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 report on 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.
Test the waters of that less bountiful ocean, and you’ll find them to be strangely acidic. The oceans continuously absorb carbon-dioxide emissions from the atmosphere, a process that helps alleviate the worst effects of climate change, but also lowers the pH of seawater. That process could reshape many ocean environments as we know them today by, for example, sapping the water of the chemical compounds necessary for marine creatures to grow shells and skeletons. After last year’s sweltering temperatures, marine experts predicted that most of the world’s coral reefs could be bleached out of existence by only 2100 thanks to acidification and high temperatures.
Eventually, the ocean may simply reach its limit and stop absorbing carbon dioxide at all. When exactly that might happen is unclear; all we know is that this absorption “will not continue forever,” Jamie Shutler, an ocean and atmospheric scientist at the University of Exeter, in England, told me in an email. That point is somewhere far down the geological line, on an Earth so far into the future that there’s almost no reason for us, now, to seriously think about it.
Strangely enough, such distant geological timelines are easier to predict than peering just a few hundred years ahead. Countless variables can change the future as measured in centuries: policy shifts, a meaningful focus on renewable energy sources, engineering solutions that pull carbon dioxide from the seas and the atmosphere. But we can be confident that “we’ll be stuck with changed oceans for thousands of years,” Turner said. Eventually—hundreds of thousands of years from now, she told me—all of the carbon dioxide that humans are currently sending into the atmosphere will become buried on the seafloor; if emissions decrease in the future, the oceans might return to their preindustrial state after that great burial. But that’s so far away, Turner said, that for us, the effects of climate change will be “effectively permanent.” Our oceans, on the grandest scale, can take a lot—but we can’t.
The idea of that permanence is chilling. Humanity’s tenure on Earth may be but a blink in our planet’s history, and yet we have made a certain cosmic choice that will affect the course of the universe. As I’ve written before, Earth has the only good oceans that we know of, despite the fact that the cosmos is excellent at forging new planets around faraway suns. Just this week, scientists released telescope observations of dozens of stars surrounded by swirling disks of gas and dust, the stuff that can eventually coalesce into whole worlds.
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.
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