Tag: Climate change

The end-Triassic mass extinction and ocean acidification

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Triassic reef limestones in the Dolomites of northern Italy. Credit: © Matteo Volpone

Four out of six mass extinctions that ravaged life on Earth during the last 300 Ma coincided with large igneous events marked by basaltic flood volcanism. But not all such bursts of igneous activity match significant mass extinctions. Moreover, some rapid rises in the rate of extinction are not clearly linked to peaks in igneous activity. Another issue in this context is that ‘kill mechanisms’ are generally speculative rather than based on hard data. Large igneous events inevitably emit very large amounts of gases and dust-sized particulates into the atmosphere. Carbon dioxide, being a greenhouse gas, tends to heat up the global climate, but also dissolves in seawater to lower its pH. Both global warming and more acidic oceans are possible ‘kill mechanisms’. Volcanic emission of sulfur dioxide results in acid rain and thus a decrease in the pH of seawater. But if it is blasted into the stratosphere it combines with oxygen and water vapour to form minute droplets of sulfuric acid. These form long-lived haze, which reflects solar energy beck into space. Such an increased albedo therefore tends to cool the planet and create a so-called ‘volcanic winter’. Dust that reaches the stratosphere reduces penetration of visible light to the surface, again resulting in cooling. But since photosynthetic organisms rely on blue and red light to power their conversion of CO­2­ and water vapour to carbohydrates and oxygen, these primary producers at the base of the marine and terrestrial food webs decline. That presents a fourth kill mechanism that may trigger mass extinction on land and in the oceans: starvation.

Palaeontologists have steadily built up a powerful case for occasional mass extinctions since fossils first appear in the stratigraphic record of the Phanerozoic Eon. Their data are simply the numbers of species, genera and families of organisms preserved as fossils in packages of sedimentary strata that represent roughly equal ‘parcels’ of time (~10 Ma). Mass extinctions are now unchallengeable parts of life’s history and evolution. Yet, assigning specific kill mechanisms involved in the damage that they create remains very difficult. There are hypotheses for the cause of each mass extinction, but a dearth of data that can test why they happened. The only global die-off near hard scientific resolution is that at the end of the Cretaceous. The K-Pg (formerly K-T) event has been extensively covered in Earth-logs since 2000. It involved a mixture of global ecological stress from the Deccan large igneous event spread over a few million years of the Late Cretaceous, with the near-instantaneous catastrophe induced by the Chicxulub impact, with a few remaining dots and ticks needed on ‘i’s and ‘t’s. Other possibilities have been raised: gamma-ray bursts from distant supernovae; belches of methane from the sea floor; emissions of hydrogen sulfide gas from seawater itself during ocean anoxia events; sea-level changes etc.

The mass extinction that ended the Triassic (~201 Ma) coincides with evidence for intense volcanism in South and North America, Africa and southern Europe, then at the core of the Pangaea supercontinent. Flood basalts and large igneous intrusions – the Central Atlantic Magmatic Province (CAMP) – began the final break-up of Pangaea. The end-Triassic extinction deleted 34% of marine genera. Marine sediments aged around 201 Ma reveal a massive shift in sulfur and carbon isotopes in the ocean that has been interpreted as a sign of acute anoxia in the world’s oceans, which may have resulted in massive burial of oxygen-starved marine animal life. However, there is no sign of Triassic, carbon-rich deep-water sediments that characterise ocean anoxia events in later times. But it is possible that bacteria that use the reduction of sulfate (SO42-) to sulfide (S2-) ions as an energy source for them to decay dead organisms, could have produced the sulfur isotope ‘excursion’. That would also have produced massive amounts of highly toxic hydrogen sulfide gas, which would have overwhelmed terrestrial animal life at continental margins. The solution ofH2S in water would also have acidified the world’s oceans.

Molly Trudgill of the University of St Andrews, Scotland and colleagues from the UK, France, the Netherlands, the US, Norway, Sweden and Ireland set out to test the hypothesis of end-Triassic oceanic acidification (Trudgill, M. and 24 others 2025. Pulses of ocean acidification at the Triassic–Jurassic boundary. Nature Communications, v. 16, article 6471; DOI: 10.1038/s41467-025-61344-6). The team used Triassic fossil oysters from before the extinction time interval. Boron-isotope data from the shells are a means of estimating variations in the pH of seawater. Before the extinction event the average pH in Triassic seawater was about the same as today, at 8.2 or slightly alkaline. By 201 Ma the pH had shifted towards acidic conditions by at least 0.3: the biggest detected in the Phanerozoic record. One of the most dramatic changes in Triassic marine fauna was the disappearance of reef limestones made by the recently evolved modern corals on a vast scale in the earlier Triassic; a so-called ‘reef gap’ in the geological record. That suggests a possible analogue to the waning of today’s coral reefs that is thought to be a result of increased dissolution of CO2 in seawater and acidification, related to global greenhouse warming. Using the fossil oysters, Trudgill et al. also sought a carbon-isotope ‘fingerprint’ for the source of elevated CO2, finding that it mainly derived from the mantle, and was probably emitted by CAMP volcanism. So their discussion centres mainly on end-Triassic ocean acidification as an analogy for current climate change driven by CO2 largely emitted by anthropogenic burning of fossil fuels. Nowhere in their paper do they mention any role for acidification by hydrogen sulfide emitted by massive anoxia on the Triassic ocean floor, which hit the scientific headlines in 2020 (see earlier link).

