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

How global warming is reshaping winter life in Canada

H. Damon Matthews, Concordia University and Mitchell Dickau, Concordia University As we begin to emerge out of yet another mild winter, Canadians are once again being reminded of just how acutely global warming has changed Canada’s winter climate.

The impacts of this mild winter were felt across the country and touched all aspects of winter culture. From melting ice castles at Québec’s winter carnival, to a dismal lack of snow at many Western Canada ski resorts, seemingly no part of Canada was unaffected. But the change that will likely be felt most keenly by many Canadians is the loss of a reliable outdoor skating season.

For the second year running, Ottawa’s Rideau Canal Skateway was closed for what should be the peak of the skating season. In 2022-2023, the Skateway did not open at all for the first time ever. This winter, a portion of the Skateway opened briefly in January, but continuing mild temperatures forced a closure again after only four days of skating. In Montréal, fewer than 40 per cent of the city’s outdoor rinks were open in the middle of February.

There is no obvious upside to this story. Outdoor skating in Canada is fast becoming the latest casualty of our failure to confront the reality of the climate crisis.

On thin ice

More than a decade ago, our research group published our first analysis of how outdoor skating was being affected by warming winter temperatures in Canada. We showed that even as of 2005, there was already evidence of later start dates, and shorter skating seasons across most of the country.

A report on the management of the Rideau Canal Skateway in 2023, produced by the CBC.

These conclusions were echoed by subsequent publications from the RinkWatch project, which has reported consistent declines in skating season length and quality in many Canadian cities.

Meanwhile in Ottawa, skating days on the Rideau Canal Skateway have been trending downwards over the last 20 years. In this time, the typical skating season has decreased by almost 40 per cent, a trend that is clearly correlated with increasing winter temperatures over the same period.

Moving in the wrong direction

Climate mitigation progress continues to be far too slow.

Global CO2 emissions reached their highest level ever recorded in 2023, and average global temperatures have now reached 1.3 C above pre-industrial temperatures. If these trends continue, we are on track to reach 1.5 C — the lower threshold of the Paris Agreement temperature target — in less than seven years.

In our 2012 paper, we estimated that suitable rink flooding days could disappear across most of southern Canada by mid-century. In a more recent analysis of Montréal’s outdoor rinks, we estimated that the number of viable skating days in Montréal could decrease to zero by as early as 2070.

In hindsight, these and other similar projections may have been far too optimistic. In a study of Rideau canal skating days published in 2015, the authors projected declining but sustained skating conditions throughout this century, even in a high future emissions scenario. The reality of the past two seasons shows that skating conditions have deteriorated far more quickly than predicted.

Global temperatures in 2023 were the highest ever recorded, as were winter temperatures in December 2023 and January 2024. Since 1950, winter temperatures in Canada have increased by more than 3 C, which is about three times the rate of global warming over this same period.

Outdoor rinks require at least three consecutive very cold days to establish a foundation of ice, followed by enough cold days to maintain a good ice surface. Temperatures above freezing are poorly tolerated by outdoor rinks, and rain is often disastrous.

A few degrees of warming in January and February temperatures can be the difference between a rink that is skatable and one that is not. As winters continue to warm, the case for building and maintaining outdoor municipal rinks will become harder to justify.

A stark and still changing new reality

As years go by without any real progress on climate mitigation, it is becoming increasingly difficult to imagine a future in which outdoor rinks will be widely available without artificial refrigeration. Other winter activities will also be affected by changing snow conditions, but outdoor skating will likely be hit first in direct response to warming winter temperatures.

Wayne Gretzky famously learned to skate and play hockey in Branford, Ont. in the 1960s on an outdoor rink built by his father. Reliable winter skating conditions in southern Ontario are already mostly a thing of the past, and are becoming more and more scarce as global warming progresses. It is increasingly unlikely that current and future generations will be able to follow Gretzky’s path.

This reality is both a tragic injustice for many young Canadians and an existential threat to a core aspect of the Canadian winter identity.

Preserving what remains of Canada’s winter skating culture will require that we rapidly step up our efforts to drive down CO2 emissions and stabilize global temperatures. Otherwise, Joni Mitchell’s “river I could skate away on” will become an increasingly wishful dream that soon will exist only in the lyrics of old songs.

H. Damon Matthews, Professor and Climate Scientist, Department of Geography, Planning and Environment, Concordia University and Mitchell Dickau, PhD Candidate, Geography, Planning, and Environment Department, Concordia University

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

Read More........→March 28, 2024