The starting point is the ocean’s distant past, which will root the stories that follow in the context of what happened over many millions of years, long before humans evolved. Keep that outlook in the back of your mind to help measure the changes unfolding in today’s ocean and grasp the magnitude of what is happening. For the centrepiece of each of the following chapters, I have picked living species and habitats that are responding in different ways to the Anthropocene. Many are familiar places and beloved animals, and I will dig deeper and uncover how their biology influences their chances of survival and dictates the course of action needed to help them thrive. We will meet species that are at the vanguard of a great modern remixing and conversion of the ocean, moving to new places and adapting to novel conditions, and that stand the greatest chances of surviving by themselves. We will also encounter a suite of species whose glory is already fading fast, the ones that will be the hardest to save and will require the most urgent action. These animals are frantically imperilled by the biggest threats to ocean life and could be the first to succumb in turn to heating seas, overfishing, and pollution. Among them are the polar animals that are disastrously sensitive to rising temperatures and cannot persist without the coldest, frozen seas; the species that grow sedately and to such grand old ages that their exploited populations cannot keep up and replace themselves fast enough; and the apex predators that balance precariously at the top of food chains, where their bodies soak up toxins from the contaminated seas. Together they pose a great challenge but not a hopeless one, of how to reverse their bad fortunes, and they serve as a warning of worse problems that need to be avoided elsewhere in the world.
Ocean wildlife has a tremendous capacity to regenerate and recover from dreadful losses and depletion. Often all that is needed is for the exploitation, destruction, and pollution to stop—a great deal of optimism about the ocean comes from this possibility. Even what seemed to be irredeemable catastrophes have been turned around, including some involving high-priced luxury fish and neglected areas of seabed.
Sometimes the recovery process needs a helping hand to kick-start natural cycles and restore balance. And some habitats face such big problems they will likely require far more deliberate steering and coaxing through the vicissitudes of the Anthropocene ocean. Scientists and conservationists are already working to engineer future-proof species and habitats that can withstand the continuing changes. Their ambitions generally are not to return the ocean to the way it was before humanity began inflicting so many troubles but rather to reach towards a resilient version of the ocean that is still healthy and abundant and, in key ways, different and new—not a rewinding that could ultimately fail but a rewilding that will last.
Together, ocean dwellers and their habitats have a lot to tell us about how the Anthropocene ocean is run through with inequalities. By no means is everything and everyone in the same boat. It is clear that some habitats and wild species—and people too—are already shaping up to be winners and some to be losers in the changing ocean. A critical part of a better future will be to find ways to undo inequalities and build an ocean that is just and fair and that benefits as many people and the greatest portion of nature as possible.
The future of the ocean matters to everyone. No matter where you live and what you do, even if you have never seen the sea in real life, you would not exist—none of us would—without the ocean. When astronomers examine other planets for signs of life, the key feature they search for is liquid water. Here on Earth, the ocean is where life began billions of years ago, and it is what has kept the planet alive ever since. Seawater covers seven-tenths of the planetary surface and plunges to many miles deep, and in its enormity absorbs huge amounts of heat beating down from the sun. Restless ocean currents swirl heat around the globe, preventing the tropics from scorching and supporting the mild mid-latitudes where many people live. The ocean is a rainmaker, providing most of the water that evaporates and falls on land. It generates weather systems and influences the climate we all experience in our daily lives.
The ocean makes the earth habitable not just by the physical presence of water but by the presence of life itself. Half the oxygen we breathe is made by multitudes of minute sun-fixing organisms, the phytoplankton that float through the seas. Phytoplankton also play their part in regulating the climate. They make clouds, which reflect the sun’s heat, by releasing particles into the atmosphere that cause water vapour to condense into droplets. Ocean life also mitigates the climate crisis by removing at least a quarter of humanity’s carbon emissions from the atmosphere. Phytoplankton absorb carbon dioxide and, when they die, create flurries of organic particles, called marine snow, which settle into the deep. A myriad of other life forms are critical in this drawdown of carbon, from underwater forests and meadows to giant whales and their plankton-fertilising faeces, and the trillions of glittering fish, called myctophids, that migrate each day between the sea surface and the shadowy twilight zone thousands of feet down.