Modelling climate change since the Devonian

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A consortium of geoscientists from Australia, Britain and France, led by Andrew Merdith of the University of Adelaide examines the likely climate cooling mechanisms that may have set off the two great ‘icehouse’ intervals in the last 541 Ma (Merdith, A.S. et al. 2025. Phanerozoic icehouse climates as the result of multiple solid-Earth cooling mechanisms. Science Advances, v. 11, article eadm9798: DOI: 10.1126/sciadv.adm9798). They consider the first to be the global cooling that began in the latter part of the Devonian culminating in the Carboniferous-Permian icehouse. The second is the Cenozoic global cooling to form the permanent Antarctic ice cap around 34 Ma and culminated in cyclical ice ages on the northern continents after 2.4 Ma during the Pleistocene. They dismiss the 40 Ma long, late Ordovician to early Silurian glaciation that left its imprint on North Africa and South America –  then combined in the Gondwana supercontinent. The data about two of the parameters used in their model – the degree of early colonisation of the continents by plants and their influence on terrestrial weathering are uncertain in that protracted event.  Yet the Hirnantian glaciation reached 20°S at its maximum extent in the Late Ordovician around 444 Ma to cover about a third of Gondwana: it was larger than the present Antarctic ice cap. For that reason, their study spans only Devonian and later times.

Fluctuation in evidence for the extent of glacial conditions since the Devonian: the ‘ice line’ is grey. The count of glacial proxy occurrences in each 10° of latitude through time is shown in the colour key. Credit: Merdith et al., Fig 2A.

Merdith et al. rely on four climatic proxies. The first of these comprises indicators of cold climates, such as glacial dropstones, tillites and evidence in sedimentary rocks of crystals of hydrated calcium carbonate (ikaite – CaCO3.6H2O) that bizarrely forms only at around 0°C . From such occurrences it is possible to define an ‘ice line’ linking different latitudes through geological time. Then there are estimates of global average surface temperature; low-latitude sea surface temperature; and estimates of atmospheric CO2. The ‘ice-line’ data records an additional, long period of glaciation in the Jurassic and early Cretaceous, but evidence does not extend to latitudes lower than 60°. It is regarded by Merdith et al. as an episode of ‘cooling’ rather than an ‘icehouse’. Their model assesses sources and sinks of CO2­ since the Devonian Period.

The main natural source of the principal greenhouse gas CO2 is degassing through volcanism expelled from the mantle and breakdown of carbonate rock in subducted lithosphere. Natural sequestration of carbon involves weathering of exposed rock that releases dissolved CO2 and ions of calcium and magnesium.   A recently compiled set of plate reconstructions that chart the waxing and waning of tectonics since the Devonian Period allows them to model the tectonically driven release of carbon over time, with time scales on the order of tens to hundreds of Ma. The familiar Milanković forcing cycles on the order of tens to hundreds of ka are thus of no significance in Merdith et al.’s  broader conception of icehouse episodes  Their modelling shows high degassing during the Cretaceous, modern levels during the late Palaeozoic and early Mesozoic, and low emissions during the Devonian. The model also suggests that cooling stemmed from variations in the positions and configuration of continents over time.  Another crucial factor is the tempo of exposure of rocks that are most prone to weathering. The most important are rocks of the ocean lithosphere incorporated into the continents to form ophiolite masses. The release of soluble products of weathering into ocean basins through time acts as a fluctuating means of ‘fertilising’ so that more carbon can be sequestered in deep sediments in the form of organisms’ unoxidised tissue and hard parts made of calcium carbonates and phosphates. Less silicate weathering results in a boost to atmospheric CO2.

Only two long, true icehouse episodes emerge from the empirical proxy data, expressed by the ‘ice-line’ plots. Restricting the modelling to single global processes that might be expected to influence degassing or carbon sequestration produces no good fits to the climatic proxy data. Running the model with all the drivers “off” produces more or less continuous icehouse conditions since the Devonian. The model’s climate-related outputs thus imply that many complex processes working together in syncopation may have driven the gross climate vagaries over the last 400 Ma or so. A planet of Earth’s size without such complexity would throughout that period have had a high-CO2 warm climate. According to Andrew Merdith its fluctuation from greenhouse to icehouse conditions in the late Palaeozoic and the Cenozoic were probably due to “coincidental combination of very low rates of global volcanism, and highly dispersed continents with big mountains, which allow for lots of global rainfall and therefore amplify reactions that remove carbon from the atmosphere”.

Geological history is, almost by definition, somewhat rambling. So, despite despite the large investment in seeking a computed explanation of data drawn from the record, the outcome reflects that in a less than coherent account. To state that many complex processes working at once may have driven climate vagaries over the last 400 Ma or so, is hardly a major advance: palaeoclimatologists have said more or less the same for a couple of decades or more, but have mainly proposed single driving mechanisms. One aspect of Merdith et al.’s  results seems to be of particular interest. ‘Icehouse’ conditions seem to be rare events interspersed with broader ice-free periods. We evolved within the mammal-dominated ecosystems on the continents during the latest of these anomalous climatic episodes. And we and those ecosystems now rely on a cool world. As the supervisor of the project commented, ‘Over its long history, the Earth likes it hot, but our human society does not’.

Readers may like to venture into how some philosophers of science deal with a far bigger question; ‘Is intelligent life a rare, chance event throughout the universe?’ That is, might we be alone in the cosmos? In the same issue of Science Advances is a paper centred on just such questions (Mills, D.B. et al. 2025. A reassessment of the “hard-steps” model for the evolution of intelligent life. Science Advances, v. 11, article eads5698; DOI: 10.1126/sciadv.ads5698). It stems from cosmologist Brandon Carter’s ‘Anthropic Principle’ first developed at Nicolas Copernicus’s 500th birthday celebrations in 1973. This has since been much debated by scientists and philosophers – a gross understatement as it knocks the spots off the Drake Equation. To take the edge off what seems to be a daunting task, Mills et al. consider a corollary of the Anthropic Principle, the ‘hard steps model’. That, in a nutshell, postulates that the origin of humanity and its ability to ponder on observations of the universe required a successful evolutionary passage through a number of hard steps. It predicts that such intelligence is ‘exceedingly rare’ in the universe. Icehouse conditions are respectable candidates for evolutionary ‘hard steps’, and in the history of Earth there have been five of them.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

The earliest known human-Neanderthal relations

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The first anatomically modern humans (AMH) known to have left their remains outside of Africa lived about 200 ka ago in Greece and the Middle East. They were followed by several short-lived migrations that got as far as Europe, leaving very few fossils or artefacts. Over that time Neanderthals were continually present. Migration probably depended on windows of opportunity controlled by pressures from climatic changes in Africa and sea level being low enough to leave their heartland: perhaps as many as 8 or 9 before 70 ka, when continuous migration out of Africa began. The first long-enduring AMH presence in Europe began around 47 ka ago.

