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Why are there waves? – Evie, age 5.
Thanks for a great question, Evie.
When you look at the waves breaking at the beach, those waves might be at the end of a long journey. The waves might have been created thousands of kilometres away, or they could have been created near you.
There are lots of types of waves in the ocean, but the waves you usually see at a beach are created by the wind. When the wind blows over a smooth ocean, it creates little waves or ripples on the surface. If the wind continues to blow, the waves grow bigger.
The faster the wind blows (like in a strong storm out at sea), the bigger the waves will grow.
The further the wind blows (or the bigger the area of the storm), the bigger the waves will grow.
And the longer the wind blows (like in a storm that lasts a long time), the bigger the waves will grow.
If the wind stops, or changes direction, the waves will stop growing, but they won’t stop travelling.
They will keep travelling away from where they were created in a straight line, sometimes for days, until they run into something like a beach where they are stopped because they break. That’s why there are still waves at the beach, even when it is not windy.
Waves trip over themselves
Imagine you were running really quickly. But then suddenly, you ran into thick gloopy mud. Your feet would slow down, but the top half of your body would still be going fast. You’d trip over.
Waves do the same thing and that is when they break.
As waves approach the shore, the water is shallower, and the bottom of the wave starts to feel the sand and rocks and seaweed. The bottom of the wave slows down, and soon, the top of the wave is going faster than the bottom part of the wave, so the top spills forward and topples over in a big splash.
Waves can travel a long way
Scientists who study the ocean (called oceanographers) have measured waves created in the Southern Ocean, and seen them travel all the way across the Pacific Ocean and break on the beaches of North America more than a week later.
Try counting the seconds between waves breaking on the beach. If the time between waves is 10 seconds or more, the waves have come from a long way away. If the waves were created nearby, the time between waves will be short, perhaps five seconds or fewer.
Sometimes when we look at the sea we might see different waves (some big, some small) all happening at the same time. These waves were created at different places, perhaps by different storms, but ended up in the same spot at the same time.
During big storms, waves can get very big. If big waves from two different storms meet together, that can create enormous waves that we call “freak waves”. The largest waves measured are around 25 metres high (that’s five giraffes standing on top of each other!) and they can tip over ships.
Large waves after the loss of sea ice can trigger Antarctic ice shelf disintegration over a period of just days, according to our new research.
With other research also published today in Nature showing that the rate of annual ice loss from the vulnerable Antarctic Peninsula has quadrupled since 1992, our study of catastrophic ice shelf collapses during that time shows how the lack of a protective buffer of sea ice can leave ice shelves, already weakened by climate warming, wide open to attack by waves.
Antarctica is covered by an ice sheet that is several kilometres thick in places. It covers an area of 14 million square kilometres – roughly twice the size of Australia. This ice sheet holds more than 90% of the world’s ice, which is enough to raise global mean sea level by 57 metres.
As snow falls and compacts on the ice sheet, the sheet thickens and flows out towards the coast, and then onto the ocean surface. The resulting “ice shelves” (and glacier tongues) buttress three-quarters of the Antarctic coastline. Ice shelves act as a crucial braking system for fast-flowing glaciers on the land, and thus moderate the ice sheet’s contribution to sea-level rise.
In the southern summer of 2002, scientists monitoring the Antarctic Peninsula (the northernmost part of mainland Antarctica) by satellite witnessed a dramatic ice shelf disintegration that was stunning in its abruptness and scale. In just 35 days, 3,250 square km of the Larsen B Ice Shelf (twice the size of Queensland’s Fraser Island) shattered, releasing an estimated 720 billion tonnes of icebergs into the Weddell Sea.
This wasn’t the first such recorded event. In January 1995, roughly 1,500 square km of the nearby Larsen A Ice Shelf suddenly disintegrated after several decades of warming and years of gradual retreat. To the southwest, the Wilkins Ice Shelf suffered a series of strikingly similar disintegration events in 1998, 2008 and 2009 — not only in summer but also in two of the Southern Hemisphere’s coldest months, May and July.
These sudden, large-scale fracturing events removed features that had been stable for centuries – up to 11,500 years in the case of Larsen B. While ice shelf disintegrations don’t directly raise sea level (because the ice shelves are already floating), the removal of shelf ice allows the glaciers behind them to accelerate their discharge of land-based ice into the ocean – and this does raise sea levels. Previousresearch has shown that the removal of Larsen B caused its tributary glaciers to flow eight times faster in the year following its disintegration.
