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Ocean circulation, including ocean currents and systems of ocean currents, such as ocean gyres and the meridional overturning circulation, play a key role in the climate system through the redistribution of heat, freshwater, carbon, and ecosystem-relevant quantities. Some of these systems of ocean currents are on a large spatial scale and of global climate relevance. For example, the basin-scale Atlantic Meridional Overturning Circulation (AMOC) plays an important role in the climate of north-west Europe (Bellomo et al., 2021; McCarthy et al., 2015). Other ocean circulation features are on a smaller spatial scale and still have an important climate relevance. For example, the regional-scale exchanges across the north-west European shelf are large and enable a disproportionately large carbon transport that plays an important role in the ocean’s sequestration of anthropogenic carbon (Legge et al., 2020). How these systems will change as the climate changes is a key focus of research.

The Atlantic Meridional Overturning Circulation (AMOC)— one of Earth’s major ocean circulation systems—redistributes heat on our planet and has a major impact on climate. Here, we compare a variety of published proxy records to reconstruct the evolution of the AMOC since about ad 400. A fairly consistent picture of the AMOC emerges: after a long and relatively stable period, there was an initial weakening starting in the nineteenth century, followed by a second, more rapid, decline in the mid-twentieth century, leading to the weakest state of the AMOC occurring in recent decades.

During the Last Glacial Maximum (LGM; ~20,000 years ago), the global ocean sequestered a large amount of carbon lost from the atmosphere and terrestrial biosphere. Suppressed CO2 outgassing from the Southern Ocean is the prevailing explanation for this carbon sequestration. By contrast, the North Atlantic Ocean—a major conduit for atmospheric CO2 transport to the ocean interior via the overturning circulation—has received much less attention. Here we demonstrate that North Atlantic carbon pump efficiency during the LGM was almost doubled relative to the Holocene. This is based on a novel proxy approach to estimate air–sea CO2 exchange signals using combined carbonate ion and nutrient reconstructions for multiple sediment cores from the North Atlantic. Our data indicate that in tandem with Southern Ocean processes, enhanced North Atlantic CO2 absorption contributed to lowering ice-age atmospheric CO2.

The overflow of deep water from the Nordic seas into the North Atlantic plays a critical role in global ocean circulation and climate. Approximately half of this overflow occurs via the Iceland-Scotland (I-S) overflow, yet the history of its strength throughout the Holocene (∼ 0-11 700 yr ago, ka) is poorly constrained, with previous studies presenting apparently contradictory evidence regarding its long-term variability. Here, we provide a comprehensive reconstruction of I-S overflow strength throughout the Holocene using sediment grain size data from a depth transect of 13 cores from the Iceland Basin. Our data are consistent with the hypothesis that the main axis of the I-S overflow on the Iceland slope was shallower during the early Holocene, deepening to its present depth by ∼ 7 ka. Our results also reveal weaker I-S overflow during the early and late Holocene, with maximum overflow strength occurring at ∼ 7 ka, the time of a regional climate thermal maximum. Climate model simulations suggest a shoaling of deep convection in the Nordic seas during the early and late Holocene, consistent with our evidence for weaker I-S overflow during these intervals. Whereas the reduction in I-S overflow strength during the early Holocene likely resulted from melting remnant glacial ice sheets, the decline throughout the last 7000 yr was caused by an orbitally induced increase in the amount of Arctic sea ice entering the Nordic seas. Although the flux of Arctic sea ice to the Nordic seas is expected to decrease throughout the next century, model simulations predict that under high emissions scenarios, competing effects, such as warmer sea surface temperatures in the Nordic seas, will result in reduced deep convection, likely driving a weaker I-S overflow. © Author(s) 2013. CC Attribution 3.0 License.