Using ERA5 Reanalysis Data Based On Real-World Observations Rather Than Climate Models Alone, The New Geophysical Research Letters Study Found That Cold Blob Cooling Extends 3,300 Feet Deep Into The Ocean; A Depth That Rules Out Surface Weather Effects And Points Directly To Slowing AMOC Circulation
According to CNN, ScienceAlert, Discover Magazine, Live Science, and a paper published in Geophysical Research Letters, a new study using reanalysis data from 1955 to 2024 has confirmed that the Atlantic Ocean’s so-called “cold blob”; a patch of water south of Greenland that has been cooling while the rest of the world warms, is primarily caused by a weakening of the Atlantic Meridional Overturning Circulation, or AMOC, rather than changes to surface conditions driven by wind or cloud patterns.
The finding is among the strongest observational evidence yet that one of Earth’s most important climate systems is losing strength, and that it may be approaching a tipping point with consequences that extend far beyond a cold patch in the North Atlantic.
What AMOC is and why it matters. The Atlantic Meridional Overturning Circulation is a system of ocean currents that functions as a planetary heat conveyor belt. Warm water at the ocean’s surface flows northward from the tropics and equator through the Atlantic.
As this warm water reaches the North Atlantic, it cools and loses heat to the atmosphere, warming Europe and the surrounding regions in the process before sinking due to increased density and flowing back southward as deep, cold water.
This loop of warm water going north and cold water returning south constitutes the AMOC, and it operates continuously, moving heat, nutrients, carbon dioxide, and salt across the entire Atlantic basin.
One of AMOC’s most consequential effects is warming the Northern Hemisphere: the system is responsible for keeping Europe and especially Britain, Scandinavia, and Ireland, considerably milder than their latitudes would otherwise suggest. Without AMOC, the Northern Hemisphere would be approximately 1.8 to 3.6 degrees Fahrenheit (1 to 2 degrees Celsius) colder. For Europe, that difference represents the distinction between a temperate, agricultural continent and one considerably less hospitable.
What the cold blob is and what it reveals. The cold blob has been observed in ocean temperature records for decades. It is a region of the subpolar Atlantic, located south of Greenland and Iceland, that has failed to warm at the rate of the surrounding ocean and in its core, has actually cooled, while global ocean temperatures have risen steadily.
The cold blob’s existence was uncontested. What scientists disagreed about was its cause. Two competing explanations emerged: either the AMOC had weakened, delivering less warm water to the region; or more heat was being lost through the sea surface due to atmospheric changes such as shifting winds and cloud cover.
The difference between these explanations matters enormously. Surface atmospheric effects are relatively benign; they can change with weather patterns and are not intrinsically linked to irreversible climate thresholds.
A weakening AMOC, by contrast, points toward a structural change in how the entire Atlantic Ocean circulates, driven by human-caused climate change, and potentially heading toward a tipping point from which recovery may not be possible on any human timescale.
The new study’s methodology and its decisive finding. To better unravel what’s happening in this part of the Atlantic, the study scientists combined real-world ocean heat data from instruments and satellites, finding that cooling in the cold blob was not just happening on the surface but also deep in the ocean, where atmospheric conditions like winds and clouds have a much weaker influence.
Specifically, the cooling extends deep into the water column, affecting roughly the top 3,300 feet (1,000 meters), which coincides with the depth of the northward-flowing AMOC layer. That depth makes it harder to explain the cold blob as a surface-weather effect alone.
The study used ERA5 reanalysis data; a dataset constructed from direct weather observations rather than pure climate model output, comparing ocean heat content records from 1955 to 2024 with surface heat flux data from 1955 to 2022.
The conclusion was clear: multidecadal heat content variations are generally larger and more tightly correlated with ocean heat transport than with surface heat flux variability. In plain language, the cold blob’s thermal pattern at depth cannot be explained by what happens at the ocean’s surface. It requires reduced heat delivery from below, from the AMOC itself.
