Science Snippets: A Century Long Thermal Burp in the Southern Ocean

Draft script:

An article in Grist correctly points out that oceans absorb and store heat created by humans. Greenhouse gas emissions have greatly increased the rate of planetary warming. Oceans of the world have absorbed and stored much of this heat.

The article in Grist was published 7 November 2025 and titled The ocean has been hoarding heat. Now it is building up a massive ‘burp.’ The subtitle indicates why we should be concerned: “When humans manage to cut emissions and eventually reduce global temperatures, new research shows the Southern Ocean could kick warming back into gear.” That’s a rather optimistic outlook: I’ve seen no indication humans will “manage to cut emissions and eventually reduce global temperatures.” Everything I’d read indicates we are accelerating global warming, not reversing or slowing it.

The opening paragraph of the Grist article provides an excellent overview that is easy to understand: “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.”

This opening paragraph is used to set up the following paragraph: “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.”

The next paragraph hits hard: “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.”

As if that’s not painful enough, the following paragraph provides clarity: “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.”

Skipping a paragraph, we read a description of the importance of the Southern Ocean: “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 article at Grist introduces a peer-reviewed, open-access paper in AGU Advances. The peer-reviewed paper was published 15 October 2025 and created by six scholars. Titled Southern Ocean Heat Burp in a Cooling World, the paper begins with an Abstract: “The ocean accumulates carbon and heat under anthropogenic CO₂ emissions and global warming. In net-negative emission scenarios, where more CO₂ is extracted from the atmosphere than emitted, we expect global cooling. Little is known about how the ocean will release heat and carbon under such a scenario. Here we use an Earth system model of intermediate complexity and show results of an idealized climate change scenario that, following global warming forced by an atmospheric CO₂ increase of 1% per year until CO₂ doubling, features subsequent sustained net-negative emissions. After several hundred years of net-negative emissions and gradual global cooling, abrupt discharge of heat from the ocean leads to a global mean surface temperature increase of several tenths of degrees that lasts for more than a century. This ocean heat ‘burp’ originates from heat that has previously accumulated under global warming in the deep Southern Ocean, and emerges to the ocean surface via deep convection. Little CO₂ is released along with the heat which is largely due to particularities of sea water carbon chemistry. As the ocean heat loss causes an atmospheric temperature increase independent of atmospheric CO₂ concentrations or emissions, it presents a mechanism that introduces a breakdown of the quasi-linear relationship of cumulative CO₂ emissions and global surface warming, a metric that underpins political decision-making. We call for assessing the robustness of how models forced with net-negative CO₂ emissions simulate durability of ocean storage of heat and CO₂, and pathways of loss to the atmosphere.”

This Abstract is clear, and it is followed by an even clearer Plain Language Summary: “The ocean accumulates carbon and heat under anthropogenic CO₂ emissions and global warming. Little is known about how the ocean will release heat and carbon under potential future ‘net-negative CO₂ emissions.’ In a net-negative emission scenario more CO₂ is extracted from the atmosphere than emitted, and one expects global cooling. We use an Earth system model which is of intermediate complexity in that its ocean is comparatively coarsely resolved and its atmosphere comparatively simple, with the advantage that it can be used for multi-centennial scale climate simulations. We expose the model to an idealized climate change scenario, with first increasing atmospheric CO₂ concentration, followed by decreasing atmospheric CO₂ that implies sustained net-negative CO₂ emissions. We find, after several centuries of global cooling under negative CO₂ emissions, global atmospheric warming that is unrelated to CO2 emissions and is caused by ocean heat release. The rate of warming is comparable to average historical anthropogenic warming rates and lasts for more than a century. The ocean heat loss originates from the deep Southern Ocean. We call for assessing the robustness of how models simulate durability of ocean storage of heat and CO₂, and pathways of loss to the atmosphere.”