Clams reveal how the North Atlantic tipped into the Little Ice Age
Posted on 31/10/2022 By: Beatriz Arellano-Nava
SEACHANGE scientists have undertaken the challenging task of understanding how oceans were before humans induced major changes. This is an important endeavour because by learning how the environment has changed in the past, we can identify potential responses of the climate system to anthropogenic global warming. For this, they turn to information recorded on all sorts of natural archives.
The ocean quahog, a species of bivalve that lives buried in the North Atlantic seabed, is the longest-lived animal known to science. Quahogs are also excellent natural historians as they keep record of environmental changes on their shells.
Every year, these clams form a new shell growth band, in which the width and chemical composition integrate information of environmental conditions - similar to the rings of a tree. SEACHANGE researchers have learnt to decipher and interpret the information stored within these natural archives.
Of particular value is to identify rapid and irreversible transitions. These changes may be unnoticed until they occur abruptly, when a threshold called a "tipping point" is reached. By finding those transitions in the relatively recent past, it is possible to identify those components of the Earth system that might exhibit tipping behaviour in the future.
A published study of three bivalve records from North Iceland reveals that an ocean circulation in the subpolar North Atlantic shows signs of having crossed a tipping point during the transition into the Little Ice Age.
The Little Ice Age was the most recent notable climate shift in Earth's history before industrialisation. It was a cooler-than-average period, particularly around the North Atlantic region and spanned several centuries from about 1300 to 1850.
Photo by Mark Olsen on Unsplash
On the quest of what triggered this transition, scientists have found evidence that suggests that ice melting in the subpolar North Atlantic diluted seawater with freshwater, leading to the weakening of ocean currents that carry warm water from the south. This, in turn, reduced heat transport towards the north, reinforcing sea-ice expansion through a positive feedback that maintained the system in a cold configuration.
The regional circulation in the subpolar North Atlantic has been put forward as a potential element of the climate system that could switch between a strong and a weak circulation configuration once a tipping point is crossed.
Near a tipping point, the environment loses stability such that any small change in the conditions can push it into a completely different state. Quahog clams have turned out to be such meticulous historians that have kept record of changes in stability on their shells.
By reading the passages recorded by these bivalves, researchers could identify two episodes of loss of stability before the transition into the Little Ice Age. These episodes were followed by rapid ocean circulation changes, a behaviour consistent with the crossing of a tipping point.
The first and most noticeable episode occurred within the last decades of a warm period known as the "Medieval Climate Anomaly". There is evidence that during those decades, melting of icebergs and glaciers from South Greenland contributed to the freshening of the surrounding ocean. This process likely caused the observed loss of stability.
A large outflow of sea ice from the Arctic was probably behind the second episode of loss of stability during the 14th century.
These findings, revealed by clams, show how additional freshwater from melting ice in the subpolar North Atlantic has the potential to destabilise regional ocean currents and can lead to rapid and long-lasting change.
Under current global warming, the Greenland ice sheet and Arctic sea ice are melting at an accelerated rate. If the associated input of freshwater in the subpolar North Atlantic continues, the crossing of an ocean circulation tipping point might again lead to rapid and long-lasting regional climate change.
