Melting Antarctic ice will slow the world’s strongest ocean current – and the global consequences are profound

Flowing clockwise around Antarctica, the Antarctic Circumpolar Current is the strongest ocean current on the planet. It’s five times stronger than the Gulf Stream and more than 100 times stronger than the Amazon River.

It forms part of the global ocean “conveyor belt” connecting the Pacific, Atlantic, and Indian oceans. The system regulates Earth’s climate and pumps water, heat and nutrients around the globe.

But fresh, cool water from melting Antarctic ice is diluting the salty water of the ocean, potentially disrupting the vital ocean current.

Our new research suggests the Antarctic Circumpolar Current will be 20% slower by 2050 as the world warms, with far-reaching consequences for life on Earth.

Why should we care?

The Antarctic Circumpolar Current is like a moat around the icy continent.

The current helps to keep warm water at bay, protecting vulnerable ice sheets. It also acts as a barrier to invasive species such as southern bull kelp and any animals hitching a ride on these rafts, spreading them out as they drift towards the continent. It also plays a big part in regulating Earth’s climate.

Unlike better known ocean currents – such as the Gulf Stream along the United States East Coast, the Kuroshio Current near Japan, and the Agulhas Current off the coast of South Africa – the Antarctic Circumpolar Current is not as well understood. This is partly due to its remote location, which makes obtaining direct measurements especially difficult.

Understanding the influence of climate change

Ocean currents respond to changes in temperature, salt levels, wind patterns and sea-ice extent. So the global ocean conveyor belt is vulnerable to climate change on multiple fronts.

Previous research suggested one vital part of this conveyor belt could be headed for a catastrophic collapse.

Theoretically, warming water around Antarctica should speed up the current. This is because density changes and winds around Antarctica dictate the strength of the current. Warm water is less dense (or heavy) and this should be enough to speed up the current. But observations to date indicate the strength of the current has remained relatively stable over recent decades.

This stability persists despite melting of surrounding ice, a phenomenon that had not been fully explored in scientific discussions in the past.

What we did

Advances in ocean modelling allow a more thorough investigation of the potential future changes.

We used Australia’s fastest supercomputer and climate simulator in Canberra to study the Antarctic Circumpolar Current. The underlying model, ACCESS-OM2-01, has been developed by Australian researchers from various universities as part of the Consortium for Ocean-Sea Ice Modelling in Australia.

The model captures features others often miss, such as eddies. So it’s a far more accurate way to assess how the current’s strength and behaviour will change as the world warms. It picks up the intricate interactions between ice melting and ocean circulation.

In this future projection, cold, fresh melt water from Antarctica migrates north, filling the deep ocean as it goes. This causes major changes to the density structure of the ocean. It counteracts the influence of ocean warming, leading to an overall slowdown in the current of as much as 20% by 2050.

Far-reaching consequences

The consequences of a weaker Antarctic Circumpolar Current are profound and far-reaching.

As the main current that circulates nutrient-rich waters around Antarctica, it plays a crucial role in the Antarctic ecosystem.

Weakening of the current could reduce biodiversity and decrease the productivity of fisheries that many coastal communities rely on. It could also aid the entry of invasive species such as southern bull kelp to Antarctica, disrupting local ecosystems and food webs.

A weaker current may also allow more warm water to penetrate southwards, exacerbating the melting of Antarctic ice shelves and contributing to global sea-level rise. Faster ice melting could then lead to further weakening of the current, commencing a vicious spiral of current slowdown.

This disruption could extend to global climate patterns, reducing the ocean’s ability to regulate climate change by absorbing excess heat and carbon in the atmosphere.

Need to reduce emissions

While our findings present a bleak prognosis for the Antarctic Circumpolar Current, the future is not predetermined. Concerted efforts to reduce greenhouse gas emissions could still limit melting around Antarctica.

Establishing long-term studies in the Southern Ocean will be crucial for monitoring these changes accurately.

With proactive and coordinated international actions, we have a chance to address and potentially avert the effects of climate change on our oceans.

The authors thank Polar Climate Senior Researcher Dr Andreas Klocker, from the NORCE Norwegian Research Centre and Bjerknes Centre for Climate Research, for his contribution to this research, and Professor Matthew England from the University of New South Wales, who provided the outputs from the model simulation for this analysis.