by Patrick Nunn
So the most parsimonious interpretation of these 21 groups of stories is that they developed independently of one another, arising from eyewitness observations – accumulated over perhaps several generations – of the rise of postglacial sea level across Australian shores. Given that this rise ceased around Australia some 7,000 years ago, these stories must have endured for at least this period of time, almost wholly as oral traditions passed on with an extraordinarily high degree of replication fidelity from one generation to another – on perhaps more than 350 occasions.89
The next chapter provides an account of how the sea level changed in the recent geological past all across our planet’s surface. At the end of that chapter, our knowledge of precisely where the sea level stood at particular times in the past is married to each of the 21 groups of stories recounted above, in order to determine – as far as will ever be possible – how long ago the observations that these stories are based on were made. This is an incredible story in its own right.
CHAPTER FOUR
The Changing Ocean Surface
The ocean covers almost three-quarters of our planet’s surface and our acquaintance with it varies. Some adults, typically those living in the centres of continents, have never seen the sea and perhaps frame their opinions of it from accounts that describe its extreme conditions – like massive waves washing across low, unprotected coasts and destroying everything and everyone foolish enough to be in their paths. Perhaps such people fear the sea unduly, knowing little of its more constant, largely benign states, which are so familiar to coastal dwellers, who also know well how the ocean surface changes. Indeed, its tidal pulse is so important in deciding the rhythms of coastal livelihoods, from fishing and foraging to trade and tourism, that it sometimes becomes information that no longer needs routine articulation. It is the unheard yet constant heartbeat of long-established coastal communities in every part of the world.
Many of us observe the daily ebb and flow of the tide – a manifestation of a short-lived yet widespread change in the ocean surface – across beaches, up the lower parts of rivers and up the sides of sea walls, without realising that such lunar modulations are in fact just one order of ocean-surface fluctuation. Yet there are innumerable higher order cycles that occur across decades, centuries and millennia. Some cycles can be detected only from millions of years of proxy observation.
Since most sea-level cycles (apart from the daily and monthly) are difficult to identify from casual observation, they remained hidden from science for a long time. Nineteenth-century geologists who believed that they had found evidence for past sea-level changes were often criticised for having mistaken these for land-level (tectonic) movements. It fell to Edmund Suess, who became Professor of Geology at the University of Vienna in 1861, to demonstrate the existence of former cyclical swings of sea level that had affected all parts of the world’s coastline. Prompted by his studies of Alpine glaciers, he was able to show that these had periodically advanced in the past, covering much wider areas than they do now, and then receded before once again advancing … and so on. Identifying four such ice ages, Suess correctly inferred that in order to build up extra ice on the land, water needed to be extracted from the oceans; conversely, when ice ages ended, the water from the melting ice was returned to the oceans, raising their levels. Thus colder periods of Earth’s history were times of comparatively low sea level, while warmer periods were times when the sea level was higher. Today we live in one of the latter periods – the Holocene Interglacial – a time much warmer (regardless of anthropogenic warming) than most of the time that modern humans have existed, and a time when the ocean surface is tens of metres higher than its average over the past few hundred thousand years. We live, in other words, in a drowned world, one that some of our ancestors 10 millennia ago might barely have recognised.
Modern humans – Homo sapiens – first appeared on Earth in the depths of tropical Africa slightly less than 200,000 years ago. They first encountered oceans about 150,000 thousand years ago.1 Since this book focuses on human observations of long-period changes in sea level, this is an appropriate point from which to commence a discussion of how and why the sea level has changed in the past. Figure 4.1 shows how the sea level has changed within the past 150,000 years, the tail-end of more than 50 climate-driven oscillations (linked to alternating ice ages and intervening interglacial periods), within the past 2.5 million years or so.2
Figure 4.1 Sea-level change over the past 150,000 years.
What we see on the left of Figure 4.1 is the sea-level rise marking the end of the Penultimate Glaciation (the ice age before the last one) that involved a comparatively rapid rise of sea level from about 140,000 to 130,000 years ago. This is typical of what happens at the end of ice ages – rapid warming causes most of the land-grounded ice to liquefy and pour into the ocean, causing its level to rise comparatively quickly. The same thing happened at the end of the last ice age, a time that considerably affected modern humans. From about 15,000 years ago until about 7,000 years ago, the sea surface rose rapidly, transforming coastal geographies in every part of the world and forcing people from the margins of the lands. What did these people do? In some places, it is suspected that their attempts to flee inland were resisted by people already occupying hinterlands, so the displaced people quickly taught themselves to build ocean-going vessels and set sail for the ocean horizon, prepared perhaps to find land or perish.3
The pattern of sea-level change – and the temperature changes that drove it – shown in Figure 4.1 is typical of most of the climate oscillations of the last few million years, in that cool periods begin comparatively slowly, often with a number of false starts when temperatures (and sea levels) rise for a few thousand years within the overall cooling period. Cooling typically takes far longer, almost 70,000 years in the case of the last ice age, to reach its coldest point, than does the subsequent warming – each of the last two deglaciations took around 10,000 years.
