Think about your morning cup of coffee. Warm the water with the heating element of the kettle or the flame on the stove, infuse it with beans and pour it into a mug. Maybe you’re busy and your cup of joe sits there for a while, releasing heat into the room’s atmosphere until it reaches equilibrium with the room temperature. In other words: It got cold.
Now, let’s consider that something similar could someday happen in the vast Southern Ocean that surrounds Antarctica. Since the beginning of the Industrial Revolution, humans have turned up the kettle to maximum temperatures, adding extraordinary amounts of heat to the atmosphere, more than 90 percent of which is absorbed by the oceans. (It also accounts for a quarter of CO2 emissions.) Under climate change, the Southern Ocean has been storing warmth, but like that morning shock, it can’t stay there forever and will someday return to the atmosphere.
A new model suggests that this fever “burp” (as scientists called it, by the way) could occur suddenly. In a scenario where humanity eventually finds a way to reduce greenhouse gas emissions and remove global warming-causing pollutants from the atmosphere and go “net negative,” the Earth’s temperature will decrease. But suddenly, the Southern Ocean is releasing its stored heat, causing planetary warming at the same rate humans are currently causing it. And heat burps will continue for at least a century.
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In other words, according to this model, at least humans will figure out a way to reverse climate change, but the Southern Ocean will end up effectively restarting climate change. There is nothing our descendants can do to stop this, as warming is already caused by stored heat, but this calculation is an even more urgent call to cut pollution as quickly and dramatically as possible.
However, this sudden vocalization is not a certainty, but rather a prediction of the model. But it is a step toward understanding how the Earth will respond as humans continue to manipulate the climate, making it warmer and cooler. “The question is: how will the climate system, and especially the oceans, respond if we remove carbon dioxide from the atmosphere and have a net global cooling effect?” said Svenja Frei, a PhD student in oceanography at the GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany, and co-author of the paper.
The Southern Ocean surrounds the frozen continent of Antarctica and is highly effective at storing heat. The Southern Ocean alone retains about 80 percent of the heat absorbed by all oceans. Some of this is due to ocean currents that carry relatively warm ocean water south, but massive upwelling in the Southern Ocean also brings cold water to the surface for warming.
The skies over the Southern Ocean are somewhat less reflective than elsewhere on Earth. Cargo ships and industry in the Northern Hemisphere spew air pollution in the form of aerosols, which themselves bounce solar energy back into space and help brighten the clouds that reflect it back. In a sense, this cooling phenomenon is competing with the warming caused by the burning of fossil fuels. “This competition is less widespread in the southern hemisphere because we have a slightly more natural atmosphere there,” said Rick Williams, an ocean and climate scientist at the University of Liverpool who studies Antarctica but was not involved in the paper.
In the scenario the researchers modeled, the concentration of CO2 in the atmosphere would increase by 1 percent every year, eventually totaling twice as much CO2 as it did on pre-industrial Earth. Second, negative emissions technology reduces carbon concentrations by 0.1% per year. (Although the study did not consider specific technologies, one option is to capture CO2 directly from the air, but this remains expensive and limited in scale.) The atmosphere, land, and oceans cool accordingly.
However, something begins to happen in the Southern Ocean. Its surface is cooler, but also saltier due to the formation of new sea ice. When seawater freezes, its salt is expelled and absorbed by the surrounding seawater, making the surface layer heavier. “At the same time, we also have warm, deep oceans,” Frey said. “At some point, the water column becomes unstable, and that’s when deep convection occurs.”
In other words, it’s a burp. This is just one way that our planet’s highly complex and intertwined systems are likely to respond to rising and falling emissions over the coming centuries. “There are huge uncertainties about the Earth system’s response to net negative emissions, which we don’t really understand,” said Kirsten Zickfeld, a climate scientist at Simon Fraser University. He is studying these dynamics but is not involved in the new paper. “As this paper shows, there’s a good chance you’ll stumble upon something surprising along the way.”
To be clear, in this scenario, even accounting for burps, removing carbon from the atmosphere would significantly reduce global temperatures. And the sooner we move away from fossil fuels, the less CO2 we will ultimately have to remove. “It’s a good thing to implement negative emissions and reduce the carbon load in the atmosphere,” Williams said. “I would add that it is better not to have a positive discharge in the first place than to have a negative discharge.”
This story was originally published by Grist. Sign up for Grist’s weekly newsletter here.
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