Evidence of minute amounts of marine life in an ancient Antarctic ice sheet helps explain a longstanding puzzle of why rising carbon dioxide (COâ‚‚) levels stalled for hundreds of years as Earth warmed from the last ice age.

Our study shows there was an explosion in productivity of marine life at the surface of the Southern Ocean thousands of years ago.

And surprisingly, this marine life once played a part regulating the climate.

Hence, this finding has big implications for future climate change projections.

Why that happened was the puzzle, but understanding it could be crucial for improving today’s climate change projections.

To our surprise, hidden in our ice samples were organic molecules – remnants of marine life thousands of years ago.

What’s exciting about finding evidence of lifẻ in ancient Antarctic ice is that, for the first time, we can reconstruct what was happening offshore in the Southern Ocean at the same time, thousands of years ago.

This increased ocean productivity coincided with the Antarctic Cold Reversal.

Our climate modelling reveals the Antarctic Cold Reversal was a time of massive change in the amount of sea ice across the Southern Ocean.

When the sea ice melts, it releases valuable nutrients into the Southern Ocean, and fuelled the explosion in marine productivity we found in the ice on the continent.

This marine life caused more carbon dioxide to be drawn from the atmosphere as it photosynthesised, similar to the way plants use carbon dioxide.

When the marine life die they sink to the floor, locking away the carbon.

The amount of carbon dioxide absorbed in the ocean was sufficiently large to register around the world.

Today, the Southern Ocean absorbs some 40% of all carbon put in the atmosphere by human activity, so we urgently need a 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.

But as the world warms with climate change, less sea ice will be formed in polar regions.

This natural carbon sink of marine life will only weaken, increasing global temperatures further.

Chris Turney, Professor of Earth Science and Climate Change, Director of the Changing Earth Research Centre and the Chronos 14Carbon-Cycle Facility at UNSW, and Node Director of the ARC Centre of Excellence for Australian Biodiversity and Heritage, UNSW and Chris Fogwill, Professor of Glaciology and Palaeoclimatology, Head of School Geography, Geology and the Environment and Director of the Institute for Sustainable Futures, Keele University