The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents, like a conveyor belt, driven by differences in temperature and salinity. It is a thermocline circulation that carries warm surface waters from the tropics towards the Northern Hemisphere, where it cools and sinks. It then returns to the tropics and then to the South Atlantic as a bottom current. From there it is distributed to all ocean basins via the Antarctic circumpolar current. This global process makes sure that the world’s oceans are continually mixed, and that heat and energy are distributed around the earth. However, the Intergovernmental Panel on Climate Change (IPCC) in its recent report highlighted that AMOC is losing its stability and is very likely to decline over the 21st century due to the following reasons:

• Melting of Arctic ice sheets: Global warming has led to an increase in freshwater due to the melting of Greenland and Arctic ice sheets. It can make circulation weaker as it reduces the salinity and density of the water, making it unable to sink to the bottom.
• Weakening of the Gulf Stream: Anthropogenic factors such as global warming have led to a weakening of the Gulf Stream, which has further led to instability and decline of AMOC.
• Increasing precipitation and river run-off: It can change the salinity and density of the ocean water, leading to unstable or not-so-strong AMOC circulation.

Impact of the Decline of AMOC

• Change in regional climate: The weakening of AMOC and Gulf Stream will trigger a cooling effect on climate and a decrease in rainfall over the North Atlantic region, as the northward heat supply is slowed down. It may also lead to an increase in winter storms over Europe and stronger hurricanes in the US.
• Sea level rise: The northward surface flow of the AMOC leads to the deflection of water masses to the right, away from the US east coast. As the current slows down, this effect weakens and more water can pile up at the US east coast, leading to an enhanced sea level rise.
• Severe consequences for Atlantic marine ecosystems: The North Atlantic ecosystem is adapted to the existence of the overturning circulation, which sets the conditions like the seasonal cycle, the temperature, and the nutrient conditions. Any changes in these conditions would disrupt fish populations and other marine life.
• Other impacts: The collapse of the AMOC may induce changes in ENSO (El Nino-Southern Oscillation) characteristics, dieback of the Amazon rainforest, and shrinking of the West Antarctic Ice Sheet due to the seesaw effect, etc.

There is a need to reconcile climate models with the presented observational evidence to assess how far or how close the AMOC really is to its critical threshold. Further, there is an urgent need to ensure the effective implementation of environmental commitments under the Paris climate deal by every country to address climate change and slow down the weakening of AMOC.