Tag: Gulf Stream

Divining the possible climatic impacts of slowing North Atlantic current patterns

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Meltwater channels and lake on the surface of the Greenland ice sheet

In August 2024 Earth-Logs reported on the fragile nature of thermohaline circulation of ocean water. The post focussed on the Atlantic Meridional Overturning Circulation (AMOC), whose fickle nature seems to have resulted in a succession of climatic blips during the last glacial-interglacial cycle since 100 ka ago. They took the form of warming-cooling cycles known as Dansgaard-Oeschger events, when the poleward movement of warm surface water in the North Atlantic Ocean was disrupted. An operating AMOC normally drags northwards warm water from lower latitudes, which is more saline as a result of evaporation from the ocean surface there. Though it gradually cools in its journey it remains warmer and less dense than the surrounding surface water through which it passes: it effectively ‘floats’. But as the north-bound, more saline stream steadily loses energy its density increases. Eventually the density equals and then exceeds that of high-latitude surface water, at around 60° to 70°N, and sinks. Under these conditions the AMOC is self-sustaining and serves to warm the surrounding land masses by influencing climate. This is especially the case for the branch of the AMOC known as the Gulf Stream that today swings eastwards to ameliorate the climate of NW Europe and Scandinavia as far as Norway’s North Cape and into the eastern Arctic Ocean.

The suspected driving forces for the Dansgaard-Oeschger events are sudden massive increases in the supply of freshwater into the Atlantic at high northern latitudes, which dilute surface waters and lower their density. So it becomes more difficult for surface water to become denser on being cooled so that it can sink to the ocean floor. The AMOC may weaken and shut down as a result and so too its warming effect at high latitudes. It also has a major effect on atmospheric circulation and moisture content: a truly complicated climatic phenomenon. Indeed, like the Pacific El Niño-Southern Oscillation (ENSO), major changes in AMOC may have global climatic implications.  QIyun Ma of the Alfred Wegner Institute in Bremerhaven, Germany and colleagues from Germany, China and Romania have modelled how the various possible locations of fresh water input may affect AMOC (Ma, Q. et al. 2024. Revisiting climate impacts of an AMOC slowdown: dependence on freshwater locations in the North Atlantic. Science Advances, v. 10, article eadr3243; DOI: 10.1126/sciadv.adr3243). They refer to such sudden inputs as ‘hosing’!

Location of the 4 regions in the northern North Atlantic used by Ma et al. in their modelling of AMOC: A Labrador Sea; B Irminger Basin; C NE Atlantic; D Nordic Seas. Colour chart refers to current temperature. Solid line – surface currents, dashed line – deep currents

First, the likely consequences under current climatic conditions of such ‘hosings’ and AMOC collapses are: a rapid expansion of the Arctic Ocean sea ice; delayed onset of summer ice-free conditions; southward shift of the Intertropical Convergence Zone (ITCZ) –  a roughly equatorial band of low pressure where the NE and SE trade winds converge, and the rough location of the sometimes windless Doldrums. There have been several attempts to model the general climatic effects of an AMOC slowdown. Ma et al. take matters a step further by using the Alfred Wegener Institute Climate Model (AWI-CM3) to address what may happen following ‘hosing’ in four regions of the North Atlantic: the Labrador Sea (between Labrador and West Greenland); the Irminger Basin (SE of East Greenland, SW of Iceland); the Nordic Seas (north of Iceland; and the Greenland-Iceland-Norwegian seas) and the NE Atlantic (between Iceland, Britain and western Norway).

