Climate explained: why coastal floods are becoming more frequent as seas rise



As sea levels rise, it becomes easier for ocean waves to spill further onto land.
from http://www.shutterstock.com, CC BY-ND

James Renwick, Victoria University of Wellington


CC BY-ND

Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.

If you have a question you’d like an expert to answer, please send it to climate.change@stuff.co.nz

I saw an article claiming that “king tides” will increase in frequency as sea level rises. I am sceptical. What is the physics behind such a claim and how is it related to climate change? My understanding is that a king tide is a purely tidal effect, related to Moon, Sun and Earth axis tilt, and is quite different from a storm surge.

This is a good question, and you are right about the tides themselves. The twice-daily tides are caused by the gravitational forces of the Moon and the Sun, and the rotation of the Earth, none of which is changing.

A “king” tide occurs around the time when the Moon is at its closest to the Earth and Earth is at its closest to the Sun, and the combined gravitational effects are strongest. They are the highest of the high tides we experience.

But the article you refer to was not really talking about king tides. It was discussing coastal inundation events.




Read more:
King tides and rising seas are predictable, and we’re not doing enough about it


When tides, storms and sea levels combine

During a king tide, houses and roads close to the coast can be flooded. The article referred to the effects of coastal flooding generally, using “king tide” as a shorthand expression. We know that king tides are not increasing in frequency, but we also know that coastal flooding and coastal erosion events are happening more frequently.

As sea levels rise, it becomes easier for ocean waves to penetrate on to the shore. The biggest problem arises when storms combine with a high tide, and ride on top of higher sea levels.

The low air pressure near the centre of a storm pulls up the sea surface below. Then, onshore winds can pile water up against the coast, allowing waves to run further inshore. Add a high or king tide and the waves can come yet further inshore. Add a bit of sea level rise and the waves penetrate even further.

The background sea level rise has been only 20cm around New Zealand’s coasts so far, but even that makes a noticeable difference. An apparently small rise in overall sea level allows waves generated by a storm to come on shore much more easily. Coastal engineers use the rule of thumb that every 10cm of sea level rise increases the frequency of a given coastal flood by a factor of three.

This means that 10cm of sea level rise will turn a one-in-100-year coastal flood into a one-in-33-year event. With another 10cm of sea level rise, it becomes a one-in-11-year event, and so on.

Retreating from the coast

The occurrence rates change so quickly because in most places, beaches are fairly flat. A 10cm rise in sea levels might translate to 30 or 40 metres of inland movement of the high tide line, depending on the slope of the beach. So when the tide is high and the waves are rolling in, the sea can come inland tens of metres further than it used to, unless something like a coastal cliff or a sea wall blocks its way.

The worry is that beaches are likely to remain fairly flat, so anything within 40 metres of the current high tide mark is likely to be eroded away as storms occur and we experience another 10cm of sea level rise. If a road or a house is on an erodible coast (such as a line of sand dunes), it is not the height above sea level that matters but the distance from the high tide mark.

Another 30cm of sea level rise is already “baked in”, guaranteed over the next 40 years, regardless of what happens with greenhouse gas emissions and action on climate change. By the end of the century, at least another 20cm on top of that is virtually certain.




Read more:
Our shameful legacy: just 15 years’ worth of emissions will raise sea level in 2300


The 30cm rise multiplies the chances of coastal flooding by a factor of around 27 (3x3x3) and 50cm by the end of the century increases coastal flooding frequency by a factor of around 250. That would make the one-in-100-year coastal flood likely every few months, and roads, properties and all kinds of built infrastructure within 200 metres of the current coastline would be vulnerable to inundation and damage.

These are round numbers, and local changes depend on coastal shape and composition, but they give the sense of how quickly things can change. Already, key roads in Auckland (such as Tamaki Drive) are inundated when storms combine with high tides. Such events are set to become much more common as sea levels continue to rise, to the point where they will become part of the background state of the coastal zone.

To ensure cities such as Auckland (and others around the world) are resilient to such challenges, we’ll need to retreat from the coast where possible (move dwellings and roads inland) and to build coastal defences where that makes sense. The coast is coming inland, and we need to move with it.The Conversation

James Renwick, Professor, Physical Geography (climate science), Victoria University of Wellington

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

Our shameful legacy: just 15 years’ worth of emissions will raise sea level in 2300



Indonesian residents wade through flood water near the Ciliwung river in Jakarta in February 2018. Our emissions in the near future will lock in sea level rise over centuries.

