Explainer: how the Antarctic Circumpolar Current helps keep Antarctica frozen



File 20181115 194516 mec002.jpg?ixlib=rb 1.1
The Antarctic Circumpolar Current keeps Antarctica cold.
Shutterstock

Helen Phillips, University of Tasmania; Benoit Legresy, CSIRO, and Nathan Bindoff, University of Tasmania

The Antarctic Circumpolar Current, or ACC, is the strongest ocean current on our planet. It extends from the sea surface to the bottom of the ocean, and encircles Antarctica.

Scientists deploying a vertical microstructure profiler (VMP-2000), which measures temperature, salinity, pressure and turbulence, from RV Investigator in the Antarctic Circumpolar Current, November 2018.
Nathan Bindoff

It is vital for Earth’s health because it keeps Antarctica cool and frozen. It is also changing as the world’s climate warms. Scientists like us are studying the current to find out how it might affect the future of Antarctica’s ice sheets, and the world’s sea levels.

The ACC carries an estimated 165 million to 182 million cubic metres of water every second (a unit also called a “Sverdrup”) from west to east, more than 100 times the flow of all the rivers on Earth. It provides the main connection between the Indian, Pacific and Atlantic Oceans.

The tightest geographical constriction through which the current flows is Drake Passage, where only 800 km separates South America from Antarctica. While elsewhere the ACC appears to have a broad domain, it must also navigate steep undersea mountains that constrain its path and steer it north and south across the Southern Ocean.




Read more:
Antarctica has lost 3 trillion tonnes of ice in 25 years. Time is running out for the frozen continent


What is the Antarctic Circumpolar Current?

A satellite view over Antarctica reveals a frozen continent surrounded by icy waters. Moving northward, away from Antarctica, the water temperatures rise slowly at first and then rapidly across a sharp gradient. It is the ACC that maintains this boundary.

Map of the ocean surface temperature as measured by satellites and analysed by the European Copernicus Marine Services. The sea ice extent around the antarctic continent for this day appears in light blue. The two black lines indicate the long term position of the southern and northern front of the Antarctic Circumpolar Current.

The ACC is created by the combined effects of strong westerly winds across the Southern Ocean, and the big change in surface temperatures between the Equator and the poles.

Ocean density increases as water gets colder and as it gets more salty. The warm, salty surface waters of the subtropics are much lighter than the cold, fresher waters close to Antarctica. We can imagine that the depth of constant density levels slopes up towards Antarctica.

The westerly winds make this slope steeper, and the ACC rides eastward along it, faster where the slope is steeper, and weaker where it’s flatter.

Fronts and bottom water

In the ACC there are sharp changes in water density known as fronts. The Subantarctic Front to the north and Polar Front further south are the two main fronts of the ACC (the black lines in the images). Both are known to split into two or three branches in some parts of the Southern Ocean, and merge together in other parts.

Scientists can figure out the density and speed of the current by measuring the ocean’s height, using altimeters. For instance, denser waters sit lower and lighter waters stand taller, and differences between the height of the sea surface give the speed of the current.

Map of how fast the waters around Antarctica are moving in an easterly direction. It is produced using 23 years of satellite altimetry (ocean height) observations as provided by the European Copernicus Marine Services.
Author provided

The path of the ACC is a meandering one, because of the steering effect of the sea floor, and also because of instabilities in the current.

The ACC also plays a part in the meridional (or global) overturning circulation, which brings deep waters formed in the North Atlantic southward into the Southern Ocean. Once there it becomes known as Circumpolar Deep Water, and is carried around Antarctica by the ACC. It slowly rises toward the surface south of the Polar Front.

Once it surfaces, some of the water flows northward again and sinks north of the Subarctic Front. The remaining part flows toward Antarctica where it is transformed into the densest water in the ocean, sinking to the sea floor and flowing northward in the abyss as Antarctic Bottom Water. These pathways are the main way that the oceans absorb heat and carbon dioxide and sequester it in the deep ocean.

Changing current

The ACC is not immune to climate change. The Southern Ocean has warmed and freshened in the upper 2,000 m. Rapid warming and freshening has also been found in the Antarctic Bottom Water, the deepest layer of the ocean.

Waters south of the Polar Front are becoming fresher due to increased rainfall there, and waters to the north of the Polar Front are becoming saltier due to increased evaporation. These changes are caused by human activity, primarily through adding greenhouse gases to the atmosphere, and depletion of the ozone layer. The ozone hole is now recovering but greenhouse gases continue to rise globally.

Winds have strengthened by about 40% over the Southern Ocean over the past 40 years. Surprisingly, this has not translated into an increase in the strength of the ACC. Instead there has been an increase in eddies that move heat towards the pole, particularly in hotspots such as Drake Passage, Kerguelen Plateau, and between Tasmania and New Zealand.

