Tag: Iceberg armadas

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.

A new twist to Pleistocene climate cycles

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The combined gravitational pulls of the sun and moon modulate variations in local tidal range. High spring tides occur when the two bodies are opposed at full moon or in roughly the same direction at new Moon. When the positions of sun and moon are at right angles (1st quarter and 3rd quarter) their gravitational pulls partly cancel each other to give neap tides. Consequently, there are two tidal cycles every lunar month.  In a similar way, the varying gravitational pulls of the planets during their orbital cycles impart a repetitive harmony to Earths astronomical behaviour. But their combined effects are on the order of tens of thousand years. Milutin Milankovich (1879-1958), a Serbian engineer, pondered on the possible causes of Earth’s climatic variations, particularly the repetition of ice ages. He was inspired by 19th century astronomers’ suggestion that maybe the gravitational effects of other planets might be a fruitful line of research. Milankovich focussed on how the shape of Earth’s orbit, the tilt of its rotational axis and the way the axis wobbles like that of a spinning top affect the amount of solar heating at all points on the surface: the effects of varying eccentricity, obliquity and precession, respectively.

 Earlier astronomers had calculated cycles of gravitational effects on Earth of the orbits of Jupiter and Saturn of the three attributes of Earth’s astronomical behaviour and found periods of about 100, 41 and 23 thousand years (ka) respectively. The other 3 inner planets and the much more distant giants Uranus and Neptune also have gravitational effects on Earth, but they are negligible compared with those of the two nearest giant planets, because gravitation force varies with mass and inversely with the square of distance. Sadly, Milankovich was long dead when his hypothesis of astronomical climate forcing was verified in 1976 by frequency analysis of the record of oxygen isotopes in foraminifera found in two ocean sediment core from the Southern Indian Ocean. It revealed that all three periods interfered in complex ways during the Late Pleistocene, to dominate variations in sea-surface temperatures and the fluctuating volume of continental ice sheets for which δ18O is a proxy (see: Odds and ends about Milankovich and climate change; February 2017).

Precession of the axis of a spinning top and that of the Earth. At present the northern end of Earth’s axis points to what we now call the Pole Star. Around 11.5 ka from now it will point to the star Vega

This was as revolutionary for climatology as plate tectonics was for geology. We now know that in the early Pleistocene glacial-interglacial cycles were in lockstep with the 41 ka period of axial obliquity, and since 700 ka followed closely – but not perfectly – the 100 ka orbital eccentricity forcing. The transitional period between 1.25 and 0.7 Ma (the Mid-Pleistocene Transition or MPT) suggested neither one nor the other. Milankovich established that axial tilt variations have the greatest influence on solar heating, so the early 41 ka cycles were no surprise. But the dominance of orbital eccentricity on the last 700 ka certainly presented a puzzle, for it has by far the weakest influence on solar heating: 10 times less than those of axial obliquity and precession. The other oddity concerns the actual effect of axial precession on climate change. There are no obvious 23 ka cycles in the climate record, despite the precession signal being clear in frequency analysis and its effect on solar heating being almost as powerful as obliquity and ten times greater than that of orbital eccentricity. Precessional wobbling of the axis controls the time of year when one hemisphere or the other is closest to the Sun. At one extreme it will be the Northern and 11.5 ka later it will be the Southern. The times of solstices and equinoxes also change relative to the calendar that we use today.

There is an important, if obvious, point about astronomical forcing of climate. It is always there, with much the same complicated interactions between the factors: human activities have absolutely no bearing on them. Climatic ‘surprises’ are likely to continue!

Changes in ice-rafted debris (IRD) since 1.7 Ma in a sediment core from the North Atlantic (orange fill) compared with its oxygen-isotope (δ18O) record of changes in continental ice cover (blue fill). At the top are the modelled variations in 23 ka axial precession (lilac) and 41 ka obliquity (green). The red circles mark major interglacial episodes, blue diamonds show the onset of significant ice rafting and orange diamonds are terminations of ice-rafting (TIR). (Credit: Barker et al., Fig. 2)

Sea temperature and ice-sheet volume are not the only things that changed during the Pleistocene. Another kind of record from oceanic sediments concerns the varying proportion in the muddy layers of abnormally coarse sand grains and even small pebbles that have been carried by icebergs; they are known as ice-rafted debris (IRD). The North Atlantic Ocean floor has plenty of evidence for them appearing and disappearing on a layer-by-layer basis. They were first recognised in 1988 by an oceanographer called Helmut Heinrich, who proposed that six major layers rich in IRD in North Atlantic cores bear witness to iceberg ‘armadas’ launched by collapse, or ablation, at the front of surging ice sheets on Scandinavia, Greenland and eastern Canada. Heinrich events, along with Dansgaard-Oeschger events (rapid climatic warming followed by slower cooling) in the progression to the last glacial maximum have been ascribed to a variety of processes  operating on a ‘millennial’ scale. However, ocean-floor sediment cores are full of lesser fluctuations in IRD, back to at least 1.7 Ma ago. That record offers a better chance of explaining fluctuations in ice-sheet ablation. A joint European-US group has investigated their potential over the last decade or so (Barker, S. et al. 2022. Persistent influence of precession on northern ice sheet variability since the early Pleistocene. Science, v. 376, p. 961-967; DOI: 10.1126/science.abm4033). The authors noted that in each glacial cycle since 1.7 Ma the start of ice rafting consistently occurred during a time of decreasing axial obliquity. Yet the largest ablation events were linked to minima in the precession cycles. In the last 700 ka, such extreme events are associated with the terminations of each ice age.

In the earlier part of the record, the 41 ka obliquity ‘signal’ was sufficient to drive glacial-interglacial cycles, hence their much greater regularity and symmetry than those that followed the Mid-Pleistocene Transition. The earlier ice sheets in the Northern Hemisphere also had consistently smaller extents than those after the MPT. Although the records show a role for precession in pre-MPT times in the form of ice-rafting events, the lesser effect of precession on summer warming at higher latitudes, compared with that of axial obliquity, gave it no decisive influence. After 700 ka the northern ice sheets extended much further south – as far as 40°N in North America – where summer warming would always have been commensurately greater than at high northern latitudes. So they were more susceptible to melting during the increased summer warming driven by the precession cycles. When maximum summer heating induced by axial precession in the Northern Hemisphere coincided with that of obliquity the ice sheets as a whole would have become prone to catastrophic collapse.

It is hard to say whether these revelations have a bearing on future climate. Of course, astronomical forcing will continue relentlessly, irrespective of anthropogenic greenhouse gas emissions. Earth has been in an interglacial for the last 11.5 ka, since the Younger Dryas; i.e. about half a precession cycle ago. The combination of obliquity- and precession-driven influences suggest that climate should be cooling and has been since 6,000 years ago, until the Industrial Revolution intervened. Can the gravitational pull of the giant planets prevent a runaway greenhouse effect, or will human effects defy astronomical forces that continually distort Earth’s astronomical behaviour?