Category Archives for waves

Climate change is making ocean waves more powerful, threatening to erode many coastlines


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Thomas Mortlock, Macquarie University; Itxaso Odériz, Universidad Nacional Autónoma de México (UNAM); Nobuhito Mori, Kyoto University, and Rodolfo Silva, Universidad Nacional Autónoma de México (UNAM)Sea level rise isn’t the only way climate change will devastate the coast. Our research, published today, found it is also making waves more powerful, particularly in the Southern Hemisphere.

We plotted the trajectory of these stronger waves and found the coasts of South Australia and Western Australia, Pacific and Caribbean Islands, East Indonesia and Japan, and South Africa are already experiencing more powerful waves because of global warming.

This will compound the effects of sea level rise, putting low-lying island nations in the Pacific — such as Tuvalu, Kiribati and the Marshall Islands — in further danger, and changing how we manage coasts worldwide.

But it’s not too late to stop the worst effects — that is, if we drastically and urgently cut greenhouse gas emissions.

An energetic ocean

Since the 1970s, the ocean has absorbed more than 90% of the heat gained by the planet. This has a range of impacts, including longer and more frequent marine heatwaves, coral bleaching, and providing an energy source for more powerful storms.

Since at least the 1980s, wave power has increased worldwide as more heat is pumped into the ocean.
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But our focus was on how warming oceans boost wave power. We looked at wave conditions over the past 35 years, and found global wave power has increased since at least the 1980s, mostly concentrated in the Southern Hemisphere, as more energy is being pumped into the oceans in the form of heat.

And a more energetic ocean means larger wave heights and more erosive energy potential for coastlines in some parts of the world than before.



Read more:
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Ocean waves have shaped Earth’s coastlines for millions of years. So any small, sustained changes in waves can have long-term consequences for coastal ecosystems and the people who rely on them.

Mangroves and salt marshes, for example, are particularly vulnerable to increases in wave energy when combined with sea level rise.

To escape, mangroves and marshes naturally migrate to higher ground. But when these ecosystems back onto urban areas, they have nowhere to go and die out. This process is known as “coastal squeeze”.

These ecosystems often provide a natural buffer to wave attack for low-lying coastal areas. So without these fringing ecosystems, the coastal communities behind them will be exposed to more wave energy and, potentially, higher erosion.

Mangrove forests are among the most imperilled ecosystems as sea levels rise and ocean waves crash harder against the coast.
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So why is this happening?

Ocean waves are generated by winds blowing along the ocean surface. And when the ocean absorbs heat, the sea surface warms, encouraging the warm air over the top of it to rise (this is called convection). This helps spin up atmospheric circulation and winds.

In other words, we come to a cascade of impacts: warmer sea surface temperatures bring about stronger winds, which alter global ocean wave conditions.



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Our research shows, in some parts of the world’s oceans, wave power is increasing because of stronger wind energy and the shift of westerly winds towards the poles. This is most noticeable in the tropical regions of the Atlantic and Pacific Oceans, and the subtropical regions of the Indian Ocean.

But not all changes in wave conditions are driven by ocean warming from human-caused climate change. Some areas of the world’s oceans are still more influenced by natural climate variability — such as El Niño and La Niña — than long-term ocean warming.

In general, it appears changes to wave conditions towards the equator are more driven by ocean warming from human-caused climate change, whereas changes to waves towards the poles remain more impacted by natural climate variability.

Ocean waves are generated by winds blowing across the ocean surface.
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How this could erode the coasts

While the response of coastlines to climate change is a complex interplay of many processes, waves remain the principal driver of change along many of the world’s open, sandy coastlines.

So how might coastlines respond to getting hit by more powerful waves? It generally depends on how much sand there is, and how, exactly, wave power increases.

For example, if there’s an increase in wave height, this may cause increased erosion. But if the waves become longer (a lengthening of the wave period), then this may have the opposite effect, by transporting sand from deeper water to help the coast keep pace with sea level rise.

Sandy beaches, including those around South Australia and Western Australia, may see greater risk of erosion in coming decades as wave power increases.
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For low-lying nations in areas of warming sea surface temperatures around the equator, higher waves – combined with sea level rise – poses an existential problem.

People in these nations may experience both sea level rise and increasing wave power on their coastlines, eroding land further up the beach and damaging property.
These areas should be regarded as coastal climate hotspots, where continued adaption or mitigation funding is needed.

It’s not too late

It’s not surprising for us to find the fingerprints of greenhouse warming in ocean waves and, consequentially, along our coastlines. Our study looked only at historical wave conditions and how these are already being impacted by climate change.

But if warming continues in line with current trends over the coming century, we can expect to see more significant changes in wave conditions along the world’s coasts than uncovered in our backward-looking research.

However, if we can mitigate greenhouse warming in line with the 2℃ Paris agreement, studies indicate we could still keep changes in wave patterns within the bounds of natural climate variability.