The species and habitats discussed in this book are all vital parts of this life-giving system. They are sentinels of the rapid changes already underway in the Anthropocene ocean, and they forewarn of greater changes to come. In the coming pages, I will focus on what could happen in the next few years and decades, with the end of the century out there on the horizon. I picked this timeframe for a few reasons. On a practical level, 2100 is commonly the date at which scientific models and predictions aim, and so there are many studies that consider what the ocean will be like by then. We will all experience changes unfolding in the next years and decades, as will many of the people we know and care about. And if you think 2100 feels far off, remember that many children born today should still be alive to see the turn of the twenty-second century.
Crucially, the next few years are when decisions made will determine—one way or another—how the next phase of the Anthropocene ocean plays out for a considerable stretch of time to come. Right now, there are choices to be made, but the window of opportunity to meaningfully act is closing. Soon the ocean will reach a point when it heads down a certain path, and it will become harder and harder to change course.
Experts predict that carbon emissions must peak by 2025, at the latest, followed by sharp global reductions, if there is to be any chance of keeping global heating to within 1.5 degrees Celsiusd and avoiding the worst-case scenarios for people and the planet. While that target is looking less likely as the present decade wears on and emissions keep climbing, the Intergovernmental Panel on Climate Change (IPCC) has laid out a road map for how those emissions reductions could happen, underscoring the fact that the necessary technologies and solutions already exist. The most promising options are solar and wind energy, as well as preserving intact forests and other carbon-rich habitats, including in the seas.
Exploring the future of the ocean demands a fair amount of mental time travel, both backwards and forwards. The trick to navigating these waters will be to keep a sense of perspective (the ocean has always changed), to not go it alone (share these stories and ideas, talk about them so more people will know what is happening), and to make sure along the way to keep seeing the glory and feeling wonderment in the ocean. I hope this book will offer an antidote to the rising tide of eco-anxiety and fears for the future of the planet. Turn that fear into commitment and initiative. What matters most now is to not look away in anguish but to confront the problems and to know how bad things are and understand why they got this way, while at the same time wanting and hoping for the future ocean to be better.
a The 2015 Living Blue Planet Report, by the conservation organisation WWF, compiled data on the 5,829 populations of 1,234 mammal, bird, fish, and reptile species that live in the ocean, and found that between 1970 and 2012 their numbers had declined by 49 per cent.
b All these discoveries, from snoozing elephant seals to giant seagrass meadows, were made within roughly a half year, in late 2022 and early 2023.
c I am asked this almost as often as that other excellent question, “What is your favourite sea creature?”
d Equivalent to 2.7 degrees Fahrenheit.
PART ONE
OCEAN CONVERSION
Chapter 1
Ancient Seas
In a desk drawer, among all sorts of things I’m saving, is a small witness to the changing ocean. The thumb-size piece of slate is etched with narrow silver lines that are smooth along one edge and serrated on the other. I found it at the base of a crumbling cliff that was folded and twisted by an aeon of orogeny. The marks look like the script of some long-lost language pencilled on the rock. In fact, these are fossils of animals that lived 430 million years ago, or there-abouts, and I like them especially for the name they’ve been given: graptolites—from ancient Greek words graptos, meaning “marked with letters,” and lithos, meaning “stone.”
The impressions graptolites left behind hold messages that took palaeontologists a long time to decipher. Many assumed these markings were fossilised plants. Others realised they were animals but had differing views on which kind, variously labelling them as corals, hydroids, or the mossy-looking creatures known as bryozoans. Ultimately the matter was settled, and it was agreed that graptolites belong among an obscure group of ocean dwellers called ptero-branchs. Each sawtooth line on my stone was once a colony of tiny animals. They lived together inside a house of interconnected tubes, which they actively built around themselves, like a spider making its web. Their homes were the only parts of graptolites that fossilised, but these are enough to tell us what their colonies looked like and what kind of lives they led. Early on in their evolution, graptolites grew on the seabed, fixed to boulders or rooted in mud. Then in time, some varieties floated away and were among the first animals to become plankton. These abundant drifters colonised open seas all around the world and swiftly evolved into flurries of different-shaped species. Some graptolites were Y-shaped, some poker-straight or two-pronged like a tuning fork, and some grew into spirals that twirled up and down as they sifted floating microbes from the water.
Now, though, the open ocean is empty of graptolites. The last of the planktonic species went extinct around three hundred million years ago. That seemed to be the end of them all, until recently, when a group of tiny animals, the Rhabdopleurida, were deemed to be living graptolites. It wasn’t a new finding but a new interpretation of animals that scientists already knew about, microscopic, seafloor-bound colonies, which live all through the ocean, from polar seas to the tropics, from coastlines to a half mile down, and are the colour and translucence of amber. Only five living species have been found, a fragmentary recollection of the once-ubiquitous graptolites, the ghosts of a vanished world.
