Atlantic circulation may become unstable after 6,500 steady years

Over the last 12,000 years, the Atlantic’s great heat mover – the AMOC – stayed mostly on an even keel. Scientists now have a quantitative picture of that long stretch, and the story mixes calm background with a few sharp bends.

The AMOC is the Atlantic Meridional Overturning Circulation, a system of currents that transports heat and freshwater between the tropics and higher latitudes.

The new research focuses on the Holocene, the relatively mild period that began about 11,700 years ago and continues today.

The reconstruction comes from an international team led by Lukas Gerber and Jörg Lippold at Heidelberg University, working with collaborators at the University of Bern and partners in Germany and Brazil.

The team combined geochemical measurements from six North Atlantic sediment sites with an Earth system model to turn isotope ratios into actual flow rates.

Why Atlantic circulation matters

The AMOC moves warm water north and returns colder, denser water south at depth, with the Gulf Stream as a major surface component that influences European climate.

That large, steady transport helps moderate temperature gradients and shapes rainfall patterns across the Atlantic basin.

Because the AMOC affects weather and climate on both sides of the ocean, scientists track its strength and structure closely. Long records help separate natural fluctuations from human-driven change.

Ocean mud records past changes

The researchers used the ratio of protactinium and thorium isotopes, buried in deep sea mud, to infer past circulation. This proxy responds to how vigorously the deep Atlantic exports protactinium relative to thorium, and it has become a key tool for reconstructing overturning changes.

Flow rates are expressed in Sverdrups, a unit equal to one million cubic meters per second, which oceanographers use to describe basin scale transport. In practical terms, the study reports a late Holocene mean near 18 Sv for the modern-like-state.

To turn these isotope ratios into flow, the team used the Bern3D model, which embeds the isotope ratio physics. This enabled the team to convert multi-site records into a single time series of overturning strength through the Holocene.

What history shows

The analysis reveals a post-Ice Age recovery of overturning, followed by a marked slowdown between roughly 9,200 and 8,000 years ago.

About 6,500 years ago, the circulation stabilized and reached a preindustrial state close to 18 Sv.

That early Holocene dip lines up with bursts of meltwater from the waning North American ice sheet, which climate models and proxy evidence identify as a trigger for temporary AMOC weakening.

The well-known 8,200-year cold event appears sharply in Greenland ice cores, while marine records tend to smooth over its shorter fluctuations.

From about 6,500 years ago to the preindustrial era, the record points to low variability around a stable mean. That long plateau provides a valuable baseline for assessing modern change.

Circulation stabilized during the Holocene

One Bermuda Rise core shows a short-lived increase in these isotope ratios around 4,200 years ago, a timing that overlaps with a widely discussed climate episode.

The authors argue that local bottom processes likely amplified the isotope signal through intense nepheloid layers, particle rich waters near the seafloor that can scavenge isotopes and mimic circulation changes.

Large compilations of global proxy data also suggest the 4.2 ka event was not uniformly expressed worldwide, which supports caution when tying a single marine signal to a basin scale overturning shift.

Together, these lines point to a stable large scale AMOC through the mid to late Holocene.

What lies ahead

Climate models participating in CMIP6 point to a 21st century slowdown of the overturning. An emergent constraint analysis suggests a decline of roughly 6 to 8 Sv by 2100 when models are calibrated against present-day observations.

“Anthropogenic climate change may result in an AMOC slowdown unprecedented for most of the ongoing Holocene interglacial,” wrote Gerber. This conclusion rests on comparing the new 12,000 year baseline with modeled futures under different emissions paths.

The paper also explains why some short-lived events are difficult to detect in sediment records. Bioturbation and limited sampling resolution smooth the signal, making perturbations lasting less than about 180 years hard to discern in the isotope ratios.

Significance of the Atlantic circulation

A weaker overturning reduces northward ocean heat transport, shifts storm tracks, and can alter rainfall belts.

Multi-model studies connect AMOC weakening to shifts in tropical Atlantic rainfall, with implications for food security, water resources, and regional climate extremes.

For roughly 6,500 years, the Atlantic’s overturning remained near a steady state, but model projections suggest it could leave that range within our lifetimes.

The study is published in the journal Nature Communications.

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