Think about your morning cup of coffee. The heating element in your kettle—or the flame on the stove—heats the water that you pour over the beans and pour into the mug. Perhaps you're busy and your cup of Joe sits there for a while, releasing its heat into the room's atmosphere until it reaches equilibrium with the room temperature. In other words: it got cold.
Now imagine that the vast Southern Ocean surrounding Antarctica could one day do the same. Since the Industrial Revolution began, humans have reached maximum capacity, adding incredible amounts of heat to the atmosphere, more than 90 percent of which has been absorbed by the sea. (It's also busy a quarter of our CO2 emissions.) With climate change, the Southern Ocean is building up heat that, like the morning tremor, cannot stay there forever and will someday return to the atmosphere.
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New modeling suggests that this “burp” of heat is scientists called it thisby the way – it could be cool. In a scenario where humanity eventually reduces greenhouse gas emissions and then goes “net negative” by finding ways to remove planet-warming pollutants from the atmosphere, global temperatures would fall. But suddenly the Southern Ocean releases its pent-up heat, resulting in rates of planetary warming similar to what humanity is causing right now. And thermal burping will continue for at least a century.
In other words: according to this modeling, at least people are finding a way to reverse climate change, only to see the Southern Ocean essentially restart it. Although our descendants will not be able to do anything to stop it (since the warming will be caused by already stored heat), the calculations are another urgent call to reduce this pollution as quickly and significantly as possible.
However, this sudden belching is not certain – it is a prediction of the model. But it is a step toward understanding how the planet might respond as humans continue to manipulate the climate, warming and cooling it at the same time. “The question is how will the climate system, and in particular the ocean, respond to scenarios where we remove CO2 from the atmosphere and where we get a net global cooling effect?” said Svenja Frey, a graduate student in oceanography at Germany's 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 retains heat very effectively: Only it holds about 80 percent heat it is swallowed up by all the oceans. This is partly due to currents that carry relatively warm waters south, and also due to the large amount of upwelling in the Southern Ocean. brings cold water to the surface to keep warm.
The skies over the Southern Ocean are also somewhat less reflective than in other parts of the globe. Cargo ships and industrial plants in the Northern Hemisphere emit air pollution in the form of aerosols, which themselves reflect the sun's energy back into space and help lighten the clouds, which reflect even more. This cooling phenomenon in some ways rivals the warming caused by burning fossil fuels. “This competition is not as common in the Southern Hemisphere because it has a slightly more pristine atmosphere,” said Rick Williams, an ocean and climate scientist at the University of Liverpool, who explores the Southern Ocean but did not participate in the work on the newspaper.
In the scenario modeled by the researchers, the concentration of CO2 in the atmosphere increases by 1 percent each year until the total amount is twice what it was on the planet before the Industrial Revolution. Negative emissions technologies then reduce carbon concentrations by 0.1 percent annually. (The study did not look at specific methods, but one option is to directly capture CO2 from the air, although this remains expensive and limited in 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 sheds its salt, which then soaks into the surrounding waters and weighs down the surface layer. “At the same time, we have warm, deep waters,” Frey said. “At some point the water column becomes unstable, and then deep convection occurs.”
In other words, burping. This is just one way in which our planet's enormously complex and intertwined systems may respond to rising and falling emissions in the coming centuries. “There is a lot of uncertainty in the Earth system's response to net negative emissions—we don't understand it very well,” said Simon Fraser University climate scientist Kirsten Zickfeld. research this dynamic, but did not participate in the new article. “As this article shows, we may well encounter surprises along the way.”
To be clear, in this scenario, removing atmospheric carbon significantly reduces global temperatures, even including burps. And the faster we move away from fossil fuels, the less CO2 we will have to remove in the future. “Negative emissions and a reduced carbon load in the atmosphere is a good thing,” Williams said. “I would simply add that instead of making negative emissions, it is better not to make positive emissions at all.”
