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.




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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.

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.




Read more:
Curious Kids: why are there waves?


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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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

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

Daylight robbery: how human-built structures leave coastal ecosystems in the shadows



Human-built structures are home to a wide variety of creatures.

Martino Malerba, Monash University; Craig White, Monash University; Dustin Marshall, Monash University, and Liz Morris, Monash University

About half of the coastline of Europe, the United States and Australasia is modified by artificial structures. In newly published research, we identified a new effect of marine urbanisation that has so far gone unrecognised.

When we build marinas, ports, jetties and coastal defences, we introduce hard structures that weren’t there before and which reduce the amount of sunlight hitting the water. This means energy producers such as seaweed and algae, which use light energy to transform carbon dioxide into sugars, are replaced by energy consumers such as filter-feeding invertebrates. These latter species are often not native to the area, and can profoundly alter marine habitats by displacing local species, reducing biodiversity, and decreasing the overall productivity of ecosystems.

Incorporating simple designs in our marine infrastructure to allow more light penetration, improve water flow, and maintain water quality, will go a long way towards curbing these negative consequences.

Pier life

We are used to thinking about the effects of urbanisation in our cities – but it is time to pay more attention to urban sprawl in the sea. We need to better understand the effects on the food web in a local context.




Read more:
Concrete coastlines: it’s time to tackle our marine ‘urban sprawl’


Most animals that establish themselves on these shaded hard structures are “sessile” invertebrates, which can’t move around. They come in a variety of forms, from encrusting species such as barnacles, to tree-shaped or vase-like forms such as bryozoans or sponges. But what they all have in common is that they can filter out algae from the water.

In Australian waters, we commonly see animals from a range of different groups including sea squirts, sponges, bryozoans, mussels and worms. They can grow in dense communities and often reproduce and grow quickly in new environments.

The sheltered and shaded nature of marine urbanisation disproportionately favours the development of dense invertebrate communities, as shown here in Port Phillip Bay.

How much energy do they use?

In our new research, published in the journal Frontiers in Ecology and the Environment, we analysed the total energy usage of invertebrate communities on artificial structures in two Australian bays: Moreton Bay, Queensland, and Port Phillip Bay, Victoria. We did so by combining data from field surveys, laboratory studies, and satellite data.

We also compiled data from other studies and assessed how much algae is required to support the energy demands of the filter-feeding species in commercial ports worldwide.

In Port Phillip Bay, 0.003% of the total area is taken up by artificial structures. While this doesn’t sound like much, it is equivalent to almost 50 soccer fields of human-built structures.

We found that the invertebrate community living on a single square metre of artificial structure consumes the algal biomass produced by 16 square metres of ocean. Hence, the total invertebrate community living on these structures in the bay consumes the algal biomass produced by 800 football pitches of ocean!

Similarly, Moreton Bay has 0.005% of its total area occupied by artificial structures, but each square metre of artificial structure requires around 5 square metres of algal production – a total of 115 football pitches. Our models account for various biological and physical variables such as temperature, light, and species composition, all of which contribute to generate differences among regions.

Overall, the invertebrates growing on artificial structures in these two Australian bays weigh as much as 3,200 three-tonne African elephants. This biomass would not exist were it not for marine urbanisation.

Colonies of mussels and polychaetes near Melbourne.

How does Australia compare to the rest of the world?

We found stark differences among ports in different parts of the world. For example, one square metre of artificial structure in cold, highly productive regions (such as St Petersburg, Russia) can require as little as 0.9 square metres of sea surface area to provide enough algal food to sustain the invertebrate populations. Cold regions can require less area because they are often richer in nutrients and better mixed than warmer waters.

In contrast, a square metre of structure in the nutrient-poor tropical waters of Hawaii can deplete all the algae produced in the surrounding 120 square metres.

All major commercial ports worldwide with associated area of the underwater artificial structures (size of grey dots) and trophic footprint (size of red borders). Trophic footprints indicate how much ocean surface is required to supply the energy demand of the sessile invertebrate community growing on all artificial structures of the port, averaged over the year. This depends on local conditions of ocean primary productivity and temperature. Ports located in cold, nutrient-rich waters (dark blue) have a lower footprint than ports in warmer waters (light blue).

Does it matter?

Should we be worried about all of this? To some extent, it depends on context.

These dense filter-feeding communities are removing algae that normally enters food webs and supports coastal fisheries. As human populations in coastal areas continue to increase, so will demand on these fisheries, which are already under pressure from climate change. These effects will be greatest in warmer, nutrient-poor waters.