This conclusion aligns with a 2025 study that used climate models rather than historical, real-world data to link the AMOC to the region of cooler water, also known as the North Atlantic warming hole. The convergence of model-based and observation-based research on the same conclusion is scientifically significant; it is the kind of cross-methodology confirmation that moves a hypothesis from plausible to strongly supported.
Why AMOC is weakening and the mechanism behind it. A raft of research suggests this system is weakening as human-driven global warming melts ice and causes a surge of freshwater into the ocean, disrupting the AMOC’s delicate balance of heat and salinity. The mechanism is straightforward but consequential. AMOC’s sinking motion in the North Atlantic depends on the water there being dense enough to sink.
Cold, salty water is dense; warm, fresh water is less so. As global warming accelerates the melting of the Greenland ice sheet and other Arctic ice, enormous volumes of freshwater pour into the North Atlantic. This freshwater dilutes the salt concentration of the surface water, reducing its density and inhibiting the sinking that drives the AMOC’s deep southward return flow.
Slow the sinking, and you slow the northward surface flow of warm water. Slow the northward flow of warm water, and you deliver less heat to the cold blob region, which begins to cool. Ancient climate records suggest the system may now be weaker than it has been in more than 1,000 years.
The tipping point concern. The AMOC has a tipping point; a threshold of weakening beyond which the system can no longer sustain itself and collapses entirely rather than simply slowing further. This is not disputed in the scientific literature; the existence of an AMOC tipping point is well-established in palaeoclimate records, which show episodes of abrupt AMOC collapse during past cold periods.
What is uncertain is exactly how close the present AMOC is to that threshold. While large uncertainty remains over how close the Earth is to this tipping point, standard CMIP6 simulations of future global warming scenarios suggest it is crossed in a substantial subset of these model simulations around the middle of this century.
The researchers are explicit about what the study implies for policy and society. “Given the well-established existence of a tipping point of the AMOC, as well as recent studies finding a range of different ‘early warning signals’ of the ocean circulation approaching such a tipping point, the strong evidence for a weakening AMOC is a serious concern for society and policy,” write the researchers in their published paper.
What an AMOC collapse would mean for the world. An AMOC shutdown would be a global catastrophe, causing accelerated sea level rise on the US East Coast, plunging Europe into a winter deep freeze and shifting the monsoon in Africa, driving prolonged droughts. The US East Coast sea level rise effect is already being observed: AMOC slowing reduces the volume of water being transported away from the US coastline, which accumulates against the shore.
The European winter scenario, while counterintuitive given global warming, reflects the enormous warming effect AMOC currently provides to Europe. Without it, British winters would resemble those of Newfoundland at the same latitude.
Shane Elipot, senior author of the related University of Miami Rosenstiel School study, described the broader impact: “A weaker AMOC can shift weather patterns, potentially leading to more extreme storms, changes in rainfall, or colder winters in some regions. It can also influence sea-level rise along coastlines, affecting communities and infrastructure.”
The distinction between slowing and collapsing and why it matters. It is important to be precise about what the evidence shows and what it does not. The current study confirms that AMOC is weakening and that the cold blob is the observational signature of that weakening.
It does not confirm that a collapse is imminent or inevitable. For Europe, the cold blob is more than an oddity on a map. If it reflects a weakening AMOC, it could point to bigger changes in the system that helps regulate the continent’s weather, rainfall, storm tracks, and agriculture.
The distinction between AMOC weakening which is already happening, measurable, and consequential and AMOC collapse, which remains uncertain in timing but potentially catastrophic in impact, is the central scientific and policy question this line of research is trying to answer.
What the 2026 study in Geophysical Research Letters contributes is methodological strength. Previous studies linking the cold blob to AMOC used climate models or proxy data. This study used ERA5 reanalysis, constructed from actual observations and confirmed the same link at depths that cannot be explained by surface atmospheric effects.
It does not answer when the AMOC will reach its tipping point. But it substantially strengthens the evidence that the AMOC is already moving in the direction of that point, and that the cold blob is the visible surface signature of a process occurring 1,000 metres down in the ocean.
To check out our previous coverage on climate science, ocean circulation, and global warming, read our articles here.