Bill Hare, Potsdam Institute for Climate Impact Research

Greenhouse gas emissions released over the first 15 years of the Paris Agreement would alone lock in 20cm of sea-level rise in centuries to come, according to new research published today.

The paper shows that what the world pumps into the atmosphere today has grave long-term consequences. It underscores the need for governments to dramatically scale up their emission reduction ambition – including Australia, where climate action efforts have been paltry.

The report is the first to quantify the sea-level rise contribution of human-caused greenhouse gas emissions that countries would release if they met their current Paris pledges.

The 20cm sea-level rise is equal to that observed over the entire 20th century. It would comprise one-fifth of the 1m sea level rise projected for 2300.

A satellite image showing meltwater ponding in northwest Greenland near the ice sheet’s edge.
EPA/NASA EARTH OBSERVATORY

The picture is bleak

The study was led by researchers at Climate Analytics and the Potsdam Institute for Climate Impact Research, and was published today by the Proceedings of the National Academy of Sciences. It estimated the sea level rise to be locked in by 2300 due to greenhouse gas emissions between 2016 and 2030 – the first pledge period on the Paris treaty.

During those 15 years, emissions would cause sea levels to rise by 20cm by 2300. Even if the world cut all emissions to zero in 2030, sea levels would still rise in 2300. These estimates do not take into account the irreversible melting of parts of the Antarctic ice sheet.




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The researchers found that just over half of the sea level rise can be attributed to the top five polluters: China, the US, the European Union, India and Russia.

The emissions of these jurisdictions under will cause seas to rise by 12cm by 2300, the study shows.

The important takeaway message is that what the world does now will take years to play out – it is a stark warning of the long-term consequences of our actions.

Severe storms at Collaroy on Sydney’s northern beaches caused major damage to beachfront homes.
UNSW WATER RESEARCH

It’s worse than we thought

Last week a separate paper in Nature Communications showed sea-level rise could affect many more people than previously thought. The authors produced a new digital elevation model that showed many of the world’s coastlines are far lower than estimated with standard methods.

In low-lying parts of coastal Australia, for example, the previous data has
overestimated elevation by an average of 2.5m.

Their projections for the millions of people to be affected by sea-level rise are frightening. Within three decades, rising sea levels could push chronic floods higher than land currently home to 300 million people. By 2100, areas home to 200 million people could be permanently below the high tide line.

But what of Australia, girt by sea?

Australia is a coastal nation: the vast majority of our population lives within 50km of the sea, and will be heavily impacted by sea-level rise. Already, we’re seeing severe coastal erosion and inundation during king tides – and that’s without factoring in the impact of storm surges.

Clearly the world needs strong climate action to reduce greenhouse gas emissions as fast as possible. The Intergovernmental Panel on Climate Change has said emissions must be lowered to 45% below 2010 levels by 2030 and to zero by mid-century.

We also know that unless the world achieves this, we will not just lose parts of our coasts but also iconic ecosystems such as the Great Barrier Reef.



Australia’s emissions comprise a relatively small proportion of the global total – 1.4% or around 5% if we count coal and liquified natural gas exports. However, we have a much bigger diplomatic and political influence on the international stage.




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Australia should use its position to push for urgent action internationally. But the federal government’s appalling record on emissions reduction – despite its efforts to claim otherwise – puts us in a very weak position on the global stage. We cannot point fingers at other nations while our emissions rise and we sell as much coal as possible to the rest of the world, while also burning as much as we can.



All the while, Australia is becoming the poster child for extreme sea-level events, more frequent and severe bushfires and other devastating climate impacts.

Governments, including Australia’s, must put forward much stronger 2030 emission reduction pledges by 2020. There should seek to decarbonise at a pace in line with the Paris Agreement’s 1.5°C temperature goal.

Otherwise, our emissions today will cause seas to rise far into the future. This process cannot be reversed – it will be our legacy to future generations.