While contemplating the future of the ocean, it’s worth pausing to turn around and look back. The ocean’s backstory matters because it provides context for what’s happening now. It lets us see what the ocean has been like and how it changed in a prehuman world. Long before us, great tides of ocean dynasties have risen and fallen, and the seas have been both a cradle of evolution and an arena for extinction. Looking back offers a chance to compare humanity against other planetary life-shaping forces. We can search for clues as to what changes lie ahead, note the warnings against misleading assumptions, and seek solace in the fact that, one way or another, life in the ocean goes on.
To think about the past requires an obvious and dizzying shift in perspective, because ocean life stretches behind us for more time than our human brains can instinctively grasp. We must briefly let go of our customary horizons of hours and days, years and decades, centuries and, at a stretch, millennia. Think instead like palaeontologists, who have learned to find ancient moments trapped in stone, then to gather them up and piece together stories that take millions of years to be told. While we try not to get overwhelmed by the scale and intricacy of it all, we can pick out details that tell a wider story of the changing ocean. From there we can begin to sense the rhythm and pace of ocean life.
Trilobites look oddly familiar, as if a pill bug scuttled under a rock and then emerged on the other side much larger, more ornate, and more than a half billion years older. Were I a more skilled and patient fossil hunter, I might find trilobites lodged in rocks not far from my graptolite-embossed slate. There are fossil trilobites on continents across the world. These animals existed in the earliest of three great chapters of complex life on earth—the Palaeozoic, meaning “ancient life,” which was followed by the Mesozoic (“middle life”) and then the Cenozoic (“new life”). Trilobites in the Palaeozoic weren’t the first large animals to evolve, but they were undoubtedly trailblazers. They worried Charles Darwin because they seemed to confound his theory of gradual evolution via natural selection. Trilobites emerged far too quickly, too completely, and too long ago to fit his theory, as perfect creatures pressed into stone.
Trilobites evolved shortly after a three-billion-year prelude in which the only living things were single-celled microbes colonising the ocean, followed in the fullness of time by enigmatic wisps of simple, mostly jelly-based creatures that palaeontologists are still trying to make sense of. Then the Palaeozoic era opened with a dramatic twist in the history of life on earth. This era is divided into six periods; the first was the Cambrian, when evolution suddenly accelerated and ran at full tilt, churning out a mob of animals, including many that looked wildly different to anything alive today, from nozzle-nosed predators to luxuriantly spiky worms. The trigger for this flurry of life, known as the Cambrian explosion, is still a matter of debate. It may have had something to do with the fact that, for a long time leading up to it, the whole planet was frozen. As snowball Earth thawed, likely due to volcanoes spewing planet-heating carbon dioxide, the climate became more favourable for life to flourish. Rocks on land, as yet devoid of living things, began to erode and release nutrients into the ocean that organisms used to grow and build their skeletons, including enormous numbers of trilobites.
Darwin needn’t have agonised over trilobites. It was partly a matter of timing. He was quite right when he surmised that ancient seas must surely have been swarming with life, though in his day nobody had yet found any evidence for it. When Darwin was writing and thinking about evolution, most of the world’s oldest animal fossils remained unfound underground, including the extraordinary variety of Cambrian life in the Burgess Shale in Canada’s Rocky Mountains, which wasn’t uncovered until after his death.
A recent study of trilobite fossils has shed light on the timing of the Cambrian explosion and strengthened the idea that evolution can run at different speeds and has sometimes been breathtakingly fast (geologically speaking). A team based at the Natural History Museum in London used a large new collection of Cambrian trilobites to track how their appearance changed over time. The fossils went through an early, short burst of frantic innovation, showing that the Cambrian explosion may have truly gone off with a bang, lasting a brief twenty million years. Once the explosion died down, the rate of evolution among the trilobites levelled off and ticked steadily along.