Consider also the amplitude of the sea-level changes shown in the figure. At the coldest time of the Last Glacial, the average global sea level was perhaps 120m (390ft) lower than it is today. Consider how that may have transformed any coastline you might be familiar with. Along continental coasts, for instance, the shoreline (where the land meets the sea) would typically have been far further seawards than it is today, maybe by thousands of kilometres. Places that are now islands offshore may then have been part of the mainland, freely accessible by people and animals. Where today coral reefs border shallow continental shelves and island platforms, during the coldest times of the last ice age such reefs were above sea level. They would have formed steep cliffs of limestone perhaps 100m (330ft) high, with only a narrow strip of coastal lowland along their bases. Geography was transformed by sea-level change.
A good example of the profound coastal transformation between the last ice age and today comes from north-west Europe, where the offshore islands now occupied by England, Ireland, Scotland and Wales were contiguous with mainland Europe during the last ice age. Where the English Channel (La Manche) now lies was a broad river valley (shown in Figure 5.1), draining westwards, across the floor of which moved people and any number of species of ice-age fauna, including the woolly mammoth, woolly rhinoceros, a straight-tusked elephant and the spectacularly antlered Irish elk (with an antler span of 3.5m/11½ft), the remains of which periodically show up at Neanderthal sites in the British Isles. One particularly rich cache of woolly rhinoceros and woolly mammoth bones occurs in the ravine of La Cotte de St Brelade on the island of Jersey in the English Channel, having accumulated there at a time when Jersey was merely an area of high ground rising above the surrounding riverine lowlands. One theory has it that herd(s) of mammoths and rhinos, which here date from at least 25,000 years ago, were periodically driven off the cliffs at La Cotte by people as part of a well-planned hunting strategy.4
Further north-east, where the North Sea Basin (between eastern England and the Low Countries of north-west Eur
ope) now lies, there was during the last ice age a land mass, named Doggerland today, the form of which has been convincingly reconstructed and the ways of life followed by its inhabitants plausibly imagined.5 Even before we knew that the sea level in the past had once been lower than it is today, we had an inkling from Doggerland that this must have been so. For centuries, commercial fishermen working the higher parts of Dogger Bank – the now-submerged island that was the last emergent vestige of Doggerland – pulled up animal (including mammoth) bones and human artefacts in their nets, wondering no doubt how these might have reached places on the ocean floor more than 100km (60 miles) from the nearest sizeable land mass.6 More recently perhaps than you might realise, a major discovery about one of Europe’s most ferocious ice-age predators was made here.
On 16 March 2000, the Dutch trawler UK33 was fishing in the North Sea about halfway between IJmuiden in the Netherlands and Lowestoft in England. In one of the nets the fishermen pulled up was part of an animal jawbone with teeth that appeared sufficiently unusual to merit scientific attention. It turned out to be part of a mandible of a sabre-toothed cat, one of the fiercest felids ever to roam northern hemisphere lands.7 While the discovery made headlines on account of its rarity, the real repercussions of this find came later, after its age was determined at around 28,000 years ago – almost 300,000 years later than the time it had previously been thought that the last sabre-toothed cat in Europe had died. The implications are that this tenacious predator merely retreated to warmer Mediterranean lands during the ice ages in Europe, then moved north once again as things warmed up, roaming coastal lowlands like Doggerland in search of prey animals sometimes many times its size.
The lower sea levels of the last ice age would have transformed the geography not only of continental margins but also of island worlds. At times of lower sea levels, islands in the world’s ocean basins would have been larger and more numerous than they are today. Several convincing models have been developed about faunal migration across the world’s oceans, demonstrating how animals used islands as stepping stones, something that would not be viable in today’s drowned world. It has even been proposed that ice-age humans crossed the entire Pacific Ocean – the world’s largest – from west to east to reach the Americas long before the better studied migrations via the Bering Strait began about 14,000 years ago.
A well-documented example of the effects of postglacial sea-level changes on island peoples comes from the Channel Islands off the coast of California, USA. They were first occupied by people who reached them by crossing the 7km (4½ mile) wide Santa Barbara Channel (which was narrower than it is today) about 13,000 years ago, when the postglacial rise of sea level in the area was well underway. While today there are four main islands in the group (San Miguel, Santa Rosa, Santa Cruz and Anacapa), at this time of lower sea level these were all one island – posthumously named Santarosae – that also incorporated a sizeable area of exposed insular shelf. Surrounded by pristine reefs, brimful with food, the sight of such an island offshore must have stimulated bolder inhabitants of the increasingly crowded mainland coast to take to their boats. Some evidently succeeded in making the crossing, but their descendants were ultimately disadvantaged by the move, for as sea levels continued rising so Santarosae became smaller and increasingly isolated from the mainland, eventually breaking up into the four constituent islands we see there today, reducing the livelihood options available for their inhabitants.
