Of all the threats looming over the planet today, one of the most alarming is the seemingly inexorable descent of the world’s oceans into ecological perdition. Over the last several decades, human activities have so altered the basic chemistry of the seas that they are now experiencing evolution in reverse: a return to the barren primeval waters of hundreds of millions of years ago.

A visitor to the oceans at the dawn of time would have found an underwater world that was mostly lifeless. Eventually, around 3.5 billion years ago, basic organisms began to emerge from the primordial ooze. This microbial soup of algae and bacteria needed little oxygen to survive. Worms, jellyfish, and toxic fireweed ruled the deep. In time, these simple organisms began to evolve into higher life forms, resulting in the wondrously rich diversity of fish, corals, whales, and other sea life one associates with the oceans today.

Yet that sea life is now in peril. Over the last 50 years -- a mere blink in geologic time -- humanity has come perilously close to reversing the almost miraculous biological abundance of the deep. Pollution, overfishing, the destruction of habitats, and climate change are emptying the oceans and enabling the lowest forms of life to regain their dominance. The oceanographer Jeremy Jackson calls it “the rise of slime”: the transformation of once complex oceanic ecosystems featuring intricate food webs with large animals into simplistic systems dominated by microbes, jellyfish, and disease. In effect, humans are eliminating the lions and tigers of the seas to make room for the cockroaches and rats.

The prospect of vanishing whales, polar bears, bluefin tuna, sea turtles, and wild coasts should be worrying enough on its own. But the disruption of entire ecosystems threatens our very survival, since it is the healthy functioning of these diverse systems that sustains life on earth. Destruction on this level will cost humans dearly in terms of food, jobs, health, and quality of life. It also violates the unspoken promise passed from one generation to the next of a better future.

LAYING WASTE

The oceans’ problems start with pollution, the most visible forms of which are the catastrophic spills from offshore oil and gas drilling or from tanker accidents. Yet as devastating as these events can be, especially locally, their overall contribution to marine pollution pales in comparison to the much less spectacular waste that finds its way to the seas through rivers, pipes, runoff, and the air. For example, trash -- plastic bags, bottles, cans, tiny plastic pellets used in manufacturing -- washes into coastal waters or gets discarded by ships large and small. This debris drifts out to sea, where it forms epic gyres of floating waste, such as the infamous Great Pacific Garbage Patch, which spans hundreds of miles across the North Pacific Ocean.

The most dangerous pollutants are chemicals. The seas are being poisoned by substances that are toxic, remain in the environment for a long time, travel great distances, accumulate in marine life, and move up the food chain. Among the worst culprits are heavy metals such as mercury, which is released into the atmosphere by the burning of coal and then rains down on the oceans, rivers, and lakes; mercury can also be found in medical waste.

Hundreds of new industrial chemicals enter the market each year, most of them untested. Of special concern are those known as persistent organic pollutants, which are commonly found in streams, rivers, coastal waters, and, increasingly, the open ocean. These chemicals build up slowly in the tissues of fish and shellfish and are transferred to the larger creatures that eat them. Studies by the U.S. Environmental Protection Agency have linked exposure to persistent organic pollutants to death, disease, and abnormalities in fish and other wildlife. These pervasive chemicals can also adversely affect the development of the brain, the neurologic system, and the reproductive system in humans.

Then there are the nutrients, which increasingly show up in coastal waters after being used as chemical fertilizers on farms, often far inland. All living things require nutrients; excessive amounts, however, wreak havoc on the natural environment. Fertilizer that makes its way into the water causes the explosive growth of algae. When these algae die and sink to the sea floor, their decomposition robs the water of the oxygen needed to support complex marine life. Some algal blooms also produce toxins that can kill fish and poison humans who consume seafood.

The result has been the emergence of what marine scientists call “dead zones” -- areas devoid of the ocean life people value most. The high concentration of nutrients flowing down the Mississippi River and emptying into the Gulf of Mexico has created a seasonal offshore dead zone larger than the state of New Jersey. An even larger dead zone -- the world’s biggest -- can be found in the Baltic Sea, which is comparable in size to California. The estuaries of China’s two greatest rivers, the Yangtze and the Yellow, have similarly lost their complex marine life. Since 2004, the total number of such aquatic wastelands worldwide has more than quadrupled, from 146 to over 600 today.

TEACH A MAN TO FISH -- THEN WHAT?

Another cause of the oceans’ decline is that humans are simply killing and eating too many fish. A frequently cited 2003 study in the journal Nature by the marine biologists Ransom Myers and Boris Worm found that the number of large fish -- both open-ocean species, such as tuna, swordfish, and marlin, and large groundfish, such as cod, halibut, and flounder -- had declined by 90 percent since 1950. The finding provoked controversy among some scientists and fishery managers. But subsequent studies have confirmed that fish populations have indeed fallen dramatically.

