Historical and natural clues suggest that Earth’s climate underwent small changes over the past 2,000 years, and variations in North Atlantic ocean circulation may have been a key driver. In a new paper, Moffa-Sánchez et al. compile recent advancements in analytical tools used to probe this period of ocean circulation, presenting a comprehensive overview of current knowledge.
The past 2 millennia have seen several centuries-long climate shifts, such as the Medieval Warm Period, followed by the Little Ice Age, which were particularly recorded around the North Atlantic. The North Atlantic is an important climatological region because of the strong interactions between ocean, atmosphere, and sea ice, as well as the overturning circulation between surface and deep-ocean currents. Scientists have traditionally proposed that changes in this Atlantic Meridional Overturning Circulation played a key role in the observed climate shifts, but this view remains under debate because of the lack of clear evidence.
The new review synthesizes 20 years’ worth of rapid progress toward understanding the role of ocean circulation in historical climate change. The authors highlight advancements in two major fields: observational data that provide proxy clues to past oceanic and climate conditions, such as sediment cores and the remains of shelled organisms, and models that simulate the past climate.
The authors unite a number of previously published proxy data sets to paint a picture of North Atlantic ocean circulation over the past 2,000 years. This compilation reveals details of past conditions in various regions. For example, the past 2,000 years around Greenland show progressively cooler and icier surface waters reaching the coldest conditions during the Little Ice Age. The compilation also underscores a need for more comprehensive data sets on deep North Atlantic waters for this period.
To gain further insight, the authors selected and compared three recently developed climate models that are particularly well suited for studying the North Atlantic’s past climate over the past millennium. They identified key areas of uncertainty and disagreement between the models that could be addressed in future research. They also demonstrated agreement between model and proxy data estimates of sea surface temperatures for some historical periods and regions but disagreement for others.
These findings highlight important advancements toward understanding how changes in ocean circulation may have affected historical climate change while emphasizing areas for improvement. The authors note the promise of using models to fill gaps in observational data but also call for continued collection and improvement of proxy data sets. (Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2018PA003508, 2019)
—Sarah Stanley, Freelance Writer