For about 7 thousand years thereafter – about 350 generations – AMH and Neanderthals co-occupied Europe. Evidence is growing that the two groups shared technology. After 40 ka there are no tangible signs of Neanderthals other than segments of their DNA that constitute a proportion of the genomes of modern non-African people. They and AMH must have interbred at some time in the last 200 ka until Neanderthals disappeared. In the same week in late 2024 two papers that shed much light on that issue were published in the leading scientific journals, Nature and Science, picked up by the world’s news media. Both stem from research led by researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. They focus on new DNA results from the genomes of ancient and living Homo sapiens. One centred on 59 AMH fossils dated between 45 and 2.2 ka and 275 living humans (Iasi, L. M. N. and 6 others 2024. Neanderthal ancestry through time: Insights from genomes of ancient and present-day humans Science, v. 386, p. 1239-1246: DOI: 10.1126/science.adq3010. PDF available by request to leonardo_iasi@eva.mpg.de). The other concerns genomes recovered from seven AMH individuals from the oldest sites in Germany and Czechia. (Sümer, A. P. and 44 others 2024. Earliest modern human genomes constrain timing of Neanderthal admixture. Nature, online article; DOI: 10.1038/s41586-024-08420-x. PDF available by request to arev_suemer@eva.mpg.de ).

Leonardo Iasi and colleagues from the US and UK examined Neanderthal DNA segments found in more than 300 AMH  genomes, both ancient and in living people, by many other researchers. Their critical focus was on lengths of such segments. Repeated genetic recombination in the descendants of those individuals who had both AMH and Neanderthal parents results in shortening of the lengths of their inherited Neanderthal DNA segments. That provides insights into the timing and duration of interbreeding. The approach used by Iasi ­et al­. used sophisticated statistics to enrich their analysis of Neanderthal-human gene flow. They were able to show that the vast majority of Neanderthal inheritance stems from a single period of such gene flow into the common ancestors of all living people who originated outside Africa. This genetic interchange seems to have lasted for about 7 thousand years after 50 ka. This tallies quite closely with the period when fossil and cultural evidence supports AMH and Neanderthals having co-occupied Europe.

Reconstruction of the woman whose skull was found at Zlatý kůň, Czechia. Credit: Tom Björklund / Max Planck Institute for Evolutionary Anthropology.

The other study, led by Arev Sümer,  has an author list of 44 researchers from Germany, the US,  Spain, Australia, Israel, the UK, France, Sweden, Denmark and Czechia. The authors took on a difficult task: extracting full genomes from seven of the oldest AMH fossils found in Europe, six from a cave Ranis in Germany and one from about 230 km away at Zlatý kůň in Czechia. Human bones, dated between 42.2 and 49.5 ka, from the Ranis site had earlier provided mitochondrial DNA that proved them to be AMH. A complete female skull excavated from Czechia site, dated at 45 ka had previously yielded a high quality AMH genome. Interestingly that carried variants associated with dark skin and hair, which perhaps reflect African origins. Neanderthals probably had pale skins and may have passed on to the incomers genes associated with more efficient production of vitamin D in the lower light levels of high latitudes and maybe immunity to some diseases. Both sites contain a distinct range of artefacts known as the Lincombian-Ranisian-Jerzmanowician technocomplex. This culture was once regarded as having been made by Neanderthals, but is now linked by the mtDNA results to early AMH. Such artefacts occur across central and north-western Europe. The bones from both sites are clearly important in addressing the issue of Neanderthal-AMH cultural and familial relationships.

The new, distinct genetic data from the Ranis and Zlatý kůň individuals reveals a mother and her child at Ranis. The female found at Zlatý kůň had a fifth- to sixth-degree genetic relationship with Ranis individuals: she may have been their half first cousin once removed. This suggests a wider range of communications than most people in medieval Europe would have had. The data from both sites suggests that the small Ranis-Zlatý kůň population – estimated at around 200 individuals – diverged late from the main body of AMH who began to populate Asia and Australasia at least 65 ka ago. Their complement of Neanderthal genetic segments seems to have originated during their seven thousand-year presence in Europe. Though they survived through 350 generations it seems that their genetic line was not passed on within and outside of Europe. They died out, perhaps during a sudden cold episode during the climatic decline towards the Last Glacial Maximum. We know that because their particular share of the Neanderthal genome does not crop up in the wider data set used by Iasi et al., neither in Europe and West Asia nor in East Asia. That they survived for so long may well have been due to their genetic inheritance from Neanderthals that made them more resilient to what, for them, was initially an alien environment. It is not over-imaginative to suggest that both populations may have collaborated over this period. But neither survived beyond about 40 ka..

Widely publicised as they have been, the two papers leave much more unanswered than they reveal. Both the AMH-Neanderthal relationship and the general migration out of Africa are shown to be more complex than previously thought by palaeoanthropologists. For a start, the descendants today of migrants who headed east carry more Neanderthal DNA that do living Europeans, and it is different. Where did they interbreed? Possibly in western Asia, but that may never be resolved because warmer conditions tend to degrade genetic material beyond the levels that can be recovered from ancient bones. Also, some living people in the east carry both Neanderthal and Denisovan DNA segments. Research Centres like the Max Planck Institute for Evolutionary Anthropology will clearly offer secure employment for some time yet!

