The Oceans Have Already Changed

Warmer seas absorb less oxygen.
That sounds bad.

“Our oceans, on the grandest scale, can take a lot—but we can’t.”

Marina Koren in the Atlantic:

Even after nearly three months of winter, the oceans of the Northern Hemisphere are disturbingly warm. Last summer’s unprecedented temperatures—remember the “hot tub” waters off the coast of Florida?—have simmered down to a sea-surface average around 68 degrees Fahrenheit in the North Atlantic, but even that is unprecedented for this time of year. The alarming trend stretches around the world: 41 percent of the global ocean experienced heat waves in January. The temperatures are also part of a decades-long hot streak in the oceans. “What we used to consider extreme is no longer an extreme today,” Dillon Amaya, a research scientist at the National Oceanic and Atmospheric Administration’s Physical Sciences Laboratory, told me.

The situation is expected to worsen. Research suggests that by the end of the century, much of the ocean could be in a permanent heat wave relative to historical thresholds, depending on the quantity of greenhouse gases that humans emit. Many other changes will unfold alongside those hot ocean temperatures: stronger hurricanes, rising sea levels, unmanageable conditions for marine life. Our seas, in other words, will be altered within decades.

Many detailed climate projections focus on the state of the oceans by 2100, a short time frame that allows for relative certainty. “That’s what policy makers want to know about,” Sandra Kirtland Turner, a paleoceanography professor at UC Riverside, told me. It’s also a year in which many people being born today will still be living, witnessing the consequences of what we’re doing currently. But Earth has many, many millennia ahead of it, and that deep future is being shaped by the burning of fossil fuels happening right now. If we continue down the path we’re on, Earth’s oceans may be irrevocably transformed over the next several hundred years. Imagine yourself in space, hovering over the planet as an astronaut would, a few centuries from now. “The ocean will still be blue and beautiful,” Amaya said. But even from space, you’d know something was different. And the closer you got to the waves, the more clearly you’d see how things went awry.

Right away, you’d notice unfamiliar water in Earth’s polar regions—“huge swaths of ocean that you wouldn’t otherwise have seen in the past, because they would have been underneath sea ice,” Amaya said. Greenland and Antarctica have been steadily losing ice for decades, and even thus far, “the changes we’ve seen are more pronounced than any we had projected,” Fiamma Straneo, a climate professor at the Scripps Institution of Oceanography, told me. If global warming reaches and stays in the range of 2 to 3 degrees Celsius above preindustrial norms, the West Antarctic ice sheet could “be lost almost completely and irreversibly” over the next several millennia, according to a recent report by an Intergovernmental Panel on Climate Change (IPCC) working group.

You could also discern, with the help of Earth-observing satellites, that the way the ocean moves has changed. Warmer temperatures and melting freshwater ice may have already weakened the conveyer-belt system of currents in the Atlantic that carries warm water north and cold water south, which is important for spreading nutrients to marine ecosystems and regulating temperatures in Europe. The potential collapse of this system, known as the Atlantic Meridional Overturning Circulation (AMOC for short), is one of the major points of no return for Earth’s climate, but experts are unsure exactly when it could happen. A 2019 IPCC reporton the future of the oceans predicted that, if high emissions continue apace, an AMOC collapse would be a toss-up by 2300; a more recent study suggests that AMOC could fall off a cliff much sooner.

Closer to Earth’s surface, familiar coastlines would be gone, buried under encroaching seas. If emissions continue as they are for another century, sea levels may be nearly 50 feet higher in the 2500s, according to some researchers. A bird’s-eye view would reveal signs of fish and marine mammals tracing new paths through once-icy waters, and quiet zones in the once-bustling tropics. Hundreds of years from now, polar seas might be particularly attractive to marine fauna for several reasons: First, warmer seas absorb less oxygen, even as slowed-down currents inhibit the natural mixing between the shallow and deep parts of the ocean, preventing the oxygen that does get absorbed from reaching the depths. Growing stratification also prevents deep-sea nutrients from rising to the marine life that needs them in the upper oceans. Hundreds of years from now, many species might adjust to these conditions by migrating poleward, toward colder waters. (Some of this redistribution is already happening.) By 2300, Earth may experience “a significant, fundamental reorganization of the ocean ecosystem,” and a “catastrophic collapse” of fisheries, Matthew Long, a climate scientist at the National Center for Atmospheric Research who runs a nonprofit dedicated to techniques for removing carbon dioxide from ocean environments, told me.
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Perhaps there are many other Earths out there, and their inhabitants have, like us, altered them. Usually, when scientists and writers imagine such modifications, they envision planet-enveloping Dyson spheres to harness solar energy, or some other megastructure meant to support the hum of life—something that signals a more enlightened and seamless existence. Humans are certainly creating impressive, life-sustaining technologies. But it seems possible that our most lasting cosmic mark will instead make things more difficult for our oceans, the beings within, and ourselves.