Researchers plug carbon sink gaps

The stormy waters south of the Cape suck carbon dioxide out of the atmosphere and are key to understanding what will happen to our climate as the Earth heats up.

The storms are part of the reason the Southern Ocean is one of the most under-researched in the world, although it absorbs almost half of the world’s man-made carbon emissions.

Last week, more than 50 researchers returned from a scientific voyage of the waters encircling Antarctica, the first leg of a two-year experiment – the third Southern Ocean Seasonal Cycle Experiment – that aims to fill in many of the blank spaces about how this ocean mitigates the effects of climate change.

A report from the United Nation’s Intergovernmental Panel on Climate Change, released last year, says the world is already experiencing climate change. “African ecosystems are already being impacted by climate change, and the future impacts are expected to be substantial,” it said.

Mediating global climate
Pedro Monteiro, chief oceanographer of the Council for Scientific and Industrial Research and head of the CSIR’s Southern Ocean Carbon-Climate Observatory programme, says: “Globally, there is a renewed interest in understanding what is going on in the Southern Ocean from a climate perspective and how it mediates global climate.

“Although anthropogenic carbon dioxide emissions – those created by humans – are well studied and account for about 10 gigatonnes per year, the natural carbon cycle between the ocean and the atmosphere is substantially larger at 100 gigatonnes per year and is less understood,” he says.

But, because of the stormy and cold weather in the Southern Ocean, researchers struggle to obtain scientific measurements during winter, something that this experiment plans to change.

The observatory’s Sandy Thomalla, who led the team of 51 scientists, says: “One of the reasons for the experiment was to measure the Southern Ocean in winter as this provides the ‘reset’ conditions that determine the food supply for phytoplankton production in the following spring.”

Phytoplankton, small microscopic organisms that live in water systems, are in effect tiny plants that absorb carbon dioxide, and are one of the two oceanic mechanisms that suck carbon dioxide out of the atmosphere. “There are two ‘pumps’ in the ocean that drive the ability of the Southern Ocean to be an effective carbon dioxide sink,” Thomalla says.

“There’s a physical pump that is driven primarily by temperature and density – the Southern Ocean is cold, and cold water is better able to absorb carbon dioxide. When it sinks to a lower layer in the ocean, it takes the carbon dioxide with it. The biological pump is driven by phytoplankton production. [These micro-organisms] absorb carbon dioxide when they photosynthesise. When these plants die and sink to the bottom of the ocean, they take the carbon with them,” she says.

“In large areas of the Southern Ocean, these pumps work in unison and are the reason the South Ocean is the most effective carbon dioxide sink of all the world’s oceans. This is why we are trying to understand it.”

The data from this voyage – which included a multidisciplinary team studying the physics, chemistry and biology of the ocean, and the birds and mammals in the ecosystem – will allow scientists to determine how the ocean changes from its “dead” time in winter to its relatively fecund summer conditions.

Massive contribution to understanding
“We have almost no observations of the dynamics of this reset period linking the physics to the biogeochemistry, so this combined ship- and robotics-based work is a massive contribution to our understanding,” Monteiro wrote in his briefing memo on the experiment.

“Whilst major achievements have been made in the last 18 years [of researching this ocean], we are only now beginning to understand the mean seasonal state and variability of the Southern Ocean and its coupling with the atmosphere.”

The new polar research ship SA Agulhas II is being used to improve this understanding, “together with robotics-based, continuous, year-round, high-resolution observations of the upper ocean”, according to the Southern Ocean Seasonal Cycle Experiment mission statement.

SA Agulhas II route of first expedition

The second leg of the experiment will leave for the Antarctic ice shelf in December and return in February next year. The experiment includes ship-based observations, deploying ocean robots and using satellites and moorings with sensors in the ocean. This data will enable scientists to improve their models of the Southern Ocean and, by extension, global climate change.

“South Africa is trying to find a [research] niche in understanding the Southern Ocean and climate change,” Thomalla says. “We think that small-scale interactions of ocean eddies and fronts, with seasonal heating and transient storms, are what drives the large-scale effectiveness of the Southern Ocean carbon sink.”

“Most models [of] climate change are not resolving those small time and space scales. We think they might be missing important characteristics of the system that we need to get a handle on.”

The experiment, which is funded by the department of science and technology through the National Research Foundation, marks a watershed for South African Southern Ocean science: it is the first time the SA Agulhas II has been employed for a purely scientific reason, according to Thomalla.

The polar vessel is usually used to service South Africa’s base on Antarctica, the South African National Antarctic Expedition, and includes scientists to undertake research en route. “Previously, we have piggy-backed on logistics cruises. This time round, this was a dedicated scientific cruise,” Thomalla says. She hopes this is also the start of being able to use the ship “in a purely research way”.

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