Many people are surprised to learn that Madrid and New York are at the same latitude, when it’s obvious that their winters are not comparable. Nor are those of Boston and Bilbao, a city in northern Spain where snow and sub-zero temperatures are a rarity. This difference between climates at similar latitudes on either side of the Atlantic is largely determined by ocean currents. But this regulatory system is under threat from climate change, and the effects of its disruption would be severe, experts warn. So how do ocean currents control the climate?
Roland Emmerich’s 2004 sci-fi blockbuster The Day After Tomorrow imagined the onset of a new ice age due to abrupt and extreme climate change caused by the collapse of the North Atlantic Current, technically known as the Atlantic meridional overturning circulation, or AMOC. The catastrophes depicted in the film were so sudden and exaggerated that several scientists were quick to point out that the whole thing was just an impossible fiction, and indeed the film made it onto some of the lists of the most scientifically inaccurate films in history.
Nevertheless, The Day After Tomorrow is often mentioned in popular science articles, and with good reason. While the timeframe and magnitude of the effects were far-fetched, as ocean physics expert Stefan Rahmstorf of the University of Potsdam wrote, the message was not: “it would be a mistake and not do the film justice if scientists simply dismiss it as nonsense”; “the film presents an opportunity to explain that some of the basic background is right: humans are indeed increasingly changing the climate and this is quite a dangerous experiment, including some risk of abrupt and unforeseen changes.” And above all, it serves to explain something that is often not well understood: how can global warming bring more cold to some regions?
A globally interconnected system
Despite the seemingly still appearance of the oceans in photographs of Earth from space, in reality the mass of saltwater covering 71% of the planet is in constant motion, and not erratically but in a highly organised manner. Factors such as winds, the Coriolis effect—resulting from Earth’s rotation—tides and others are involved in this movement. In particular, there exists a globally interconnected system that moves due to a temperature and salinity gradient, hence it is known as thermohaline circulation.
The colder and saltier the water, the denser it is and thus the deeper it sinks. From the Gulf of Mexico, the Gulf Stream, part of the AMOC, flows across the surface of the Atlantic, carrying warm waters towards northern Europe, making winters in the Old Continent milder than at similar latitudes in North America. Near the pole, the cooled water sinks, forming the deep water mass of the North Atlantic, which returns south to emerge in the Antarctic and Pacific Oceans. In turn, the warmed equatorial waters flow into the Indian Ocean and back into the Atlantic, while the Southern meridional overturning circulation (SMOC), the second major branch of the thermohaline, forms around Antarctica.
This system of currents, known as the ocean conveyor belt because of its global nature, distributes not only energy (solar heat) around the planet, but also nutrients and other dissolved solids, as well as gases trapped by the water, including atmospheric CO2; given that the oceans absorb 25-30% of our CO2 emissions and 90% of the resulting excess heat, the relationship between ocean currents and climate is understandable. Warming waters melt polar ice, which in turn releases more freshwater into the oceans by reducing the salinity gradient, and these two effects tend to weaken and slow down the global ocean current system.
Accelerated collapse of the AMOC
Broadly speaking, a weakening of the AMOC leads to colder conditions in Europe—up to 20 degrees cooler in Norway—and warmer conditions in the southern hemisphere, less rainfall in the mid-latitudes and altered monsoons in the tropics, a reduction in nutrients and dissolved oxygen in the ocean, a rise in sea level along the North American coast and stronger hurricanes in the Atlantic. As such a complex system, scientific models are subject to uncertainty. But although the AMOC has weakened by 15% since the mid-20th century, the most widely used projections, such as that of the UN’s Intergovernmental Panel on Climate Change (IPCC), did not predict a full collapse until perhaps the next century.
But in 2023, a study by the University of Copenhagen, based on early warning signs that go beyond natural variations and had not been taken into account, warned of a possible accelerated collapse of the AMOC in the middle of this century. In 2024, another study from Utrecht University confirms this: according to René van Westen and his colleagues, there is a tipping point in the salinity decline at the southern boundary of the Atlantic, and “once a threshold is reached, the tipping point is likely to follow in one to four decades.” For Ramhstorf, “the new study confirms past concerns that climate models systematically overestimate the stability of the AMOC.”
What’s more, another study from 2023 adds that the deep, cold, salty waters of Antarctica are becoming warmer and less salty, weakening another driver of the global conveyor belt. Van Westen and his colleagues add: “It might seem counterintuitive to worry about extreme cold as the planet warms, but if the main Atlantic Ocean circulation shuts down from too much meltwater pouring in, that’s the risk ahead.”
Comments on this publication