Scientists used centuries-old clam shells to reconstruct how the North Atlantic climate system reached a “tipping point” prior to the Little Ice Age.
The Little Ice Age was a period of regional cooling, particularly in the North Atlantic, that lasted several centuries and ended around 1850.
A long-held theory holds that the initial cooling during this period was sustained by “sea-ice to ocean feedbacks”—sea ice expanded, slowing ocean currents, and thus reducing the flow of warm water from the south.
The shells of quahog clams, which can live for hundreds of years, were used in the new study by the University of Exeter to understand how the ocean has evolved and responded to external changes over recent centuries.
The findings show that prior to the Little Ice Age, the North Atlantic climate system destabilized and lost resilience (the ability to recover from external changes), potentially causing it to “tip” into a new, colder state.
The North Atlantic may be approaching a new tipping point, with major implications for the region’s climate, according to the researchers.
With scientists warning that multiple tipping points may be approaching around the world as a result of human-caused climate change, the study helps us understand when and how tipping points occur.
“One sign that a system is approaching a sudden transition is when it becomes slow to respond to perturbations (external changes),” said lead author Beatriz Arellano-Nava of the Global Systems Institute at the University of Exeter.
“In other words, instead of returning to its average state, a system can ‘tip’ into a new state.”
“In the case of the North Atlantic before the Little Ice Age, this loss of resilience made the system vulnerable to an abrupt switch, potentially heralding the transition to Little Ice Age conditions,” said co-author Dr. Paul Halloran.
The new study warns that the North Atlantic system’s vulnerability is a critical issue today, with recent analysis indicating that it has destabilised over the last century and may be approaching a tipping point.
“Our latest analysis suggests that the system of ocean currents in the northern North Atlantic could be at risk of a tipping point again now due to global warming,” said Professor Tim Lenton, Director of the Global Systems Institute.
Oxygen and carbon isotopes, as well as shell growth, were studied in clam shells as indicators of environmental variability.
The paper, titled “Destabilisation of the Subpolar North Atlantic Prior to the Little Ice Age,” was published in the journal Nature Communications.