The ocean around ice shelves is typically covered by a very different (but equally important) type of ice, called sea ice. This is formed from frozen seawater and is generally no more than a few metres thick. But it stretches far out into the ocean, doubling the area of the Antarctic ice cap when at its maximum extent in winter, and varying in extent throughout the year.
The response of Antarctic sea ice to climate change and variability is complex, and differs between regions. Around the Antarctic Peninsula, in the Bellingshausen and northwestern Weddell seas, it has clearly declined in extent and annual duration since satellite monitoring began in 1979, at a similar rate to the Arctic’s rapidly receding sea ice.
The Southern Ocean is also host to the largest waves on the planet, and these waves are becoming more extreme. Our new study focuses on “long-period” swell waves (with swells that last up to about 20 seconds). These are generated by distant storms and carry huge amounts of energy across the oceans, and can potentially flex the vulnerable outer margins of ice shelves.
The earliest whalers and polar pioneers knew that sea ice can damp these waves — Sir Ernest Shackleton reported it in his iconic book South!. Sea ice thus acts as a “buffer” that protects the Antarctic coastline, and its ice shelves, from destructive ocean swells.
Strikingly, all five of the sudden major ice shelf disintegrations listed above happened during periods when sea ice was abnormally low or even absent in these regions. This means that intense swell waves crashed directly onto the vulnerable ice shelf fronts.
The straw that broke the camel’s back
The Antarctic Peninsula has experienced particularly strong climate warming (roughly 0.5℃ per decade since the late 1940s), which has caused intense surface melting on its ice shelves and exacerbated their structural weaknesses such as fractures. These destabilising processes are the underlying drivers of ice shelf collapse. But they do not explain why the observed disintegrations were so abrupt.
Our new study suggests that the trigger mechanism was swell waves flexing and working weaknesses at the shelf fronts in the absence of sea ice, to the point where they calved away the shelf fronts in the form of long, thin “sliver-bergs”. The removal of these “keystone blocks” in turn led to the catastrophic breakup of the ice shelf interior, which was weakened by years of melt.
Our research thus underlines the complex and interdependent nature of the various types of Antarctic ice – particularly the important role of sea ice in forming a protective “buffer” for shelf ice. While much of the focus so far has been on the possibility of ice shelves melting from below as the sea beneath them warms, our research suggests an important role for sea ice and ocean swells too.
In July 2017 an immense iceberg broke away from the Larsen C Ice Shelf, just south of Larsen B, prompting fears that it could disintegrate like its neighbours.
Our research suggests that four key factors will determine whether it does: extensive flooding and fracturing across the ice shelf; reduced sea ice coverage offshore; extensive fracturing of the ice shelf front; and calving of sliver-bergs.
If temperatures continue to rise around the Antarctic, ice shelves will become weaker and sea ice less extensive, which would imply an increased likelihood of future disintegrations.
However, the picture is not that clear-cut, as not all remaining ice shelves are likely to respond in the same way to sea ice loss and swell wave impacts. Their response will also depend on their glaciological characteristics, physical setting, and the degree and nature of surface flooding. Some ice shelves may well be capable of surviving prolonged absences of sea ice.
Irrespective of these differences, we need to include sea ice and ocean waves in our models of ice sheet behaviour. This will be a key step towards better forecasting the fate of Antarctica’s remaining ice shelves, and how much our seas will rise in response to projected climate change over coming decades. In parallel, our new findings underline the need to better understand and model the mechanisms responsible for recent sea ice trends around Antarctica, to enable prediction of likely future change in the exposure of ice shelves to ocean swells.
Luke Bennetts, Lecturer in applied mathematics, University of Adelaide; Rob Massom, Leader, Sea Ice Group, Antarctica & the Global System program, Australian Antarctic Division and Antarctic Climate and Ecosystems CRC, and Vernon Squire, Deputy Vice-Chancellor Academic, Professor of Applied Mathematics
At first glance it may not seem so, but the story of the now-vanished island of Nahlapenlohd, a couple of kilometres south of Pohnpei Island in Micronesia, holds some valuable lessons about recent climate change in the western Pacific.