Climate Analytics researcher Alexander Nauels was lead author of the study.The Conversation

Bill Hare, Director, Climate Analytics, Adjunct Professor, Murdoch University (Perth), Visiting scientist, Potsdam Institute for Climate Impact Research

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

Water may soon lap at the door, but still some homeowners don’t want to rock the boat



Storm-damaged beachfront homes along Pittwater Road at Collaroy on the northern beaches of Sydney in June 2016.
Dean Lewins/AAP

Vanessa Bowden, University of Newcastle; Christopher Wright, University of Sydney, and Daniel Nyberg, University of Newcastle

It is becoming increasingly possible that sea-level rise of a metre or more will occur this century. You might expect this threat to preoccupy coastal homeowners. But many deny the need to act, for fear their property values will fall.

This particular brand of climate denial presents a conundrum for governments and local councils, which must plan urgently for climate change. The very act of officials identifying homes exposed to sea-level rise can be vehemently opposed by the owners, let alone policies to deal with it.

This is an urgent problem. As long as we keep failing to reduce global carbon emissions, adapting to the inevitable changes in our climate is vital. But winning cooperation from coastal property owners requires more than just talking about the science.

A car covered in sand near Bondi Beach, Sydney after heavy storms in 2015.
David Moir/AAP

A tide of irrefutable facts

An Intergovernmental Panel on Climate Change report released this month warned sea levels are rising faster than we thought. This will lead to more flooding, storm surges and inundation than previously modelled.

In Australia, 85% of people live within 50km of the coast. In 2009, a federal assessment estimated that up to 247,600 Australian homes were at risk of inundation under a 1.1m sea-level rise scenario.




Read more:
Rising seas threaten Australia’s major airports – and it may be happening faster than we think


Authorities must manage this threat, which might include limiting development, protecting properties, or planning a retreat from some areas.

Yet our research shows that getting community support for such measures can be contentious and time-consuming.

Waterfront properties in Point Piper, Sydney. Some 85% of Australians live near the coast.
JOEL CARRETT/AAP

Property values are king

We researched Lake Macquarie in New South Wales, a council area of about 200,000 residents. Lake Macquarie City Council is a recognised leader in climate adaptation policy.

Lake Macquarie is a large coastal estuary vulnerable to sea-level rise. It has been identified as one of six council areas in Australia at highest risk of inundation. Up to 6,800 buildings in the area – about 10% – could be at risk from sea-level rise and storm surges this century.

In response, the council limited development in the most vulnerable areas and in 2012 began community consultation. This included working with residents to develop an adaptation plan, released in 2016.




Read more:
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In 2017 and 2018, we interviewed current and former councillors and council staff, local businesspeople and residents about the consultation process.

We found there was initially strong resistance to the council’s policy attempts. Community members expressed concern that acknowledging the need to adapt to sea-level rise would reduce property prices and increase home insurance costs.

The potential worst-case scenario, being required to abandon one’s home, was strongly resisted by the community.

Heavy machinery moves onto Palm Beach on the Gold Coast to repair cliffs carved out of the front yards of beachfront homes.
Tony Bartlett/AAP

Such community opposition is common across Australia. The Queensland property industry lobbied against state requirements that would have barred new development until climate adaptation plans were in place. At Lakes Entrance in Victoria, coastal residents have complained that adaptation measures are “taking away people’s money … because they’re going to suffer financial loss”.

The problem of climate denialism

In 2012 when community consultation began, property developer Jeff McCloy told the Sydney Morning Herald he was considering suing the council over its policies, describing concern over sea-level rise as “unjustified, worldwide idiocy”.

People have a tendency to want to see or feel the impacts of climate change before they agree to actions they see as conflicting with their priorities.

Property owners who live near oceans or lakes may not have observed rising sea levels or other climate change effects, and sometimes hesitate to believe it will be a future problem, even if flood map modelling shows otherwise.

The proliferation of climate scepticism in public discourse provides ready-made arguments to which some property owners, fearful of climate change impacts, can attach themselves.




Read more:
Climate change: sea level rise could displace millions of people within two generations


We found that these broader debates around climate change impeded Lake Macquarie council’s ability to reach agreement with residents. Those opposing the policy arranged for prominent climate sceptics to speak at public meetings, and published anti-science opinion pieces in the local newspaper.

A yacht washed up on a beach at Little Manly Cove in Sydney in 2012 after wild storms.
AAP/Mick Tsikas

Where to now?

