The Evolution of the Atlantic Meridional Overturning Circulation for more than 1000 Years

Abstract:

The Atlantic Meridional Overturning Circulation (AMOC) is a major mechanism for heat redistribution on our planet and an important factor in climate variability and change. The AMOC is a sensitive non-linear system dependent on subtle thermohaline density differences in the ocean, and major AMOC transitions have been implicated e.g. in millennial climate events during the last glacial period (Rahmstorf, 2002). There is evidence that the AMOC is slowing down in response to anthropogenic global warming (Caesar et al., 2018; Chemke et al., 2020), but as direct measurements of the AMOC only started in 2004 (Smeed et al., 2018), longer term reconstruction must be based on proxy data.

Using several different and largely independent proxy indicators of the AMOC evolution over the last one hundred to nearly two thousand years that have been published in recent years (Sherwood et al., 2011; Rahmstorf et al., 2015; Cheng et al., 2017; Thibodeau et al., 2018; Caesar et al., 2018; Thornalley et al., 2018; Osman et al., 2019; Spooner et al., 2020), we compare the evolution of the AMOC as represented by the different proxy data. Despite the different locations, time scales and processes represented by these proxies, they provide a highly consistent picture of the AMOC evolution since about 400 AD, giving strong evidence that the AMOC decline in the 20th Century is unprecedented and that over the last decades the AMOC is in its weakest state for over a millennium.