Scientists stunned by discovery in world's coldest ocean waters: 'Signals from the past'

Tens of thousands of years ago, a new study suggests, a warming event led to the release of carbon dioxide from deep in the Southern Ocean. Now, scientists say, these findings could shed light on the potential fallout from continued warming in the Antarctic.

What's happening?

Led by scientists at the Laoshan Laboratory in Qingdao, China, and Germany's GEOMAR Helmholtz Centre for Ocean Research Kiel, the study looked at sediment cores from the Southern Ocean to understand the behavior of Antarctic Bottom Water, or AABW. 

​​"We wanted to understand how the influence of Antarctic Bottom Water, the coldest and densest water mass in the global ocean, changed during the last deglaciation, and what role it played in the global carbon cycle," said Huang Huang, who led the study, in a GEOMAR press release.

The researchers' findings, published recently in the journal Nature Geoscience, suggest that AABW expansion helped bring the last Ice Age to an end.

"As the planet warmed and the ice sheets melted between about 18,000 and 10,000 years ago, the volume of Antarctic Bottom Water expanded in two distinct phases," according to the release. 

"These phases coincided with known warming events in Antarctica. As vertical mixing in the Southern Ocean increased, the carbon that had been stored in the deep ocean was able to return to the atmosphere."

The research team found that the deepest waters stayed fairly stagnant and out of touch with shallower waters until the Earth began to warm. When the water was stirred up by the expansion created by the warming of the planet, carbon stores were released into the Earth's atmosphere, forcing temperatures higher. 

Why is this research important?

The team's study goes back tens of thousands of years and thousands of meters down into the ocean. But its insights could be relevant today and on land where countless communities are vulnerable to the impacts of rising global temperatures.

"Comparisons with the past are always imperfect, but ultimately it comes down to how much energy is in the system," said GEOMAR geochemist Marcus Gutjahr, per the release. "If we understand how the ocean responded to warming in the past, we can better grasp what is happening today as Antarctic ice shelves continue to melt." 

Deep Southern Ocean waters have warmed faster than most others in the past half-century. If the warming waters and thawing ice repeat the patterns of the past, huge amounts of carbon could be released into the atmosphere again and contribute to subsequent temperature rise.

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"I want to properly understand the modern ocean in order to interpret signals from the past," Gutjahr said. "If we can trace how Antarctic Bottom Water has changed over the last few thousand years, we can assess more accurately how rapidly the Antarctic Ice Sheet may continue to lose mass in the future."

Other studies have shown that sea ice levels have dropped alarmingly over the last 50 years. 

What can be done?

Advances in this field could help world leaders and global organizations prepare for the potentially catastrophic consequences of continued planetary warming. Evidence about the threats of sea level rise, food system disruptions, and disease spread can inform adaptation strategies.

But such alarming findings could also help stir meaningful public action now to mitigate rising temperatures, such as transitioning to cleaner home energy systems and transportation options.

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