As the world has warmed since the last ice age, carbon dioxide levels have stalled for nearly 2,000 years. It̵
The detection of tiny amounts of marine life in an ancient Antarctic ice sheet explains a long-standing riddle as to why the rise in carbon dioxide (CO₂) levels has stalled for hundreds of years since the last ice age as the earth warmed.
Our study shows that thousands of years ago there was an explosion in the productivity of marine life on the surface of the Southern Ocean.
And surprisingly, this marine life once played a role in regulating the climate. Therefore, this finding has a major impact on future climate change forecasts.
Go back in time
Our research led us to a four-hour flight from Chile to the Weddell Sea at the far southern end of the Atlantic Ocean to land on an ice rink in a cold latitude of 79 ° South.
The Weddell Sea is often clogged with sea ice and has been dangerous to ships since the earliest explorers ventured south.
In 1914, Anglo-Irish explorer Ernest Shackleton and his men were stuck here for two years, 1,000 kilometers from civilization. They were exposed to isolation, hunger, freezing temperatures, burns, migratory icebergs and the danger of cannibalism.
Surviving here is difficult, as is science.
We spent three weeks in the nearby Patriot Hills and drilled through ice to collect samples.
When scientists collect ice samples, they usually drill a deep core vertically through the annual layers of snow and ice. We did something completely different: we went horizontally by drilling a series of shorter cores across the ice landscape.
This is because the Patriot Hills are a wild place that is hit by hurricanes from the Weddell Sea that shed heavy snow, followed by strong cold winds (called catabatic winds) that flow from the polar plateau.
These catabatic winds blew heavily.
As the winds blow all year round, they remove the surface ice in a process called sublimation. Older, deeper ice is pulled to the surface. This means that a stroll across the blue ice towards Patriot Hills is practically like a journey through time.
The exposed ice shows what happened during the transition from the last ice age about 20,000 years ago to today’s warmer world, the Holocene.
The Antarctic reversal of cold
As the earth warmed, the carbon dioxide content in the atmosphere quickly rose from about 190 to 280 ppm.
But the warming trend was not one-way.
Around 14,600 years ago, there was a 2000-year cooling period in the southern hemisphere. This period is known as the Antarctic cold reversal and is the place where the CO₂ values are around 240 ppm.
Why this happened was a mystery, but understanding it could be critical to improving today’s climate change forecasts.
Find life in the ice
We fought wind and snow for over three weeks to create a detailed collection of ice samples from the end of the last ice age.
To our surprise, organic molecules were hidden in our ice samples – remains of marine life thousands of years ago. They came from the hurricanes off the Weddell Sea, which swept organic molecules from the surface of the sea and threw them ashore to keep them in the ice.
Antarctic ice, which is formed from snowfall, usually only tells scientists about the climate. The exciting thing about finding evidence of life in the ancient ice of Antarctica is that for the first time thousands of years ago we were able to reconstruct what was happening off the coast in the Southern Ocean.
We found an unusual time with high concentrations and a diverse spectrum of marine microplankton. This increased ocean productivity coincided with the reversal of the Antarctic.
The melting of sea ice in summer preserves marine life
Our climate models show that the Antarctic cold reversal was a time of massive changes in the amount of sea ice over the Arctic Ocean.
When the world came out of the last ice age, the summer heat destroyed large amounts of sea ice that had formed in winter. When the sea ice melts, it releases valuable nutrients into the Arctic Ocean, fueling the explosion in marine productivity that we found in continental ice.
This marine life resulted in more carbon dioxide being removed from the atmosphere during photosynthesis, similar to how plants use carbon dioxide. When the sea creatures die, they sink to the bottom and lock the carbon away. The amount of carbon dioxide absorbed in the ocean was large enough to register worldwide.
What this means for climate change today
Today, the Southern Ocean absorbs about 40% of all carbon released into the atmosphere through human activities. So we urgently need to better understand the drivers of this important part of the carbon cycle.
Marine life in the Southern Ocean still plays an important role in regulating the amount of atmospheric carbon dioxide.
If the world warms up with climate change, less sea ice will form in the polar regions. This natural carbon sink of marine life will only weaken and global temperatures will continue to rise.
It is a timely reminder that although Antarctica appears remote, its effects on our future climate are closer and closer together than we might think.
Written by Chris Turney, Professor of Earth Sciences and Climate Change, Director of the Changing Earth Research Center and Chronos 14Carbon-Cycle Facility at UNSW and Node Director of the ARC Center of Excellence for Australian Biodiversity and Heritage, UNSW and Chris Fogwill. Professor of Glaciology and Paleoclimatology, Head of School Geography, Geology and Environment and Director of the Institute for Sustainable Future at Keele University.
Originally published on The Conversation.