The Global Ocean

The consequences of our taking resources from the sea were once limited to local scales. Today, exploitation, depletion, and loss affect us all.

Long ago, the ocean was a barrier to human movement, a place where the land ended and further progress was impossible. But it was also a provider of nourishment, so we lived by the shore and learned the ways of the sea. Some of the earliest traces of modern humans are from coastal caves in South Africa. Shell remains dating back 164,000 years show that seafood was often on the menu.

Over time, we figured out ways to catch fish and other more elusive marine prey, using traps, hook and line, and nets. The emergence of people from Africa around 70,000 years ago appears to have been propelled by a bundle of maritime skills and technologies, enabling an almost explosive spread of people across the world. The ocean became our path for dispersal and settlement. Within 10,000 years people occupied Asia and Indonesia, and then moved onward to Australia 10,000 years later. They leapfrogged to the Americas perhaps by following plentiful seafood on a kelp forest highway around the Pacific rim.

Oral stories that evolved into Homeric literature; frescos, painted pottery sherds; and even wooden wrecks—remarkably preserved in the anoxic, deep Black Sea—show great advances in seafaring by the time of ancient Greece. The sea had become a route to new discoveries, fame, and fortune. By the end of the first millennium, long-distance voyaging had emerged in cultures of the Pacific, Asia, and Middle East. Our worldview expanded as the ocean and new landmasses were mapped through the great Renaissance Age of Exploration.

Exploiters of natural resources followed swiftly on the heels of explorers, drawn by tales of extraordinary wildlife abundance in faraway places. Whales, walrus, seals, seabirds, and fish became targets of industrial scale exploitation for meat, oil, ivory, feathers, fur, and myriad other useful products derived from their carcasses (seal whiskers made excellent pipe cleaners, for example). The slaughter was so intense that seal haulouts numbering millions of animals disappeared within the space of a few decades, and whole regions of ocean were emptied of their whales. Few places were too remote or experienced conditions too savage to save them. By the 17th century whalers had established themselves in the Arctic, and by the early 19th century, whalers and seal hunters were attacking the fringes of Antarctica.

The impacts of such concentrated plunder were soon visible in falling catches and unprofitable voyages. A belief widely held in the 19th century was that, if there was such a thing as overfishing or hunting overkill (a point much debated) it would be self-limiting. Falling profits would cause people to abandon the pursuit and populations would recover. It didn’t work out that way. By devising ever better ways to catch animals, technological innovation constantly tilted the odds in favor of continued exploitation even as populations plummeted. And as the favored species became scarce we switched targets to others, picking off the original species whenever they were encountered. Less valuable quarry subsidized continued capture of now rare species.

Exploration and discovery led to new opportunities for trade and travel. By the early 20th century, the ocean was crisscrossed by shipping highways that connected the world, including new routes punched through the Suez and Panama canals. World trade grew in lockstep with economic growth. Today, ocean-going container ships carry 90% of internationally traded goods and have reached such monumental sizes you could fit 15 Titanics into their volume.

The ocean’s role as a connector in human affairs is mirrored in its influence on the planetary environment. The greatest river in the world does not pour across any continent but runs between and around them. The global ocean conveyor is a giant current that carries the water of 20,000 Niagara Falls as it descends from the surface to the deep Atlantic, before looping the globe on a thousand-year journey that takes in the whole ocean. The surface arm of this great current is driven by winds whipped up by temperature differences between tropics and polar regions. The atmosphere and ocean moderate the climate by transporting heat poleward and cold toward the tropics. If these winds did not exist, the tropics would be so torrid as to be uninhabitable and the poles far colder.

The climate-controlling role of the sea has long been appreciated. Heat carried by the Gulf Stream leg of this circumglobal current warms the winter streets of London and Paris. At a much bigger scale, the ocean is a planetary air conditioner without which Death Valley temperatures would already be the norm almost everywhere. Since the onset of the Industrial Revolution, the ocean has absorbed 93% of the excess heat trapped by our greenhouse gas emissions. It’s hard to grasp what that means, but here is a telling analogy: The rate of heat trapped by the ocean since 1871 was equivalent to the underwater detonation of one and a half atomic bombs—of the sized dropped on Hiroshima—every second. Over the past two decades, the rate of heat capture has risen to three to six bombs per second.

Global warming affects everything. It powers stronger winds as there is more heat to redistribute from tropics to poles, which means larger and more powerful waves. Wave energy has increased by 31% since 1960, and the tallest waves have increased in height by more than 1 foot in just over 30 years. Such waves compound the threat from sea level rise, beating harder on our shores and now regularly inundating low-lying coral islands of the western Pacific.

There is another dimension to the ocean’s climate pacifying ability. Fish and whales move carbon from the surface to the deep sea, locking it out of harm’s way and slowing climate change. The biggest players in this ocean carbon shuttle are billions upon billions of tiny fish smaller than the palm of your hand. They live in the twilight zone, between 660 feet and 3,300 feet down, at night migrating toward the surface to feed and by day retreating to the depths, where they excrete surface carbon: eat shallow, poop deep. It is estimated that without them there would be 50% more carbon dioxide in the atmosphere and the planet would be oppressively hot. These fish are precious but the fishing industry, running out of other options, views them as a cheap feed for burgeoning aquaculture. Cheap they are not when it comes to their value to keeping our world habitable.

