Heat from El Niño can warm oceans off West Antarctica – and melt floating ice shelves from below

AndreAnita/Shutterstock Maurice Huguenin, UNSW Sydney; Matthew England, UNSW Sydney, and Paul Spence, University of Tasmania

As snow falls on Antarctica, layers build up and turn to ice. Over time, this compressed snow has become a continent-sized glacier, or ice sheet. It’s enormous – almost double the size of Australia and far larger than the continental United States.

As the weight of ice builds up, the ice sheet begins to move towards the oceans. When it reaches the sea, the ice floats. These floating extensions are known as ice shelves. The largest is over 800 kilometres wide.

When the ocean water has a temperature close to 0°C, these ice shelves can persist for a long time. But when temperatures rise, even a little, the ice melts from below. Antarctic ice shelves are now losing an alarming 150 billion tons of ice per year, adding more water to the ocean and accelerating global sea level rise by 0.6 mm per year. Ice shelves in West Antarctica are particularly prone to melting from the ocean, as many are close to water masses above 0°C.

While the melting trend is clear and concerning, the amount can vary substantially from year-to-year due to the impact of both natural climate fluctuations and human-made climate change. To figure out what is going on and to prepare for the future, we need to tease apart the different drivers – especially El Niño-Southern Oscillation, the world’s largest year-to-year natural climate driver.

Our new research explores how heat brought by El Niño can warm the ocean around West Antarctica and increase melting of the ice shelves from below.

Antarctic Ice Mass Loss 2002-2023. Credit: NASA Climate Change.

How can El Niño-Southern Oscillation affect Antarctica?

Australians are very familiar with the two phases of this climate driver, El Niño and La Niña, as they tend to bring us hotter, dryer weather and cooler, wetter weather, respectively. But the influence of this cycle is much larger, affecting weather and climate all around the Pacific.

Can it reach through Antarctica’s cold, fast currents of air and water? Yes.

Giant convective thunderstorms in the Pacific’s equatorial regions move east during El Niño and intensify in the West during La Niña. As these storm systems change, they excite ripples in the atmosphere that are able to travel large distances, just as waves can cross oceans. Within two months, these atmospheric waves reach the Antarctic continent, where their energy can affect the coastal atmosphere and ocean circulation. During El Niño, the energy from these waves weakens the easterly winds off West Antarctica (and vice versa for La Niña).

Using satellite data, researchers recently found that West Antarctic ice shelves actually gain height but lose mass during El Niño. That’s because more low-density snow falls at the top of the ice shelves, while at the same time more warm water flows under the ice shelves where it melts compressed high-density ice from underneath.

What we don’t yet know is how this warmer water (above zero) comes up from below. Similarly, we don’t know what happens during La Niña.

Answering these questions with the few observations we have from Antarctica is challenging because this climate driver doesn’t happen in isolation. Storms, tides, large eddy currents and other climate drivers such as the Southern Annual Mode can change the temperatures of the water under ice shelves too, and they can occur at the same time as El Niño.

Finding a needle in the ice stack

So how did we do it? Modelling.

We take a high-resolution global ocean circulation model and added El Niño and La Niña events to the baseline simulation. By doing so, we can examine what these anomalies do to the currents and temperatures around Antarctica.

The energy brought by El Niño’s atmospheric waves to West Antarctica weakens the prevailing easterly winds along the coasts.

Normally, most of the warm water reservoir is located off the continental shelf rather than on the continental shelf. As the winds weaken, more of this warmer water – known as Circumpolar Deep Water – is able to flow onto the continental shelf and near the base of the floating ice shelves.

During El Niño, weaker winds along the coasts push less cold Antarctic surface waters towards the continent, allowing warmer Circumpolar Deep Water to flow to the base of the ice shelves. During La Niña, stronger winds drive a wedge of cold water up towards the continent, reducing the inflow of warm water. Maurice Huguenin, CC BY-SA

We call this water mass “warm”, but that’s relative – it’s only 1–2°C above freezing, and the heat only warms the water on the continental shelf by about 0.5°C. But that’s enough to begin melting ice shelves, which are at or below freezing point.

As you’d expect, the longer the warm water stays on the shelf and the hotter it is, the more melting occurs.

During La Niña, the opposite occurs and the ice rebounds. Winds along the coast strengthen, pushing more cold surface water onto the continental shelf and preventing warm water from flowing under the ice shelves.

What does this mean for the near future?

Researchers have found El Niño and La Niña have already become more frequent and more extreme.

If this trend continues, as climate projections suggest, we can expect warming around West Antarctica to get even stronger during El Niño events, accelerating ice shelf melting and speeding up sea level rise.

More frequent and stronger El Niño events could also push us closer to a tipping point in the West Antarctic ice sheet, after which accelerated melting and mass loss could become self-perpetuating. That means the ice wouldn’t melt and reform but begin to steadily melt.

More bad news? Unfortunately, yes. The only way to stop the worst from happening is to get to net zero carbon emissions as quickly as humanly possible.