But there is a flip side. Ports and urban coastlines are often polluted with increased nutrient inputs, such as sewage effluents or agricultural fertilisers. The dense populations of filter-feeders on the structures near these areas may help prevent this nutrient runoff from triggering problematic algal blooms, which can cause fish kills and impact human health. But we still need to know what types of algae these filter-feeding communities are predominantly consuming.




Read more:
Explainer: what causes algal blooms, and how we can stop them


Our analysis provides an important first step in understanding how these communities might affect coastal production and food webs.

In places like Southeast Asia, marine managers should consider how artificial structures might affect essential coastal fisheries. Meanwhile, in places like Port Phillip Bay, we need to know whether and how these communities might affect the chances of harmful algal blooms.The Conversation

Mussels in the port of Hobart.

Martino Malerba, Postdoctoral Fellow, Monash University; Craig White, Head, Evolutionary Physiology Research Group, Monash University; Dustin Marshall, Professor, Marine Evolutionary Ecology, Monash University, and Liz Morris, Administration Manager, Monash University

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

Rising seas allow coastal wetlands to store more carbon



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Carbon storage in Australian mangroves can help mitigate climate change.
Shutterstock

Kerrylee Rogers, University of Wollongong; Jeffrey Kelleway, Macquarie University, and Neil Saintilan, Macquarie University

Coastal wetlands don’t cover much global area but they punch well above their carbon weight by sequestering the most atmospheric carbon dioxide of all natural ecosystems.

Termed “blue carbon ecosystems” by virtue of their connection to the sea, the salty, oxygen-depleted soils in which wetlands grow are ideal for burying and storing organic carbon.

In our research, published today in Nature, we found that carbon storage by coastal wetlands is linked to sea-level rise. Our findings suggest as sea levels rise, these wetlands can help mitigate climate change.

Sea-level rise benefits coastal wetlands

We looked at how changing sea levels over the past few millennia has affected coastal wetlands (mostly mangroves and saltmarshes). We found they adapt to rising sea levels by increasing the height of their soil layers, capturing mineral sediment and accumulating dense root material. Much of this is carbon-rich material, which means rising sea levels prompt the wetlands to store even more carbon.

We investigated how saltmarshes have responded to variations in “relative sea level” over the past few millennia. (Relative sea level is the position of the water’s edge in relation to the land rather than the total volume of water within the ocean, which is called the eustatic sea level.)




Read more:
Mangrove forests can rebound thanks to climate change – it’s an opportunity we must take


What does past sea-level rise tell us?

Global variation in the rate of sea-level rise over the past 6,000 years is largely related to the proximity of coastlines to ice sheets that extended over high northern latitudes during the last glacial period, some 26,000 years ago.

As ice sheets melted, northern continents slowly adjusted elevation in relation to the ocean due to flexure of the Earth’s mantle.

Karaaf Wetlands in Victoria, Australia.
Boobook48/flickr, CC BY-NC-SA

For much of North America and Europe, this has resulted in a gradual rise in relative sea level over the past few thousand years. By contrast, the southern continents of Australia, South America and Africa were less affected by glacial ice sheets, and sea-level history on these coastlines more closely reflects ocean surface “eustatic” trends, which stabilised over this period.

Our analysis of carbon stored in more than 300 saltmarshes across six continents showed that coastlines subject to consistent relative sea-level rise over the past 6,000 years had, on average, two to four times more carbon in the upper 20cm of sediment, and five to nine times more carbon in the lower 50-100cm of sediment, compared with saltmarshes on coastlines where sea level was more stable over the same period.

In other words, on coastlines where sea level is rising, organic carbon is more efficiently buried as the wetland grows and carbon is stored safely below the surface.

Give wetlands more space

We propose that the difference in saltmarsh carbon storage in wetlands of the southern hemisphere and the North Atlantic is related to “accommodation space”: the space available for a wetland to store mineral and organic sediments.

Coastal wetlands live within the upper portion of the intertidal zone, roughly between mean sea level and the upper limit of high tide.

These tidal boundaries define where coastal wetlands can store mineral and organic material. As mineral and organic material accumulates within this zone it creates layers, raising the ground of the wetlands.

The coastal wetlands of Broome, Western Australia.
Shutterstock

New accommodation space for storage of carbon is therefore created when the sea is rising, as has happened on many shorelines of the North Atlantic Ocean over the past 6,000 years.

