Tag: Fresh water

Changing Atlantic Ocean currents may threaten Gulf Stream warming of Europe

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Climate during the last Ice Age was continually erratic. Generally fine-grained muds cored from the floor of the North Atlantic Ocean show repeated occurrences of layers containing gravelly debris. These have been ascribed to periods when ice sheets on Greenland and Scandinavia calved icebergs at an exceptionally fast rate, to release coarse debris as they melted while drifting to lower latitudes. These ‘iceberg armadas’ (known as Heinrich events) left their unmistakable signs as far south as Portugal. Their timing correlates with short-lived (1 to 2 ka) warming-cooling episodes (Dansgaard-Oeschger events) recorded in Greenland ice cores that involved variations in air temperature of up to 15°C. The process that resulted in these sudden climate shifts seems to have been changing ocean circulation brought about by vast amounts of fresh water flooding into the Arctic and North Atlantic Oceans. This lowered seawater density to the extent that its upper parts could not sink when cooled. It is this thermohaline circulation that drags warmer surface water northwards, known as the Atlantic Meridional Overturning Circulation (AMOC), part of which is the Gulf Stream. When it fails or slows the result is plummeting temperatures at high latitudes. The last major AMOC shutdown was after 8 ka of warming that followed the last glacial maximum. Between 12.9 and 11.7 ka major glaciers grew again north of about 50°N in the period known as the Younger Dryas, almost certainly in the aftermath of a flood to the Arctic Ocean of glacial meltwater from the Canadian Shield. Around 8.2 thousand years ago human re-colonisation of Northern Europe was set back by a similar but lesser cooling event.

The Atlantic Meridional Overturning Circulation (AMOC). Red – warm surface currents; cyan – cold deep-water flow. (Credit: Stefano Crivellari)

Three researchers at Utrecht University, the Netherlands have issued an early warning that the AMOC may have reached a critical condition (Van Westen, R.M., Kliphuis, M & Dijkstra, H.A. 2024. Physics-based early warning signal shows that AMOC is on tipping course. Science Advances, v. 10, article adl1189; DOI: 10.1126/sciadv.adk1189). Previous modelling of AMOC has suggested that only rapid, massive decreases in the salinity of North Atlantic surface water near the Arctic Circle could shut down the Gulf Stream in the manner of Younger Dryas and Dansgaard-Oeschger events. René van Westen and colleagues have simulated the effects of steady, long-term addition of fresh water from melting of the Greenland ice sheet. They ran a sophisticated Earth System model for six months on the Netherlands’ Snellius super computer. Their model used a slowly increasing influx of glacial meltwater to the Atlantic at high northern latitudes.

The various feedbacks in the model eventually shut down the AMOC, predicted to result in cooling of NW Europe by 10 to 15 °C in a matter of a few decades. Yet to achieve that required the model to simulate more than 2000 years of change. It took 1760 years for a persistent AMOC transport of 10 to 15 million m3 s-1 to drop over a century or so and reach near-zero. That collapse involved around 80 times more melting of Greenland’s ice sheet than at present. Yet their modelling does not take into account global warming: including that factor would have exceeded their budgeted supercomputer time by a long way. Melting of the Greenland ice sheet is, however, accelerating dramatically

Van Westen et al. have shown the possibility that steadily increasing ice-sheet melting can, theoretically, ’flip’  the huge current system associated with the Atlantic Ocean, and with it regional climate patterns. The tangible fear today is of a more than 1.5°C increase in global surface temperature, yet a warming-induced failure of AMOC may cause local annual temperatures to fall by up to ten times that. Rather than the currently heralded disappearance of sea-ice from the Arctic Ocean, it may spread in winter to as far south as the North Sea. The only way of forecasting in detail what may actually happen – and where – is ever-more sophisticated and costly modelling of ocean currents and ice melting in a warming world. Uncertain as it stands, the work by van Westen and colleagues may well be ignored: perhaps as a ‘thing we dinnae care to speak aboot’.

See also: Le Page, M. 2024. Atlantic current shutdown is a real danger, suggests simulation. New Scientist, 9 February 2024; Watts, J. 2024. Atlantic Ocean circulation nearing ‘devastating’ tipping point, study finds. The Guardian, 9 February 2024.

