial melting is causing massive amounts of freshwater to enter the North Atlantic, lowering the surface salinity and preventing the seawater from sinking. Without sufficient descension, warmer water will not be pulled poleward, and the current will ultimately stagnate.

Recent studies have already shown an ‘exceptional’ slowdown of this current in the last century, with researchers citing Greenland melt-water as a potential contributor. If this circulation was to shut down completely, it could cause cooling in Northern America, Europe and the North Atlantic, with a profound reduction in temperature noted in the British Isles, Nordic countries and France. It could also cause widespread, major flooding and storm events, as well as warming in the tropics and Antarctica, and more intense El Niño events. Possibly the most vexatious of these consequences comes in the form of collapsing plankton stocks, as this would lead to mass fish population decline and a subsequent food crisis (something that we would likely struggle with considering that as of 2016, 815 million people were undernourished even with fish as a potential food source).

Whilst perhaps not as dramatic as the film 'The Day After Tomorrow' depicted, western Europe and eastern North America would nevertheless experience a noticeable shift in climate. Palaeoclimate modelling using Greenland ice core data has now shown past instances of circulation shut down. From this data, we can ascertain that within several decades Greenland had cooled by roughly 7 degrees Celsius. The difference between this instance and what we're facing now relates to cause disparity. The past event was natural, and as such occurred over a long period of time (the melting of ice covering Northern America during the last ice age). What we're experiencing now is a profound increase in the speed of emission dispersion coupled with an ensuing temperature increase, so it stands to reason that the shut-down in oceanic currents would be more far-reaching in its effects this time around, too.

With ocean levels still rising and global temperatures doing the same, only time will tell whether the current rectifies itself or gets worse. Reasonable logic dictates that whilst the rising global temperature is exacerbating melt-water runoff, it also increases the rate of evaporation – removing some of this deleterious fresh water from the current circulation. Whilst this should limit the possibility of an irreversible shut down, this eventuality cannot be excluded, and policy changes should reflect the seriousness and unpredictable nature of the issue at hand. In the meantime, lowering our carbon footprints to help slow its progression is an admirable step in the right direction.


Photo: Derry Stock

Derry Stock BSc (Hons)
Environmental Consultant Intern

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