To confirm this theory we analysed changes in carbon storage within a unique wetland that has experienced rapid relative sea-level rise over the past 30 years.




Read more:
Without wetlands, what will protect the Great Barrier Reef?


When underground mine supports were removed from a coal mine under Lake Macquarie in southeastern Australia in the 1980s, the shoreline subsided a metre in a matter of months, causing a relative rise in sea level.

Following this the rate of mineral accumulation doubled, and the rate of organic accumulation increased fourfold, with much of the organic material being carbon. The result suggests that sea-level rise over the coming decades might transform our relatively low-carbon southern hemisphere marshes into carbon sequestration hot-spots.

How to help coastal wetlands

The coastlines of Africa, Australia, China and South America, where stable sea levels over the past few millennia have constrained accommodation space, contain about half of the world’s saltmarshes.

Saltmarsh on the shores of Westernport Bay in Victoria.
Author provided

A doubling of carbon sequestration in these wetlands, we’ve estimated, could remove an extra 5 million tonnes of CO₂ from the atmosphere per year. However, this potential benefit is compromised by the ongoing clearance and reclamation of these wetlands.

Preserving coastal wetlands is critical. Some coastal areas around the world have been cut off from tides to lessen floods, but restoring this connection will promote coastal wetlands – which also reduce the effects of floods – and carbon capture, as well as increase biodiversity and fisheries production.




Read more:
As communities rebuild after hurricanes, study shows wetlands can significantly reduce property damage


In some cases, planning for future wetland expansion will mean restricting coastal developments, however these decisions will provide returns in terms of avoided nuisance flooding as the sea rises.

Finally, the increased carbon storage will help mitigate climate change. Wetlands store flood water, buffer the coast from storms, cycle nutrients through the ecosystem and provided vital sea and land habitat. They are precious, and worth protecting.


The authors would like to acknowledge the contribution of their colleagues, Janine Adams, Lisa Schile-Beers and Colin Woodroffe.The Conversation

Kerrylee Rogers, Associate Professor, University of Wollongong; Jeffrey Kelleway, Postdoctoral Research Fellow in Environmental Sciences, Macquarie University, and Neil Saintilan, Head, Department of Environmental Science, Macquarie University

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

Coastal seas around New Zealand are heading into a marine heatwave, again



File 20190121 100292 m58bim.jpg?ixlib=rb 1.1
This summer, coastal seas to the north and east of New Zealand are even warmer than during last year’s marine heat wave.
from http://www.shutterstock.com, CC BY-ND

Craig Stevens, National Institute of Water and Atmospheric Research and Ben Noll, National Institute of Water and Atmospheric Research

As New Zealanders are enjoying their days at the beach, unusually warm ocean temperatures look to be a harbinger of another marine heatwave.

Despite the exceptional conditions during last year’s heatwave in the Tasman Sea, this summer’s sea surface temperatures to the north and east of New Zealand are even warmer.

The latest NIWA climate assessment shows that sea surface temperatures in coastal waters around New Zealand are well above average. Marine heatwave conditions are already occurring in parts of the Tasman Sea and the ocean around New Zealand and looking to become the new normal.




Read more:
Marine heatwaves are getting hotter, lasting longer and doing more damage


Changing sea surface temperature anomalies (conditions compared to average) in the oceans around New Zealand during the first two weeks of January – comparing 2009 to 2019. Source: NIWA

What’s in a name

Currently, marine heatwaves are defined as periods that last for five or more days with temperatures warmer than the 90th percentile based on a 30-year historical baseline. Given we are likely to experience many more such events as the oceans continue to warm, it is time to understand and categorise the intensity of marine heat.

The names Hurricane Katrina, tropical cyclone Giselle (which sank the ferry Wahine 50 years ago), tropical cyclone Winston give a malevolent personality to geophysical phenomena. Importantly they get graded into categories, so we can rapidly assess their potential impact.




Read more:
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An Australian team has developed a classification scheme for marine heatwaves. The team used an approach similar to that used for hurricanes and cyclones – changing conditions can be slotted into to a sequence of categories. At the moment it looks like we are in marine heat wave category one conditions, but potentially entering category two if it continues to warm.

Turning the heat up on marine life

A marine heatwave is potentially devastating for marine ecosystems. It is also an indication that the hidden buffer in the climate system – the fact that the oceans have absorbed 93% of the excess heat – is starting to change. Individual warm seasons have always occurred, but in future there will be more of them and they will keep getting warmer.

