Second Regional Scientific Forum: Bridging Research and Operations in an Earth-System Approach: Increasing Water and Heat Resilience in RA VI. Addressing Gulf Stream Weakening Risks.

The Atlantic Meridional Overturning Circulation (AMOC) is a major component of the global ocean circulation system and plays a crucial role in regulating Earth’s climate. In policy discussions, it is often confused with the Gulf Stream; however, the Gulf Stream represents only one surface current within the much larger AMOC system. Understanding the dynamics and future evolution of the AMOC is therefore essential for assessing climate risks, particularly for Europe. The global ocean performs critical climate-regulating functions. It absorbs approximately 25% of
anthropogenic carbon dioxide emissions and stores around 93% of the excess heat generated by global warming. In addition, the ocean produces about half of the oxygen in Earth’s atmosphere and supports key economic sectors, including fisheries, maritime transport and coastal tourism.
At the same time, the ocean is undergoing significant changes, including rising sea levels, decreasing ocean pH due to acidification, and shifts in marine species distributions. These changes have implications for marine ecosystems and global food systems. Within this broader context, the AMOC redistributes heat, carbon and nutrients across the Atlantic Ocean. Warm, salty surface waters flow northward from the South Atlantic toward the North Atlantic, releasing heat to the atmosphere and contributing to the relatively mild climate of Europe. As these waters cool and become denser, they sink and return southward at depth. This circulation transports massive amounts of heat and carbon and helps store them in the deep ocean. Climate model simulations consistently project a weakening of the AMOC under continued global warming, primarily due to changes in ocean density and freshwater input. While a complete collapse during the twenty-first century is considered unlikely in most models, a gradual weakening is widely expected and could have important regional climate consequences. Since 2004, sustained observation programmes have significantly improved understanding of AMOC variability. In particular, the RAPID Array at 26°N and the Overturning in the Subpolar North Atlantic Program (OSNAP) in the subpolar North Atlantic have provided continuous
measurements of Atlantic circulation. These observations reveal that the AMOC varies on timescales from daily to decadal and is influenced not only by density changes but also by wind forcing, indicating that it does not behave as a simple, steady “conveyor belt.” Observations indicate a long-term decline of approximately 0.9 sverdrups1 per decade, broadly consistent with model projections of around 1 sverdrup per decade. However, due to large natural variability in the system, detecting a statistically robust long-term trend will require roughly 30 years of continuous observations. A striking example of AMOC variability occurred in 2009–2010, when the circulation weakened
sharply. This event reduced northward heat transport by approximately 30%, contributed to a temporary sea-level rise of about 13 cm along the north-eastern coast of the United States, and coincided with extremely cold European winters during 2009/2010 and 2010/2011, generating significant societal and economic impacts. Looking ahead, a sustained weakening of the AMOC could lead to cooler but more variable climatic conditions in Europe, increased storms, flooding, and droughts, pressures on agriculture and fisheries, stronger tropical cyclones, and accelerated regional sea-level rise. It could also reduce the ocean’s capacity to absorb atmospheric carbon dioxide. The AMOC interacts with several global climate tipping elements, and some studies suggest it could approach a critical threshold at around 2 °C of global warming. These risks highlight the importance of sustained ocean observations, improved modelling and
comprehensive climate risk assessments to better understand and anticipate future changes in Atlantic circulation.