How changes in the Earth System have affected human evolution, migration and culture

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Refugees from the Middle East migrating through Slovenia in 2015. Credit: Britannica

During the Pliocene (5.3 to 2.7 Ma) there evolved a network of various hominins, with their remains scattered across both the northern and southern parts of that continent. The earliest, though somewhat disputed hominin fossil Sahelanthropus tchadensis hails from northern Chad and lived  around 7 Ma ago, during the late Miocene, as did a similarly disputed creature from Kenya Orrorin tugenensis (~5.8 Ma). The two were geographically separated by 1500 km, what is now the Sahara desert and the East African Rift System.  The suggestion from mtDNA evidence that humans and chimpanzees had a common ancestor, the uncertainty about when it lived (between 13 to 5 Ma) and what it may have looked like, let alone where it lived, makes the notion debateable. There is even a possibility that the common ancestor of humans and the other anthropoid apes may have been European. Its descendants could well have crossed to North Africa when the Mediterranean Sea had been evaporated away to form the thick salt deposits that now lie beneath it: what could be termed the ‘Into Africa’ hypothesis. The better known Pliocene hominins were also widely distributed in the east and south of the African continent. Wandering around was clearly a hominin predilection from their outset. The same can be said about humans in the general sense (genus Homo) during the Early Pleistocene when some of them left Africa for Eurasia. Artifacts dated at 2.1 Ma have been found on the Loess Plateau of western China, and Georgia hosts the earliest human remains known from Eurasia. Since them H. antecessor, heidelbergensis, Neanderthals and Denisovans roamed Eurasia. Then, after about 130 ka, anatomically modern humans progressively populated all continents, except Antarctica, to their geographic extremities and from sea level to 4 km above it.

There is a popular view that curiosity and exploration are endemic and perhaps unique to the human line: ‘It’s in our genes’. But even plants migrate, as do all animal species. So it is best to be wary of a kind of hominin exceptionalism or superior motive force. Before settled agriculture, simply diffusion of populations in search of sustenance could have achieved the enormous migrations undertaken by all hominins: biological resources move and hunter gatherers follow them. The first migration of Homo erectus from Africa to northern China by way of Georgia seems to taken 200 ka at most and covered about ten thousand kilometres: on average a speed of only 50 m per year! That achievement and many others before and later were interwoven with the evolution of brain size, cognitive ability, means of communication and culture. But what were the ultimate drivers? Two recent papers in the journal Nature Communications make empirically-based cases for natural forces driving the movement of people and changes in demography.

The first considers hominin dispersal in the Palaearctic biogeographic realm: the largest of eight originally proposed by Alfred Russel Wallace in the late 19th century that encompasses the whole of Eurasia and North Africa (Zan, J. et al. 2024. Mid-Pleistocene aridity and landscape shifts promoted Palearctic hominin dispersals. Nature Communications, v. 15, article 10279; DOI: 10.1038/s41467-024-54767-0). The Palearctic comprises a wide range of ecosystems: arid to wet, tropical to arctic. After 2 Ma ago, hominins moved to all its parts several times. The approach followed by Zan et al. is to assess the 3.6 Ma record of the thick deposits of dust carried by the perpetual westerly winds that cross Central Asia. This gave rise to the huge (635,000 km2) Loess Plateau. At least 17 separate soil layers in the loess have yielded artefacts during the last 2.1 Ma. The authors radiocarbon dated the successive layers of loess in Tajikistan (286 samples) and the Tarim Basin (244 samples) as precisely as possible, achieving time resolutions of 5 to 10 ka and 10 to 20 ka respectively. To judge variations in climate in these area they also measured the carbon isotopic proportions in organic materials preserved within the layers. Another climate-linked metric that Zan et al. is a time series showing the development of river terraces across Eurasia derived from the earlier work of many geomorphologists. The results from those studies are linked to variations through time in the numbers of archaeological sites across Eurasia that have yielded hominin fossils, stone tools and signs of tool manufacture, many of which have been dated accurately.

The authors use sophisticated statistics to find correlations between times of climatic change and the signs of hominin occupation. Episodes of desertification in Palaearctic Eurasia clearly hindered hominins’ spreading across the continent either from west to east of vice versa. But there were distinct, periodic windows of climatic opportunity for that to happen that coincide with interglacial episodes, whose frequency changed at the Mid Pleistocene Transition (MPT) from about 41 ka to roughly every 100 ka. That was suggested in 2021 to have arisen from an increased roughness of the rock surface over which the great ice sheets of the Northern Hemisphere moved. This suppressed the pace of ice movement so that the 41 ka changes in the tilt of the Earth’s rotational axis could no longer drive climate change during the later Pleistocene, despite the fact that the same astronomical influence continued. The succeeding ~100 ka pulsation may or may not have been paced by the very much weaker influence of Earth changing orbital eccentricity. Whichever, after the MPT climate changes became much more extreme, making human dispersal in the Palearctic realm more problematic. Rather than hominin’s evolution driving them to a ‘Manifest Destiny’ of dominating the world vastly larger and wider inorganic forces corralled and released them so that, eventually, they did.

Much the same conclusion, it seems to me, emerges from a second study that covers the period since ~ 9 ka ago when anatomically modern humans transitioned from a globally dominant hunter-gatherer culture to one of ‘managing’ and dominating ecosystems, physical resources and ultimately the planet itself. (Wirtz, K.W et al. 2024. Multicentennial cycles in continental demography synchronous with solar activity and climate stability. Nature Communications, v. 15, article 10248; DOI: 10.1038/s41467-024-54474-w). Like Zan et al., Kai Wirtz and colleagues from Germany, Ukraine and Ireland base their findings on a vast accumulated number (~180,000) of radiocarbon dates from Holocene archaeological sites from all inhabited continents. The greatest number (>90,000) are from Europe. The authors applied statistical methods to judge human population variations since 11.7 ka in each continental area. Known sites are probably significantly outweighed by signs of human presence that remain hidden, and the diligence of surveys varies from country to country and continent to continent: Britain, the Netherlands and Southern Scandinavia are by far the best surveyed. Given those caveats, clearly this approach gives only a blurred estimate of population dynamics during the Holocene. Nonetheless the data are very interesting.