The Great Barrier Reef has already been hit hard by a succession of marine heatwave events, bleaching the iconic corals and changing the structure of the ecosystem it supports.




Read more:
The 2016 Great Barrier Reef heatwave caused widespread changes to fish populations


Further south, off Tasmania’s east coast, a number of species that normally occur in tropical waters have extended their range further south. A number of fish species, lobster and octopus species have also taken up residence along the Tasmanian coast, displacing some of the species that call this coast home. Mobile species can escape the warmer temperatures, but sedentary plants and animals are hardest hit.

In New Zealand, aquaculture industries will find it more difficult to grow fish or mussels as coastal waters continue to warm. If the same trends seen off Tasmania occur here, areas with substantial kelp canopies will struggle and start to be replaced by species normally seen further north. But the impacts will likely be very variable because the warming will be heavily influenced by wind and ocean currents and different locations will feel changes to a greater or lesser extent.

NIWA’s research vessel Kaharoa has deployed Argo floats in the Southern Ocean and in waters around New Zealand.
NIWA, CC BY-ND

Predicting the seasons

As important as it is to identify a marine heatwave at the time, reliable predictions of developing conditions would help fishers, aquaculture companies and local authorities – and in fact anyone living and working around the ocean.

Seasonal forecasting a few months ahead is difficult. It falls between weather and climate predictions. In a collaboration between the National Institute of Water and Atmospheric Research and the Australian Bureau of Meteorology, we are examining how well long-term forecasts of ocean conditions around New Zealand stack up. Early forecasts suggested this summer would not be as warm as last year. But it now looks like this summer will again be very warm in the ocean.




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This summer’s sea temperatures were the hottest on record for Australia: here’s why


One of the important points to keep in mind is that when we are at the beach, we are sampling only the surface temperature. The same is true of satellites – they monitor less than the top millimetre of the ocean.

Sea surface temperatures are several degrees above normal at the moment. But in deeper waters, because of the high heat content of water, even a tenth of a degree is significant. Temperature in the deeper ocean is monitored by a network of moored buoys on and off the continental shelf along the Australian coast. New Zealand has almost nothing that would be comparable.

Measuring temperature in real time

What we can look to, in the absence of moored buoys, is a fleet of ocean robots that monitor temperature in real time. Argo floats drift with ocean currents, sink to two kilometres every ten days and then collect data as they return to the surface.

These data allowed us to identify that the 2017/18 marine heatwave around New Zealand remained shallow. Most of the warmer water was in the upper 30 metres. Looking at the present summer conditions, one Argo robot off New Zealand’s west coast shows it is almost four degrees above normal in the upper 40 metres of the ocean. On the east coast, near the Chatham Islands, another float shows warmed layers to 20 metres deep. To the south, the warming goes deeper, down to almost 80 metres.

Our work using the Australian Bureau of Meteorology forecast model highlights how variable the ocean around New Zealand is. Different issues emerge in different regions, even if they are geographically close.

The research on categories of marine heatwaves shows we will have to keep shifting what we regard as a heat wave as the ocean continues to warm. None of this should come as a surprise. We have known for some time that the world’s oceans are storing most of the additional heat and the impacts of a warming ocean will be serious.The Conversation

Craig Stevens, Associate Professor in Ocean Physics, National Institute of Water and Atmospheric Research and Ben Noll, Meteorologist/forecaster, National Institute of Water and Atmospheric Research

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

Australia’s coastal living is at risk from sea level rise, but it’s happened before



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Australia’s coastline has moved before thanks to changes in sea level.
Flickr/Travellers travel photobook, CC BY

Sean Ulm, James Cook University; Alan N Williams, UNSW; Chris Turney, UNSW, and Stephen Lewis, James Cook University

With global sea levels expected to rise by up to a metre by 2100 we can learn much from archaeology about how people coped in the past with changes in sea level.

In a study published this week in Quaternary Science Reviews, we looked at how changes in sea level affected different parts of Australia and the impact on people living around the coast.

The study casts new light on how people adapt to rising sea levels of the scale projected to happen in our near future.


Read more: Cave dig shows the earliest Australians enjoyed a coastal lifestyle


Coastal living

More than eight out of every ten Australians live within 50km of the coast.

The Intergovernmental Panel on Climate Change says global sea levels are set to increase by the equivalent of 12mm/year, four times the average of the last century.

