Cause and Effect | The slowdown of global ocean currents

ByTannu Jain

Apr 21, 2023 03:16 PM IST

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Ocean currents are slowing due to melting ice, with dire consequences for the planet. Here is what we know

A rather complicated consequence of warming oceans and melting ice is the slowdown of deep ocean currents that carry heat, oxygen, carbon and nutrients for marine life across the globe.

The Southern Ocean system moves nutrient-dense water north from Antarctica, past New Zealand into the North Pacific Ocean, the North Atlantic and Indian Ocean. (Reuters)

At the present level of temperature rise, this system of currents in the Antarctic, known as the Antarctic Overturning Circulation, or the abyssal overturning circulation, is on track to slow by 42% by 2050, a new study has warned.

As suggested earlier, the study, Abyssal ocean overturning slowdown and warming driven by Antarctic meltwater, published in the journal, Nature, on March 29, said that deep ocean currents, or overturnings, in the northern and southern hemispheres, have been relatively stable for thousands of years, but they are now being disrupted by the warming climate.

And it isn’t just the Antarctic Ocean. In 2018, another research paper, Observed fingerprint of a weakening Atlantic Ocean overturning circulation, warned of a similar slowing down of the Atlantic Ocean circulation system, which the study said had weakened significantly over the last 150 years.

Research has warned the Atlantic Ocean circulation system has weakened significantly over the last 150 years. (Shutterstock)

But what are these ocean systems?

The global ocean is made of five connected basins, where water flows in streams called currents. These ocean currents facilitate the interbasin exchange of water heat, salt, dissolved gases and nutrients.

There are two types of currents: Surface, which depend on wind, tide and spin of the Earth; and deep currents, which depend on differences in the density of water.

With deep currents, the water goes from the surface to the sea floor in a process called thermohaline circulation: cold, salty oxygen-rich water is dense and sinks to the ocean abyss, leaving the warmer water on the surface.

After a long, complicated process, warm water reaches the poles where it forms ice, leaving the salt in the ocean, which in turn increases the water density and hence, causes it to sink to the floor.

As it sinks, it pushes water from underneath to the surface to replace the water that sank in a process called upwelling. This water also brings nutrients to the surface for marine life to sustain.

The ocean currents facilitate the interbasin movement of nutrients sustaining marine life. (Reuters)

This cyclical movement of water, dubbed the global conveyor belt by scientists, controls the global climate and has sustained life for thousands of years.

But, as the planet warms, melting more ice, this circulation system is slowing down. Its implications would range from faster melting of the ice to changes in rain patterns, the scientists said.

The larger focus till now was on the Atlantic Meridional Overturning Circulation (AMOC), the system of currents that carry warm water from the tropics into the North Atlantic. The Southern Ocean system, which moves nutrient-dense water north from Antarctica, past New Zealand into the North Pacific Ocean, the North Atlantic and the Indian Ocean, remains less studied.

In the latest research, scientists explained the slowing of the circulation as such: “We find that meltwater input around Antarctica drives a contraction of Antarctic Bottom Water (AABW), opening a pathway that allows warm Circumpolar Deep Water greater access to the continental shelf. The reduction in AABW formation results in warming and ageing of the abyssal ocean, consistent with recent measurements.”

The abyssal warming is projected to continue at an increasing rate by 2050 and “to spread more broadly through all Southern Ocean basins”, they said, adding that there is evidence that “AABW has warmed, freshened and reduced in volume”.

"As the meltwater release from Greenland and Antarctica increases over time, the AABW overturning and AMOC strength both weaken by 2050,” the report said, pegging the decline in AMOC at 19%.

This weakening of the circulations would then act as a tipping point, with ranging implications.

For the AMOC, a slowdown would cause cooling around the northern hemisphere, particularly around Eastern Europe and the east coast of North America, and a 0.2°C cooling in global mean surface temperature by 2061–2080.

For the Antarctic Overturning Circulation, on the other hand, one implication would be the nutrient-rich seawater will build up on the seafloor, hurting marine ecosystems.

A second implication would be more heat getting to the ice, accelerating global sea-level rise.

Third, the slowdown could reduce the ocean’s ability to absorb carbon dioxide, leaving more greenhouse gas in the atmosphere, and leading to more warming.

It could also shift rain bands in the tropics by as much as 1,000 kilometres, thus changing the monsoon patterns across the world and closer home in India (the country typically witnesses monsoon between June to September), having a cascading impact on water availability and food production.

Rainfall patterns near the tropics are at a risk of changing. (Satish Bate/HT Photo)

“Our projections were run under a 'business as usual' scenario. Deep and urgent emissions reductions will give us a chance of avoiding an ocean overturning collapse. But the time is running out fast. And 2050 is just 26 years, 9 months and 2 days away,” Matthew England, deputy director of the Australian Research Council’s Centre for Excellence in Antarctic Science, who coordinated the study, said on Twitter.