Overturning Circulation in Antarctica

Overturning Circulation in Antarctica

Q. Context :

·    Antarctica sets the stage for the world’s greatest waterfall. Trillions of tonnes of cold, dense, oxygen-rich water cascade off the continental shelf and sink to great depths. This Antarctic “bottom water” then spreads north along the sea floor in deep ocean currents, before slowly rising, thousands of kilometres away.

Q. What is Overturning circulation?

·     Antarctica drives a global network of ocean currents called the “overturning circulation” that redistributes heat, carbon and nutrients around the globe.

·        The overturning is crucial to keeping the earth’s climate stable.

·        It’s also the main way oxygen reaches the deep ocean.

Q. What is the issue?

·    There are signs this circulation is slowing down and it’s happening decades earlier than predicted.

·    This slowdown has the potential to disrupt the connection between the Antarctic coasts and the deep ocean, with profound consequences for the earth’s climate, sea level and marine life.

·      Measurements show the overturning circulation has slowed by almost a third (30%) and deep ocean oxygen levels are declining. This is happening even earlier than climate models predicted.

·    Melting of Antarctic ice is disrupting the formation of Antarctic bottom water. The meltwater makes Antarctic surface waters fresher, less dense, and therefore less likely to sink. This puts the brakes on the overturning circulation.

Q. Why does this matter?

·   As the flow of bottom water slows, the supply of oxygen to the deep ocean declines. The shrinking oxygen-rich bottom water layer is then replaced by warmer waters that are lower in oxygen, further reducing oxygen levels.

·    Ocean animals, large and small, respond to even small changes in oxygen. Deep-ocean animals are adapted to low oxygen conditions but still have to breathe. Losses of oxygen may cause them to seek refuge in other regions or adapt their behaviour. Models suggest we are locked in to a contraction of the “viable” environment available to these animals with an expected decline of up to 25%

·  Slowdown of the overturning may also intensify global warming. The overturning circulation carries carbon dioxide and heat to the deep ocean, where it is stored and hidden from the atmosphere. As the ocean storage capacity is reduced, more carbon dioxide and heat are left in the atmosphere. This feedback accelerates global warming.

·    Reductions in the amount of Antarctic bottom water reaching the ocean floor also increases sea levels because the warmer water that replaces it takes up more space (thermal expansion).

Q. Way Forward?

·     Ice loss from Antarctica is expected to continue, even accelerate, as the world warms. We are almost certain to cross the 1.5℃ global warming threshold by 2027.

·    More ice loss will mean more freshening, so we can anticipate the slowdown in circulation and deep oxygen losses will continue.

·   The consequences of a slowdown will not be limited to Antarctica. The overturning circulation extends throughout the global ocean and influences the pace of climate change and sea level rise. It will also be disruptive and damaging for marine life.

·      It is  another reason to work harder – and faster – to reduce greenhouse gas emissions.