A recent paper in Science Advances examines how massive volcanic eruptions during the last glacial period drove rapid collapses and slow recoveries of the Atlantic Meridional Overturning Circulation, a system that once determined the pace and intensity of abrupt climate shifts across the Northern Hemisphere. After this brief reference, the focus turns entirely to the physical events themselves and the conditions that made them possible.
During the last glacial period, the climate of the North Atlantic region was controlled by a circulation engine that moved warm water northward and returned dense cold water to the depths. When this engine ran strongly, heat reached the high latitudes and reduced winter sea ice. When it slowed, the region cooled and sea ice advanced. The speed of these shifts defied expectations. Greenland ice cores record temperature jumps of ten to fifteen degrees Celsius in less than twenty years. These jumps divided long stretches of cold from shorter periods of warmth. The pacing of these events points to a circulation system always close to a boundary, able to shift rapidly when pushed hard enough.
Massive volcanic eruptions provided that push. During the glacial period, eruptions far larger than anything in recorded history injected enormous amounts of sulfur into the stratosphere. Eruptions releasing more than two hundred thirty teragrams of sulfur produced widespread cooling almost immediately. For comparison, the Tambora eruption of 1815, which caused severe global impacts, released roughly one tenth of that amount. The Samalas eruption of 1257, one of the largest in the past millennium, reached less than half the sulfur mass required to match the events of the glacial period. Eruptions on the scale seen in the glacial record occupied a different category, capable of altering the temperature structure of the atmosphere, shifting pressure systems, and driving long lasting reorganizations of ocean circulation.
When such an eruption occurred, sunlight reached the surface at reduced intensity. Global temperatures dropped. The cooling was strongest at high latitudes. As temperatures fell, the distribution of sea level pressure changed. The North Atlantic entered a pattern with weakened westerlies and reduced transport of moisture into the regions where deep water normally formed. With less precipitation and lower runoff, surface waters in the subpolar gyre became saltier. Cold salty water sinks more easily than cold fresh water, and this created a short period where deep mixing intensified. As a result, the overturning circulation showed a brief increase in strength. This increase did not represent stability. It was a short response produced by cold dense water sinking through the upper ocean.
The next stage began as cooling deepened and sea ice expanded. Under glacial conditions, sea ice had the ability to grow quickly once temperatures dropped. As the ice advanced across the deep water formation zones, the upper layers of the ocean began to change. Winter ice formation expelled brine into the surrounding water, raising salinity. During summer, meltwater accumulated at the surface. This produced a strong contrast between the fresh surface layer and the saltier layers below. When this structure formed, vertical mixing weakened. Without vigorous mixing, the warm salty waters transported northward could no longer sink efficiently.
The decline in mixing triggered a reduction in the overturning circulation. Once this reduction began, several reinforcing processes followed. The surface layer, strengthened by meltwater from the expanded sea ice, became more stable with each passing year. That stability prevented the deeper layers from rising to release their heat. With less heat escaping into the atmosphere, sea ice continued to grow. Larger ice cover produced more meltwater during summer. Each cycle reinforced the structure of the stable surface layer. The system that had once relied on sinking dense water to maintain its circulation shifted into a long lasting weakened state.
This weakened state persisted for centuries. The glacial period contained multiple episodes where the overturning circulation collapsed and then later recovered. During collapse phases, the North Atlantic cooled sharply. Greenland entered long cold intervals known as stadials. Sea ice reached farther south. Subsurface waters stored heat that could not escape. Over time, the accumulation of this heat created conditions for a later recovery. When the system eventually returned to a stronger mode, the release of stored ocean heat caused another abrupt warming. The entire cycle depended on the sensitivity of the overturning circulation to disruptions in surface density and mixing.
The volcanic eruptions of the glacial period operated within this sensitive environment. Their impact depended on the state of the circulation at the moment of the eruption. When the overturning was already close to its upper limit, the shock from cooling and shifting salinity patterns could push it toward a collapse. When the overturning was farther from that threshold, the same eruption produced only a short term disturbance followed by recovery. The glacial climate contained enough internal variability for both outcomes to occur. Temperature patterns, sea ice coverage, salinity distributions, and atmospheric pressure all varied naturally over decades. These natural fluctuations influenced how strongly the system reacted to each eruption.
The scale of the eruptions matters for understanding the forcing involved. To alter deep water formation in the glacial North Atlantic, an eruption needed to inject sulfur into the stratosphere at levels far above the largest historical eruptions. Sulfur masses around two hundred thirty teragrams produced global cooling of approximately two and a half degrees Celsius. Cooling of this magnitude altered the hydrological cycle, reduced evaporation, lowered precipitation in key regions, and expanded high latitude sea ice. Weaker cooling events did not produce the same pattern of pressure shifts and sea ice responses. Under glacial climate conditions, only the largest eruptions had the ability to change the overturning circulation at the required scale.