A major challenge for managing such a large increase in sea level is our limited understanding of what impact this scale of change might have on humanity.

While there are excellent online resources to model the local physical impacts of sea level rise, the recent geological past can provide important insights into how humans responded to dramatic increases in sea level.

The last ice age

At the height of the last ice age some 21,000 years ago, not only were the Greenland and Antarctic ice sheets larger than they are today, but 3km-high ice sheets covered large parts of North America and northern Europe.

This sucked vast amounts of water out of our planet’s oceans. The practical upshot was sea level was around 125m lower, making the shape of the world’s coastlines distinctly different to today.

As the world lurched out of the last ice age with increasing temperatures, the melting ice returned to the ocean as freshwater, dramatically increasing sea levels and altering the surface of our planet.

Arguably nowhere experienced greater changes than Australia, a continent with a broad continental shelf and a rich archaeological record spanning tens of millennia.

A bigger landmass

For most of human history in Australia, lower sea levels joined mainland Australia to both Tasmania and New Guinea, forming a supercontinent called Sahul. The Gulf of Carpentaria hosted a freshwater lake more than twice the size of Tasmania (about 190,000km2).

Our study shows that lower sea levels resulted in Australia growing by almost 40% during this time – from the current landmass of 7.2 million km2 to 9.8 million km2.

The coastlines also looked very different, with steep profiles off the edge of the exposed continental shelf in many areas forming precipitous slopes and cliffs.

Imagine the current coastline where the Twelve Apostles are on Victoria’s Great Ocean Road and then extend them around much of the continent. Many rivers flowed across the exposed shelf to the then distant coast.

The steep cliffs at the Apostles, off Victoria’s Great Ocean Road, look like parts of the ancient coastline of Australia.
Flickr/portengaround, CC BY-SA

When things warmed up

Then between 18,000 and 8,000 years ago, global climate warmed, leading to rapid melting of the ice sheets, and seeing sea levels in the Australian region rising from 125m below to 2m above modern sea levels.

Tasmania was cut off with the flooding of Bass Strait around 11,000 years ago. New Guinea was separated from Australia with the flooding of Torres Strait and creation of the Gulf of Carpentaria around 8,000 years ago.

We found that 2.12 million square km, or 20-29% of the landmass – a size comparable to the state of Queensland – was lost during this inundation. The location of coastlines changed on average by 139km inland. In some areas the change was more than 300km.

Much of this inundation occurred over a 4,000-year period (between 14,600 and 10,600 years ago) initiated by what is called Meltwater Pulse 1A, a period of substantial ice sheet collapse releasing millions of cubic litres of water back into the oceans.

During this period, sea levels rose by 58m, equivalent to 14.5mm per year. On the ground, this would have seen movement of the sea’s edge at a pace of about 20-24m per year.

Impacts of past sea level rise

The potential impacts of these past sea-level changes on Aboriginal populations and societies have long been a subject of speculation by archaeologists and historians.

Map of Australia showing sea-level change and archaeological sites for selected periods between 35,000 and 8,000 years ago. PMSL=Present Mean Sea Level.
Sean Ulm, Author provided

In his 1970s book Triumph of the Nomads: A History of Aboriginal Australia, the Australian historian Geoffrey Blainey hypothesised that:

Most tribal groups on the coast 18,000 years ago must have slowly lost their entire territory […] a succession of retreats must have occurred. The slow exodus of refugees, the sorting out of peoples and the struggle for territories probably led to many deaths as well as new alliances.

Archaeologists have long recognised that Aboriginal people would have occupied the now-drowned continental shelves surrounding Australia, but opinions have been divided about the nature of occupation and the significance of sea-level rise. Most have suggested that the ancient coasts were little-used or underpopulated in the past.

Our data show that Aboriginal populations were severely disrupted by sea-level change in many areas. Perhaps surprisingly the initial decrease in sea level prior to the peak of the last ice age resulted in people largely abandoning the coastline, and heading inland, with a number of archaeological sites within the interior becoming established at this time.

Cross-section profiles of the continental shelf at Port Stephens, NSW (top) and Cape Otway, Vic (bottom). PMSL=Present Mean Sea Level.
Sean Ulm, Author provided

During the peak of the last ice age, there is evidence on the west coast that shows people continued to use marine resources (shellfish, fish etc) during this time, albeit at low levels.