The long oscillatory fall of the sea level from the end of the Last Interglacial to the Last Glacial Maximum (see below) transformed the Earth’s climates and its coastal geographies. Yet it was the cooling that drove this sea-level fall that had more widespread effects, bringing lower temperatures to places where plants and animals were accustomed to warmer conditions. A glimpse into this cooling world is provided by the hypersaline Dead Sea, the surface of which lies more than 400m (1,310ft) below the ocean surface. Bounded on the east by the Jordan Plateau and to the west by the Judean Mountains, the Dead Sea is a terminus for water entering along the Jordan River. For this reason, the sediments accumulated on the floor of the Dead Sea over tens of thousands of years have the potential to tell us a lot about the changes in climate that have affected the region in the past.
We know that during the cooler periods of the past few million years – the ice ages – the region became much wetter, causing the Dead Sea to expand to accommodate double today’s rainfall. Conversely, during interglacial periods such as the one we live in today, the region was drier and the Dead Sea contracted. Examination of the lake-floor sediments has allowed insights into the shorter term changes in the lake level driven by precipitation changes across the region. Thus we learn from recent research that around 87,000–93,000 years ago there was a rapid drop in the lake level probably driven by the abrupt onset of some 6,000 years of drier, warmer conditions. Because this warm interval occurred within the overall cooling marking the early part of the last ice age, it is referred to as a stadial. The Dead Sea came close to drying up completely. Then the ice age resumed in earnest, the period between 75,000 and 87,000 years ago being one of conspicuously high lake levels, corresponding to a time of cooler conditions marked by ice-sheet build-up across much of the northern hemisphere. This period of cooler conditions within the overall cooling is known as an interstadial. Long interstadials punctuated by shorter stadials are characteristic of the long, slow cooling that marked Earth’s transition from the Last Interglacial to the Last Glacial Maximum.
At last, around 22,000 years ago, 70,000 years of climatic prevarication came to an end and the world was plunged into the coldest time of the last ice age – a time known as the Last Glacial Maximum (LGM). Lasting for perhaps only a few millennia, depending on where you were in the world, the LGM would inevitably have posed challenges to the way our ancestors had been living up to that point. Take those in Eurasia, from Ireland to Kamchatka, a region for which there are excellent data about the links between LGM climates and vegetation. The dominance of temperate forests in the region came to a comparatively swift end at the start of the LGM. Cold-adapted forests and tundra dominated during the LGM but were gradually pushed northwards after its end, being replaced in lower latitude areas by temperate forests.
The LGM was not only the coldest time within the past 150,000 years, but also one when the sea level was lowest – at an average of around 120m (400ft) below today’s levels. What this meant was that land masses were bigger than they are today, often with land connections where now there is ocean. This world was generally easier for terrestrial plants and animals to get around in, although due to the cold conditions many of those we are most familiar with today were often confined to refugia, places where environmental conditions were fortuitously configured to allow a particular species – or group of species – to survive.
Consider the thinhorn or dall sheep (Ovis dalli), a wild sheep inhabiting cooler parts of North America to which it is native. Study of thinhorn sheep genes shows that this species survived through the LGM in this region by occupying ice-free refugia. Research suggests that different subspecies of the sheep evolved at this time within different refugia, and that when the ice was gone they reunited and interbred.
It was not only ice that pushed animal and plant species into refugia, but also the lower sea level that transformed many coastlines, so that particular habitats temporarily disappeared in certain places, becoming re-established only after the end of the LGM when the sea level began rising. A good example is provided by tidal estuarine habitats along the coasts of California and Baja California (Mexico). Before the LGM (as is the case today) these habitats were occupied by several species of estuarine fish, but during the LGM, when the sea level in this part of the world dropped to 130m (430ft) below its present level, these habitats vanished – and the fish along with them. Similarly to the thinhorn sheep, the fish species survived in two refugia north and south of the area, 1,000km (625 miles) apart, coming back together only after the sea level ro
se and tidal estuarine habitats were restored to coastal California.
While we think of the LGM as the coldest time of the last ice age, it is also important to appreciate that cold did not affect every part of the Earth’s surface. While polar and temperate climate zones may have expanded towards the Equator, the effect of this was to reduce the size of tropical regions – concertinaing them, if you like – but they were still comparatively warm and hosted most tropical species of plant and animal that had been able to move there from higher latitudes. Yet in many mid-latitude regions (not Australia), the last ice age was marked by wetter climates that in many such places opened up new opportunities for living things, including humans.8
As the LGM drew to its close and Earth’s surface temperatures began to rise, living things may have drawn a collective sigh of relief that the millennia of hard times were finally at an end … and that a warming world would be more suffused with promise and opportunity. But this was not to be, for living things (as a rule) deplore environmental change, especially when it happens quickly. This was to be inevitable in the postglacial world, in which temperatures and sea levels not only rose comparatively rapidly, but also did so in bursts punctuated by times of stasis or even short-lived cooling and sea-level falls. It was not to be an easy road ahead for many species, and some of the conspicuous megafaunal extinctions (see Chapter 6) that occurred during the half-dozen millennia following the end of the LGM have been blamed on rapid changes of climate.9
The idea that rising temperatures in the aftermath of the LGM would have melted most of the massive continental ice sheets, with the meltwater flowing down rivers into the oceans, the surfaces of which would rise in response, is fine for conceptual and illustrative purposes – but not ultimately realistic. There are two main difficulties.