We have observed much change already. The question now is how this increased transfer of heat across the ACC will impact the stability of the Antarctic ice sheet, and consequently the rate of global sea-level rise.The Conversation

Helen Phillips, Senior Research Fellow, Institute for Marine and Antarctic Studies, University of Tasmania; Benoit Legresy, , CSIRO, and Nathan Bindoff, Professor of Physical Oceanography, Institute for Marine and Antarctic Studies, University of Tasmania

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Advertisements

‘Keep it in the ground’: what we can learn from anti-fossil fuel campaigns


Fergus Green, London School of Economics and Political Science

From the fossil fuel divestment movement to the Stop Adani campaign, in recent years we’ve seen a wave of climate activism that directly targets fossil fuels — both the infrastructure used to produce, transport and consume them, and the corporations that finance, own and operate that infrastructure.

What makes targeting fossil fuels so attractive for activists, and can we learn anything from them?




Read more:
The fossil fuel divestment game is getting bigger, thanks to the smaller players


Failure to launch

Climate change became a topic of mainstream international concern in the early 1990s. For the first two decades of international climate cooperation, until the failed Copenhagen climate conference in 2009, the international environment movement embraced a more “technocratic” approach. Professionally-staffed environment groups made technical arguments aimed at persuading politicians and the public to adopt global climate treaties, national greenhouse gas emission reduction targets, and complex market-based policy mechanisms such as emissions trading schemes.




Read more:
The too hard basket: a short history of Australia’s aborted climate policies


All of these things, if sufficiently stringent, would have been great if they were politically possible. But the groups advocating them were politically weak; they had few political resources. Consequently, in the competition to influence policy they were systematically outgunned by the fossil fuel industry.

Not only did the environment movement lack money and power over the economy, they lacked public support for their policy agenda. While public concern for climate change throughout this period was widespread, it was shallow. It was a political priority for few people, and fewer still were willing to take to the streets to demand strong, urgent action.

A protestor at the coal port in Newcastle.
BREAK FREE NEWCASTLE

Why fossil fuels resonate

Compared with such ineffective climate activism, the present wave of anti-fossil fuel politics has an important advantage: it resonates better with ordinary people.

First, fossil fuels and associated infrastructure are readily understood by lay audiences. In contrast, concepts such as greenhouse gases, “2°C average warming”, and “350 ppm” are abstract, technical constructions not readily grasped by laypersons.




Read more:
A matter of degrees: why 2C warming is officially unsafe


Second, whereas the harms caused by climate change are hard to understand and (perceived to be) remote from their cause in time and space, the production, transport and consumption of fossil fuels cause and are popularly associated with a range of other harms on top of climate change.

These include: local environmental, health and other socio-economic impacts, as well as corruption, repression, human rights abuses and other injustices along the supply chain. Most of these affect people living or working close to fossil fuel infrastructure such as mines, pipelines and coal-fired power stations.

Local communities faced health problems when the Hazelwood coal mine caught fire in 2014.
COUNTRY FIRE AUTHORITY

Surveys about energy sources in the US and Australia, for example, support the claim that fossil fuels are unpopular. In China, local air pollution caused by fossil fuels is one of the biggest public concerns. And case studies from various countries indicate the potential for proposed fossil fuel infrastructure to generate strong local opposition, social conflict, and wider media attention.

Third, targeting fossil fuels helps to personalize the causes of climate change. One of the reasons climate change is not psychologically salient to most people is that it is typically perceived to be an unintentional side-effect of the everyday actions of billions of people. This makes it hard for us to attribute blame.




Read more:
Unburnable carbon: why we need to leave fossil fuels in the ground


But the fossil fuel industry is disproportionately responsible for our dependence on emissions-intensive energy. Targeting the industry helps to concentrate moral pressure on these more culpable agents and stokes the indignation that fuels climate activism.

Among anti-fossil fuel campaigns, the fossil fuel divestment movement aims most directly and explicitly to delegitemise the fossil fuel industry.
Studies show that the divestment movement has, in a very short time, had a revitalising effect on climate activism through the mobilisation of young people, and improved wider public discourse toward climate change action, among other beneficial effects.

Divestment protesters at UNSW in Sydney.
DANNY CASEY

Targeting fossil fuels also has advantages when it comes to the other elements of successful social movement activism — resource accumulation, alliance-building, and sustaining participants’ enthusiasm over time.

A necessary part of climate politics

Targeting fossil fuels is not the only way to build more successful movements around climate action. Campaigns providing a more positive vision around renewable energy, for example, have also been successful in mobilising grassroots support, and are a crucial component in contemporary climate activism. And successful grassroots mobilisation is not everything: elite politics and international relations also greatly affect climate policy.

But building wide and deep social movements committed to urgent climate action is a necessary element of the political task before us. As the rising tide of anti-fossil fuel activism shows, if campaigners work with the grain of ordinary human motivation, drawing on what we know about the psychology and sociology of social movements, then they are in with a fighting political chance.The Conversation

Fergus Green, PhD Candidate in Political Theory, Department of Government, London School of Economics and Political Science

This article is republished from The Conversation under a Creative Commons license. Read the original article.