Prolonged freshwater flow into the Irminger Basin has the most pronounced effect on AMOC weakening, largely due to a U-bend in the AMOC where the surface current changes from northward to south-westward flow parallel to the East Greenland Current. The latter carries meltwater from the Greenland ice sheet whose low density keeps it near the surface. In turn, this strengthens NE and SW winds over the Labrador Sea and Nordic Seas respectively, which slow this part of the AMOC. In turn that complex system slows the entire AMOC further south. Since 2010 an average 270 billion tonnes of ice has melted in Greenland each year. This results in an annual 0.74 mm rise in global sea level, so the melted glacial ice is not being replenished. When sea ice forms it does not take up salt and is just as fresh as glacial ice. Annual melting of sea ice therefore temporarily adds fresh water to surface waters of the Arctic Ocean, but the extent of winter sea ice is rapidly shrinking. So, it too adds to freshening and lowering the density of the ocean-surface layer. The whole polar ocean ‘drains’ southwards by surface currents, mainly along the east coast of Greenland potentially to mix with branches of the AMOC. At present they sink with cooled more saline water to move at depth. To melting can be added calving of Greenlandic glaciers to form icebergs that surface currents transport southwards. A single glacier (Zachariae Isstrom) in NE Greenland lost 160 billion tonnes of ice between 1999 and 2022. Satellite monitoring of the Greenland glaciers suggests that a trillion tonnes have been lost through iceberg formation during the first quarter of the 21st century. Accompanying the Dansgaard-Oeschger events of the last 100 ka were iceberg ‘armadas’ (Heinrich events) that deposited gravel in ocean-floor sediments as far south as Portugal.

 The modelling done by Ma et al. also addresses possible wider implications of their ‘hosing’ experiments to the global climate. The authors caution that this aspect is an ‘exploration’ rather than prediction. Globally increased duration of ‘cold extremes’ and dry spells, and the intensity of precipitation may ensue from downturns and potential collapse of AMOC. Europe seems to be most at risk. Ma et al. plea for expanded observational and modelling studies focused on the Irminger Basin because it may play a critical role in understanding the mechanisms and future strength of the AMOC.

 See also: Yirka, R. 2024. Greenland’s meltwater will slow Atlantic circulation, climate model suggests. Phys Org, 21 November 2024

The gross uncertainty of climate tipping points

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That the Earth has undergone sudden large changes is demonstrated by all manner of geoscientific records. It seems that many of these catastrophic events occurred whenever steady changes reach thresholds that trigger new behaviours in the interlinked atmosphere, hydrosphere, atmosphere, biosphere and lithosphere that constitute the Earth system. The driving forces for change, both steady and chaotic, may be extra-terrestrial, such as the Milankovich cycles and asteroid impacts, due to Earth processes themselves or a mixture of the two. Our home world is and always has been supremely complicated; the more obviously so as knowledge advances.  Abrupt transitions in components of the Earth system occur when a critical forcing threshold is passed, creating a ‘tipping point’. Examples in the geologically short term are ice-sheet instability, the drying of the Sahara, collapse of tropical rain forest in the Amazon Basin, but perhaps the most important is the poleward transfer of heat in the North Atlantic Ocean. That is technically known as the Atlantic Meridional Overturning Circulation with the ominous acronym AMOC.

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

As things stand today, warm Atlantic surface water, made more saline and dense by evaporation in the tropics is transferred northwards by the Gulf Stream. Its cooling at high latitudes further increases the density of this water, so at low temperatures it sinks to flow southwards at depth. This thermohaline circulation continually pulls surface water northwards to create the AMOC, thereby making north-western European winters a lot warmer than they would be otherwise. Data from Greenland ice cores show that during the climatic downturn to the last glacial maximum, the cooling trend was repeatedly interrupted by sudden warming-cooling episodes, known as Dansgaard-Oeschger events, one aspect of which was the launching of “armadas” of icebergs to latitudes as far south as Portugal (known as Heinrich events), which left their mark as occasional gravel layers in the otherwise muddy sediments on the deep Atlantic floor (see: Review of thermohaline circulation; February 2002).

These episodes involved temperature changes over the Greenland icecap of as much as 15°C.  They began with warming on this scale within a matter of decades followed by slow cooling to minimal temperatures, before the next turn-over. Various lines of evidence suggest that these events were accompanied by shutdowns of AMOC and hence the Gulf Stream, as shown by variations in the foraminifera species in sea-floor sediments. The culprit was vast amounts of fresh water pouring into the Arctic and northernmost Atlantic Oceans, decreasing the salinity and density of the surface ocean water. In these cases that may have been connected to repeated collapse of circumpolar ice sheets to launch Heinrich’s iceberg armadas. A similar scenario has been proposed for the millennium-long Younger Dryas cold spell that interrupted the onset of interglacial conditions. In that case the freshening of high-latitude surface water was probably a result of floods released when glacial barriers holding back vast lakes on the Canadian Shield burst.

