In order to shed some light on this topic and establish evidence for the weakening of Atlantic’s circulation, researchers used paleoclimate markers, which take past changes in the Earth’s climate to see how recent changes fit into that picture. In his paper “Atlantic Ocean Circulation at Weakest Point in 1,600 Years.”, Thornalley lays out the details of his studies in which he used  sediment cores collected from the ocean floor along the U.S. east coast to reveal how deep ocean currents connected to the AMOC have changed over time. The internal logic of this measurement is based on the fact that stronger currents deposit larger grains of sediment. Tiny fossilized creature in sediment cores were also observed to see how ocean temperature changed as the AMOC diminished in strength.
  Rahmstorf’s study used precise measurements of ocean temperatures dating back to the nineteenth century. Both studies came to generally similar results: The AMOC is in a very weakened state. The studies however differed on the timing of when that diminishment in strength of the AMOC began. Thornaley’s record, which encompasses a 1,600 year’s timeline, posits that the weakening started at the end of the little ice age, a period from about A.D. 1350 to 1850. During this period, solar and volcanic influenced depressed temperatures across the Northern Hemisphere and glaciers and ice sheets expanded. With the ending of the little ice age, as temperatures warmed, ice melted and freshwater flooded into the North Atlantic. The results propose that the current state of the AMOC is the weakest that it ever was over the entire period mentioned earlier. Whether today’s state of the AMOC is no more than a continuation of that reaction or whether global warming has also started to contribute to the development is not clear, according to Thornalley. Caesar, meanwhile, puts the turning point toward a weaker AMOC in the mid-20th century, suggesting it is due to the influence of human-caused warming. However, her records do not extend to this point. According to statistics, the Gulf Maine is at its warmest temperature in the last 1600 years, and it is also said that the US is on the verge of experiencing rapid increment in sea level.

  There is a doubt among researchers that climate models could be missing something in the AMOC process because they do not catch significant weakening. If these concerns are confirmed, there is a huge potential that the models are not adequately sensitive to the modifications in the ocean freshwater or not all the factors that influenced the circulation are considered. If the climate models have not been capturing the sensitivity of the system correctly so far, it means that scientists have wrong imagination about the pace of AMOC response, which is greater concern.