The changes in population growth rates show distinct cyclicity during the Holocene, which Wirtz et al. suggest are signs of booms and busts in population on all six continents. Matching these records against a large number of climatic time series reveals a correlation. Their chosen metric is variation in solar irradiance: the power per unit area received from the Sun. That has been directly monitored only over a couple of centuries. But ice cores and tree rings contain proxies for solar irradiance in the proportions of the radioactive isotopes 10Be and 14C contained in them respectively. Both are produced by the solar wind of high-energy charged particles (electrons, protons and helium nuclei or alpha particles) penetrating the upper atmosphere. The two isotopes have half-lives long enough for them to remain undecayed and thus detectable for tens of thousand years. Both ice cores and tree rings have decadal to annual time resolutions. Wirtz et al. find that their crude estimates of booms and busts in human populations during the Holocene seem closely to match variations in solar activity measured in this way. Climate stability favours successful subsistence and thus growth in populations. Variable climatic conditions seem to induce subsistence failures and increase mortality, probably through malnutrition.

A nice dialectic clearly emerges from these studies. ‘Boom and bust’ as regards populations in millennial and centennial to decadal terms stem from climate variations. Such cyclical change thus repeatedly hones natural selection among the survivors, both genetically and culturally, increasing their general fitness to their surroundings. Karl Marx and Friedrich Engels would have devoured these data avidly had they emerged in the 19th century. I’m sure they would have suggested from the evidence that something could go badly wrong – negation of negation, if readers care to explore that dialectical law further . . . And indeed that is happening. Humans made ecologically very fit indeed in surviving natural pressures are now stoking up a major climatic hiccup, or rather the culture and institutions that humans have evolved are doing that.

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

A major breakthrough in carbon capture and storage?

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Carbon capture and storage is in the news most weeks and is increasingly on the agenda for some governments. But plans to implement the CCS approach to reducing and stopping global warming increasingly draws scorn from scientists and environmental campaigners. There is a simple reason for their suspicion. State engagement, in the UK and other rich countries, involves major petroleum companies that developed the oil and gas fields responsible for unsustainably massive injection of CO2 into the atmosphere. Because they have ‘trousered’ stupendous profits they are a tempting source for the financial costs of pumping CO2 into porous sedimentary rocks that once contained hydrocarbon reserves. Not only that, they have conducted such sequestration over decades to drive out whatever petroleum fluids remaining in previously tapped sedimentary strata. For that second reason, many oil companies are eager and willing to comply with governmental plans, thereby seeming to be environmentally ‘friendly’. It also tallies with their ambitions to continue making profits from fossil-fuel extraction. But isn’t that simply a means of replacing the sequestered greenhouse gas with more of it generated by burning the recovered oil and natural gas; i.e. ‘kicking the can down the road’? Being a gas – technically a ‘free phase’ – buried CO2 also risks leaking back to the atmosphere through fractures in the reservoir rock. Indeed, some potential sites for its sequestration have been deliberately made more gas-permeable by ‘fracking’ as a means of increasing the yield of petroleum-rich rock. Finally, a litre of injected gas can drive out pretty much the same volume of oil. So this approach to CCS may yield a greater potential for greenhouse warming than would the sequestered carbon dioxide itself.

Image of calcite (white) and chlorite (cyan) formed in porous basalt due to CO2-charged water-rock interaction at the CarbFix site in Iceland. (Credit: Sandra Ósk Snæbjörnsdóttir)

Another, less widely publicised approach is to geochemically bind CO2 into solid carbonates, such as calcite (CaCO­3), dolomite (CaMgCO3), or magnesite (MgCO3). Once formed such crystalline solids are unlikely to break down to their component parts at the surface, under water or buried. One way of doing this is by the chemical weathering of rocks that contain calcium- and magnesium-rich minerals, such as feldspar (CaAl2Si2O8), olivine ([Fe,Mg]2SiO4) and pyroxene ([Fe,Mg]CaSi2O6) . Mafic and ultramafic rocks, such as basalt and peridotite are commonly composed of such minerals. One approach involves pumping the gas into a Icelandic borehole that passes through basalt and letting natural reactions do the trick. They give off heat and proceed quickly, very like those involved in the setting of concrete. In two experimental field trials 95% of injected CO2 was absorbed within 18 months. Believe it or not, ants can do the trick with crushed basalt and so too can plant roots. There have been recent experiments aimed at finding accelerants for such subsurface weathering (Wang, J. et al. 2024. CO2 capture, geological storage, and mineralization using biobased biodegradable chelating agents and seawater. Science Advances, v. 10, article eadq0515; DOI: 10.1126/sciadv.adq0515). In some respects the approach is akin to fracking. The aim is to connect isolated natural pores to allow fluids to permeate rock more easily, and to release metal ions to combine with injected CO2.

Chelating agents are biomolecules that are able to dissolve metal ions; some are used to remove toxic metals, such as lead, mercury and cadmium, from the bodies of people suffering from their effects. Naturally occurring ones extract metal ions from minerals and rocks and are agents of chemical weathering; probably used by the aforesaid ants and root systems. Wang and colleagues, based at Tohoku University in Japan, chose a chelating agent GLDA (tetrasodium glutamate diacetate –  C9H9NNa4O8) derived from plants, which is non-toxic, cheap and biodegradable. They injected CO2 and seawater containing dissolved GDLA into basaltic rock samples. The GDLA increases the rock’s porosity and permeability by breaking down its minerals so that Ca and Mg ions entered solution and were thereby able to combine with the gas to form carbonate minerals. Within five days porosity was increased by 16% and the rocks permeability increased by 26 times. Using electron microscopy the authors were able to show fine particles of carbonate growing in the connected pores. In fact these carbonate aggregates become coated with silica released by the induced mineral-weathering reactions. Calculations based on the previously mentioned field experiment in Iceland suggest that up to 20 billion tonnes of CO2 could be stored in 1.3 km3 of basalt treated in this way: about 1/25000 of the active rift system in Iceland (3.3 x 104 km2 covered by 1 km of basalt lava). In 2023 fossil fuel use emitted an estimated 36.6 bllion tons of CO2 into the atmosphere.

