As Key Atlantic Currents Decelerate, US East Coast Confronts Rising Sea Levels

The decline in significant Atlantic currents is contributing to flooding linked to rising sea levels in the northeastern United States, which are already affected by climate change. As global temperatures increase, a total collapse of the Atlantic Meridional Overturning Circulation (AMOC) could exacerbate sea level rise.

“If AMOC collapses, this will greatly increase flood frequency along the US coastline, independent of major storms,” states Liping Chan from the Geophysical Fluid Dynamics Laboratory at the US National Oceanic and Atmospheric Administration (NOAA) in New Jersey. “Even a partial reduction in current strength can have significant consequences.”

The warm waters melting ice sheets and rising sea levels are influenced by climate change, which leads to uneven rates of sea level rise across different regions. For instance, some coastal areas have subsided, increasing the relative rate of sea level rise there. Local sea levels are also affected by the circulation of heat, water, and salt in the ocean, with warm, fresh water occupying more volume than cold, salty water.

Over the past few decades, sea levels along the northeastern US coast have risen 3-4 times faster than the global average. The slowing of AMOC—responsible for transporting warm water from lower latitudes to the North Atlantic, where it cools and sinks—has long been considered a potential cause of this phenomenon. As this circulation weakens, warm deep water expands, pushing more water onto the shallow continental shelf.

AMOC strength varies naturally over different timescales, and climate change has contributed to its slowdown as the North Atlantic and its waters have become warmer and clearer in recent decades. However, it remained uncertain whether this decrease significantly affected sea levels.

Chang and her team utilized tidal gauge measurements from the New England coast to reconstruct local sea levels dating back over a century. Alongside a steady rise due to climate change, they identified significant fluctuations between low and high sea levels every few decades. Low sea levels correlated with periods of weak AMOC, while high sea levels were also aligned with these intervals, which brought more frequent coastal flooding.

The researchers then employed two distinct ocean models to quantify the impact of AMOC intensity variations on local sea levels. While the primary driver of change was the steady rise due to climate change, they discovered that weakened AMOCs significantly increased sea-level-related flooding. In multiple coastal regions, they noted that the slowdown in AMOC has contributed to delaying flooding by 20-50% since 2005.

Given that the natural cycle of AMOC strength is largely predictable, Zhang asserts that these findings enable researchers to forecast potential flooding events up to three years in advance. This foresight can guide long-term infrastructure planning and emergency preparedness.

“This highlights the critical role of AMOC in [sea level rise],” remarks Chris Hughes, who was not involved in the research, from the University of Liverpool in the UK. “It’s not merely theoretical; it’s evident in the real world.”

It remains unclear how much of the recent AMOC weakening is attributable to climate change versus natural variability. Nevertheless, the findings bolster predictions that if AMOC were to completely collapse due to climate change, significant portions of the US East Coast could experience a surge in sea levels.

Hughes warns that if AMOC nearly collapses, sea levels could rise by around 24 centimeters. “While it may not seem dramatic, even a small increase can have a substantial effect.”