The global ocean conveyor current also ventilates the deep sea, carrying oxygen from the polar surface on its descent. The intense polar chill maximizes the oxygen payload because cold water carries more oxygen than warm. That temperature dependence also means that global warming is hindering the ocean’s ability to hold oxygen, with some unexpected consequences. Lower oxygen slows the metabolism of fish and other ocean creatures, so they grow more slowly to smaller sizes. Since the number of eggs a fish spawns depends on how big it is, populations in a future warmer world may struggle to replenish themselves, hitting fisheries production just when the expanding human population needs more food.

That the ocean is a great connector is starkly visible in other ways. The beaches of the farthest reaches of the planet are piled high with plastics, and plastic particles are embedded in the bodies of even the world’s deepest inhabitants. In 2020 a new species of giant amphipod crustacean was discovered 22,000 feet down in the Mariana Trench with plastic in its gut. To mark this grim milestone, scientists named it Eurythenes plasticus.

Countries still look upon the ocean as a source of wealth, despite growing signs that its resources are in trouble. The high seas fishing fleet has grown rapidly in recent decades—China alone has launched 17,000 vessels—taking advantage of less exploited waters in weakly regulated international space. Fueled by national subsidies and, in some cases, geopolitical ambition, these fleets are still stripping the world of its ocean megafauna, coercing poorer nations into selling their fish too cheaply, and stealing from the waters of others. Meanwhile, the deep ocean may soon be carved up by mining corporations for the benefit of a few at a cost that will be paid by the whole world. Deep sea scientists reckon that removal of polymetallic nodules from the deep seabed with gargantuan underwater excavators will, at the very least, obliterate the fauna over thousands of square miles. Worse, it will probably cause the extinction of tens or hundreds of species and would release vast quantities of carbon that could accelerate global change. The island nation of Nauru, its surface long ago laid waste by phosphate miners, now wants to extend mining into surrounding seas. The country has asked the International Seabed Authority to authorize a deep-sea mining operation that has no prospect of containing damage whose magnitude we can only guess at. Experimental mining trenches dug 13,000 feet down in the deep eastern Pacific in 1989 and revisited 26 years later looked as fresh as if they were dug only months before. Life moves slowly at these extreme depths and recovery from large-scale mining is probably impossible on meaningful human timescales.

And so we find ourselves at a crossroads in human history. For most of our existence, the consequences of our resource use have been limited to local scales. Today, the impacts of resource exploitation and depletion affect us all. What one nation does in its own interests has consequences that spill far beyond its borders. If deep-sea mining goes ahead, its scars will last for thousands of years and its effects will spread with the global ocean conveyor current to every corner of the globe. Never has there been a greater need for the world to act as one, governing itself in the interests of all its citizens and all life on Earth.

The clear message from the ocean, and from that other global common space, the atmosphere, is that a new form of world governance is needed to rein in national self-interest in favor of collective action to safeguard our shared living space. Our struggles to regulate greenhouse gases to address climate change show we are still far off this goal. Some psychologists contend that evolution has wired our psyche for selfish tribalism, handicapping efforts to manage human affairs at the global scale. But those other great attributes of human success—intelligence, adaptability, and cooperation—make me optimistic that collective interest will eventually prevail. How long we take to get there will dictate the shape of the future for countless generations to come.

The Intergovernmental Panel on Climate Change has acknowledged in recent reports that climate change is no longer avoidable; we are in the thick of it. Some impacts are irreversible on meaningful human timescales, like sea level rise or ocean acidification from carbon dioxide dissolution. Regardless of what we do now, a rise of 6 or even 10 feet in sea level is locked in based on emissions already in the atmosphere. For some, this represents local inconvenience, for others, such as low-lying coral nations like the Maldives, it could wipe them out forever.

As some look to the stars in search of escape from earthly destruction, we should instead look deep into the sea. The ocean is the beating heart of planet Earth, which so far as we know, is the only habitable place in the universe. Although there are many reasons for concern, there is also hope. Ocean life is proving resilient and can bounce back given the right protection, as we have seen again and again when endangered species were protected from hunting and fishing, and in the resurgence of life inside newly created marine protected areas. The legacy of industrial hunting, for example, can be undone in under a century. Humpback whales off Australia are increasing at the theoretical maximum rate, while elephant seals off the west coast of North America have rebounded from a low of 100 animals early in the 20th century to over 100,000 today. Efforts to restore lost and damaged habitats like seagrass, salt marsh, and mangrove forest are growing rapidly in number and scale, in part through recognition that they are worth more to us as intact, biologically rich and vibrant habitats, than reclaimed land or prawn ponds. Fisheries have recovered in the U.S. and other countries by the simple expedient of following scientific advice, rather than awarding excessive quotas for short-term political or economic gain.

It is nature that keeps Earth alive and habitable. In the past, we took for granted that this natural life support system would always ensure our own well-being. But as human agency to reshape Earth expanded, nature retreated and that assurance has crumbled. Ambitious new protected area targets at last recognize our complete dependence on the living world. They could see protection expand to 30% of Earth’s surface, above and below water, by 2030, and perhaps secure half of Earth for nature by 2050. This new mindset is pushing out the old orthodoxy that conservation is a luxury affordable only to the rich. There is only one ocean, one Earth, and we are the guardians of the future.

Callum Roberts is professor of marine conservation at the University of Exeter, a 2000 Pew marine fellow, and the author of The Unnatural History of the Sea, The Ocean of Life, and Reef Life: An Underwater Memoir.