The Atlantic Ocean contains a system of currents known as the Atlantic Meridional Overturning Circulation (AMOC), which acts as a vast conveyor belt for heat and energy across the globe. This circulation includes the Gulf Stream, which transports warm, tropical water north along the ocean surface. The AMOC is a fundamental regulator of Earth’s climate, driving significant heat transfer from the equator toward the poles. Scientific observation suggests this system is weakening, a trend observed over the past century. This slowdown threatens to disrupt established heat distribution patterns, leading to substantial consequences for global climate stability.
How the Ocean Current Works
The mechanism that powers the AMOC is thermohaline circulation, driven by differences in water density determined by temperature and salinity. The Gulf Stream forms the upper, northward-flowing limb, carrying warm, salty surface water from the tropics toward the North Atlantic. This current transports an immense amount of heat, equivalent to about 50 times the energy consumption of all humankind.
As the warm water reaches the subpolar regions near Greenland and the Nordic Seas, it releases heat into the atmosphere, which significantly moderates the climate of Western Europe. The water then cools, and as ice forms, the remaining water becomes saltier. This cold, salty water is extremely dense, causing it to sink thousands of meters to the ocean floor. This sinking process pulls more warm water from the south, completing the overturning circulation as the cold, deep water flows back toward the equator.
Why the Gulf Stream is Slowing Down
The observed slowing of the AMOC is attributed to human-caused climate change, which interferes with the balance of temperature and salinity required for circulation. Data from proxy measurements and direct observation systems, such as the RAPID array, suggest the AMOC has weakened by approximately 15% since the mid-twentieth century. This weakening is associated with a distinct sea surface temperature pattern: a cold patch south of Greenland that has resisted the general warming trend of the Atlantic.
The primary mechanism for the slowdown involves the massive influx of fresh water, mainly from the melting of the Greenland ice sheet and increased precipitation in the North Atlantic. Fresh water is less dense than salty water, and when it enters the North Atlantic, it reduces the overall density of the surface water. This density reduction inhibits the sinking process that drives the AMOC. Analyses of ocean temperature and salinity confirm that only climate models simulating a weakening AMOC can account for the observed changes in the North Atlantic.
Impacts on Global Weather Patterns
The reduced strength of the AMOC means less heat is transported northward, leading to direct atmospheric consequences, particularly over the North Atlantic and Europe. A weaker current reduces the heat reaching the subpolar region, resulting in a regional cooling effect that contrasts with global warming trends. This disruption in ocean temperatures also affects the path and behavior of the atmospheric jet stream, which guides weather systems across North America and Europe.
Changes in the jet stream can lead to more persistent weather patterns, increasing the likelihood of extreme events such as prolonged heatwaves, droughts, or severe cold snaps. Climate modeling suggests a weakened AMOC could alter precipitation patterns across the Euro-Atlantic sector. This could lead to drier conditions in southern Europe and an increase in wet days over parts of northwestern Europe.
Consequences for Coastal Regions and Marine Ecosystems
A slowing current impacts coastlines and marine life. A weakened AMOC affects sea level along the Atlantic coast of North America by altering the horizontal distribution of water masses. As the current slows, water that is usually pushed away from the coast can “pile up,” contributing to a regional sea level rise independent of global thermal expansion.
For the U.S. Northeast coast, this dynamic change has accelerated sea level rise and increased the frequency of coastal flooding. Research indicates that the AMOC’s decline may be responsible for 20% to 50% of the increase in flood days observed in the region over the last two decades. In the marine environment, the slowdown disrupts the vertical mixing of water, which brings deep-sea nutrients to the surface layers. This reduction in nutrient supply negatively affects the base of the marine food web, leading to shifts in the distribution and abundance of commercially important fish species like cod and haddock.