Maurice Huguenin, Postdoctoral research associate in Physical Oceanography, UNSW Sydney; Matthew England, Scientia Professor and Deputy Director of the ARC Australian Centre for Excellence in Antarctic Science (ACEAS), UNSW Sydney, and Paul Spence, Associate professor of oceanography, University of Tasmania

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Read More........→April 09, 2024

Emperor penguins face a bleak future – but some colonies will do better than others in diverse sea-ice conditions

Sara Labrousse/French Polar Institute, CC BY-SA Sara Labrousse, Sorbonne Université and Michelle LaRue, University of Canterbury

The long-term future looks bleak for Emperor penguins, but our new research shows some birds may be able to survive in certain conditions, depending on where they live, at least for the next few decades.

Over the past two years, Antarctic sea ice has declined dramatically, prompting scientists to suggest it could reach a “new state”.

A study based on satellite images shows that sea ice broke out early in Antarctica’s Bellingshausen Sea in 2022, potentially resulting in breeding failures across several Emperor penguin colonies in that region.

Our research shows Emperors form colonies in surprisingly diverse environmental conditions that vary depending on location around the continent. Within each of these regions, there is little difference between where birds make their homes and other sites, suggesting they could shift if they had to. This provides a ray of hope in an otherwise bleak outlook.

Emperor penguins may be the only birds to rarely set foot on land. They are unique among penguin species in that they breed on sea ice during the harsh Antarctic winter.

Male Emperor penguins incubate eggs and raise the chicks on sea ice during the Antarctic winter. Sara Labrousse/French Polar Institute, CC BY-SA

We know that they need “fast ice” – the coastal sea ice attached to the Antarctic continent or ice shelves. But they actually inhabit a range of fast-ice locations that differ in the timing of ice formation, how much ice forms and breaks, and even how close they get to other penguin species.

Depending on where they are along the Antarctic coast, Emperors make use of the habitat available to them. Their behaviour may be flexible enough to allow some colonies to cope better in a warming world.

Why fast ice is important

Emperor penguins rely on fast ice as a stable platform for their breeding season. Female Emperors lay their eggs and the males incubate them for about two and a half months.

Even though Antarctica’s sea ice is diminishing, this refers to a measure known as “sea ice extent”, which includes all sea ice covering the polar ocean, whether it is fast ice or drifting pack ice.

A decrease in sea ice extent is not necessary linearly linked to a drop in the area covered by fast ice (although the reverse is true).

If fast ice were to disappear, we would expect more than 90% of Emperor colonies to become functionally extinct by the end of the century. However, our study suggests that in the short to medium term, we should consider the differences in the penguins’ breeding habitats when we think about ways to protect them.

Emperors are unlikely to move far

By looking a little closer at different fast-ice habitats, we found Emperor penguins have certain preferences. The persistence of the ice (how long it lasts into the summer) was important because chicks had more time to develop their water-proof swimming feathers.

In some cases, being close to Adélie penguins made a difference. In other cases, Emperors preferred sites with shallow ocean depths below the colony.

Our results suggest that two of these habitat conditions support larger colonies: stable fast ice that lasts throughout the breeding season (with only small changes in the growth and retreat seasonal cycle) and a good balance between a fast-ice platform that is wide enough to raise chicks but close enough to the ocean to get food for them.

Emperor penguins need access to the ocean to feed their chicks during the breeding season. Sara Labrousse/French Polar Institute, CC BY-SA

We need further studies to clarify these links and the relationship between population size and habitat quality. In our study, we weren’t able to consider prey availability and there may be other factors that play an important role.

Previous research has already shown that Emperor penguins have limited capacity to disperse to find more suitable climate refuges. This is supported by the genetic partitioning among the penguin populations in different Antarctic regions we studied.

It is therefore unlikely Emperors would move far to avoid more severe climate impacts, even if “better” habitats existed and could host larger colonies.

Emperors don’t easily move to other breeding sites, even if the conditions are better. Sara Labrousse/French Polar Institute, CC BY-SA

Protecting penguin habitat

Climate change is currently one of the main pressures driving Emperor penguins closer to extinction.

However, the latest global assessment by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) clearly identified fishing activities as historic and current drivers of the erosion of marine biodiversity worldwide.

This is also true for Antarctica. While fishing pressure there is limited to a fraction of the global fishing fleet, some of the largest vessels target krill, a tiny shrimp-like crustacean consumed by many Antarctic predators, including Emperor penguins.

With climate models predicting further reductions in sea ice extent, new fishing grounds could open and amplify pressure on other Antarctic wildlife.

If we want to live in a world with Emperor penguins, the most important thing to do would be to cut greenhouse gas emissions steeply. Another key action could be to prevent fishing in areas where climate change will have the most impact.

In this respect, truly protected areas are one conservation tool at our disposal. Now that our research provides more detailed information about penguin habitats, we can begin the process of more careful planning for conservation.

The world’s largest marine protected area exists in the Ross Sea, which is home to about 25% of the world’s Emperor penguins. Lessons we learn from protection there could help mitigate future declines of Emperors around Antarctica.

Sara Labrousse, Chercheuse en écologie polaire, Sorbonne Université and Michelle LaRue, Associate Professor in Conservation Biology, University of Canterbury

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Read More........→October 04, 2023