The Gulf Stream is slowing down primarily because freshwater from melting ice sheets is disrupting the engine that drives deep ocean circulation in the North Atlantic. This isn’t just about the surface current itself. The real concern is a weakening of the broader system it belongs to: the Atlantic Meridional Overturning Circulation, or AMOC, a massive conveyor belt that moves warm water northward and cold water southward through the Atlantic Ocean. Measurements at 26°N in the Atlantic show that northward heat transport has dropped from 1.32 petawatts before 2009 to 1.15 petawatts after, a reduction linked to a measurable decline in the AMOC’s strength.
The Gulf Stream and the AMOC
The Gulf Stream is the part of the system most people have heard of: a powerful surface current that carries warm water from the Gulf of Mexico up along the U.S. East Coast and across toward Europe. It moves roughly 32 million cubic meters of water per second through the Florida Straits, a flow rate monitored by NOAA since 1982. But the Gulf Stream is just one piece of a much larger circulation loop.
The AMOC works like a conveyor belt. Warm, salty water flows north along the surface. As it reaches the subpolar North Atlantic, it cools, becomes denser, and sinks deep into the ocean. That cold, dense water then flows south along the ocean floor, completing the loop. When scientists say the Gulf Stream is “slowing down,” they’re usually referring to a weakening of this entire overturning process, not just the wind-driven surface current. The surface Gulf Stream is partly pushed by winds and would continue flowing even if the AMOC weakened significantly. What’s changing is the deeper, density-driven component that powers the full conveyor belt.
How Freshwater Disrupts the System
The sinking of water in the North Atlantic depends on a precise balance of temperature and salinity. Seawater needs to be cold and salty enough to become dense enough to sink. When large volumes of freshwater pour into this region, they dilute the surface water, making it lighter. Lighter water doesn’t sink, and the conveyor belt loses its driving force.
The Greenland ice sheet is the biggest source of that freshwater. As it melts at accelerating rates, huge volumes of meltwater flow into the North Atlantic. Climate simulations show that at moderate meltwater inputs, surface salinity in the North Atlantic drops measurably, and at higher volumes the freshening becomes dramatic, with salinity plunging by more than a full unit on the standard scale. That freshening penetrates deep into the ocean in the northern high latitudes, directly interfering with the formation of the cold, dense water that powers the AMOC’s lower limb.
Increased rainfall over the North Atlantic, another consequence of a warming climate, adds to the freshwater problem. So does the accelerating loss of Arctic sea ice, which releases stored freshwater as it melts. All of these sources compound the same basic issue: too much freshwater in the wrong place, weakening the density contrast that makes the system work.
The Labrador Sea: Where the Engine Stalls
The Labrador Sea, between Greenland and Canada, is the main site where deep convection happens in the North Atlantic. During harsh winters, surface water loses so much heat to the atmosphere that it becomes extremely dense and plunges to intermediate depths. This process creates Labrador Sea Water, which then feeds into the Deep Western Boundary Current, a southward-flowing river of cold water hugging the ocean floor along the east coast of Canada.
Research shows that when deep convection in the Labrador Sea is strong, more Labrador Sea Water enters the deep boundary current within about a year, and the local overturning circulation strengthens in response. The reverse is also true. When conditions suppress deep convection, whether through surface freshening, milder winters, or both, less dense water forms, less water sinks, and the overturning weakens. The Labrador Sea is essentially where you can watch the AMOC’s engine speed up or stall, and recent decades have shown periods of notably reduced deep water formation there.
How Much Has It Weakened?
Direct measurements of the AMOC have only been available since 2004, when an array of instruments was deployed across the Atlantic at 26°N. Those measurements revealed something striking: the circulation is more variable than anyone expected, and it has weakened over the monitoring period. The overturning dropped by about 2.5 million cubic meters per second between the pre-2009 and post-2009 periods. That corresponds to the 0.17 petawatt reduction in northward heat transport, roughly equivalent to the output of a hundred large power plants no longer warming the northern Atlantic.
The Florida Current, the portion of the Gulf Stream flowing through the Florida Straits, has been measured even longer, since 1982, using submarine cables and ship surveys. Its long-term average sits around 32 million cubic meters per second. While this surface flow hasn’t shown the same clear decline as the broader AMOC (it’s partly sustained by winds regardless of deep circulation changes), it provides a crucial baseline for detecting future shifts.
Rising Seas Along the U.S. East Coast
One of the most immediate consequences of a weakening AMOC is already hitting the U.S. East Coast. When the overturning circulation slows, deep water along the current’s path warms and expands, pushing more water onto the shallow continental shelf. Years with a weak AMOC line up closely with years of higher sea levels and more frequent coastal flooding along the eastern seaboard.
Modeling studies estimate that the weakening AMOC has been responsible for 20 to 50 percent of coastal flooding along different parts of the U.S. East Coast since 2005. That’s on top of the baseline sea level rise from global warming. If the AMOC were to collapse nearly completely, it could raise sea levels along this coastline by around 24 centimeters, independent of all other factors driving sea level rise. For low-lying coastal communities, that additional water would be the difference between manageable tidal flooding and chronic inundation.
What It Means for European Weather
The AMOC delivers an enormous amount of heat to Northern Europe. London sits at roughly the same latitude as Calgary, Canada, yet enjoys far milder winters, largely thanks to the warmth carried across the Atlantic by this circulation. A significant weakening would shift European temperatures downward, even as the rest of the planet continues to warm.
Climate simulations paint a complex picture. Under a strongly reduced AMOC combined with moderate global warming of 2°C or less, Northwestern Europe would experience profound cooling with more intense cold extremes. Winter storms would strengthen, bringing larger day-to-day temperature swings. Somewhat counterintuitively, less winter precipitation is expected despite stormier conditions, because the reduced ocean heat supply would limit the moisture available to weather systems. The net effect would be a Europe that feels colder, more volatile, and drier in winter than its latitude and the global temperature trend would suggest.
How Close Is a Tipping Point?
The most alarming aspect of the AMOC isn’t gradual weakening. It’s the possibility that the system could cross a tipping point, a threshold beyond which collapse becomes inevitable regardless of what happens next. Unlike a slow decline that could theoretically be reversed, a tipping point would commit the ocean to a fundamentally different circulation state, one that might take centuries to recover from.
A 2025 study found that this risk is higher than previously thought. Under continued rising emissions, 70 percent of model simulations led to AMOC collapse. Under intermediate emissions, 37 percent collapsed. Even in a low-emission scenario consistent with the Paris Agreement, 25 percent of models still produced a shutdown. The researchers estimated that the tipping point, the moment when eventual collapse becomes locked in, is likely within the next 10 to 20 years, though the actual collapse would unfold over 50 to 100 years after that threshold is crossed.
These numbers prompted the study’s authors to conclude that an abrupt AMOC collapse can no longer be classified as a “low-likelihood, high-impact event,” the category used in the most recent report from the Intergovernmental Panel on Climate Change. A separate study using different methods reached a similar conclusion, placing the likely tipping point around mid-century. The scientific community hasn’t reached full consensus on exact timing, but the direction of recent findings has been consistent: the risk is higher and closer than the last major assessments suggested.