How Ocean Warming is Changing Our World

David Wallace-Wells

The longest-living vertebrate on Earth isn’t the giant tortoise, the blue whale, or the saltwater crocodile. It’s the Greenland shark, a creature that can live up to 300 years or more, thanks to the cold waters of the northern oceans. These sharks don’t even reach sexual maturity until they’re about 150 years old. This means that some of the sharks swimming today were born around the time of New England’s whaling peak in the 19th century. Since then, 90% of the largest ocean creatures by weight have vanished.

This isn’t just a story about warming seas. By the 1960s, during the peak of global whaling, around 80,000 whales were still being hunted each year, long after many species were nearly driven to extinction. Today, 90% of global marine fish stocks are either fully exploited or overfished, and 81% of monitored migratory freshwater populations have declined since 1970. Although humans only make up about 0.4 metric gigatons of the Earth’s mass, we are responsible for the loss of about 2.7 metric gigatons of marine life, according to physicist and oceanographer Helen Czerski in her book “The Blue Machine.”

The warming of the oceans is a staggering phenomenon. Over 90% of the excess heat trapped by the greenhouse effect is absorbed by the oceans. While this might seem like a good thing for life on land, it means that the oceans have absorbed nine times more global warming than the land.

This is a big problem for what Czerski calls the “blue machine,” an enormous engine that drives and distributes heat, energy, life, and nutrients around the globe. This system keeps our climate relatively stable. However, shifts in ocean temperatures, especially in the tropical Pacific, have caused some of the greatest ecological disasters in modern history. For example, poorly managed El Niño events in the late 19th century may have led to the deaths of 50 million people, a number comparable to 320 million deaths today.

The oceans cover 70% of the Earth’s surface. The Pacific Ocean alone is so vast that if you look at a globe from the right angle, you can barely see any land. Susan Casey, in her book “Underworld,” notes that the Pacific could swallow every landmass and still have room for another South America. The oceans dominate even more when you consider life by volume.

The oceans are also expanding, not just because of melting ice but also due to “thermal expansion.” Heat causes water to expand, contributing to at least one-third of all sea-level rise.

In January, over 40% of the world’s oceans were experiencing marine heat waves. By the end of the century, these heat waves could become permanent in many parts of the world’s seas. Coral reefs, which support a quarter of all marine life, are particularly vulnerable. Even with rapid decarbonization, it’s likely that nearly all coral reefs will die, affecting up to a billion people who rely on them for food and other benefits.

A recent El Niño event has pushed sea-surface temperatures to record highs for over a year. Scientists are particularly concerned about the Atlantic, where hurricane activity is expected to reach historic levels. Some ocean scientists talk about “regime shifts,” while others warn of impending chaos. As Marina Koren of The Atlantic puts it, “The oceans we knew are already gone.”

Last year, a study examined the risk of the collapse of the Atlantic Meridional Overturning Circulation (A.M.O.C.), a system that transfers heat from the tropics to Europe and Greenland. If this system collapses, parts of Western Europe could become 15 degrees Celsius colder. The study suggested that this tipping point could occur between 2025 and 2095, with a central estimate of mid-century.

While some scientists questioned the study’s methodology, the episode highlighted the catastrophic risks of continued warming. Stefan Rahmstorf, a scientist, noted that we should aim to be 100% sure that the A.M.O.C. won’t collapse, but new research has increased the estimated risk.

These potential changes would be catastrophic, but for now, the alarm is often muted. Olive Heffernan, in her book “The High Seas: Greed, Power and the Battle for the Unclaimed Ocean,” points out that the high seas are a remote realm, far from most people’s minds. These waters, which make up two-thirds of the planet’s oceans, are largely ungoverned. Even after a landmark U.N. treaty, only 1% of the world’s oceans are protected, with a goal to increase this to 30%. The high seas, with average depths of nearly 13,000 feet, make up 95% of the planet’s living space, a vast area where we often dump pollution and neglect our responsibilities.