In fact, if one looks back further than 1950, the 90 percent figure turns out to be conservative. As historical ecologists have shown, we are far removed from the days when Christopher Columbus reported seeing large numbers of sea turtles migrating off the coast of the New World, when 15-foot sturgeon bursting with caviar leaped from the waters of the Chesapeake Bay, when George Washington’s Continental army could avoid starvation by feasting on swarms of shad swimming upriver to spawn, when dense oyster beds nearly blocked the mouth of the Hudson River, and when the early-twentieth-century American adventure writer Zane Grey marveled at the enormous swordfish, tuna, wahoo, and grouper he found in the Gulf of California.

Today, the human appetite has nearly wiped those populations out. It’s no wonder that stocks of large predator fish are rapidly dwindling when one considers the fact that one bluefin tuna can go for hundreds of thousands of dollars at market in Japan. High prices -- in January 2013, a 489-pound Pacific bluefin tuna sold for $1.7 million at auction in Tokyo -- make it profitable to employ airplanes and helicopters to scan the ocean for the fish that remain; against such technologies, marine animals don’t stand a chance.

Nor are big fish the only ones that are threatened. In area after area, once the long-lived predatory species, such as tuna and swordfish, disappear, fishing fleets move on to smaller, plankton-eating fish, such as sardines, anchovy, and herring. The overexploitation of smaller fish deprives the larger wild fish that remain of their food; aquatic mammals and sea birds, such as ospreys and eagles, also go hungry. Marine scientists refer to this sequential process as fishing down the food chain.

The problem is not just that we eat too much seafood; it’s also how we catch it. Modern industrial fishing fleets drag lines with thousands of hooks miles behind a vessel, and industrial trawlers on the high seas drop nets thousands of feet below the sea’s surface. In the process, many untargeted species, including sea turtles, dolphins, whales, and large sea birds (such as albatross) get accidentally captured or entangled. Millions of tons of unwanted sea life is killed or injured in commercial fishing operations each year; indeed, as much as a third of what fishermen pull out of the waters was never meant to be harvested. Some of the most destructive fisheries discard 80 to 90 percent of what they bring in. In the Gulf of Mexico, for example, for every pound of shrimp caught by a trawler, over three pounds of marine life is thrown away.

As the oceans decline and the demand for their products rises, marine and freshwater aquaculture may look like a tempting solution. After all, since we raise livestock on land for food, why not farm fish at sea? Fish farming is growing faster than any other form of food production, and today, the majority of commercially sold fish in the world and half of U.S. seafood imports come from aquaculture. Done right, fish farming can be environmentally acceptable. But the impact of aquaculture varies widely depending on the species raised, methods used, and location, and several factors make healthy and sustainable production difficult. Many farmed fish rely heavily on processed wild fish for food, which eliminates the fish-conservation benefits of aquaculture. Farmed fish can also escape into rivers and oceans and endanger wild populations by transmitting diseases or parasites or by competing with native species for feeding and spawning grounds. Open-net pens also pollute, sending fish waste, pesticides, antibiotics, uneaten food, diseases, and parasites flowing directly into the surrounding waters.

Tangled up in blue: an Antarctic fur seal caught in a fishing net, South Georgia Island. Yva Momatiuk & John Eastcott / Minden Pictures / Corbis

DESTROYING THE EARTH’S FINAL FRONTIER

Yet another factor driving the decline of the oceans is the destruction of the habitats that have allowed spectacular marine life to thrive for millennia. Residential and commercial development have laid waste to once-wild coastal areas. In particular, humans are eliminating coastal marshes, which serve as feeding grounds and nurseries for fish and other wildlife, filter out pollutants, and fortify coasts against storms and erosion.

Hidden from view but no less worrying is the wholesale destruction of deep-ocean habitats. For fishermen seeking ever more elusive prey, the depths of the seas have become the earth’s final frontier. There, submerged mountain chains called seamounts -- numbering in the tens of thousands and mostly uncharted -- have proved especially desirable targets. Some rise from the sea floor to heights approaching that of Mount Rainier, in Washington State. The steep slopes, ridges, and tops of seamounts in the South Pacific and elsewhere are home to a rich variety of marine life, including large pools of undiscovered species.

Today, fishing vessels drag huge nets outfitted with steel plates and heavy rollers across the sea floor and over underwater mountains, more than a mile deep, destroying everything in their path. As industrial trawlers bulldoze their way along, the surfaces of seamounts are reduced to sand, bare rock, and rubble. Deep cold-water corals, some older than the California redwoods, are being obliterated. In the process, an unknown number of species from these unique islands of biological diversity -- which might harbor new medicines or other important information -- are being driven extinct before humans even get a chance to study them.