When considering the full sequence triggered by these eruptions, several stages stand out. The first stage involved rapid atmospheric cooling. The second involved changes in sea level pressure that reduced moisture transport into the high latitudes. The third involved a short surge in deep convection because of higher salinity and colder temperatures. The fourth involved the expansion of sea ice and the creation of a stable stratification that prevented mixing. The fifth involved a long lasting decline in the overturning circulation. Each stage built on the previous one. No single stage could produce the full effect on its own. The combined effect created the abrupt shifts found in the glacial record.
These shifts were not minor adjustments. The overturning circulation controls the distribution of heat across the Atlantic. When the system entered a weak state, the northward flow of warm water decreased. The reduction in heat transport allowed cold air masses to dominate the high latitudes. Sea ice expanded into regions normally free of ice. Storm tracks shifted. The atmosphere reorganized itself around a colder North Atlantic. The changes influenced temperature, precipitation, and wind patterns across much of the Northern Hemisphere. Greenland recorded the steepest temperature drops, but the influence extended far beyond the Arctic.
As the ocean settled into the weakened mode, heat began to accumulate below the surface. The warm salty waters that once sank efficiently were now trapped beneath the fresh stable layer. Over centuries, this subsurface heat increased. At some point, the heat content grew large enough to destabilize the cold surface layer. When that occurred, the barrier preventing deep mixing broke down. Once mixing resumed, the overturning circulation recovered. The release of subsurface heat produced a rapid warming in the North Atlantic region. This warming marked the transition from a cold stadial to a warm interstadial. The pattern repeated throughout the glacial period.
This structure reveals how the overturning circulation functioned as a switch controlling abrupt climate shifts. Volcanic eruptions served as the external jolt capable of pushing the system into the cold phase when conditions aligned. The background climate determined how easily that jolt could succeed. Large ice sheets, extensive winter sea ice, and the delicate density balance in deep water formation zones all contributed to the system’s sensitivity. The glacial climate created a narrow window where large enough forcing could disrupt the circulation. Outside that window, eruptions could cool the planet but could not produce millennial scale reorganizations.
The Holocene, which followed the glacial period, did not contain abrupt shifts of the same magnitude. Even when large eruptions occurred, the overturning circulation remained stable. Without the extensive ice sheets and the persistent sea ice of the glacial period, the system did not sit near the threshold required for rapid transitions. The eruptions still cooled the atmosphere, altered pressure patterns, and affected regional climates, but they did not replicate the chain of feedbacks needed to force a collapse. This contrast between the glacial period and the Holocene highlights the role of background climate in determining sensitivity.
The Atlantic Meridional Overturning Circulation remains a central component of the climate system. It transports heat, influences weather patterns, and supports the stability of temperature gradients across the Atlantic. Changes in this system have large consequences for regions bordering the ocean. During the glacial period, the overturning operated near a threshold that allowed abrupt collapses when pushed by extreme forcing. Those collapses reshaped the climate for centuries until recovery occurred. The mechanism involved changes in density, salinity, ice cover, and vertical mixing. All of these factors govern the strength of the circulation.
Understanding the sequence of events that unfolded in the glacial North Atlantic provides a clear picture of how the overturning circulation behaves when placed under strong external stress. Large volcanic eruptions supplied that stress in the distant past. Once a collapse began, the transition unfolded over roughly two hundred years. The circumstances that allowed these collapses involved a sensitive climate state with extensive ice cover and a finely balanced density structure. In that context, the overturning circulation responded strongly to large disruptions in surface conditions.
The system recovered only after subsurface heat accumulated enough to destabilize the cold surface layer. The release of this heat produced rapid warming and restored the strong overturning mode. This cycle of collapse and recovery defined the rhythm of the glacial climate. Each event left a clear signature in ice core records and in proxy data from the North Atlantic. Temperature rose quickly, remained high for centuries, then dropped abruptly into cold conditions once the circulation weakened. Volcanic eruptions acted as triggers at several points in this sequence.
The interaction between extreme eruptions and a sensitive overturning circulation represents a key feature of past climate behavior. The magnitude of forcing required to collapse the system was large, but the existence of such eruptions in the glacial record confirms that the conditions for these events were present. The dependence of the circulation on density contrasts made it vulnerable to the surface changes produced by cooling, sea ice growth, and altered freshwater fluxes. The resulting reorganizations shaped the climate for long intervals.
The overturning circulation remains a dominant factor in the climate system. Its sensitivity under certain configurations has been demonstrated by past events. The glacial record shows how quickly the system can fall into a weaker mode when surface conditions shift and how slowly it can recover. Volcanic activity supplied the forcing that initiated several of these transitions. The resulting temperature swings were rapid, intense, and long lasting. The central role of the overturning circulation in these events confirms its importance in shaping large scale climate patterns across the North Atlantic region.
Source:
Science Advances (2026). “Volcanism-induced collapse and recovery of the Atlantic meridional overturning circulation under glacial conditions.”
https://www.science.org/doi/10.1126/sciadv.adx2124
