Past, present and future of the Atlantic meridional overturning circulation

Abstract

The global climate has undergone substantial palaeoclimatic changes, such as the rhythmicity between glacial and interglacial periods, during the last three million years. These changes imply the existence of major variations in the latitudinal distribution of heat, with a changing Atlantic meridional overturning circulation (AMOC) likely playing a major role in the Earth’s shift between different climatic states. The anthropogenic-driven warming and melting of the subpolar North Atlantic is now turning its surface waters more buoyant, opening the possibility of an AMOC breakdown that could drive the Earth towards a new state. The debate remains open: available instrumental data tell us that the AMOC has remained fairly stable during the last two decades, while global coupled climate models predict a moderate weakening during this century. Additionally, complex-theory statistics of some regional ocean indexes, assumed to be fingerprints of the AMOC’s intensity, suggest that the AMOC may be approaching a tipping point leading to collapse in the coming decades. Furthermore, the Earth’s growing radiative energy imbalance, caused primarily by the increase in atmospheric greenhouse gases, causes a huge amount of energy to be stored in the ocean, which will be distributed and released in yet unknown manners that will likely affect the AMOC. Though the available in situ data are robust, the diversity of analyses warns us that transitions may take place faster than projected by numerical models, emphasizing the urgent need to reduce pressures that risk undermining the climate system and the well-being of future generations.