Antarctica's Southern Ocean might be gearing up for a thermal 'burp' that could last a century

Consider your morning cup of coffee. Your kettle's heating element — or flame on a stove — warms up water that you infuse with beans and pour into a mug. Maybe you get busy and the cup of joe sits there for a while, releasing its heat into the atmosphere of the room, until it reaches equilibrium with the indoor temperature. In other words: It got cold.

Now consider that the expansive Southern Ocean, which wraps around Antarctica, could one day do much the same thing. Since the Industrial Revolution kicked off, humans have dialed up the kettle to its max, adding extraordinary amounts of heat into the atmosphere, more than 90 percent of which has been absorbed by the sea. (It's also taken up a quarter of our CO₂ emissions.) Under climate change, the Southern Ocean has been storing warmth which, like your morning jolt, can't stay there forever, and will someday return to the atmosphere.

New modeling suggests that this "burp" of heat — the scientists called it that, by the way — could be abrupt. In a scenario where humanity eventually reduces its greenhouse gas emissions and then goes "net negative," finding ways to remove those planet-warming pollutants from the atmosphere, global temperatures fall. But suddenly the Southern Ocean belches its accumulated heat, leading to a rate of planetary warming similar to what humanity is causing right now. And the thermal burping would continue for at least a century.

Put another way: According to this modeling, at least, humans figure out a way to reverse climate change, only to see the Southern Ocean essentially restart it. While there would be nothing our descendants could do to stop this — since the warming would be driven by already stored heat — the calculations are yet another urgent call to reduce that pollution as quickly and dramatically as possible.

This sudden eructation is not a sure thing, however — it's the prediction of a model. But it's a step toward understanding how the planet could respond as humans continue to manipulate the climate, both warming and cooling it. "The question is: How will the climate system, and specifically the ocean, react to scenarios where we remove CO₂ from the atmosphere, and when we have a net global cooling effect?" said Svenja Frey, an oceanography PhD student at Germany's GEOMAR Helmholtz Centre for Ocean Research Kiel and coauthor of the paper.

The Southern Ocean may encircle the frozen continent of Antarctica, but it's very effective at storing heat: It alone holds around 80 percent of the warmth that's taken up by all the oceans. Some of this comes from currents that transport relatively toasty waters south, but also lots of upwelling in the Southern Ocean brings cold water to the surface to be warmed up.

The skies above the Southern Ocean are also somewhat less reflective than elsewhere around the globe. Cargo ships and industries in the Northern Hemisphere spew air pollution in the form of aerosols, which themselves bounce solar energy back into the cosmos and help brighten clouds, which reflect still more. That cooling phenomenon has vied, in a sense, with the warming that's come from the burning of fossil fuels. "That competition hasn't been as prevalent over the Southern Hemisphere, because it's this slightly more pristine atmosphere," said Ric Williams, an ocean and climate scientist at the University of Liverpool, who studies the Southern Ocean but wasn't involved in the paper.

In the scenario the researchers modeled, the atmospheric concentration of CO₂ increases by 1 percent every year until the total amount is double what the planet had before the Industrial Revolution. Then negative emissions technologies reduce the carbon concentration by 0.1 percent annually. (The study didn't look a specific techniques, but one option is direct air capture of CO₂, though this remains expensive and limited in scale.) In response, the atmosphere, land, and oceans cool.

But something starts brewing in the Southern Ocean. Its surface becomes colder, but also saltier due to the formation of new sea ice: When sea water freezes, it rejects its salt, which is then absorbed into the surrounding waters and makes the surface layer heavier. "At the same time, we have these warm, deeper waters," Frey said. "At some point, the water column becomes unstable, and that's when we have the deep convection event."

In other words, a burp. It's just one way that our planet's extraordinarily complex and intertwining systems might respond to rising and falling emissions in the centuries ahead. "There's very large uncertainty in the Earth system response to net-negative emissions — we don't understand that very well," said Simon Fraser University climate scientist Kirsten Zickfeld, who studies these dynamics but wasn't involved in the new paper. "We may well encounter surprises along the way, as this paper shows."

To be clear, in this scenario, removing atmospheric carbon significantly reduces global temperatures, even factoring in the burp. And the faster we move away from fossil fuels, the less CO₂ we'll have to remove down the line. "Doing negative emissions and reducing our carbon load in the atmosphere is a good thing," Williams said. "I would just add that, rather than do negative emissions, it's better not to do the positive emissions in the first place."

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