At present the Greenland icecap is melting rapidly. Rising sea level may undermine the ice sheet’s coastal edges causing it to surge seawards and launch an iceberg armada. This may be critical for AMOC and the continuance of the Gulf Stream, as predicted by modelling: counter-intuitive to the fears of global warming, at least for NW Europe. In August 2024 scientists from Germany and the UK published what amounts to a major caution about attempts to model future catastrophes of this kind (Ben-Yami, M. et al 2024, Uncertainties too large to predict tipping times of major Earth system components from historical data, Science Advances, v. 10, article  eadl4841; DOI 10.1126/sciadv.adl4841). They focus on records of the AMOC system, for which an earlier modelling study predicted that a collapse could occur between 2025 and 2095: of more concern than global warming beyond the 1.5° C currently predicted by greenhouse-gas climate models .

Maya Ben-Yami and colleagues point out that the assumptions about mechanisms in Earth-system modelling and possible social actions to mitigate sudden change are simplistic.  Moreover, models used for forecasting rely on historical data sets that are sparse and incomplete and depend on proxies for actual variables, such as sea-surface and air temperatures. The further back in geological time, the more limited the data are. The authors assess in detail data sets and modelling algorithms that bear on AMOC. Rather than a chance of AMOC collapse in the 21st century, as suggested by others, Ben Yami et al. reckon that any such event  lies between 2055 and 8065 CE, which begs the question, “Is such forecasting  worth the effort?”, however appealing it might seem to the academics engaged in climatology. The celebrated British Met Office and other meteorological institutions, use enormous amounts of data, the fastest computers and among the most powerful algorithms on the planet to simulate weather conditions in the very near future. They openly admit a limit on accurate forecasting of no more than 7 day ahead. ‘Weather’ can be regarded as short-term climate change.

It is impossible to stop scientists being curious and playing sophisticated computer games with whatever data they have to hand. Yet, while it is wise to take climate predictions with a pinch of salt because of their gross limitations, the lessons of the geological past do demand attention. AMOC has shut down in the past – the last being during the Younger Dryas – and it will do so again. Greenhouse global warming probably increases the risk of such planetary hiccups, as may other recent anthropogenic changes in the Earth system. The most productive course of action is to reduce and, where possible, reverse those changes. In my honest opinion, our best bet is swiftly to rid ourselves of an economic system that in the couple of centuries since the ‘Industrial Revolution’ has wrought these unnatural distortions.

Sudden climate change: a warning from 8 millennia ago

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Mesolithic hunter-gatherers in Britain must have had a very hard time around 8.2 thousand years age. The whole area around the North Atlantic experienced sudden climatic cooling of around 3.3°C together with drought that lasted about 70 years. To make things worse shortly afterwards, coasts around the North were devastated by a tsunami generated by a submarine landslide off western Norway. That event exceeded the maximum coast ‘run up’ of both the 26 December 2004 Indian Ocean tsunami and that in NW Japan on 11 March 2011. Doggerland, then in the central North Sea was devastated by a catastrophic event of a few days duration. It littered the seabed with the bones of its megafauna and even Mesolithic tools recovered by trawlers from its surviving relic the shallow Dogger Bank. It seems the tsunami arrived just as climate was warming back to ‘normal’ Holocene conditions: for many foragers, surely, a last straw.

The cooling episode has been attributed to perturbation of the Atlantic Meridional Overturning Circulation (AMOC) as a result of meltwater discharge during the deglaciation of the Laurentide Ice Sheet (see: Just when you think it’s going to turn out alright… November 2009).The event may have unfolded in a similar fashion to the trigger for the Younger Dryas and the succession of warming-cooling episodes known as Dansgaard-Oeschger events that interrupted the otherwise relentless global cooling towards the last glacial maximum (see: Review of thermohaline circulation; February 2002). The physics that set off such climatic ‘hiccups’ is that freshening of surface seawater reduces its density, so that it cannot sink to be replaced by denser saline water ‘dragged’ northwards from warmer latitudes. That currently takes the form of the Gulf Stream with its warming influence, particularly in the eastern North Atlantic and even beyond Norway’s North Cape, responsible for much warmer winters than at similar latitudes on the western side. The culprit  had long been suggested to be the drainage of a huge lake dammed by the ice sheet that covered most of eastern Canada during late stages of deglaciation. Seemingly the best candidate was Lake Agassiz trapped by the early Holocene ice front in Manitoba – the largest proglacial lake known anywhere.