Relatively new problems present additional challenges. Invasive species, such as lionfish, zebra mussels, and Pacific jellyfish, are disrupting coastal ecosystems and in some cases have caused the collapse of entire fisheries. Noise from sonar used by military systems and other sources can have devastating effects on whales, dolphins, and other marine life. Large vessels speeding through busy shipping lanes are also killing whales. Finally, melting Arctic ice creates new environmental hazards, as wildlife habitats disappear, mining becomes easier, and shipping routes expand.

IN HOT WATER

As if all this were not enough, scientists estimate that man-made climate change will drive the planet’s temperature up by between four and seven degrees Fahrenheit over the course of this century, making the oceans hotter. Sea levels are rising, storms are getting stronger, and the life cycles of plants and animals are being upended, changing migration patterns and causing other serious disruptions.

Global warming has already devastated coral reefs, and marine scientists now foresee the collapse of entire reef systems in the next few decades. Warmer waters drive out the tiny plants that corals feed on and depend on for their vivid coloration. Deprived of food, the corals starve to death, a process known as “bleaching.” At the same time, rising ocean temperatures promote disease in corals and other marine life. Nowhere are these complex interrelationships contributing to dying seas more than in fragile coral ecosystems.

The oceans have also become more acidic as carbon dioxide emitted into the atmosphere dissolves in the world’s water. The buildup of acid in ocean waters reduces the availability of calcium carbonate, a key building block for the skeletons and shells of corals, plankton, shellfish, and many other marine organisms. Just as trees make wood to grow tall and reach light, many sea creatures need hard shells to grow and also to guard against predators.

On top of all these problems, the most severe impact of the damage being done to the oceans by climate change and ocean acidification may be impossible to predict. The world’s seas support processes essential to life on earth. These include complex biological and physical systems, such as the nitrogen and carbon cycles; photosynthesis, which creates half of the oxygen that humans breathe and forms the base of the ocean’s biological productivity; and ocean circulation. Much of this activity takes place in the open ocean, where the sea and the atmosphere interact. Despite flashes of terror, such as the Indian Ocean earthquake and tsunami of 2004, the delicate balance of nature that sustains these systems has remained remarkably stable since well before the advent of human civilization.

But these complex processes both influence and respond to the earth’s climate, and scientists see certain recent developments as red flags possibly heralding an impending catastrophe. To take one example, tropical fish are increasingly migrating to the cooler waters of the Arctic and Southern oceans. Such changes may result in extinctions of fish species, threatening a critical food source especially in developing countries in the tropics. Or consider that satellite data show that warm surface waters are mixing less with cooler, deeper waters. This reduction in vertical mixing separates near-surface marine life from the nutrients below, ultimately driving down the population of phytoplankton, which is the foundation of the ocean’s food chain. Transformations in the open ocean could dramatically affect the earth’s climate and the complex processes that support life both on land and at sea. Scientists do not yet fully understand how all these processes work, but disregarding the warning signs could result in grave consequences.

A WAY FORWARD

Governments and societies have come to expect much less from the sea. The base lines of environmental quality, good governance, and personal responsibility have plummeted. This passive acceptance of the ongoing destruction of the seas is all the more shameful given how avoidable the process is. Many solutions exist, and some are relatively simple. For example, governments could create and expand protected marine areas, adopt and enforce stronger international rules to conserve biological diversity in the open ocean, and place a moratorium on the fishing of dwindling fish species, such as Pacific bluefin tuna. But solutions will also require broader changes in how societies approach energy, agriculture, and the management of natural resources. Countries will have to make substantial reductions in greenhouse gas emissions, transition to clean energy, eliminate the worst toxic chemicals, and end the massive nutrient pollution in watersheds.

These challenges may seem daunting, especially for countries focused on basic survival. But governments, international institutions, nongovernmental organizations, scholars, and businesses have the necessary experience and capacity to find answers to the oceans’ problems. And they have succeeded in the past, through innovative local initiatives on every continent, impressive scientific advances, tough environmental regulation and enforcement, and important international measures, such as the global ban on the dumping of nuclear waste in the oceans.

So long as pollution, overfishing, and ocean acidification remain concerns only for scientists, however, little will change for the good. Diplomats and national security experts, who understand the potential for conflict in an overheated world, should realize that climate change might soon become a matter of war and peace. Business leaders should understand better than most the direct links between healthy seas and healthy economies. And government officials, who are entrusted with the public’s well-being, must surely see the importance of clean air, land, and water.

The world faces a choice. We do not have to return to an oceanic Stone Age. Whether we can summon the political will and moral courage to restore the seas to health before it is too late is an open question. The challenge and the opportunity are there.