The Lake Macquarie experience shows intensive, long-term, early efforts at community engagement can overcome some community opposition to climate adaptation. After four years of consultation, the council reached agreement with residents in two areas that affected land would be filled in over time, and there would be no forced retreat from homes.

The council is continuing to plan, with community involvement. It is developing suburb-specific adaptation plans designed so residents understand the science and embrace the solutions – including the chance to identify adaptation options themselves.

But across Australia, much work remains. As global carbon emissions continue to rise and the window to act closes, it is crucial that councils, governments and communities plan for whatever the future holds. This includes implementing adaptation plans that get property owners on board.The Conversation

Vanessa Bowden, Postdoctoral research fellow, University of Newcastle; Christopher Wright, Professor of Organisational Studies, University of Sydney, and Daniel Nyberg, Professor of Management, Newcastle Business School, University of Newcastle

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

If warming exceeds 2°C, Antarctica’s melting ice sheets could raise seas 20 metres in coming centuries



During the Pliocene, up to one third of Antarctica’s ice sheet melted, causing sea-level rise of 20 metres.
from http://www.shutterstock.com, CC BY-ND

Georgia Rose Grant, GNS Science and Timothy Naish, Victoria University of Wellington

We know that our planet has experienced warmer periods in the past, during the Pliocene geological epoch around three million years ago.

Our research, published today, shows that up to one third of Antarctica’s ice sheet melted during this period, causing sea levels to rise by as much as 20 metres above present levels in coming centuries.

We were able to measure past changes in sea level by drilling cores at a site in New Zealand, known as the Whanganui Basin, which contains shallow marine sediments of arguably the highest resolution in the world.

Using a new method we developed to predict the water level from the size of sand particle moved by waves, we constructed a record of global sea-level change with significantly more precision than previously possible.

The Pliocene was the last time atmospheric carbon dioxide concentrations were above 400 parts per million and Earth’s temperature was 2°C warmer than pre-industrial times. We show that warming of more than 2°C could set off widespread melting in Antarctica once again and our planet could be hurtling back to the future, towards a climate that existed three million years ago.




Read more:
Not convinced on the need for urgent climate action? Here’s what happens to our planet between 1.5°C and 2°C of global warming


Overshooting the Paris climate target

Last week we saw unprecedented global protests under the banner of Greta Thunberg’s #FridaysForFuture climate strikes, as the urgency of keeping global warming below the Paris Agreement target of 2°C hit home. Thunberg captured collective frustration when she chastised the United Nations for not acting earlier on the scientific evidence. Her plea resonated as she reminded us that:

With today’s emissions levels, that remaining CO₂ budget [1.5°C] will be entirely gone in less than eight and a half years.

At the current rate of global emissions we may be back in the Pliocene by 2030 and we will have exceeded the 2°C Paris target. One of the most critical questions facing humanity is how much and how fast global sea levels will rise.

According to the recent special report on the world’s oceans and cryosphere by the Intergovernmental Panel on Climate Change (IPCC), glaciers and polar ice sheets continue to lose mass at an accelerating rate, but the contribution of polar ice sheets, in particular the Antarctic ice sheet, to future sea level rise remains difficult to constrain.

If we continue to follow our current emissions trajectory, the median (66% probability) global sea level reached by the end of the century will be 1.2 metres higher than now, with two metres a plausible upper limit (5% probability). But of course climate change doesn’t magically stop after the year 2100.




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Drilling back to the future

To better predict what we are committing the world’s future coastlines to we need to understand polar ice sheet sensitivity. If we want to know how much the oceans will rise at 400ppm CO₂, the Pliocene epoch is a good comparison.

Back in 2015, we drilled cores of sediment deposited during the Pliocene, preserved beneath the rugged hill country at the Whanganui Basin. One of us (Timothy Naish) has worked in this area for almost 30 years and identified more than 50 fluctuations in global sea level during the last 3.5 million years of Earth’s history. Global sea levels had gone up and down in response to natural climate cycles, known as Milankovitch cycles, which are caused by long-term changes in Earths solar orbit every 20,000, 40,000 and 100,000 years. These changes in turn cause polar ice sheets to grow or melt.

While sea levels were thought to have fluctuated by several tens of metres, up until now efforts to reconstruct the precise amplitude had been thwarted by difficulties due to Earth deformation processes and the incomplete nature of many of the cycles.

