The formation of deep waters in the North Atlantic play a key role in sustaining todays climate.
Changes in this circulation have been relatively small over the past 7kyr, the time during which human civilizations developed, compared to those found during ice age climates of the geological past.
This recent stability has contributed to the idea that rapid changes in this circulation are a low-probability but high-impact tipping point in the climate system.
High impact because if changes did happen, they would have widespread consequences for climate and sea level around the Atlantic and impact fisheries, agriculture, and even the ocean’s ability to soak up the excess carbon dioxide we emit to the atmosphere.
With such consequences it is important to know whether the circulation really is stable in warm climates. How can this be verified/checked?
A fresh look at stability
A team of scientist led from the Bjerknes Centre for Climate Research (Eirik Vinje Galaasen, Ulysses Ninnemann, Augustin Kessler, Nil Irvali, Jerry Tjiputra, Helga Kleiven) provide fresh evidence that circulation may in fact become more unstable in warmer climate conditions.
The international team, including researchers from U.S, France, Netherlands, and U.K, studied traces of deep ocean properties imprinted in the sediments on the seafloor accumulated over 450 thousand years.
Within this timespan there were a number of periods when the North Atlantic was as warm or a few degrees Celsius warmer than today. Using mud that was cored from the seabed south of Greenland—a sensitive location for tracking changes in this circulation—the researchers analyzed how the properties of the abyssal ocean varied.
The sediments here accumulate many times faster than in most locations at the seafloor allowing the researchers to look at much finer detail changes than had ever been attempted. What the researchers found was surprising. Each of these previous warm periods contained evidence of large changes in ocean circulation patterns.
Again and again, there were abrupt shifts between waters formed in the North and those formed around Antarctica that often lasted a few centuries before changing back. Through advanced computer simulations the team was able to show how these changes in deep chemistry could be linked to circulation and might arise spontaneously as the climate gradually changed.
"It was shocking to see how circulation could suddenly become highly chaotic under climate conditions similar to those we may soon be facing" says Ulysses Ninnemann.
Might cross a threshold
The research shows not just that dramatic changes in circulation are possible, but they are common, in these past warm periods that were only slightly warmer than now.
So while circulation may not collapse, confirming other previous studies, it might cross a threshold where it suddenly becomes much more variable and unstable than we are used to.
These types of variations would create serious challenges for societies who would have to grapple with the sudden climate shifts, draughts, and sea level changes that could accompany them.
"In a time when we see clearly the devastating effects of societal upheaval it is scary to think that the relative stability we have enjoyed in terms of ocean circulation may be more easy to disrupt than we thought", Ulysses Ninnemann adds.