So, why do such means of efficiently reducing the greenhouse effect not receive wide publicity by governments or the Intergovernmental Panel on Climate Change? Answers on a yellow PostIt™ please . . .

A new timeline for modern humans’ colonisation of Europe

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Aurignacian sculptures: ‘Lion-Man’ and ‘Venus’ from the Hohlenstein-Stadel and Hohle Fels caves in Germany.

The earliest culture (or techno-complex) that can be related to anatomically modern humans (AMH) in Europe is called the Aurignacian. It includes works of art as well as tools made from stone, bone and antler. Perhaps the most famous are the ivory sculptures of ‘Lion-Man’ and Venus of the Hohlenstein-Stadel  and Hohle Fels caves in Germany,  and also the stunning cave art, of Chauvet Cave in France. Aurignacian artefacts that are dated at 43 to 26 ka occur at sites throughout Europe south of about 52°N. It was this group of people who interacted with the original Neanderthal population of Europe and finally replaced them completely. There is a long standing discussion over who ‘invented’ the stone tools, both human groups apparently having used similar styles of manufacture (Châtelperronian). Likewise, as regards the subsistence methods deployed by each; in one approach Neanderthals may have largely restricted their activities to roughly fixed ranges, whereas the incomers were generally seasonal nomads. As yet it has not been possible to show if the interbreeding between the two, which ancient and modern genetic data show, preceded the Aurignacian influx or continued when the met in Europe. Whatever, Neanderthals as a distinct human group had disappeared from the geological record by 40 ka. (Note that the three thousand years of coexistence is as long as the time between now and the end of the Bronze Age, about 150 generations at least.) But that aspect of European human development is not the only bone of contention about the spread into Europe. How did the Aurignacian people fare during and after their entry into Europe?

Despite continuing discovery of AMH sites in Europe, and reappraisal of long-known ones, there are limits to how much locations, dates, bones and artifacts can tell us. The actual Aurignacian dispersal of people across Europe is confounded by the limited number of proven occupation sites. These were people who, like most hunter gatherers, must have moved continually in response to variations in the supply of resources that depend on changing climatic conditions. They probably travelled ‘light’, occupied many temporary camp sites but few places to which they returned generation after generation. Temporary ‘stopping places’ are difficult to find, showing little more than evidence of fire and a ‘litter’ of shards from retouched stone tools (debitage), together with discarded bones that show marks left by butchery. A group of archaeologists and climate specialists from the University of Cologne, Germany have tried to shed some light on the completely ‘invisible’ aspects of Aurignacian dispersal and subsistence using what they have called – perhaps a tribute to Frank Sinatra! – the ‘Our Way Model’ (Shao, Y. et al. 2024. Reconstruction of human dispersal during Aurignacian on pan-European scale. Nature Communications, v. 15, Article 7406; DOI: 10.1038/s41467-024-51349-y. Click link to download a PDF).

The reality of hunter-gatherer life during a period of repeated rapid change in climate would clearly have been complex and sometimes precarious. To grasp it also needs to take account of human population dynamics as well as climatic and ecological drivers. The team’s basic strategy was to combine climate and archaeological data to model the degree to which human numbers may have fluctuated and the extent and direction of their migration. Three broad factors would have driven both: environmental change; culture – social change, curiosity, technology; and human biology. Really, environmental change is the only one that can be addressed with any degree of precision through records of climate change, such as Greenland ice cores. Archaeological data from known sites should provide some evidence for technological change, but only for two definite phases in Aurignacian culture (43-38 ka and 38-32 ka). Dating of   Aurignacian sites establishes some time calibration for episodes of occupation, abandonment and resettlement. Issues of human biology can be addressed to some extent from ancient genetics, where suitable bones are available. However, the ‘Our Way Model’ is driven by climate modelling and archaeology. It outputs an historical estimate of ‘human existence potential’ (HEP) that includes predictions of carbon storage in plants and animals – i.e.  potential food resources – expressed as regional population density in Europe. The technical details are complex, but Shao et al.’s conclusions are quite striking.

Maps of estimated anatomically modern human population density during the first six thousand years of Aurignacian migration and palaeoclimate record from the Greenland NGRIP ice core, with shaded warm episodes – red spots indicate the time of the population estimates above. (Credit: Shao et al. Fig. 1)

Climate change in the later stages of cooling towards the last glacial maximum at ~20 ka was cyclical, with ten Dansgaard-Oeschger cold stadial events capable of ‘knocking back’ both population density and the extent of settlement. In the first two millennia expansion from the Levant into the Balkans was slow. From 43 to 41 ka the pace quickened, taking the Aurignacian culture into Western Europe, with an estimate total European AMH population of perhaps 60 thousand. A third phase (41 to 39 ka) shrank the areas and densities of population during a prolonged cold period. The authors suggest that survival was in Alpine refuge areas that AMH people had occupied previously. Starting at around 38 ka, a lengthy climatic warm period allowed the culture to spread to its maximum extent reaching southern Britain and the north and east of the Iberian Peninsula. Perhaps by then the AMH population had evolved better strategies to adapt to increasing frigid conditions. But by that time the Neanderthals had disappeared from Europe freeing up territory and food resources. That too may have contributed to the expansion and the sustenance of an AMH total population of between 80 and 100 thousand during the second phase of the Aurignacian.