Our research used a well-established theoretical relationship between the size of the particles transported by waves on the continental shelf and the depth to the seabed. We then applied this method to 800 metres of drill core and outcrop, representing continuous sediment sequences that span a time period from 2.5 to 3.3 million years ago.

We show that during the Pliocene, global sea levels regularly fluctuated between five to 25 metres. We accounted for local tectonic land movements and regional sea-level changes caused by gravitational and crustal changes to determine the sea-level estimates, known as the PlioSeaNZ sea-level record. This provides an approximation of changes in global mean sea level.

Antarctica’s contribution to sea-level rise

Our study also shows that most of the sea-level rise during the Pliocene came from Antarctica’s ice sheets. During the warm Pliocene, the geography of Earth’s continents and oceans and the size of polar ice sheets were similar to today, with only a small ice sheet on Greenland during the warmest period. The melting of the Greenland ice sheet would have contributed at most five metres to the maximum 25 metres of global sea-level rise recorded at Whanganui Basin.

Of critical concern is that over 90% of the heat from global warming to date has gone into the ocean. Much of it has gone into the Southern Ocean, which bathes the margins of Antarctica’s ice sheet.




Read more:
New research shows that Antarctica’s largest floating ice shelf is highly sensitive to warming of the ocean


Already, we are observing warm circumpolar deep water upwelling and entering ice shelf cavities in several sites around Antarctica today. Along the Amundsen Sea coast of West Antarctica, where the ocean has been heating the most, the ice sheet is thinning and retreating the fastest. One third of Antarctica’s ice sheet — the equivalent to up to 20 metres of sea-level rise — is grounded below sea level and vulnerable to widespread collapse from ocean heating.

Our study has important implications for the stability and sensitivity of the Antarctic ice sheet and its potential to contribute to future sea levels. It supports the concept that a tipping point in the Antarctic ice sheet may be crossed if global temperatures are allowed to rise by more than 2℃. This could result in large parts of the ice sheet being committed to melt-down over the coming centuries, reshaping shorelines around the world.The Conversation

Georgia Rose Grant, Postdoctoral Research Assistant, Paleontology Team, GNS Science and Timothy Naish, Professor, Victoria University of Wellington

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

A landmark report confirms Australia is girt by hotter, higher seas. But there’s still time to act



Aerial imagery revealing the extent of storm damage in Dee Why on Sydney’s Northern Beaches in 2016 following wild weather.
NEARMAP/AAP

Jess Melbourne-Thomas, CSIRO; Kathleen McInnes, CSIRO; Nathan Bindoff, University of Tasmania, and Nerilie Abram, Australian National University

A landmark scientific report has confirmed that climate change is altering the world’s seas and ice at an unprecedented rate. Australia depends on the ocean that surrounds us for our health and prosperity. So what does this mean for us, and life on Earth?

The Intergovernmental Panel on Climate Change (IPCC) findings were launched in Monaco on Wednesday night. They provide the most definitive scientific evidence yet of warmer, more acidic and less productive seas. Glaciers and ice sheets are melting, causing sea level to rise at an accelerating rate.

The implications for Australia are serious. Extreme sea level events that used to hit once a century will occur once a year in many of the world’s coastal places by 2050. This situation is inevitable, even if greenhouse gas emissions are dramatically curbed.

The findings, titled the Special Report on the Ocean and Cryosphere in a Changing Climate, strengthen the already compelling case for countries to meet their emission reduction goals under the 2015 Paris agreement.

Beachgoers cool off in the water at Bondi Beach in Sydney, February 2019. Australia’s coast dwellers must adapt to the inevitable effects of climate change.
Joel Carrett/AAP



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A rapid and dramatic cut in greenhouse gas emissions would prevent the most catastrophic damage to the ocean and cryosphere (frozen polar and mountain regions). This would help protect the ecosystems and people that rely on them.

The report entailed two years of work by 104 authors and review editors from 36 countries, who assessed nearly 7,000 scientific papers and responded to more than 30,000 review comments.

The picture is worse than we thought

Mountain glaciers and polar ice sheets are shrinking and, together with expansion of the warming ocean, are contributing to an increasing rate of sea level rise.

During the last century, global sea levels rose about 15cm. Seas are now rising more than twice as fast – 3.6mm per year – and accelerating, the report shows.