Colour coded topographic elevation of North America showing the maximum extent of Lake Agassiz and four possible routes for its drainage: north-west to the Arctic Ocean via the Mackenzie River; south to the Gulf of Mexico via the Mississippi valley; east to the North Atlantic via the Great Lakes and St Laurence River; north to the North Atlantic via Hudson Bay. (Credit: ©Sheffield University)

The present landforms of central Canada show evidence for several outflow directions at different times, Including to the northwest to reach the Arctic Ocean at the onset of the Younger Dryas. Until recently there was little detailed evidence for the flow volume and timing of its drainage around 8 to 9 ka. Providing the details in the context of the short-lived event around 8.2 ka requires accurate data over a mere 200 years able to reveal a change in sea level to a precision of better than a few tens of centimetre. Any site on the shores of the North Atlantic would do, provided it satisfies these criteria. Geographers from universities in York, Leeds, Sheffield and Oxford, UK selected the small estuary of the River Ythan in NE Scotland. There, a continuous sand unit just above fine-grained intertidal tidal muds marks the knife-sharp time datum of the Storegga tsunami (Rush, G. et al. 2023. The magnitude and source of meltwater forcing of the 8.2 ka climate event constrained by relative sea-level data from eastern Scotland. Quaternary Science Advances, v. 12, article 100119; DOI: 10.1016/j.qsa.2023.100119).

Cores of the intertidal sediments from beneath the present Ythan salt marsh contain plant remains that yielded precise radiocarbon dates at several stratigraphic levels from which to derive an age-depth model for the age range of interest. The buried sediments are also rich in marine microfossils (foraminifera and diatoms) that thrive in estuaries at a variety of depths.  These enabled fluctuations in relative sea level during the build-up of the intertidal sediments to be constrained at unprecedented resolution and precision for a three thousand year period from 9.5 to 6.5 ka. The authors show that there were two episodes of rapid sea-level rise over that time: between 8.53 and 8.37 ka (~2.4 m at 13 mm yr-1) and 8.37 to 8.24 ka (~ 0.6 m at 4 mm yr-1) – these would have been global increases in sea level.

Despite its vast size, it turns out that Lake Agassiz would have been unable to result in sea-level rises of that magnitude so quickly merely through outflow. Rush et al. suggest that the huge  and rapid addition of fresh water to the North Atlantic involved flow of lake water towards Hudson Bay, beneath the ice sheet, causing it to collapse and melt, followed by completion of Lake Agassiz’s emptying in the second stage. It took a long drawn-out ‘freshening’ of the North Atlantic surface water ultimately to shut down the Atlantic Meridional Overturning Circulation, thereby depriving high latitudes of its east-side warming effect by the Gulf Stream.

Sea level has been rising since the early 20th century mainly through the melting of Greenland’s ice cap together with a substantial amount of thermal expansion while global climate has been warming. Between 1901 and 2018 the rise has amounted to 15 to 25 cm at a rate of 1 to 2 mm yr-1. The AMOC is possibly weaker now than at any time during the last millennium (Zhu, C. et al. 2023. Likely accelerated weakening of Atlantic overturning circulation emerges in optimal salinity fingerprint. Nature Communications, v. 14, article 1245; DOI: 10.1038/s41467-023-36288-4). Yet increases in freshening of the northernmost parts of the North Atlantic are now being added to by annual increases in the melting of polar sea ice, which is salt-free. The AMOC may be approaching a tipping point, because warming is accelerating over Greenland at around 1.5°C each year: faster than most of the rest of the world. In 2021 it rained for the first time ever recorded at the ice cap’s summit (3.2 km above sea level). A ‘perturbation’ of the AMOC would add chaos to the dominantly linear view of global warming taken by climatologists. That could launch frigidity and drought at mid northern latitudes as it did eight millennia ago: the opposite of what is currently feared.

See also: Unlocking Ancient Climate Secrets – Melting Ice Likely Triggered Climate Change Over 8,000 Years Ago. Scitechdaily 16 September 2023.