The Southern Ocean Could Release a Huge Heat Burp After Fossil Fuels End

The Southern Ocean has faithfully absorbed a century’s worth of carbon dioxide and heat produced by human activity. Yet once humanity finally abandons fossil fuels, that service could ultimately return to trouble us.

In a new study, German climate scientists forecast that, once people stop emitting greenhouse gases and the planet eventually begins to cool, the Southern Ocean could abruptly release an enormous ‘burp’ of heat. This would trigger global warming again for at least a century.

Their forecast relies on an established climate framework that brings together models of atmospheric energy and moisture balance, ocean circulation and sea ice, the land biosphere, and ocean biochemistry. Using it, the researchers modelled an idealised future climate-change scenario.

Southern Ocean heat absorption in a future climate scenario

The scenario unfolds as follows: human activity keeps increasing greenhouse gas emissions until they reach a high point within 70 years, by which time atmospheric carbon dioxide has doubled.

Emissions are then cut sharply - perhaps through human ingenuity, or possibly human extinction. Several centuries of net-negative carbon emissions follow, gradually cooling the world.

During global warming, heat builds up in the ocean for several reasons. First, deep waters beneath the surface are ventilated by warmer water from above. Second, the natural processes through which the ocean lost heat before industrialisation - including the upwelling of deep circumpolar waters in the Southern Ocean - become weaker.

In the model, the ocean goes on taking in heat long after atmospheric CO₂ reaches its peak and net-negative emissions begin, because temperatures in the surface atmosphere are also slow to decline. This future ocean can also absorb far more shortwave solar radiation, as much of the sea ice that once reflected this heat has melted.

“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,” explains the team led by biogeochemist Ivy Frenger from the GEOMAR Helmholtz Centre for Ocean Research.

“The rate of warming is comparable to average historical anthropogenic warming rates and lasts for more than a century.”

Put simply, the ‘buffer’ currently provided by the Southern Ocean, as it absorbs heat trapped in the atmosphere by greenhouse gases, cannot last indefinitely.

Global warming could return after net-negative emissions

Once released, this stored heat would not be spread evenly around the planet. Although the consequences would be global, the researchers found that warming would be “greatest and longest-lasting in the Southern Hemisphere, suggesting a greater impact on today's more vulnerable countries of the global south.”

Models of this kind can be relatively simplified and idealised. However, when other modelling configurations were used to test the same scenario, they produced consistent results. This calls into question the idea that cumulative CO₂ emissions and global warming always move in step - an assumption that, as the authors note, “underpins political decision-making”.

The findings suggest that expectations about the time needed to see the benefits of climate action may need to be reconsidered, potentially extending to centuries after net-negative emissions are achieved.

According to these models, the world would keep warming long after humanity gives up its fossil fuel habit. And the longer this is delayed, the larger that ‘burp’ could become.

The research was published in AGU Advances.