The IPCC’s projections are more dire than in its 2014 oceans report. It has revised upwards by 10% the effect of the melting Antarctic ice sheet on sea level rise by 2100. Antarctica appears to be changing more rapidly than was thought possible even five years ago, and further work is needed to understand just how quickly ice will be lost from Antarctica in future.

Key components and changes of the ocean and cryosphere, and their linkages in the Earth system.
IPCC, 2019

If you live near the Australian coast, change is coming

By 2050, more than one billion of the world’s people will live on coastal land less than 10 metres above sea level. They will be exposed to combinations of sea level rise, extreme winds, waves, storm surges and flooding from intensified storms and tropical cyclones.

Many of Australia’s coastal cities and communities can expect to experience what was previously a once-in-a-century extreme coastal flooding event at least once every year by the middle of this century.

Our island neighbours in Indonesia and the Pacific will also be hit hard. The report warns that some island nations are likely to become uninhabitable – although the extent of this is hard to assess accurately.

Some change is inevitable and we will have to adapt. But the report also delivered a strong message about the choices that still remain. In the case of extreme sea level events around Australia, we believe a marked global reduction in greenhouse as emissions would buy us more than 10 years of extra time, in some places, to protect our coastal communities and infrastructure from the rising ocean.

Indonesian residents wade through flood water in Jakarta. The northwestern part of Jakarta is rapidly sinking.
MAST IRHAM/EPA

More frequent extreme events are often occurring at the same time or in quick succession. Tasmania’s summer of 2015-16 is a good example. The state experienced record-breaking drought which worsened the fire threat in the highlands. An unprecedented marine heatwave along the east coast damaged kelp forests and caused disease and death of shellfish, and the state’s northeast suffered severe flooding.

This string of events stretched emergency services, energy supplies and the aquaculture and manufacturing industries. The total economic cost to the state government was an estimated A$445 million. The impacts on the food, energy and manufacturing sectors cut Tasmania’s anticipated economic growth by about half.

Reefs and fish stocks are suffering

The ocean has taken a huge hit from climate change – taking up heat, absorbing carbon dioxide that makes the water more acidic, and losing oxygen. It will bring ocean conditions unlike anything we have seen before.

Marine ecosystems and fisheries around the world are under pressure from this barrage of stressors. Overall, the fisheries potential around Australia’s coasts is expected to decline during this century.

Heat build-up in the surface ocean has already triggered a marked rise in the intensity, frequency and duration of marine heatwaves. Ocean heatwaves are expected to become between four and ten times more common this century, depending on how rapidly global warming continues.

The report said coral reefs, including the Great Barrier Reef, are already at very high risk from climate change and are expected to suffer significant losses and local extinctions. This would occur even if global warming is limited to 1.5℃ – a threshold the world is set to overshoot by a wide margin.




Read more:
Extreme weather caused by climate change has damaged 45% of Australia’s coastal habitat


Our choices now are critical for the future

This report reinforces the findings of earlier reports on the importance of limiting global warming warming to 1.5℃ if we are to avoid major impacts on the land, the ocean and frozen areas.

Even if we act now to drastically reduce greenhouse gas emissions, some damage is already locked in and our ocean and frozen regions will continue to change for decades to centuries to come.

Mertz Glacier in east Antarctica. IPCC scientists say the expected effect of melting Antarctic ice on sea level rise is worse than projected five years ago.
Australian Antarctic Division

In Australia, we will need to adapt our coastal cities and communities to unavoidable sea level rise. There are a range of possible options, from building barriers to planned relocation, to protecting the coral reefs and mangroves that provide natural coastal defences.

But if we want to give adaptation the best chance of working, the clear message of this new report is that we need to reduce greenhouse gas emissions as quickly as possible.The Conversation

Jess Melbourne-Thomas, Transdisciplinary Researcher & Knowledge Broker, CSIRO; Kathleen McInnes, Senior research scientist, CSIRO; Nathan Bindoff, Professor of Physical Oceanography, Institute for Marine and Antarctic Studies, University of Tasmania, and Nerilie Abram, ARC Future Fellow, Research School of Earth Sciences; Chief Investigator for the ARC Centre of Excellence for Climate Extremes, Australian National University

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

Climate change may change the way ocean waves impact 50% of the world’s coastlines


Mark Hemer, CSIRO; Ian Young, University of Melbourne; Joao Morim Nascimento, Griffith University, and Nobuhito Mori, Kyoto University

The rise in sea levels is not the only way climate change will affect the coasts. Our research, published today in Nature Climate Change, found a warming planet will also alter ocean waves along more than 50% of the world’s coastlines.