By the Ordovician, the Palaeozoic period after the Cambrian, the ocean was brimming with trilobites. They ranged from flea-size swimmers to shovel-shaped diggers two and a half feet long, although most species were neatly pocket-size, measuring between one and three inches. Their basic anatomy was a head, thorax, and tail, with a ridged shell divided lengthwise into three sections, hence the name trilobite, and multiple pairs of legs underneath, like a centipede. These simple creatures were moulded and embellished into a phenomenal variety of forms. Many trilobites sprouted impressive spines and barbs, elegant quills, and devil horns. Some were smooth and rounded, like Bumastus, which looked just like an armadillo if you popped off its head and hid its tail. And like armadillos and pill bugs, most trilobites could curl up into a ball when they were scared.
From their fossilised remains, it’s possible to interpret the ways many trilobites lived their lives. Masses of them scurried across the seabed, leaving footprints as if they had walked over wet cement; these imprints were preserved by rapid burial in sediment and then slowly turned into stone. Fossil trails captured the details of a hunting foray: a line of worm tracks joined by those of a trilobite, and then the trilobite walking off by itself, worm presumably in belly. Cryptolithus evolved to be filter-feeding trilobites that stirred up the sediment by scrabbling the seabed with their forelegs, then straining suspended food particles through their perforated, colander-like heads. Others never set a foot down but chased after prey through the water, aided by hydrodynamic shells. Chunks bitten out of their shells show that trilobites were prey for other animals, such as the giant sea scorpions that also roamed the Palaeozoic seas. Planktonic trilobites floated in great midwater swarms, occupying a pelagic niche similar to the one that krill occupy today. In shallow tropical seas, trilobites were beetling around the world’s first true coral reefs, which had been built in the Ordovician by horn-and honeycomb-shaped corals. Some trilobites ventured between the tides and foraged on exposed tidal flats, but it seemed they never moved into rivers or lakes or made a permanent move onto land.
Uniquely among animals, trilobites’ eyes were made from crystals of the hard mineral calcite, which means they were often exquisitely preserved, and their shapes and arrangements tell us even more about these creatures’ lives. From their inception, Cambrian trilobites had complex, multifaceted eyes, similar in general form to the compound eyes of living insects and crustaceans. Some had eyes on long stalks, which scanned for prey while their bodies lay hidden in the mud. Erbenochile trilobites had columnar eyes that gave them almost 360-degree vision, each eye with a small, overhanging brow that shaded it in bright light. In deep waters of the twilight zone, where sunlight is dim, Cyclopyge trilobites soaked up rare photons with enormous eyes that occupied much of their heads, like the helmet eyes of dragonflies. Deeper still, trilobites evolved to be eyeless and blind, vision serving no purpose in the dark midnight zone.
Some trilobites resembled their nearest living relatives, the horseshoe crabs. Olenellus had a rounded, helmetlike head, rearward-pointing body spines, and a long prong for a tail. Not in fact crustaceans, the trilobites and horseshoe crabs are more closely aligned with spiders.
In all, more than twenty-five thousand species of trilobites are known, and more are constantly being found. (For comparison, there are roughly one thousand named dinosaurs.) They were fossilised in the millions, thanks in part to their tough exoskeletons. Trilobites periodically moulted their outer layer, growing new, bigger ones and tossing the casts into the fossil record, duplicating themselves and increasing the chances of being remembered through the passage of time.
The enormous diversity of species and ecology of trilobites show what very early ocean ecosystems were like, with habitats and food webs that are broadly recognisable in the contemporary ocean. More than five hundred million years ago, although the shape of the global ocean was very different, ocean ecology was already working, in many similar ways, as it does today.
Being so prolific and dotted around the planet, trilobites have also helped scientists reconstruct what the entire global ocean used to look like. For much of the Palaeozoic, the Northern Hemisphere was covered in the huge Panthalassic (meaning “all-sea”) Ocean, and the continents were located mostly in the Southern Hemisphere. The world was warm, with little ice locking up water, and many of the continents were flooded in shallow seas, each home to a unique assortment of trilobites that didn’t cross the deeper ocean in between. Later, when they were long dead and rockbound, fossil trilobites travelled around the planet, pushed by the forces of tectonic drift. Mapping the range of trilobites across modern-day continents is one way palaeontologists have worked out how landmasses moved and the ocean reshaped around them. Through much of the Palaeozoic, a supercontinent, Gondwana, was made up of many of today’s continents and subcontinents all clustered together, including Australia, Antarctica, Africa, India, and Madagascar. Today, their shared trilobite fauna is testament to that earlier convergence. For instance, the same tropical species of trilobites have been chipped out of rocks in western Newfoundland, in New York State, and in the Inner Hebrides archipelago in Scotland, showing these lands were all once part of the same ancient continent.
