Ocean currents are a key mechanism of the climate system. As a result, climate change-induced changes to oceans currents will have a profound impact on earth’s climate overall, particularly at regional scales. To illustrate this point, this article will focus on one current system in particular: the Atlantic Meridional Overturning Circulation (AMOC).
The AMOC is a system of currents in the Atlantic Ocean. It has surface currents (e.g., the Gulf Stream in the northern cell) travelling towards the poles, and deepwater currents (e.g., the North Atlantic Deep Water in the northern cell) returning cool water to equatorial regions. Surface currents are driven by winds, such as the Gulf Stream being pushed northeast by Westerlies. As the surface current moves north, water is cooled by the atmosphere, which reduces its volume and, since the mass is constant, this increases its density. The smaller volume of water, with no reduction in the mass of dissolved salt, also means higher salinity and even greater density. This denser water sinks into the cold deepwater current. As the current reaches equatorial waters, it is drawn to the surface to replace the surface water above moved by trade winds back towards the poles, and this process repeats in a cyclical fashion.[1]
The AMOC is a critical aspect of the climate system in the Atlantic and especially for the UK’s climate. London is at roughly the same latitude as Labrador in Canada, where the average daily high temperature in January is -16°C.[2] London does not see such extreme cold because the North Atlantic Drift (which emanates from the Gulf Stream) brings warm water to the higher latitudes. Since water has a higher specific heat capacity than land, the ocean around the UK remains relatively warm, despite weak insolation of the winter months. The UK climate is thus moderated by the North Atlantic Drift, causing much milder winters than would occur otherwise. The same process occurs in summer, but now the ocean is relatively cooler than the land, and so the opposite influence is observed, with UK summers being relatively mild.
However, the warming which the climate system is currently experiencing threatens to change this. Recent research has found that the AMOC is now the weakest it has been for a thousand years.[3] Whilst natural variations may be affecting the strength of the AMOC, climate change is also a likely cause. As a result of global warming, the ice sheets at both poles are melting, causing freshwater to enter the North Atlantic and Southern Ocean. Since the salinity of water is also a factor in its density, freshening the water at the poles inhibits its ability to sink. Thus, the circulation of currents is slowed.
This causes greater extremes in both weather and climate. As mentioned above, the AMOC regulates the UK’s climate to a great extent. This slowing reduces the volume of warm water carried towards Europe, and therefore the moderating impact is reduced. As a consequence, the UK climate will become more extreme. Winter storms will become more frequent, and summer and winter average temperature will drift further apart to the extent that Britain’s climate may become more akin to that of Southern Alaska, albeit slightly warmer on average.[4]
Low latitude climates will also be altered, particularly semi-arid regions like the Sahel. Researchers have shown a close correlation between precipitation in the Sahel and the strength of the AMOC.[5] One reason for this is that a reduction in the strength of the AMOC causes a rise in the surface sea temperature in the South Atlantic, as warm equatorial water is not removed as quickly. Consequently, the temperature gradient between the land and sea in the Sahel region is reduced, meaning the strength of the Sahelian monsoon declines. Semi-arid regions rely on monsoon rains for much of their precipitation, and so a reduction in the monsoon strength means droughts become more likely in such places.
Tipping points are often talked about in climate, and the AMOC is no different. Researchers suggest that currently, if all warming were to stop, the AMOC would recover its strength. However, they fear the AMOC will eventually reach such a tipping point past which, even if we reversed the damage being done to the climate, the AMOC would still not recover. The uncertainty around where that tipping point is highlights the risk involved in further global warming.
As Maslin says in A Very Short Introduction to Climate, ‘Climate affects everything we do.’[6] Changes to regional climates caused by the weakening of the AMOC have significant wider effects. For example, UK infrastructure is designed for its mild climate. Houses are designed to retain heat, and rarely contain air conditioning units.[7] Thus, when heatwaves strike, cooling becomes difficult, creating a hazard to public health – the British heatwave in August 2022 caused an estimated 1458 deaths.[8] Britain is equally unprepared for extreme cold, with transport infrastructure in particular grinding to a halt in heavy snow, and so the increased incidence of such storms will take an ever-larger toll on the UK economy. Agriculture may also be impacted. More extreme weather will likely also damage crop yields, putting further strain on a food supply chain which the Russian invasion of Ukraine has shown to be highly fragile. While it can be all too easy to think of the damage of climate change in the abstract terms of warming figures, these changing physical patterns can be seen to have real human impacts that are affecting many in the UK already.
The final point to note is that the response of ocean currents to global warming reflects the reality of climate change. It is not simply the case that average temperatures rise evenly across the globe, with no other changes except warming. The climate is a system, with all its mechanisms finely tuned, and one small change can throw everything else into discord. While this article has focused on the AMOC, other ocean currents are also impacted significantly, and the nature of these impacts can vary immensely from region to region in both type and magnitude. The AMOC is just one cog in the machine, but the consequences of its changing patterns are profound.
[1] Met Office, no date.
[2] NOAA, no date.
[3] Caesar, L et al., 2021.
[4] Razor Science Show, 2021, 6:30.
[5] Mulitza, S et al., 2008.
[6] Maslin, 2013, p.1
[7] Victor, 2022.
[8] UK Health Security Agency, 2022.
Bibliography
Caesar, L et al. (2021) ‘Current Atlantic Meridional Overturning Circulation weakest in last millennium’ Nature Geoscience. Available at: https://doi.org/10.1038/s41561-021-00699-z-
Maslin, M (2013). Climate: A Very Short Introduction. Oxford: Oxford University Press.
Met Office (no date). What is the Atlantic Meridional Overturning Circulation? Available at: https://www.metoffice.gov.uk/weather/learn-about/weather/oceans/amoc (Accessed: 31st October 2022).
Mulitza, S et al. (2008) ‘Sahel megadroughts triggered by glacial slowdowns of Atlantic meridional overturning’. Paleoceanography and Paleoclimatology. Available at: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2008PA001637 (Accessed: 31st October 2022).
NOAA (no date) Labrador City, NL, Canada. Weather averages. Available at: https://www.google.com/search?&safe=active&q=labrador+climate&rlz=1C1ONGR_en-GBGB980GB980&oq=labrador+climate&aqs=chrome..69i57.329j0j7&sourceid=chrome&ie=UTF-8 (Accessed: 31st October 2022).
Razor Science Show (2021) How ocean currents reveal new secrets about climate change, 6 August. Available at: https://www.youtube.com/watch?v=QejJLcltkuU&t=385s (Accessed: 31st October 2022).
Victor, D (2022) ‘British homes were built to retain heat. That’s becoming a problem.’ New York Times. Available at: https://www.nytimes.com/2022/07/19/world/europe/britain-homes-heatwave.html (Accessed: 31st October 2022).
UK Health Security Agency (2022) UKHSA and ONS release estimates of excess deaths during summer of 2022. Available at: https://www.gov.uk/government/news/ukhsa-and-ons-releaseestimates-of-excess-deaths-during-summer-of-2022 (Accessed: 31st October 2022).
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