The Southern Ocean plays a major role in capturing CO2

Sea ice in the Southern Ocean played a key role in regulating atmospheric CO2 levels during times of climate change in the past. A new study from UNSW shows that in a warming climate, the seasonal cycles of sea ice enhance marine life as well as the amount of carbon stored in the ocean. 

“The Southern Ocean occupies 14% of the Earth’s surface and plays a fundamental role in the global carbon cycle and climate,” said study lead author Professor Chris Fogwill.

“It has captured around half of all human-related carbon that has entered the ocean to date, and is therefore crucial for regulating carbon dioxide levels resulting from human activity – and understanding the processes that determine its effectiveness as a carbon sink through time are essential to reducing uncertainty in future projections.”

The research was focused on a previous time in Earth’s history around 18,000 years ago when atmospheric carbon dioxide levels changed rapidly after the Last Ice Age. At this time, the world naturally transitioned into the warm interglacial conditions that exist today.

Over the course of about 7,000 years, carbon dioxide spiked from around 190 parts per million (ppm) to 280 ppm. However, there was one period during this timeframe when carbon dioxide levels plateaued at a nearly constant level of 240 ppm for 1,900 years.

Understanding the cause of this plateau could be crucial for improving climate change projections. To investigate, Professor Fogwill’s team analyzed ice cores in the Patriot Hills Blue Ice Area of Antarctica. 

“Blue ice areas are the perfect laboratory for Antarctic scientists due to their unique topography. Created by fierce, high-density winds, the top layer of snow is effectively eroded, exposing the ice below. As a result, ice flows up to the surface, providing access to ancient ice below,” explained study co-author Professor Chris Turney.

“Instead of drilling kilometers into the ice, we can simply walk across a blue ice area and travel back through time.”

“Organic biomarkers and DNA from the Southern Ocean are blown onto Antarctica and preserved in the ice, providing a unique record in a region where we have few scientific observations.”

Using a specialized technique to look at the natural fluorescence of organic molecules found in the ice, the researchers discovered that there was a substantial increase in the number and diversity of marine organisms during the 1,900 years when carbon dioxide plateaued.

“This is the first recorded evidence of increased biological productivity in the Antarctic zone during that period, and suggests that Southern Ocean processes may have caused the carbon dioxide plateau,” said study co-author Dr. Laurie Menviel.

The team developed a climate model to investigate the cause of the increase in marine life. 

“The modeling revealed that the plateau period coincided with the greatest seasonal changes in sea ice, which occurred during a pronounced cold phase across the Southern Ocean known as the Antarctic Cold Reversal,” said Dr. Menviel.

“During this period, sea ice grew extensively across the Southern Ocean, but as the world was warming rapidly, each year the sea ice would be rapidly destroyed during the summer.”

“Our results imply that during periods of Southern Ocean sea ice expansion, high variability in winter and summer sea ice extent may result in increased marine life activity and therefore enhanced carbon capture.”

The cause of the extended plateau in global carbon dioxide levels is essential for learning the potential of the Southern Ocean to moderate CO2 levels in the future.

“We need to understand the ways in which carbon dioxide levels have been stabilized by natural processes in the past,” said Professor Turney.

“Under future warming, less sea ice will likely mean a weakening of this part of the carbon sink, making it increasingly difficult to meet Australia’s commitment to the Paris Agreement.”

Study co-author Dr. Zoë Thomas said the paper is an exciting example of multidisciplinary collaboration.

“A lot of science can just focus on one specific process such as a physical, biological, or chemical aspect, but this work is a true interdisciplinary collaboration,” said Dr. Thomas. 

“It’s only when you look at the combination of all these different roles that you get the main story – while it can be tempting to just go down one route, we got together a big team with a range of different expertise which allowed us to get a broader understanding of the system.”

The findings will be used to develop improved Earth system models. Accounting for the ecosystem dynamics of the Southern Ocean will be crucial for more accurate climate projections and for anticipating future warming.

The study is published in the journal Nature Geoscience.

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By Chrissy Sexton, Earth.com Staff Writer