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.
Test Yourself: Mains:
Q11. How do ocean currents and water masses differ in their impacts on
marine life and coastal environment? Give suitable examples. (2019)
Q12. What are the forces that
influence ocean currents? Describe their role in fishing industry of the world.
(2022) |