If the climate warms by more than 2℃ beyond pre-industrial levels, southern Australia is likely to see longer, more southerly waves that could alter the stability of the coastline.

Scientists look at the way waves have shaped our coasts – forming beaches, spits, lagoons and sea caves – to work out how the coast looked in the past. This is our guide to understanding past sea levels.




Read more:
Rising seas threaten Australia’s major airports – and it may be happening faster than we think


But often this research assumes that while sea levels might change, wave conditions have stayed the same. This same assumption is used when considering how climate change will influence future coastlines – future sea-level rise is considered, but the effect of future change on waves, which shape the coastline, is overlooked.

Changing waves

Waves are generated by surface winds. Our changing climate will drive changes in wind patterns around the globe (and in turn alter rain patterns, for example by changing El Niño and La Niña patterns). Similarly, these changes in winds will alter global ocean wave conditions.




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Further to these “weather-driven” changes in waves, sea level rise can change how waves travel from deep to shallow water, as can other changes in coastal depths, such as affected reef systems.

Recent research analysed 33 years of wind and wave records from satellite measurements, and found average wind speeds have risen by 1.5 metres per second, and wave heights are up by 30cm – an 8% and 5% increase, respectively, over this relatively short historical record.

These changes were most pronounced in the Southern Ocean, which is important as waves generated in the Southern Ocean travel into all ocean basins as long swells, as far north as the latitude of San Francisco.

Sea level rise is only half the story

Given these historical changes in ocean wave conditions, we were interested in how projected future changes in atmospheric circulation, in a warmer climate, would alter wave conditions around the world.

As part of the Coordinated Ocean Wave Climate Project, ten research organisations combined to look at a range of different global wave models in a variety of future climate scenarios, to determine how waves might change in the future.

While we identified some differences between different studies, we found if the 2℃ Paris agreement target is kept, changes in wave patterns are likely to stay inside natural climate variability.

However in a business-as-usual climate, where warming continues in line with current trends, the models agreed we’re likely to see significant changes in wave conditions along 50% of the world’s coasts. These changes varied by region.

Less than 5% of the global coastline is at risk of seeing increasing wave heights. These include the southern coasts of Australia, and segments of the Pacific coast of South and Central America.

On the other hand decreases in wave heights, forecast for about 15% of the world’s coasts, can also alter coastal systems.

But describing waves by height only is the equivalent of describing an orchestra simply by the volume at which it plays.

Some areas will see the height of waves remain the same, but their length or frequency change. This can result in more force exerted on the coast (or coastal infrastructure), perhaps seeing waves run further up a beach and increasing wave-driven flooding.

Similarly, waves travelling from a slightly altered direction (suggested to occur over 20% of global coasts) can change how much sand they shunt along the coast – important considerations for how the coast might respond. Infrastructure built on the coast, or offshore, is sensitive to these many characteristics of waves.

While each of these wave characteristics is important on its own, our research identified that about 40% of the world’s coastlines are likely to see changes in wave height, period and direction happening simultaneously.

While some readers may see intense waves offering some benefit to their next surf holiday, there are much greater implications for our coastal and offshore environments. Flooding from rising sea levels could cost US$14 trillion worldwide annually by 2100 if we miss the target of 2℃ warming.




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Droughts and flooding rains already more likely as climate change plays havoc with Pacific weather


How coastlines respond to future climate change will be a response to a complex interplay of many processes, many of which respond to variable and changing climate. To focus on sea level rise alone, and overlooking the role waves play in shaping our coasts, is a simplification which has great potential to be costly.


The authors would like to acknowledge the contribution of Xiaolan Wang, Senior Research Scientist at Environment and Climate Change, Canada, to this article.The Conversation

Mark Hemer, Principal Research Scientist, Oceans and Atmosphere, CSIRO; Ian Young, Kernot Professor of Engineering, University of Melbourne; Joao Morim Nascimento, PhD Candidate, Griffith University, and Nobuhito Mori, Professor, Kyoto University

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