When the Arctic Ocean was filled with fresh water

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The salinity of surface water at high latitudes in the North Atlantic is a critical factor in its sinking to draw warm, low-latitude water northwards in the Gulf Stream while contributing to the southwards flow of North Atlantic Deep Water along the ocean floor. One widely supported hypothesis for rapid cooling events, such as the Younger Dryas, is the shutdown of this thermohaline circulation (Review of thermohaline circulation, February 2002). That may happen when surface seawater at high latitudes is freshened and made less dense by rapid melting or break-up of continental ice sheets, or through the release of vast amounts of fresh water from glacially dammed lakes. The climatic decline leading to the last glacial maximum at around 20 ka was punctuated by irregular episodes known as Dansgaard-Oeschger and Heinrich Events that have been attributed to such hiccups in thermohaline processes. In this context, a whole new barrel of fish has been opened up by a geochemical study of the top few metres of sediments on the Arctic Ocean floor (Geibert, W. et al. 2021. Glacial episodes of a freshwater Arctic Ocean covered by a thick ice shelf. Nature, v. 590, p. 97–102; DOI: 10.1038/s41586-021-03186-y), particularly their content of an isotope of thorium (230Th).

Being radioactive (half-life ~75 ka), 230Th is useful in working out sediment deposition rates, especially as it is insoluble and adheres to dust grains. The isotope is a decay product of uranium, yet it not only forms on land from uranium in hard rocks, eventually to be transported into marine sediments, but from uranium dissolved in seawater too. Interestingly, the amount of uranium that can enter seawater in solution depends on water salinity. Fresh water, especially that locked up in glacial ice, has very low concentrations of uranium. Consequently, ordinary seawater adds additional 230Th to sediments whereas fresh water does not. An excess of the isotope in marine sediments signifies their deposition from salty water, but those deposited in fresh water carry no excess. In the course of analysing deep-sea cores from the floors of the Arctic Ocean and the northernmost part of the North Atlantic, Walter Geibert and colleagues at the Alfred Wegener Institute in Bremerhaven, and the University of Bremen, Germany revealed a series of sediment layers that were devoid of excess 230Th. This suggests that twice, probably in periods between 150 to 131 and 70 to 62 ka, water in the Arctic Ocean and the connected Nordic Sea was entirely fresh. In two cores the evidence suggests a third, restricted occurrence of fresh water fill at about 15 ka.

The most likely explanation is that the fresh-water episodes marked the development of major ice shelves, similar to those still present around Antarctic; i.e. floating or grounded ice of glacial origin (not sea ice). That had been anticipated, but not previously proved for the northern polar region. The outlets from the Arctic Ocean basin to the Pacific and North Atlantic Oceans are marked by barriers of shallow seabed. One is the Bering Straits, which became the Beringia land bridge that facilitated animal and human migrations from Siberia to North America when sea level fell as continental ice sheets grew. The other is the Greenland-Scotland Ridge formed by volcanism connected to the Icelandic hot spot as the North Atlantic opened. It is possible that the suggested ice shelves grounded on these ridges, to effectively dam and isolate the Arctic Ocean. Fresh water from melting land ice would ‘pond’ beneath the ice shelves, floating on denser salt water and eventually expelling it from much of the polar marine basin. A side effect of this would have been partially to accumulate and isolate the oxygen-isotope proportions that characterise snow and glacial ice. Remember that the light 16O isotope is preferentially extracted from sea water during evaporation, to become stored in glacial ice sheets so that the proportion of the heavier 18O increases in ocean water; δ18O is therefore an important proxy for glacial waxing and waning and thus the fluctuations of global sea level. Trapping a proportion of water of glacial origin in isolated Arctic Ocean water and ice shelves would explain discrepancies in the oxygen-isotope records of successive ice ages. Also, if the ice shelves periodically broke up, fresh water derived from them and ponded in the deepest Arctic Ocean basin could change the salinity of surface ocean water elsewhere – being lower density that fresh water would ‘float’.

The work of Geibert and colleagues may well result in a great deal of head scratching among palaeoclimatologists and perhaps new ideas on the dynamics of ice age climates.

See also: Hoffmann, S. 2021. The Arctic Ocean might have been filled with freshwater during ice ages. Nature, v. 590, p. 37-38; DOI: 10.1038/d41586-021-00208-7