In 1850, Nahlapenlohd was so large that not only did it support a sizeable coconut forest, but it was able to accommodate a memorable battle between the rival kingdoms of Kitti and Madolenihmw. The skirmish was the first in Pohnpeian history to involve the European sailor-mercenaries known as beachcombers and to be fought with imported weapons like cannons and muskets.
Today the island is no more. The oral histories tell that so much blood was spilled in this fierce battle that it stripped the island of all its vegetation, causing it to shrink and eventually disappear beneath the waves.
Like many oral tales, this one tries to explain island disappearance post-1850 by making reference to an historical event. But in light of what we know today, the more plausible cause of the island’s disappearance is the sea-level rise in the western Pacific since the early 19th century, which has accelerated significantly over the past few decades. The disappearance of islands in the Solomon Islands in the southwest Pacific has recently been attributed to sea level rise. Further north, the same is true of several reef islands off Pohnpei.
Surveys of 12 of these islands have shown that not only have some – like Nahlapenlohd – completely disappeared, but that most others have shrunk over the past decade. Islands such as Laiap and Ros, which have lost two-thirds of their land area over this time, are likely to disappear completely within the coming decade.
Why are islands in the western Pacific becoming the earliest casualties of sea-level rise? Partly because sea levels in this region have risen at two to three times the global average over the past few decades.
In parts of Micronesia, sea level has risen by 10-12mm each year between 1993 and 2012, far outpacing the global average of 3.1mm a year. While this rate is unlikely to be sustained indefinitely, the current trend would raise sea levels by a further 30-40cm by mid-century if it were to continue.
What’s more, reef islands are particularly vulnerable to erosion by rising seas, being made almost entirely of sand and gravel. Whole islands – even some island nations with which we are familiar today – are likely to be rendered uninhabitable or even disappear within the next 30 years. These include islands in nations like Kiribati, the Marshall Islands, Tokelau and Tuvalu, as well as some in other island nations that comprise mostly larger islands, such as the Federated States of Micronesia, of which Pohnpei is one.
Yet we should note that not all of Pohnpei’s reef islands are disappearing, at least not at the same rate, and some have fortuitously evolved protection that will likely help them outlive their neighbours.
The coasts of some islands – like Kehpara and Nahlap – are “armoured” by beaches of huge boulders left there by large storms, often along their most exposed coasts. Other reef islands off Pohnpei’s leeward coast, such as Dawahk, are becoming “skeletonized” as waves wash across the island removing the sand and leaving only rocks, held in place by a maze of giant mangrove roots.
Whether or not the islands themselves succumb or survive, sea-level rise is a clear threat to their habitability for humans. Short-term interventions – either natural fortifications such as boulder beaches, or human-built defences such as seawalls – are unlikely to change the long-term outcome.
This underscores the fact that low-lying reef islands are transient – most Pacific reef islands formed only in the past 4,000 years after sea levels fell and sediment began to pile up on exposed reef platforms. The sea will remove today’s islands, just as it has washed away countless others before.
But of course we cannot ignore the human dimension. While only a few dozen people today call the reef islands of Pohnpei home, they are similar to many larger reef islands in Micronesia from which people may well be involuntarily displaced during the next few decades. Where these people might go, and how they can be accommodated in ways that preserve their dignity as well as their unique cultures, are very real questions for community leaders.
People first reached the islands of Micronesia from the Philippines, about 3,500 years ago after an unbroken ocean crossing of 2,300km. It’s an extraordinary achievement when you consider that people in most other parts of the world at that time rarely sailed out of sight of land. To have survived on islands in the middle of the ocean for more than three millennia, Micronesians and other Pacific islanders must have developed considerable resilience.
On high islands in Micronesia, the evidence for this is manifest. Ancient stonework constructions line many parts of the coastline, testament to a long
history of resisting shoreline change, and sometimes of manipulating it for human advantage.
Perhaps nowhere is more evocative of this today than Nan Madol, a megalithic complex built 1,000 years ago on 93 artificial islands off southeast Pohnpei. There are many explanations about why Nan Madol was created. Perhaps the truth is that it is an expression of dogged human resilience – one of hundreds along Micronesian coasts – in the face of an unruly nature.
I thank my co-researchers on the project focused on Pohnpei’s reef islands, Augustine Kohler from the Department of National Archives, Culture and Historic Preservation of the Government of the Federated States of Micronesia, and my colleague Roselyn Kumar from the University of the Sunshine Coast’s Sustainability Research Centre.