The stories of trilobites have much to tell us about what the ocean used to be like, and together they refute a wider misconception about evolution. Trilobites are proof that life has not simply been advancing from primitive towards ever more advanced forms. They show that since early times, some organisms have been remarkably specialised and sophisticated. And perhaps the most important message from the trilobites is that their early abundance and diversity weren’t enough to protect them from the changing ocean. Look all through the seas today, and not a single living trilobite is to be found.
After almost three hundred million years of scurrying and swimming, drifting, digging, and rolling up in balls, trilobites went extinct. They were among the species wiped out by the catastrophic Permian extinction event, which drew the Palaeozoic era to a close. This was the most devastating of the five ancient mass extinctions.a The cause was likely a spell of runaway global warming, triggered by immense volcanic eruptions, which filled the atmosphere with so much carbon dioxide the ocean was cooked, acidified, and sapped of oxygen until most aquatic life suffocated. That was the end of the trilobites, although in fact they had been in decline for much longer.
Trilobite diversity peaked in the late Cambrian and into the early Ordovician, and thereafter this group’s splendour had been fading away. For the rest of the Palaeozoic—through the Silurian, Devonian, Carboniferous, and finally Permian periods—trilobites had been relinquishing their dominance in the ocean. Steadily, their diversity diminished until a single family remained, containing a handful of species that were quite plain and small compared to their predecessors.
We have some clarity as to why trilobites were knocked back. For instance, at the end of the Ordovician, the supercontinent Gondwana drifted over the South Pole and became covered in giant ice sheets, pushing the earth deep into an ice age. Sea levels dropped, and when continental seas dried out, crowds of tropical trilobites lost their habitat and went extinct. Those species that happened to be better able to cope with the cold survived. What continues to mystify is why trilobites didn’t rebound once the ice age was over and conditions on the earth became more agreeable. New trilobite species were still evolving but not fast enough to replace the older species that were going extinct. No doubt the ocean filling up with new predators had an effect, including the first fish and squid, which were busy chasing after trilobites. Another suggestion is that trilobites weren’t very good at shedding their exoskeletons. Fossils show that many injured themselves trying to climb out of their old shells, emerging with damaged eyes and misshapen heads.
Nobody has yet found a convincing, single explanation for the trilobites’ long-term demise, which suggests it was likely a mix of changes and challenges emerging in their world. Whatever the ultimate causes were, from the end of the Ordovician onwards, trilobites suffered repeated setbacks from which they never fully recovered. Their former success did not predict their future survival.
The second great chapter in prehistoric life, the Mesozoic era, got underway around 250 million years ago in the aftermath of the mass extinction that devastated the earth’s biosphere. Trilobites were gone. Huge coral and sponge reefs were gone. So were sea scorpions and spiny sharks called acanthodians. Many other groups of organisms, while not entirely lost, were stripped back to a tiny portion of their former diversity and abundance. In all, fewer than one in ten species survived. For millions of years after the extinction, a disaster fauna, as palaeontologists refer to it, existed on the seabed, made up of species that were just holding on and by no means thriving. The situation was better up in the open water, where shoals of conodonts—eel-like, two-inch-long fish with bulging eyes and no jaws—proliferated. There were also spiral-shelled cephalopods called ammonoids, which looked similar to living chambered nautiluses, as well as an increasing diversity of bony fishes. These animals all became prey for a group of animals whose ancestors had left the ocean more than a hundred million years earlier and in the Mesozoic made a spectacular return to the sea.
Back in the Palaeozoic, a group of fishes had gradually adapted to life beyond the tideline. They already had four legs, which they used while still living at the shallow edges of the sea, and some of them walked out onto land and became the ancestors of all the land-living vertebrates alive today: the amphibians, reptiles, birds, and mammals. Collectively, these vertebrates are known as tetrapods, even the ones that later turned some of their four legs into wings or flippers—and reptiles did both.
By the time the Mesozoic was underway, the reptiles known as dinosaursb were famously ruling the land, and pterosaurs had taken to the skies. Meanwhile, the ocean was dominated by different groups of reptiles. Having lost their ancestral fishy gills, these animals drew gulps of air into their lungs and then leapt, slithered, and strutted back into the sea and very soon were well acclimatised to their revamped aquatic life. Within a few million years of the end-Permian mass extinction, reptilian apex predators were swimming through all the seas and making a major impression on the rest of ocean life.