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Still, one thing is abundantly clear: the impacts of climate change on waves is not a thing of the future, and is already occurring in large parts of the world’s oceans.

The extent to which these changes continue and the risk this poses to global coastlines will be closely linked to decarbonisation efforts over the coming decades.

This story is part of Oceans 21

Our series on the global ocean opened with five in depth profiles. Look out for new articles on the state of our oceans in the lead up to the UN’s next climate conference, COP26. The series is brought to you by The Conversation’s international network.The Conversation

Thomas Mortlock, Senior Risk Scientist, Risk Frontiers, Adjunct Fellow, Macquarie University; Itxaso Odériz, Research assistant, Universidad Nacional Autónoma de México (UNAM); Nobuhito Mori, Professor, Kyoto University, and Rodolfo Silva, Professor, Universidad Nacional Autónoma de México (UNAM)

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

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The story of a wave: from wind-blown ripples to breaking on the beach



By the time a wave reaches shore, it may have travelled tens of thousands of kilometres.
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Shane Keating, UNSW

It’s a cliché, but Aussies love the beach. And little wonder: with 36,000 kilometres of coastline, Australia is blessed with some of the best beaches in the world.

Around 20 million Australians live within 50 kilometres of the coast. As summer temperatures soar, we flock to the ocean to splash, swim, surf, paddle, and plunge in the waves.

But where do those waves come from? How do they form, and why do they break? As it turns out, what we see at the shore is just the last few moments of an epic journey.



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Great waves from tiny ripples grow

The waves we see crashing on the beach can begin their lives tens of thousands of kilometres away. Surface waves, as they are known, are born when the wind blows over the ocean, amplifying small ripples and transferring momentum from the atmosphere to the water.

The height of the wave depends on how long the wind is blowing and the distance – or fetch – over which it blows. The largest waves are created by distant storms, which churn up the surface of the ocean and radiate waves outwards like ripples in a pond.

Surface waves don’t move the water itself very far – each water molecule travels forward and back in a circle a few meters across and ends up back at its starting point.

As the wave crest rises, water molecules gather gravitational potential energy that is released as kinetic energy when the water descends into the trough of the wave. This energy is then passed onto the next crest in a see-saw of kinetic and potential energy that can propagate across an entire ocean basin.

The mounting wave

Once a wave leaves the open ocean and approaches land, the sea floor begins to exert its influence. Surface waves transmit their energy more slowly in shallow water than in deep water. This causes energy to pile up near the shore. Waves start to shoal, becoming taller, steeper, and more closely spaced.

Once a wave grows too steep to hold together, it breaks. Breaking waves come in different varieties.

Spilling breakers, which crumble gently into white water, occur when the sea floor rises relatively slowly.

By contrast, plunging breakers – the classic rolling waves favoured by surfers – form when the sea floor rises sharply, particularly near reefs and rocky headlands.

Finally, surging waves occur when the shore is almost vertical. These waves don’t produce breakers but rather a rhythmic rise and fall of the sea surface.

Bend it like bathymetry

The shape or topography of the sea floor – called bathymetry – can have remarkable effects on breaking waves. If the depth of the sea floor changes parallel to the coast, incoming waves will refract or bend so their crests line up with the shoreline.

The effect can be clearly seen near headlands: waves close to the headland move slowly because the water is shallow, while waves further out move more quickly. This causes waves to curl around the headland like a marching band rounding a corner.

Bathymetry is also responsible for some of the biggest waves on Earth. Famous big wave surf spots like Mavericks in Northern California and Nazaré in Portugal benefit from undersea canyons that refract incoming waves and focus them into monsters. The Nazaré wave originates from an undersea canyon almost 5 kilometres deep to produce waves as tall as an eight-storey building.



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Don’t risk the rip

The story of a wave doesn’t end when it breaks, however. Breaking waves push water towards the shore, raising the water level. This water will try to flow back offshore via the lowest point along the beach. The result is a rip current: a swift, narrow current that flows out to sea.

Rip currents are Australia’s number one coastal hazard, responsible for more fatalities per year than shark attacks, bush fires, floods, and cyclones combined. Inexperienced swimmers caught in a rip can panic and try to swim against the current, which is a dangerous recipe for exhaustion. Yet most Australians are unable to identify a rip current, and two-thirds of those who think they can get it wrong.

Purple dye traces the path of a rip current.
Rob Brander

To spot a rip, look for a gap in the waves, a dark channel, or ripples surrounded by smoother water. The safest thing to do is to stick to patrolled beaches and swim between the flags. If you do find yourself caught in a rip, Surf Lifesaving Australia advises you to stay calm and conserve your energy.

Rip currents are usually quite narrow, so swim at right angles to the current until you are outside the rip. If you are too tired to swim, tread water and let yourself go with the flow until the rip weakens and you can signal for help.

Above all, if you are unsure, don’t risk the rip. Sit back and enjoy the waves from a safe distance instead.The Conversation

Shane Keating, Senior Lecturer in Mathematics and Oceanography, UNSW

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