Australia: The Land Where Time Began

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The Southern Ocean has been slowing Global Warming by Absorbing Carbon, But that Could Change

During a research expedition in 1994 in the Southern Ocean it was noticed that the surface waters were low in oxygen, had a high carbon content and were extremely acidic – which were surprising signs that deep ocean waters rich in nutrients, that are typically found in the deep ocean, were at the surface. The water the research vessel was passing through were actually ancient water that had been in the deep ocean for centuries.

The upwelling in the Southern Ocean was controversial in 1994 but has now been recognised as a hallmark of the Southern Ocean which is driven by the strongest sustained winds in the world to move continuously around the landmass of Antarctica. Large amounts of heat and carbon dioxide transfers from the atmosphere to the waters of the Southern Ocean, and this has slowed the progress of global warming. Much of the global circulation of the global ocean is driven by the powerful currents of the Southern Ocean.

It has been difficult for scientists to carry out research in the ocean around Antarctica for decades but at last it has become possible for the research to get under way. It has become possible for the research to be carried out with floats, moorings, ships, gliders, satellites, computer models and sensors that are attached to seals. This research is increasing understanding of Southern Ocean and the global climate by supplying data to fill very large data gaps. According to Tollefson this research could be key to improving estimates of how quickly the world will warm, the length of time the Antarctic ice will survive and the rate of sea level rise that could be expected.

It is suggested by early data from an array of floats that the amount of CO_2­ that could be absorbed per year could be limited by the upwelling waters. This data is raising new questions about how effective these waters will be in the future in slowing global warming.

Carbon tracking

Tollefson says the unique geography of the Antarctic region makes it a perilous place for ships. There are no landmasses to slow the winds and waves that circle Antarctica at high speed at around the latitude of 60^oS. And there is a history of ships being engulfed by Antarctic ice, which includes Endurance, the ship of Ernest Shackleton in 1915.

The importance of the Antarctic region in controlling global climate was began to be realised in the 1980s when several groups were looking for an explanation of what had caused the atmospheric concentration of CO₂ to decrease by about 33 % during the last ice age and rise again following the close of the ice age. It was realised several decades later that circulation and biology changes in the Southern Ocean could help to cool and warm the Earth (Sarmiento & Toggweller, 1984).

Some of the first data that has resulted from the new research indicate that CO₂ fluxes into the atmosphere are much higher than expected, especially in winter. It has been suggested that these results would imply that the Southern Ocean is a much weaker sink than has been estimated.

The unpublished data that has been analysed was from 13 floats that have been in the water for at least 1 year, now it needs to be determined if the high CO₂ emissions from the Southern Ocean are representative of larger trends across the entire Southern Ocean

Previously there have been hints of something similar, such as a study that was published in2007 in Science (Le Quéré et al., 2007). This study found that the rate at which the Southern Ocean took up CO₂ decreased between 1981 and 2004. The authors of that study blamed the changes on the winds that encircle Antarctica. Over that period of time the speed of the winds had increased, probably as a result of the ozone hole in the stratosphere above Antarctica and possibly because of global warming. Stronger winds are better able to drag up the deep, ancient water, which releases CO₂ as it reaches the surface, which would cause a net weakening of the carbon sink.

Atmospheric CO₂ levels would rise even faster in the future if that trend were to continue. Though a study published in Science (Landschützer et al., 2015) found that the carbon sink began to strengthen in the early 2000s (see “The unreliable sink’).

It is not clear if this CO₂ increased absorption is a return to normal or a deviation from the long-term weakening of the sink. Whichever it is it does show that the Southern Ocean may be much more fickle as a sink than has been believed.

Warming Ocean

Carbon is not the only factor affecting the Southern Ocean. The fate of all the heat that is absorbed by the Southern Ocean is beginning to be determined.

A network of currents carrying water, heat and nutrients throughout the ocean basins has its starting point in the Southern Ocean. Surface waters near Antarctica usually become cold and dense enough to sink to the ocean floor where they form abyssal currents that stay close to the ocean floor as they flow to the north into all the other major oceans, The Pacific, Atlantic and the Indian Oceans.

Ship surveys that have been conducted every decade or so since the early 1990s are sources of much of the knowledge that has been accumulated. When data from the surveys were analysed in 2010 it was found that there was a pronounced warming trend in abyssal waters, which were thus shown to be absorbing about 10 % of the excess heat generated by global warming (Purkey & Johnson, 2010).

Several proposals have been suggested to explain the level of warming in the deep ocean that was unexpected, centring on the Southern Ocean. The decrease in salt content of surface waters around Antarctica partially results from increased rainfall over the ocean during summer. The supply of cold water to the sea floor to feed the bottom currents could be reduced as a result of change in the fresher surface waters which have become less dense. As a result of not getting as much replenishment by cold water could lead to warming of the deep water.

According to an analysis that is yet to be published, that was based on the data from the 3^rd round of ship surveys, similar trends were found, though for a fuller picture requires more frequent measurements.

Elsewhere moorings have been used to monitor deep water flows. Since 1999 an array of moorings have been maintained in the Weddell Sea which is one of the main areas where cold surface waters sink to form the ocean bottom currents. These have recorded a decrease in the salt content of the water in some areas, though the trend in the long term is not clear (Gordon, Huber, McKee & Visbeck, 2010).

Along the edge

In January 2015 the Australian icebreaker Aurora Australis followed a crack in the ice off Antarctica and reached the edge of the Totten Glacier, which is one of the biggest drainage points of the East Antarctic ice sheet. Previously other expeditions had not been able to get to closer than 50 km of this glacier.

Floats and gliders were deployed into the waters around and beneath the glacier, the front edge of which is 200 m thick. They made the shocking discovery that the water at the front of the glacier was 3^oC warmer than the freezing point at the base of the glacier. Prior to this discovery it had been believed that the glacier was too far from warm water to be susceptible, but their instruments had reported that the water was warm all over the shelf in their study area.

It had already been found (Joughin, Smith & Medley, 2014; Rignot et al., 2014) that warm water currents are in the process of undercutting the West Antarctic ice sheet in many areas along the Antarctic Peninsula where the glaciers extend into the ocean. This expedition found some of the first evidence that the East Antarctic ice sheet was being affected by the same processes, which raises new questions about long the ice sheets covering the Antarctic continent will remain.

At present there is no clear answer for what is driving the warming of these currents that are near the surface. Changes in the winds over the Southern Ocean and the upwelling of warm waters have been invoked by some explanations. The focus of other explanations is on fresher surface waters and expanding sea ice in some areas. A kind of cap on the ocean formed by a combination of extra sea ice and fresher surface waters which funnels some of the warmer upwelling waters towards the coast.

There are plans for the team at the British Antarctic Survey to attach sensors to Weddell seals to collect water measurements as they forage beneath the sea ice along the continental shelf. As this zone is where cold water begins its descent into the abyss it is a particularly important zone.

As well as the Weddell seals autonomous gliders will be sent under the ice on pre-programmed routes in order to collect temperature and salinity data to depths of 1,000 m. There will also be measurements taken from ships to make the picture more complete of what is happening in this crucial region around Antarctica, and its relationship with the remainder of the circulation of the global ocean.

It has been found by analysing data from ship surveys that upwelling ocean water does not rise in a simple pattern near Antarctica. Once the water of the upwelling begins its journey to the surface it only reaches the surface after swirling around Antarctica 1.5 times. It has been found that high resolution models are required to capture accurately that behaviour.

Sources & Further reading

1. Tollefson, Jeff, 2016, How much longer can Antarctica’s hostile ocean delay global warming? Nature, 539, 346–348 (17 November 2016)