In 1751, Captain Henry Ellis of an English slave-trading ship lowered a bucket with a built-in thermometer while traveling to 25°N in the North Atlantic Ocean. At that time, water temperature was measured at various sites and depths by sailors sailing around the world.
The captain was shocked by the findings. The deep water of the sea was icy cold. He wrote in a letter, “The cold increased regularly in proportion to the depth, until it got down to 3,900 feet: whence the mercury in the thermometer came to 53 degrees (Fahrenheit); and though I afterwards sank it to a depth of 5,346 feet, that is, a mile and 66 feet, it came up no less than this.”
it AccountAccording to Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, this was the first recorded deep-sea temperature measurement. Captain’s discovery became the basis on which scientists now understand the functioning of water on Earth – deeper waters are colder, and warmer waters are closer to the surface.
But just south of Greenland, in the North Atlantic Ocean, there is a large patch of water that is cooling even as the rest of the ocean is warming. On modern temperature maps, it appears as a blue spot in brushes of red and orange that cover much of the globe.
Known as the ‘Cold Blob’ or ‘North Atlantic Warming Hole’, the patch defying the global warming trend is located in the subpolar North Atlantic at about 25°W-45°W, 50°N-60°N and has been cooling at a rate of 0.15°C per century from 1900 to 2014.
NASA’s GISTEMP data confirms a long-term cooling trend in the region from 1880 to 2025 — and the reason for it, research published last month, indicates has a lot to do with the Atlantic Ocean current.
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What do scientists make of the cold drop?
The cold blob was not so much discovered as gradually recognized.
This variation was present in temperature records captured by sailors and datasets in the 19th century – visible, retroactively, in the same NASA GISTEMP data now used to map it. But, for decades, this discrepancy was considered an anomaly, perhaps just ‘noise’ in the data. While the last century saw temperatures rise by an average of 1 degree Celsius worldwide, the patch quietly cooled by about 0.9 degrees Celsius.
The first modern study that took a systematic, long-term approach was conducted by Mihai Dima and Gerrit Lohmann in 2010. It analyzed sea surface temperature patterns since 1870 and found that the blob may be linked to the Atlantic Meridional Overturning Circulation, or AMOC.
The researchers reported that the AMOC, a belt of water that influences weather across continents, had been weakening since the 1930s.
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earth’s ocean conveyor belt
The AMOC is a system of ocean currents in the Atlantic Ocean that bring warm water to the north and cold water to the south.
One of its circulation patterns is the movement of warm surface water northward via the Gulf Stream, where the water cools and forms sea ice. As ice forms, salt is left behind, causing water to condense. That dense, cold, salty water then sinks and is carried southward through the deep ocean. Eventually, it is pulled back towards the surface in a process called ‘upwelling’, where it warms and the cycle begins again.
The entire circulation loop takes an estimated 1,000 years and involves a flow of water in a volume of about 18–20 Sverdrups – the unit oceanographers use for ocean current flow, where one Sverdrup is equal to one million cubic meters of water per second. To put it into context, this is equivalent to about 90-100 times the flow of the Amazon River at its mouth.
The AMOC is important for regulating the European climate – the UK and north-western Europe experience mild winters compared to polar regions partly because the AMOC brings warmth from southern latitudes.
In the south, after sinking, cold deep water flows along the ocean floor towards the Antarctic. From there, they spread into the Indian and Pacific Oceans, where mixing of the ocean in the Southern Ocean and westerly winds slowly pull them back to the surface – completing the cycle.
Beyond heat, the water belt transports nutrients that support marine ecosystems and plays a role in the carbon cycle, transporting carbon-rich surface waters to the deep ocean. If this sinking process slows down, the ocean cannot transfer carbon dioxide from the atmosphere to the deep ocean quickly enough, leaving large amounts of greenhouse gases in the atmosphere and accelerating climate change.
