Photograph by Ronja-Elina Kappl / Connected Archives
words by jason p. dinh
Consider this: It’s the year 3000. We got our act together and reached net-zero emissions by 2050. Carbon dioxide has been creeping out of the atmosphere for nearly a thousand years; air temperature and precipitation have been stable over that period, too. Calamity avoided, right?
Wrong. There’s a longer story playing out in the seas—one whose chapters span thousands of years, and we’ve already kicked it into high gear.
Even if fossil fuel emissions end today and we meet the 2°C goal outlined in the Paris Agreement, a growing body of research suggests that the deep oceans will warm and rise some 20 to 25 meters over the next one to two millennia, said Peter Clark, a climate scientist at Oregon State University and lead author of one such study. “We’re going to be constantly adapting to sea level rise for the next 1,000 or 2,000 years,” he said. “Everything coastal is going to be in a constant state of adaptation.”
Under a more pessimistic outlook, if we continue business as usual, the seas could rise by an additional 25 meters by the year 3000. The Bahamas, Shanghai, Bangkok, Kolkata, and Amsterdam would be entirely submerged. Parts of the ocean could be 5°C warmer than they were in 2100. Under unchecked global warming, major ocean currents could halt, extreme weather could become commonplace, and temperatures in some parts of the world could change by tens of degrees over the course of just decades. Huge swaths of Earth will become extremely hostile to human and biological life, said John Abraham, a climate scientist at University of St. Thomas.
“It’s going to make a lot of losers,” Abraham said. “It’s safe to say there’ll be tremendously negative consequences, and some areas of the planet are going to be unrecognizable from how they are today.”
Over the past 150 years, we’ve laid our foot on the pedal of ocean warming as the seas have absorbed 91% of the heat trapped by greenhouse gases. Halting the heating is like pumping the breaks on a full-speed train. Its inertia is too great to stop on a dime.
In part, this is due to the physical properties of seawater and the sheer vastness of the ocean. As anyone who has boiled water knows, it takes a while to heat and to cool. After turning off a burner, the air above quickly returns to room temperature, but the boiled liquid slowly creeps back to equilibrium. Large vats like a pot of soup take longer to cool than small ones like a cup of tea. Now, imagine a 3.5-quintillion gallon tub, one that covers 71% of our planet.
“Some areas of the planet are going to be unrecognizable from how they are today.”
After heat is absorbed, it must descend into the deep ocean, often miles down, before rising and returning to the atmosphere. Each ocean layer slowly heats the one below—a sluggish process that will take thousands of years to conclude, said Kirsten Zickfeld, a climate scientist at Simon Fraser University. She explained that once heat is far enough from the surface, it’s there for good. “The inner ocean is so disconnected from the surface, it doesn’t really realize what’s going on at the surface. Even if you see cooling at the surface, the mid and deeper ocean still keep warming,” she said.
And when that water warms, it also expands, causing sea level rise through a process called thermal expansion. This, too, is locked in for at least a thousand years. In a study published in 2017, Zickfeld showed that even if emissions end in 2050, the thermal expansion caused by carbon dioxide would double by 2150 and quadruple by 2550.
The prolonged trajectory is, in part, because carbon dioxide lingers in the atmosphere for hundreds to thousands of years, but the ocean’s inertia is so strong that even short-lived greenhouse gases like methane, which has a lifespan of only a decade, causes thermal expansion for centuries. Zickfeld’s study found that 75% of methane-caused thermal expansion will last 100 years after emissions end; 40% persists 500 years after. The gas, though famously ephemeral, leaves an enduring mark on the seas.
Deep ocean warming and sea level rise are truly irreversible, Zickfeld said, even if we reach net-zero—even if we can remove massive amounts of greenhouse gas from the atmosphere. “It is still warming because it lives in a different era. It lives in the warming era,” she said. Unfortunately for us, she added, “the ocean does not forget.”
While the timescale of deep sea warming is certain, the extent of it is not, Zickfeld said. Largely, that depends on our decisions in the next century or two. The difference between immediate action and no action could be tens of meters of sea level rise.
Early climate mitigation is especially helpful. There’s a direct correlation between sea level rise and carbon dioxide emissions, research shows. And that relationship is most sensitive when atmospheric carbon dioxide is relatively low: In other words, the sooner we decarbonize, the greater the impact we can have. As Clark put it, “For every amount we emit, the more we commit.”
Halting the heating is like pumping the breaks on a full-speed train. Its inertia is too great to stop on a dime.
What’s more, while the heat that has penetrated the deep sea is lost to us, the sea surface remains firmly in our grasp. The top-layer of the ocean controls our climate, weather, and precipitation. “Hurricanes—what they care about is the temperature in the upper, I don’t know, five meters,” said Zickfeld. “The surface-air warming is not baked in. If we reach net-zero, then there’s a good chance that we can actually stabilize our climate.”
Of course, these surface conditions are indirectly related to deep-ocean warming—for example, currents of warm water flowing beneath the Greenland and West Antarctic Ice Sheets are melting them from below, which is slowing down major ocean currents. If those halt, it could cause extreme temperature drops, strong storms, and profound shifts in precipitation. But these things depend on the surface as much as they do the deep ocean. Disaster on that front, then, can still be averted.
The world of tomorrow—of year 3000—depends on decisions today. Of course, even a best-case scenario means tens of meters of sea level rise over thousands of years. This will cost coastal countries their land and millions of people their homes. That’s already locked in. “What people need to realize is that there is not going to be an endpoint to adaptation to sea level rise, but that task will have to be passed on from generation to generation.” Zickfeld said.
The extent of that change, however, and whether more abrupt climate impacts accompany it, remains in our hands. When every tick of the thermometer could save countless lives, the issue is, in more ways than one, a matter of degree.
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I’ve Been to the Year 3000. Not Much Has Changed, but They Lived Underwater.
