How ocean warming is warping the world

David Wallace-Wells

The world’s longest-living vertebrate is not the friendly giant tortoise, the breathtaking blue whale or the saltwater crocodile, which can terrorize the imagination of toddlers and centenarians alike. It’s the shuddersome, floppy Greenland shark, which can live to 300, perhaps even longer, its life span slowed and distended by the deep cold of the northern oceans. Greenland sharks do not even reach sexual maturity until about age 150, which means that today there are, swimming slowly through the waters of the far North Atlantic, the equivalent of preteenagers born not long after the 19th-century heyday of New England whaling, as the Industrial Revolution was just metastasizing beyond the Anglosphere. Since then, measured by weight, 90 percent of the largest creatures sharing the oceans with them have disappeared.

This is not just a parable about the warming of the seas. By the global peak of whaling, in the 1960s, roughly 80,000 whales were still being harvested for their meat each year, more than a half-century after the bowhead, right and gray whales were brought close to extinction for their blubber and oil. Ninety percent of global marine fish stocks have now been fully exploited or overfished; 81 percent of monitored migratory freshwater populations have declined since 1970. And although the total mass of humans on earth is only about 0.4 metric gigatons, the physicist and oceanographer Helen Czerski writes in her hypnotic tribute “The Blue Machine,” we are collectively responsible for about 2.7 metric gigatons of life going missing from the seas which are, after all, the only known oceans of water anywhere in the universe and the primal source of all known biology.

But the story of that warming is nevertheless astonishing, even for those of us anesthetized by exposure to the world’s rapid ecological transformation. More than 90 percent of all the excess heat trapped in the atmosphere by the greenhouse effect goes into the oceans, and while climate-conscious humans may regard this as a lucky break for life on land, the math implies a different and less narcissistic emphasis: that the planet’s water, home also to a majority of its life, has absorbed nine times as much global warming as the world above the surface we know so well and worry over so much.

This is a problem for the blue machine “an engine the size of a planet,” Czerski writes, driving and distributing unimaginable scales of heat and energy, life and nutrients, around the globe, while also keeping the whole climate system (and the human civilization built on it) relatively stable. Most of the time, that is: Many of modern history’s greatest ecological disasters were produced by the flickering of ocean temperatures in the tropical Pacific as it shifted between El Niño and La Niña years, most significantly toward the end of the 19th century. A string of fallow El Niño harvests were so poorly managed by out-of-touch governments that they may have killed 50 million people (a share of the global population comparable to 320 million deaths today) and were later called, by the radical environmentalist Mike Davis, “late Victorian Holocausts.” This, mind you, is the “preindustrial” period we now use as a climate base line, against which are marked the perturbations of warming.

Famously, the oceans occupy 70 percent of the earth’s surface, with the Pacific alone so vast that if you consider a classroom globe from the right angle, you can see only the thinnest slivers of land. “The Pacific alone could swallow every landmass, every continent and island, and still have room for another South America,” Susan Casey writes in “Underworld,” her tour of the “shadow kingdom” of the deep seas and the 80 percent of the ocean floor whose details remain unmapped. When you look below the surface to consider life on the planet by volume, the oceans dominate even more.

The vastness is also growing not just because of melting Arctic and Antarctic ice, which could raise global sea levels by several feet this century and many more in the millenniums to come, but also because of what is known as “thermal expansion.” Heat expands the volume of water too and to date is responsible for at least one-third of all sea-level rise.

Last January, more than 40 percent of the planet’s oceans were experiencing marine heat waves, and by the end of the century, given continued warming, those heat waves could be permanent in much of the world’s seas. In shallow waters, coral reefs endured the temperatures of hot tubs, prompting the creation of three new levels of risk above what had been the highest level on the coral-bleaching scale. In a few decades, even in a rapidly decarbonizing world, it is considered likely that bleaching will kill nearly all the ocean’s coral reefs, which support a quarter of all marine life and provide food and other benefits to as many as one billion people.

A now-subsiding El Niño in the Pacific has helped push sea-surface temperatures above previous records for more than a year straight, with temperatures in the waters of the northern hemisphere so freakishly high they have been described as nine-standard-deviation anomalies. If you have read about scientists expressing alarmed confusion about recent records in global surface temperatures, they are generally more perplexed about what’s happening in the Atlantic, where hurricane activity is predicted to reach historic levels this summer and fall. Many ocean scientists now talk openly of “regime shifts”; others warn, “Expect chaos.” The Atlantic’s Marina Koren has put it more elegiacally: “The oceans we knew are already gone.”

Last year, an especially disquieting study examined the risk of the most famous of those possible shifts, the possible collapse of the large ocean system that transfers heat from the tropics up toward Europe and Greenland called the Atlantic Meridional Overturning Circulation, or A.M.O.C. This conveyor is what makes Western Europe comfortably warm, among other planet-stabilizing effects; with a total cessation, average temperatures in parts of the continent might be 15 degrees Celsius colder than they are today. The paper projected the system would reach a tipping point somewhere between 2025 and 2095, with a central estimate of midcentury, just a few decades from now.

Plenty of other scientists rushed to put the study in more reassuring context, calling into question its methodology and pointing to a whole literature of more stable projections. But the ultimate lesson of the episode was, for me, less a matter of outlier science or hysterical media than the catastrophic tail risks of continued warming. “This is not about being 100 percent or even just 50 percent sure that the A.M.O.C. will pass its tipping point this century,” the scientist Stefan Rahmstorf wrote in April in the journal Oceanography. “The issue is that we’d like to be 100 percent sure that it won’t.”

Unfortunately, the new research “greatly” elevated his estimate of that risk once understood to be effectively zero and revised upward more recently to less than 10 percent. In February, another paper suggested that the A.M.O.C. was on a “tipping course,” and ongoing research suggests other ocean-circulation systems may be facing a one-in-three risk of collapse this century perhaps higher.

These would be catastrophic transformations. But for now, we typically hear the alarm only when we strain our ears. “For most, the high seas are a remote realm, far offshore, that we have neither the chance, nor the desire, to visit,” writes Olive Heffernan, the founding editor of the journal Nature Climate Change, in her gripping book “The High Seas: Greed, Power and the Battle for the Unclaimed Ocean.”

“The high seas” is not a poetic term but a legal one, Heffernan reminds us, one that refers to “wild west” waters beyond the close reach of shore up to two-thirds of the planet’s oceans, which remain ungoverned by any sovereign system of law. Even after the landmark U.N. treaty signed last year, barely 1 percent of the world’s oceans is protected, a share the U.N. hopes to raise to 30 percent. The rest remains mostly out of sight and out of mind. With average depths of nearly 13,000 feet, Heffernan calculates, the high seas make up 95 percent of the planet’s total available living space a vast and distant elsewhere, into which we can casually dump not just pollution and heat but our sense of responsibility, as well.