Antarctica’s Southern Ocean might be gearing up for a thermal ‘burp’ that could last a century
Think about your morning cup of coffee. The heating element in your kettle – or the flame on a stove – warms the water, which you brew with beans and pour into a cup. Maybe you’re busy and the cup of coffee 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 was cold.
Now consider that the vast Southern Ocean, which surrounds Antarctica, could one day do much the same thing. Since the start of the Industrial Revolution, humans have cranked the kettle up to maximum, adding extraordinary amounts of heat to the atmosphere, more than 90% of which has been absorbed by the sea. (This also consumes a quarter of our CO2 emissions.) Due to climate change, the Southern Ocean has stored heat that, like your morning jolt, cannot stay there forever and will one day return to the atmosphere.
New modeling suggests that this heat “burp” – scientists call it that – could be abrupt. In a scenario where humanity ultimately reduces its greenhouse gas emissions and then goes “net negative,” by finding ways to remove these planet-warming pollutants from the atmosphere, global temperatures would fall. But suddenly the Southern Ocean spews out its accumulated heat, leading to global warming similar to what humanity is causing now. And thermal burping would continue for at least a century.
In other words: according to this modeling, at least, humans find a way to reverse climate change, only to see the Southern Ocean restart it. Although there is nothing our descendants can do to stop this – since warming would be caused by already stored heat – the calculations provide an urgent new call to reduce this pollution as quickly and radically as possible.
This sudden belching is not a sure thing, however: it is a model’s prediction. But it’s a step toward understanding how the planet might respond as humans continue to manipulate the climate, both warming and cooling it. “The question is: how will the climate system, and in particular the ocean, respond to scenarios in which we remove CO2 of the atmosphere, and when we will have a net global cooling effect? said Svenja Frey, a doctoral student in oceanography at the German GEOMAR Helmholtz Center for Ocean Research in Kiel and co-author of the paper.
The Southern Ocean may surround the frozen continent of Antarctica, but it is very efficient at storing heat: it alone retains about 80% of the heat absorbed by all the oceans. Part of this comes from currents carrying relatively warm waters south, but also many cold upwellings in the Southern Ocean bring cold water to the surface to be warmed.
The sky over the Southern Ocean is also a little less reflective than elsewhere in the world. Cargo ships and industries in the Northern Hemisphere release air pollution in the form of aerosols, which in turn reflect solar energy back into the cosmos and help brighten clouds, which reflect even more of it. This cooling phenomenon rivaled, in a sense, the warming caused by the burning of fossil fuels. “This competition hasn’t been as prevalent in the southern hemisphere because it’s a slightly more pristine atmosphere,” said Ric Williams, an ocean and climate scientist at the University of Liverpool who studies the Southern Ocean but was not involved in the study.
In the scenario modeled by the researchers, the atmospheric concentration of CO2 increases by 1 percent each year until the total amount is double what the planet had before the industrial revolution. Negative emissions technologies then reduce the carbon concentration by 0.1% per year. (The study did not look at specific techniques, but one option is direct air capture of CO.2although this remains expensive and on a limited scale.) In response, the atmosphere, land and oceans cool.
But something is starting to brew in the Southern Ocean. Its surface becomes colder, but also saltier due to the formation of new sea ice: when seawater freezes, it releases its salt, which is then absorbed into the surrounding waters and weighs down the surface layer. “At the same time, we have warmer, deeper waters,” Frey said. “At some point, the water column becomes unstable, and that’s when we have a deep convection event.”
In other words, a burp. This is just one way our planet’s extraordinarily complex and intertwined systems could respond to rising and falling emissions over the coming centuries. “There is very large uncertainty in the response of the Earth system to net negative emissions — we don’t understand that very well,” said Kirsten Zickfeld, a climate scientist at Simon Fraser University who studies these dynamics but was not involved in the new paper. “We may well encounter surprises along the way, as this document shows.”
To be clear, in this scenario, removing atmospheric carbon significantly reduces global temperatures, even accounting for burp. And the faster we move away from fossil fuels, the less CO there will be2 we will have to delete down the line. “Reducing negative emissions and reducing our carbon load in the atmosphere is a good thing,” Williams said. “I would just add that rather than doing negative shows, it’s better not to do positive shows in the first place.”
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