Cold drop as a symptom of weak AMOC
Starting in 2012, several studies linked the cold blob to a weakened AMOC. The researchers say that cold patch of water sits exactly where the AMOC delivers its northward-flowing heat. Slower conveyor belts mean less heat, and as a result, cooler areas.
But over the past few years a section of scientists have pointed to other possible causes of the cooling effect, among them surface heat loss due to atmospheric effects and the effects of aerosol pollution.
Most of this has now been countered.
Last year, researchers at the University of California, Riverside (UCR) analyzed 20 years of AMOC observations and a century of ocean temperature and salinity data and tested them with 94 different climate models. They came to the same conclusion.
“If you look at the observations and compare them with all the simulations, only the weak-AMOC scenario reproduces the cooling in this region. That’s a very strong correlation,” co-author Kai-Yuan Li said in a statement. The study, titled ‘Weak Atlantic Meridional Overturning Circulation Causes Historic North Atlantic Warming Hole’, was published in the journal Communications Earth & Environment in May 2025.
The latest study by Rahmstorf and colleagues was published In Geophysical Research Letters in 2026. By analyzing the temperature data, this study concluded that the cold blob was likely caused by a reduction in heat transport into the water as surface heat loss declined over the years.
“To explain the cooling trend in the cold blob region from surface heat loss when the AMOC is stable, this heat loss would need to increase to outweigh the AMOC heat supply. The opposite is seen in the ERA5 data (a global weather dataset): surface heat loss over the cold blob has actually decreased (since 1993, slightly since 1955). The latter is expected when the AMOC region supplies less heat to the atmosphere and thus releases less into the atmosphere,” read the study.
If AMOC collapses
The consequences of continued weakening or collapse of the AMOC are virtually science fiction. It was the basis for the 2004 film The Day After Tomorrow, which was inspired by the book The Coming Global Superstorm.
Stefan Rahmstorf, the scientist cited above who has researched the topic extensively wrote about a scene The film shows the hero-researcher addressing a climate conference in Delhi.
“On the other hand, given the rules and constraints of the genre, it is remarkable to what extent the filmmakers have attempted to incorporate some realistic background. The beginning of the film shows the UN climate conference in Delhi where Jack Hall talks about the potential risk of the closure of the North Atlantic Stream. I gave a similar speech at such a UN conference in Buenos Aires in 1998 – I also showed the same picture.
In the film discussion, Hall said that the (AMOC) shutdown could happen in a hundred years, or in a thousand years, or not at all. Many actual climate scientists have said the same thing. “In this way, what climatologists think is presented in a realistic manner in the film, and it is very clear that the intense drama that unfolds later is the opposite of what any climatologist would expect – this is where the imagination begins,” Rahmstorf wrote in a review.
Much of this narrative is supported by science.
A 2023 study by Peter Ditlevsen of the Niels Bohr Institute at the University of Copenhagen and colleagues, published in Nature Communications, used statistical modeling and over a century of ocean temperature data to estimate that under current greenhouse gas emissions, the AMOC is likely to collapse this century.
The Potsdam Institute for Climate Impact Research (PIK) found in an August 2025 study that under high-emissions scenarios, the AMOC could shut down after the year 2100, cutting the ocean’s northward heat supply and leading to “summer dryness and severe winter extremes in northwestern Europe” as well as “shifts in the tropical rain belt.”
In another research, scientists estimated that under AMOC collapse the annual temperature in Bremen, Germany could drop by 10 degrees Celsius, rainfall could reduce in the US and the monsoon system in South Asia including India could be disrupted.
A warmer Southern Hemisphere will alter the temperature of the Pacific Ocean, indirectly causing El Niño episodes – which are known to suppress the Indian monsoon – to become stronger. Because more than half of India’s cultivable land depends on monsoon rains for irrigation, the livelihoods of millions of farmers and the country’s food security will be affected by any disturbances in this season.
A weakened AMOC could cause Antarctica’s ice sheets to melt faster and reduce the ocean’s ability to absorb atmospheric carbon, releasing more greenhouse gases into an already warming world.
