The heart of the global ocean

In August, 300 scientists from 25 countries met in Tasmania, Australia, for a conference about the Southern Ocean Observing System (SOOS) and released a statement about a critical need for more study as the world warms up.

“To a large extent, the Southern Ocean controls the uptake of human-generated heat and carbon into the ocean and keeps our planet liveable,” SOOS Chair Dr Sian Henley from the University of Edinburgh said.

“However, despite the efforts of long-term programs carried out by several nations, the Southern Ocean remains one of the most under-observed regions on our planet.

“As the extent of winter sea ice crashes and penguin populations shift dramatically, it is more pressing than ever to have a sustained and coordinated Southern Ocean observing system to understand current conditions and inform predictions of future states.”

The ‘global ocean’

The call was echoed by participants in the University of Tasmania’s Public Talks Series, in a presentation called “Unsung Heroes: Southern Ocean vs climate change”.

The talk was presented by Dr Edward Doddridge from the university’s Institute for Marine and Antarctic Studies (IMAS), a physical oceanographer who studies physical conditions and processes within the ocean, such as its interaction with the atmosphere and how it stores heat.

Doddridge says the Southern Ocean is “the heart of the global ocean”.

“We have one global ocean,” he says. “We talk of it in terms of the Pacific or the Atlantic, but they are all connected. And the place that they connect is the Southern Ocean. This is why the Southern Ocean is such a crucial ocean to understand, and why it is crucial to understand it in order to be able to predict climate change.”

Part of the “heartbeat” analogy of the Antarctic is the seasonal swelling and shrinkage of the sea ice around the South Pole.

The cold, salty brine the freezing process creates, Doddridge says, spreads north. Along the way, it gathers oxygen and nutrients that sustain ecosystems around the globe.

But there have been recent large and disturbing anomalies in the amount of sea ice in the Southern Ocean.

“There has been much less sea ice than expected,” Doddridge says. “This is particularly true this year, where we have set multiple records almost every day for a new record low in sea ice.”

The CO2 effect

The Southern Ocean also acts as a significant absorber of CO2, according to the University of Tasmania’s Dr Tamara Schlosser, a research associate with IMAS.

Through a process known as “the solubility pump,” CO2 dissolves into the water, combining with water molecules to produce carbonic acid, as well as bicarbonate ions and hydrogen ions. This process is temperature-dependent.

“The colder the temperatures get, the more carbon dioxide can dissolve in the water,” Schlosser says. “And this is why our polar regions are so important, because the waters there are colder … the Southern Ocean is about 10 degrees or lower.

“And so our polar regions have much more importance when it comes to removing carbon from the atmosphere.”

The Southern Ocean is home to great quantities of phytoplankton, drifting plant-like organisms that act in the same way plants do on land, releasing CO2 back into the water at night.

Then there is the complicated play of movement of CO2 within the ocean, including:

• the gravity pump, where “marine snow” – a fine fall of dead plant and animal material and faeces containing carbon – falls deep down into the ocean and acts as a food source for deep sea creatures.
• ocean mixing, where the energetic motion of the wind and tides forces carbon downward into the deep ocean
• the migrant pump, where organisms feed near the surface at night and dive down deep during the day to avoid predation, excreting waste containing carbon in the depths.

The bad news, Schlosser says, is that warmer temperatures mean less carbon dioxide dissolving in the water, and shrinking sea ice means a decline in marine life and a reduced migrant pump effect.

The good news is that predictions of stronger winds mean more downward movement in the ocean, and a warmer climate could mean more phytoplankton.

Schlosser says all of this adds up to a kind of lifeline for the planet – the deeper the carbon sinks into the ocean, the longer it will take to be released back into the atmosphere.

“If it is 100m, it might be a decade before it’s back in the atmosphere,” she says. “If it’s able to sink to 1,000m or more, it might be a millennium.”