Context: 

A new research, published in the journal Nature Climate Change has pointed at signs of slowing down overturning circulation of Antarctica and it’s happening decades earlier than predicted.

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. 
• In Antarctica, 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.
• The overturning is crucial to keeping Earth’s climate stable. It’s also the main way oxygen reaches the deep ocean.

Findings of the report:

• Increased rate of slowdown: 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. 
• Disruption in Antarctic cycle: 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.

Impacts of the slowdown 

• Oxygen reduction: 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. 
• Deep Ocean animals: Losses of oxygen may cause them to seek refuge in other regions or adapt their behaviour.
• Intensification of global warming: 
  1. The overturning circulation carries carbon dioxide and heat to the deep ocean, where it is stored and hidden from the atmosphere. 
  2. As the ocean storage capacity is reduced, more carbon dioxide and heat are left in the atmosphere. This feedback accelerates global warming.

• Sea level rise: 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).
• Pervasive consequences: 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.

News Source: The Hindu