It’s as well to remember that this work is based on a model, albeit sophisticated, based on currently known data. Palaeoanthropology is extremely prone to surprises as field- and lab work progresses …

See also: New population model identifies phases of human dispersal across Europe. EurekaAlert, 4 September 2024; Kambani, K. 2024. The Dynamics of Early Human Dispersal Across Europe: A New Population Model. Anthropology.net, 4 September 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.

Ocean-floor sediments reveal the influence of Mars on long-term climate cycles

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In 1976 three scientists from Columbia and Brown (USA) and Cambridge (UK) Universities published a paper that revolutionised the study of ancient climates (Hays J.D., Imbrie J. and Shackleton N.J. 1976. Variations in the Earth’s Orbit: Pacemaker of the Ice Ages. Science, v. 194, p. 1121-1132;  DOI: 10.1126/science.194.4270.1121). Using variations in oxygen isotopes from foraminifera through two cores of sediments beneath the floor of the southern Indian Ocean they verified Milutin Milankovich’s hypothesis of astronomical controls over Earth’s climate. This centred on changes in Earth’s orbital parameters induced by gravitational effects from the motions of other planets: its orbit’s eccentricity, and the tilt and precession of its rotational axis. Analysis of the frequency of isotopic variations in the resulting time series yielded Milankovich’s predictions of ~100, 41 and 21 ka periodicities respectively. The time spanned by the cores was that of the last 500 ka of the Pleistocene and thus the last 5 glacial-interglacial cycles. Subsequently, the same astronomical climate forcing  has been detected  for various climate-induced changes in the earlier sedimentary record, including the glacial cycles of the Carboniferous and Neoproterozoic, Jurassic climate changes due to oceanic methane emissions and many other types of cyclicity during the Phanerozoic.

One hemisphere of Mars captured by ESA’s Mars Express. Credit: ESA / DLR / FU Berlin /

As well as time series based on isotopic and other geochemical changes in marine cores, other variables such as thickness of turbidite beds or cyclical repetitions of short rock sequences such as the ‘cyclothems’ of Carboniferous age (repetitions of a  limestone, sandstone, soil, coal sequence) have also been subject to frequency analysis. Sedimentary features that have not been tried are gaps or hiatuses in stratigraphic sequences where strata are missing from a deep-sea sequence. These signify erosion of sediment due to vigorous bottom currents in sequences otherwise dominated by continuous deposition under low-energy conditions. Three geoscientists from the University of Sydney, Australia and the Sorbonne University, France, have subjected records of gaps in Cenozoic sedimentation from 293 deep-sea drill cores to time-series analysis to discover what such ‘big data’ might reveal as regards climate fluctuations on the order of millions of years (Dutkiewicz, A., Boulila, S. & Müller, R.D. 2024. Deep-sea hiatus record reveals orbital pacing by 2.4 Myr eccentricity grand cycles. Nature Communications, v. 15, article 1998; DOI: 10.1038/s41467-024-46171-5).

In theory gravitational interrelationships between all the orbiting planets should have an effect on the orbital parameters of each other, and thus the amount of received solar radiation and changes in global climate. As well as the Milankovich effect, longer astronomical ‘grand cycles’ may therefore have been reflected somehow in Earth’s climatic history (Laskar, J. et al. 2004. A long-term numerical solution for the insolation quantities of the Earth. Astronomy & Astrophysics, v. 428, p. 261-285; DOI: 10.1051/0004-6361:20041335). Based on Laskar et al.’s calculations Adriana Dutkiewicz and colleagues sought evidence for two predicted ‘grand cycles’ that result from orbital interactions between Earth and Mars. These are a 2.4 Ma period in the eccentricity of Earth’s orbit and one of 1.2 Ma in the tilt of its axis.

The authors were able to detect cyclicity in the hiatus time series that is close to the 2.4 Ma Mars-induced waxing and waning of solar heating. Warming would increase mixing of ocean water through cyclones and hurricanes. That would then induce more energetic deep ocean currents and more erosion on the deep ocean floor: more gaps in sedimentation. Cooler conditions would ‘calm’ deep ocean currents so that deposition would outweigh evidence of erosion. The 1.2 Ma axial tilt cyclicity is not apparent in the data. Interestingly, the ~2.4 Ma cyclicity underwent a significant deviation at the Palaeocene-Eocene Boundary’ (56Ma), seemingly predicted by Laskar et al’s  astronomical solutions as a chaotic orbital transition between 56 and 53 Ma. Dutkiewicz et al. also chart the relations between the sedimentary-hiatus time series and major tectonic, oceanographic, and climatic changes during the Cenozoic Era, and found that terrestrial processes did disrupt the Mars-related orbital eccentricity cycles.

The findings suggest that long-term astronomical climate forcing needs to be borne in mind for better understanding the future response of the ocean to global warming. Also, if Mars had such an influence so must have Venus, which is more massive and closer. That remains to be investigated, and also the effects of the giant planets. In the very distant past there behaviour may have resulted in unimaginable astronomical changes. According to the bizarrely named Nice Model a back and forth shuffling of the Giant Planets was probably responsible for the Late Heavy Bombardment 4.1 to 3.8 billion years (Ga) ago. Such errant behaviour may even have triggered the flinging of some of the Sun’s original planetary complement out of the solar system and changed the outward order of the existing eight. Fortunately, the present planetary set-up seems to be stable …

See also: Dutkiewicz, A., & Müller, R. D. 2022. Deep-sea hiatuses track the vigor of Cenozoic ocean bottom currents. Geology, v. 50, p. 710–715; DOI: 10.1130/G49810.1; Mars drives deep-ocean circulation in Earth’s oceans, study suggests. Sci News, 13 March 2024.

The ‘Anthropocene Epoch’ bites the dust?

