In 1751, Captain Henry Ellis of an English slave-trading ship lowered a bucket with a built-in thermometer while sailing 25°N in the north Atlantic Ocean.
But just south of Greenland, in the northern Atlantic Ocean, is a large patch of water that has been cooling even as the rest of the ocean warms.
Known as the 'cold blob' or the 'North Atlantic warming hole', the patch that belies the global warming trend is located roughly 25°W–45°W, 50°N–60°N in the subpolar North Atlantic and has been cooling at a rate of 0.15 degrees Celsius per century from 1900 to 2014.
In the south, after sinking, the cold deep waters flow along the sea floor towards the Antarctic.
AMOC belts against a backdrop of sea surface temperature trend since 1993.
In 1751, Captain Henry Ellis of an English slave-trading ship lowered a bucket with a built-in thermometer while sailing 25°N in the north Atlantic Ocean. In those times, water temperatures at different sites and depths were captured by seafarers sailing across the globe. The cold blob south of Greenland, visible on this temperature map. The visualisation shows temperature changes from 1880 to 2015 as a rolling five-year average. Orange represents areas warmer than the 1951-80 baseline, and blue, areas cooler than the baseline. (Courtesy: Nasa) The captain was startled by the findings. The deep water in the ocean was icy cold. “The cold increased regularly, in proportion to the depths, till it descended to 3,900 feet: from whence the mercury in the thermometer came up at 53 degrees (Fahrenheit); and tho’ I afterwards sunk it to the depth of 5,346 feet, that is a mile and 66 feet, it came up no lower,” he wrote in a letter. This account, according to Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, was the first recorded temperature measurement of the deep ocean. The captain’s finding went on to become the basis on which scientists now understand workings of waters on Earth – deep waters are cold, and warmer waters are closer to the surface. But just south of Greenland, in the northern Atlantic Ocean, is a large patch of water that has been cooling even as the rest of the ocean warms. On modern temperature maps, it shows up as a blue spot in brushes of red and orange that cover most of the world. Known as the 'cold blob' or the 'North Atlantic warming hole', the patch that belies the global warming trend is located roughly 25°W–45°W, 50°N–60°N in the subpolar North Atlantic and has been cooling at a rate of 0.15 degrees Celsius per century from 1900 to 2014. Nasa’s GISTEMP data confirms a long-term cooling trend from 1880 to 2025 in this region – and the reasons for it, research published last month, indicates it has much to do with an Atlantic Ocean current. Also read: The climate promise that won’t survive geopolitics What scientists make of the cold blob The cold blob was not discovered so much as gradually recognised. The variance was present in temperature records captured by seafarers and datasets going back to the 19th century — visible, in retrospect, in the same Nasa GISTEMP data now used to map it. But, for decades, it was believed to be an anomaly, perhaps just 'noise' in the data. While the world saw temperatures increase by an average of 1°C over the past century, the patch had quietly cooled about 0.9°C. The first modern study that took a systematic, long-period view was by Mihai Dima and Gerrit Lohmann in 2010. It analysed patterns of sea surface temperatures since 1870 and found that the blob may be linked to the Atlantic Meridional Overturning Circulation, or AMOC. AMOC, a belt of water that influences weather across continents, had been weakening since the 1930s, the researchers flagged. Also read: What India must do as climate crisis worsens
Sea-surface temperature from 1880 to 2025, (Courtesy: Nasa GISTEMP)
The Earth's ocean conveyor belt AMOC is a system of ocean currents in the Atlantic Ocean that brings warm water to the north and cold water to the south. One of its circulation patterns is the movement of warm surface water through the Gulf Stream to the north, where water cools and forms sea ice. As ice is formed, salt is left behind, making the water denser. That dense, cold, salty water then sinks and is carried southward through the deep ocean. Eventually, it is pulled back up toward the surface in a process called 'upwelling', where it warms and the cycle restarts. The entire circulation loop takes an estimated 1,000 years and involves moving water at a volume of roughly 18–20 sverdrups — the unit oceanographers use for ocean current flow, where one sverdrup equals one million cubic metres of water per second. To put it in context, this is equivalent to approximately 90–100 times the flow of the Amazon river at its mouth. AMOC is critical for regulating the European climate — milder winters experienced in the UK and northwestern Europe compared to the polar regions happen partly because the AMOC brings heat up from southern latitudes. In the south, after sinking, the cold deep waters flow along the sea floor towards the Antarctic. From there, they circulate into the Indian and Pacific Oceans, where ocean mixing and westerly winds in the Southern Ocean gradually pull them back to the surface — completing the loop. 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 as quickly, leaving larger amounts of greenhouse gases in the atmosphere and accelerating climate change.
AMOC belts against a backdrop of sea surface temperature trend since 1993. (Courtesy: Copernicus Climate Change Service)
