Tag: Melting

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.

A cometary air-burst over South America 12 thousand years ago

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Earth-logs has previously covered quite a few hypotheses involving catastrophic astronomical events of the past, often returning to them as new data and ideas emerge. They range from giant impacts, exemplified in the mass extinction at the K-Pg boundary to smaller-scale events that may have coincided with important changes in climate, such as the sudden onset of the Younger Dryas, and a few that have been suggested as agencies affecting local human populations such as the demise of Sodom by a cosmogenic air-burst. Some of the papers that spurred the Earth-pages posts have been widely regarded in the geoscience community. Yet there have been others that many have doubted, and even condemned. For instance, data used by the consortium that suggested an extraterrestrial event triggered the frigid millennium of the Younger Dryas (YD) have been seriously and widely questioned. A sizeable number of the team that were under close scrutiny in 2008 joined others in 2019 to back the YD air-burst hypothesis again, using similarly ‘persuasive’ data from Chile. Members of the original consortium of academics also contributed to the widely disputed notion of a cosmic air-burst having destroyed a Bronze Age urban centre in Jordan that may, or may not, have been the site of the Biblical Sodom. Again, they cited almost the ‘full monty’ of data for high-energy astronomical events, but again no crater or substantial melt glass, apart from tiny spherules. Now another paper on much the same theme, but none of whose authors contributed to those based on possibly ‘dodgy’ data, has appeared in Geology (Schultz, P.H. et al. 2021. Widespread glasses generated by cometary fireballs during the Late Pleistocene in the Atacama Desert, Chile. Geology, published online November 2, 2021; doi: 10.1130/G49426.1).

Peter Schultz of Brown University, USA and colleagues from the US and Chile make no dramatic claims for death and destruction or climate destabilisation, and simply report a fascinating discovery. In 2012 one of the authors, Nicolas Blanco of the Universidad Santo Tomás in Santiago, Chile, found slabs made of glassy material up to half a metre across. They occurred in several 1 to 3 km2 patches over a wide area of the Atacama Desert. Resting on Pleistocene glacio-fluvial sediments, they had been exposed by wind erosion of active sand dunes. The glass is dark green to brown and had been folded while still molten. For the glass slabs to be volcanic bombs presupposes a nearby volcano, but although Chile does have volcanoes none of the active vents are close enough to have flung such large lumps of lava into the glass-strewn area. The glassy material also contains traces of vegetation, and varies a great deal in colour (brown to green). Its bulk chemical composition suggests melting of a wide variety of surface materials: quite unlike volcanic glasses.

Chilean glass occurrence: panorama of large glass fragments in the Atacama Desert; a specimen of the glass; thin section of glass showing bubbles and dusty particles (Credit: Schultz et al. 2021; Figs 1B, 2D and 2C)

Microscopic examination of thin sections of the glasses also reveals nothing resembling lava, except for gas bubbles. The slabs are full of exotic fragments, some of which closely resemble mineral assemblages found in meteorites, including nickel-rich sulfides embedded in ultramafic material. Others are calcium-, aluminium- and titanium-rich inclusions, such as corundum (Al2O3) and perovskite (CaTiO3), thought to have originated as very-high temperature condensates from the pre-solar nebula: like the celebrated ‘white inclusions’ in the Allende meteorite. Some minute grains resemble dust particles recovered by the NASA Stardust mission to Comet 81P/Wild-2 which returned samples to Earth in 2006. Zircon grains in the glasses, presumed to be locally derived, have been decomposed to zirconium oxide (baddeleyite), suggesting melting temperatures greater than 1670°C: far above the highest temperature found in lavas (~1200°C). Interestingly, the green-yellow silica glass strewn over the Sahara Desert around the southern Egypt-Libya border also contains baddeleyite and cometary dusts, together with anomalously high platinum-group elements and nanodiamonds that are not reported from the Chilean glass. Much prized by the elite of pharaonic Egypt and earlier makers of stone tools, the Saharan glass is ascribed to shock heating of the desert surface by a cometary nucleus that exploded over the Sahara. Unsurprisingly, Schultz et al. come to the same conclusion.

Any object entering the Earth’s atmosphere does so at speeds in excess of our planet’s escape velocity (11.2 km s-1). Not only does that result in heating by friction with the air, but much of the kinetic energy of hypersonic entry goes into compressing air through shock waves, especially with objects larger than a few tens of metres. Such adiabatic compression can produce temperatures >>10 thousand °C. Hence the ‘fireballs’ associated with large meteorites. With very large air-bursts the flash of radiant energy would be sufficient to completely melt surface materials in microseconds, though rugged topography could protect areas shadowed from the air-burst by mountains, perhaps explaining the patchy nature of the glass occurrences.  (Note: the aforementioned papers on the YD and Sodom ‘air-bursts’ do not mention large glass fragments, whereas some surface melting would be expected). Some of the Chilean glass contains carbonised remnants of vegetation. Radiocarbon dating of four samples show that the glass formed at some time between 16.3 to 12.1 ka. Yes, that does include the age of the start of the YD (12.9 ka) and human migrants had established themselves in northern Chile and coastal Peru after 14.2 ka. Yet the authors, perhaps wisely, do no more than mention the coincidence, as well as that with the disappearance of South American Pleistocene megafaunas – more severe than on any other continent. With a very distinctive product, probably spanning a far larger area of South America, and attractive to humans as an ornament or a resource for sharp tools, expect follow-up articles in the future.

See also: http://www.sci-news.com/space/atacama-desert-comet-10247.html, Science News, 8 November 2021; Vast patches of glassy rock in Chilean desert likely created by ancient exploding comet, Eureka Alert, 2 November 2021.