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21 February 2023 5 min read

Antarctica's barren icy landscapes, immense skies and churning seas captivate and inspire the imagination.

The place is remote, difficult to get to and not exactly welcoming. For many of us, the ends of the Earth are a mystery we won’t get to experience in our lifetimes. And from those who have been to the southern pole—it’s otherworldly, oftentimes difficult to describe.

Journeys to Antarctica are treacherous but visitors are greeted upon arrival by massive, frozen landscapes that stretch to the horizon, eerie soundscapes, as well as waters teeming with wildlife including leopard seals and penguins. As ships approach, predators and their prey alike flee—either across the floating sea ice being broken up by the hull, or into the waters below it.

Deep below and around the vast continent lie many other mysteries: underwater canyons and mountain ranges; a hidden lake beneath the ice; ancient fossils dating back millions of years to when the now-frozen continent was home to tropical rainforests; and a host of unique creatures we don’t see in our everyday lives—including krill, whales, squid and, in the deeper waters, some almost nightmarish looking fish.

The mysteries of Antarctica aren’t just limited to its unique and raw beauty. The dynamics of the region, across ice sheets and the Southern Ocean, play a key role in the global climate system. At the same time, it is one of the most sensitive areas on Earth to a warming climate.

But given how difficult it can be to reach and observe this part of the world, its interplay with the climate system is poorly understood. Indeed, there is still a lot about Antarctica and its surrounds that remain a mystery, even in science.

The dynamics of Antarctica, across ice sheets and the Southern Ocean, play a key role in the global climate system. Image: CSIRO, Esmee van Wijk

The ENSO effect

The El Niño Southern Oscillation (ENSO) coupled ocean-atmosphere phenomena in the tropical Pacific is a key driver of climate variability worldwide.

A new study published today reveals that projected changes in the tropical Pacific can influence the Southern Ocean: specifically, an increasingly stronger El Niño may accelerate the melting of Antarctica’s ice shelves and ice sheets.

Ice shelves are ice that formed on land but float over the ocean while still attached to the land ice. Ice sheets are the ice that remains on the land.

The latter lock up a huge amount of water—the Antarctic ice sheets contain the equivalent of about 60 metres of global sea level.

Lead author of the study, Dr Wenju Cai from CSIRO Oceans and Atmosphere, is a global expert on the relationship between climate change and ENSO. He says about 80% of climate models are predicting an increase in ENSO variability between the 20th and 21st century, with an average 15% increase.

“This translates to a doubling in strong El Niño and strong La Niña from about one event every 20 years in the 20th century to one event every 10 years in the 21st century,” Dr Cai said.

Co-author Dr Ariaan Purich, from Securing Antarctica’s Environmental Future at Monash University, says ENSO has far reaching influences.

“It can affect the atmospheric circulation, air temperatures and snowfall over the continent,” Dr Purich said. “It can affect the freezing and melting of sea ice and it can affect the ocean circulation beneath the floating ice shelves.”

The predicted increase in ENSO variability will not only see an increase in the number of extreme events such as droughts and bushfires for Australia during El Niño, but also has the potential to fast-track sea level rise through its impact on Antarctica.

Heat from below

The findings of this new study indicate that an increase in ENSO variability leads to an accelerated warming of Antarctica’s deeper waters.

This is caused by ENSO-related winds slowing down the ocean upwelling, which affects the surface and deep waters differently. While the ocean’s surface will warm more slowly in response to global warming, the deeper ocean around Antarctica will warm more quickly due to the influence of increased ENSO.

These deeper waters around the Antarctic margins interact with the ice shelves and ice sheets—so an accelerated warming could melt them faster.

Dr Purich says the ice shelves provide buttressing support to ice sheets.

“Changes occurring around the Antarctic margins have the potential to raise global sea levels, and thus impact coastal communities around the world,” Dr Purich said.

“When ice shelves are melted by a warming ocean below, they can break away. And this reduces the buttressing support for the ice sheets, which allows them to slide into the ocean and melt faster—raising sea levels.”

However, she also says that due to the extreme nature and remoteness of Antarctica, there are still many unknowns on the processes influencing the rate of Antarctic melts and related sea level projections.

The IPCC predicts global sea levels will continue to rise well after 2100, mostly because of melting glaciers and ice sheets.

“To adapt to sea level rise, we need to better constrain sea level projections. For this, we need to improve our understanding of large-scale processes influencing ocean temperature on the Antarctic shelf, including those described in this new study.”

The findings of this new study indicate that an increase in ENSO variability leads to an accelerated warming of Antarctica’s deeper waters. Image: CSIRO, Clothilde Langlais

Not just the melt

A lot of what happens in Antarctica has impacts across the globe. The Southern Ocean, which lies south of 30°S, surrounds the continent. It is the most active of all the world’s oceans in heat carbon drawdown, absorbing around 70% of heat uptake and 45% of anthropogenic carbon dioxide uptake by the global ocean over the historical period.

Dr Purich says sea ice cover is also important for the climate system, as these bright surfaces reflect sunlight back into space. Without reflective surfaces, more solar energy gets absorbed on Earth—and its ocean temperatures rise.

“The sea ice is really important for the surface radiation balance,” Dr Purich said. “It also plays an important role in influencing the deep ocean circulation, which in turn impacts global climate.”

Where to from here?

Dr Cai says future climate research needs more focused attention on the Antarctic region.

“The impact of ENSO changes on Antarctic climate change was not previously clear,” Dr Cai said.

“And previous projections of sea level rise have not taken into account the impact from increased ENSO variability, which is therefore likely under-estimated.”

He says integrating interactive ice sheet models within Earth system models will increase their accuracy and remove some of the Antarctic mystery.

“More work is needed to understand the magnitude of effects on overall ice loss,” Dr Cai said.

“Our research is a critical step in further understanding how Antarctica will be affected by climate change.”

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