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The International Commission on Stratigraphy (ICS) issues guidance for the division of geological history that has evolved from the science’s original approach: that was based solely on what could be seen in the field. That included: variations in lithology and the law of superposition; unconformities that mark interruptions through deformation, erosion and renewed deposition; the fossil content of sediments and the law of faunal succession; and more modern means of division, such as geomagnetic changes detected in rock over time. That ‘traditional’ approach to relative time is now termed chronostratigraphy, which has evolved since the 19th century from the local to the global scale as geological research widened its approach. Subsequent development of various kinds of dating has made it possible to suggest the actual, absolute time in the past when various stratigraphic boundaries formed – geochronology. Understandably, both are limited by the incompleteness of the geological record – and the whims of individual geologists. For decades the ICS has been developing a combination of both approaches that directly correlates stratigraphic units and boundaries with accurate geochronological ages. This is revised periodically, the ICS having a detailed protocol for making changes.  You can view the Cenozoic section of the latest version of the International Chronostratigraphic Chart and the two systems of units below. If you are prepared to travel to a lot of very remote places you can see a monument – in some cases an actual Golden Spike – marking the agreed stratigraphic boundary at the ICS-designated type section for 80 of the 93 lower boundaries of every Stage/Age in the Phanerozoic Eon. Each is a sonorously named Global Boundary Stratotype Section and Point or GSSP (see: The Time Lords of Geology, April 2013). There are delegates to various subcommissions and working groups of the ICS from every continent, they are very busy and subject to a mass of regulations …

Chronostratigraphic Chart for the Cenozoic Era showing the 5 tiers of stratigraphic time division. The little golden spikes mark where a Global Boundary Stratotype Section and Point monument has been erected at the boundary’s type section.

On 11 May 2011, the Geological Society of London hosted a conference, co-sponsored by the British Geological Survey, to discuss evidence for the dawn of a new geological Epoch: the Anthropocene, supposedly marking the impact of humans on Earth processes. There has been ‘lively debate’ about whether or not such a designation should be adopted. An Epoch is at the 4th tier of the chronostratigraphic/geochronologic systems of division, such as the Holocene, Pleistocene, Pliocene and Miocene, let alone a whole host of such entities throughout the Phanerozoic, all of which represent many orders of magnitude longer spans of time and a vast range of geological events. No currently agreed Epoch lasted less than 11.7 thousand years (the Holocene) and all the others spanned 1 Ma to tens of Ma (averaged at 14.2 Ma). Indeed, even geological Ages (the 5th tier) span a range from hundreds of thousands to millions of years (averaged at 6 Ma). Use ‘Anthropocene’ in Search Earth-logs to read posts that I have written on this proposal since 2011, which outline the various arguments for and against it.

In the third week of May 2019 the 34-member Anthropocene Working Group (AWG) of the ICS convened to decide on when the Anthropocene actually started. The year 1952 was proposed – the date when long-lived radioactive plutonium first appears in sediments before the 1962 International Nuclear Test-Ban Treaty. Incidentally, the AWG proposed a GSSP for the base of the Anthropocene in a sediment core through sediments in the bed of Crawford Lake an hour’s drive west of Toronto, Canada.   After 1952 there are also clear signs that plastics, aluminium, artificial fertilisers, concrete and lead from petrol began to increase in sediments. The AWG accepted this start date (the Anthropocene ‘golden spike’) by a 29 to 5 vote, and passed it into the vertical ICS chain of decision making. This procedure reached a climax on Monday 4 March 2024, at a meeting of the international Subcommission on Quaternary Stratigraphy (SQS): part of the ICS. After a month-long voting period, the SQS announced a 12 to 4 decision to reject the proposal to formally declare the Anthropocene as a new Epoch. Normally, there can be no appeals for a losing vote taken at this level, although a similar proposal may be resubmitted for consideration after a 10 year ‘cooling off’ period. Despite the decisive vote, however, the chair of the SQS, palaeontologist Jan Zalasiewicz of the University of Leicester, UK, and one of the group’s vice-chairs, stratigrapher Martin Head of Brock University, Canada have called for it to be annulled, alleging procedural irregularities with the lengthy voting procedure.

Had the vote gone the other way, it would marked the end of the Holocene, the Epoch when humans moved from foraging to the spread of agriculture, then the ages of metals and ultimately civilisation and written history. Even the Quaternary Period seemed under threat: the 2.5 Ma through which the genus Homo emerged from the hominin line and evolvd. Yet a pro-Anthropocene vote would have faced two more, perhaps even more difficult hurdles: a ratification vote by the full ICS, and a final one in August 2024 at a forum of the International Union of Geological Sciences (IUGS), the overarching body that represents all aspects of geology.  

There can be little doubt that the variety and growth of human interferences in the natural world since the Industrial Revolution poses frightening threats to civilisation and economy. But what they constitute is really a cultural or anthropological issue, rather than one suited to geological debate. The term Anthropocene has become a matter of propaganda for all manner of environmental groups, with which I personally have no problem. My guess is that there will be a compromise. There seems no harm either way in designating the Anthropocene informally as a geological Event. It would be in suitably awesome company with the Permian and Cretaceous mass extinctions, the Great Oxygenation Event at the start of the Proterozoic, the Snowball Earth events and the Palaeocene–Eocene Thermal Maximum. And it would require neither special pleading nor annoying the majority of geologists. But I believe it needs another name. The assault on the outer Earth has not been inflicted by the vast majority of humans, but by a tiny minority who wield power for profit and relentless growth in production. The ‘Plutocracene’ might be more fitting. Other suggestions are welcome …

See also: Witze, A. 2024. Geologists reject the Anthropocene as Earth’s new epoch — after 15 years of debate. Nature, v. 627, News article; DOI: 10.1038/d41586-024-00675-8; Voosen, P. 2024. The Anthropocene is dead. Long live the Anthropocene. Science, v. 383, News article, 5 March 2024.