The Atlantic meridional overturning circulation (AMOC) is an ocean current that redistributes heat globally and is the source of much of the ocean’s deep water. The AMOC has weakened in recent years but the lack of longer term data has made it uncertain if this is a recent phenomenon or a longer term trend. A recent study provides palaeo-oceanographic evidence that the AMOC has been anomalously weak since approximately 1850 CE compared with the preceding 1,500 years. The palaeoclimate reconstructions suggest that the slowdown in the AMOC may have happened as either an abrupt shift towards the end of the Little Ice Age (LIA) or as a more gradual decline over the past 150 years. It is suggested that enhanced freshwater fluxes from the Arctic and Nordic seas towards the end of the LIA from melting glaciers and thickened may have contributed to this weakening. The persistence of a weak AMOC during the twentieth century, when there was pronounced Northern Hemisphere and global warming, suggests that climate forcings, such as greenhouse gas warming, were more important than the warming attributed to the AMOC during this period and it is inferred that the AMOC has responded to recent centennial-scale climate change rather than having driven it.
Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years, David J. R. Thornalley, Delia W. Oppo, Pablo Ortega, Jon I. Robson, Chris M. Brierley, Renee Davis, Ian R. Hall, Paola Moffa-Sanchez, Neil L. Rose, Peter T. Spooner, Igor Yashayaev & Lloyd D. Keigwin, NATURE, VOL 556, 12 APRIL 2018, PP 229-231, https://doi.org/10.1038/s41586-018-0007-4