A shrinking landmass

With the onset of the massive inundation after the end of the last ice age people evacuated the coasts causing markedly increased population densities across Australia (from around 1 person for every 355 square km 20,000 years ago, to 1 person every 147 square km 10,000 years ago).

Rising sea levels had such a profound impact on societies that Aboriginal oral histories from around the length of the Australian coastline preserve details of coastal flooding and the migration of populations.

We argue that this squeezing of people into a landmass 22% smaller – into inland areas that were already occupied – required people to adopt new social, settlement and subsistence strategies. This may have been an important element in the development of the complex geographical and religious landscape that European explorers observed in the 18th and 19th centuries.

Following the stabilisation of the sea level after 8,000 years ago, we start to see the onset of intensive technological investment and manipulation of the landscape (such as fish traps and landscape burning).

We also see the formation of territories (evident by marking of place through rock art) that continues to propagate up until the present time. All signs of more people trying to survive in less space.


Read more: Buried tools and pigments tell a new history of humans in Australia for 65,000 years


So what are the lessons of the past for today? Thankfully, we can show that past societies survived rapid sea level change at rates slightly greater than those projected in our near future, albeit with population densities far lower than today.

But we can also see that sea level rise resulted in drastic changes to where people lived, how they survived, what technology they used, and probable modifications to their social, religious and political ways of life.

The ConversationIn today’s world with substantially higher population densities, managing the relocation of people inland and outside Australia, potentially across national boundaries, may provide to be one of the great social challenges of the 21st century.

Sean Ulm, Deputy Director, ARC Centre of Excellence for Australian Biodiversity and Heritage, James Cook University; Alan N Williams, Associate Investigator, ARC Centre of Excellence for Australian Biodiversity and Heritage, UNSW; Chris Turney, ARC Centre of Excellence for Australian Biodiversity and Heritage, University of New South Wales, UNSW, and Stephen Lewis, Principal Research Officer, James Cook University

This article was originally published on The Conversation. Read the original article.

As a coastal defence, the Great Barrier Reef’s value to communities goes way beyond tourism



File 20170724 24759 1vumpv
Parts of the Great Barrier Reef’s outer reefs can form a natural barrier to coastal recession, thus protecting urban centres.
AAP

Mark Gibbs, Queensland University of Technology

Rising sea levels are widely recognised as a threat to coastal communities worldwide. In Australia, the Climate Council estimates that at least A$226 billion of assets and infrastructure will be exposed to inundation if sea levels rise by 1.1 metres. Another report recommended that global mean sea level rise of up to 2.7 metres this century should be considered in planning processes.

The Queensland state government has commissioned the QCoast2100 program. This program aims to help with the development of coastal climate adaptation plans for Queensland communities exposed to sea-level rise.

Although the largest population centres in Queensland are in the state’s southeast, several of the most populous regional centres in Australia are located along the Great Barrier Reef coastline between Gladstone and Cape York. These include Townsville, Cairns, Gladstone, Mackay and Port Douglas.

A major task in developing coastal adaptation plans under the QCoast2100 program is to model inundation from a range of scenarios for sea-level rises and assess how assets will be inundated in the future. However, another threat is on the horizon.


Further reading: What’s the value of the Great Barrier Reef? It’s priceless


How urban centres are protected

Urban centres along the reef’s coastline, which forms the majority of the Queensland coast, are protected from major ocean storms by natural deposits of coastal sediments. These include dunes and associated vegetation such as coastal forests, wetlands and mangrove systems.

These natural features continue to exist largely because the Great Barrier Reef’s outer reefs dampen incoming ocean waves. Although exposed to the occasional cyclone – which can lead to short-term erosion at specific locations – much of the coastal zone inside the reef is slowly growing out into the sea.

This increasing buffer zone can form a natural barrier to coastal recession.

A recently released report estimated the total economic, social and icon asset value of the Great Barrier Reef at A$56 billion. By design, this report did not include many of the ecosystem services the reef provides. One of these is its role in reducing the energy of waves that impact the coastline behind the reef.

However, an earlier assessment of the total economic value of ecosystem services delivered by the reef estimated the present coastal protection benefit is worth at least A$10 billion.

Despite the inherent uncertainties in such assessments, it is clear the reef acts to reduce incoming wave energy and its impacts on cities and towns along much of the Queensland coastline. The total economic value of these benefits is in the billions of dollars.


Further reading: Coastal communities demand action on climate threats


What role is bleaching playing?

The Great Barrier Reef’s ability to keep protecting the Queensland shoreline, and communities living along it, depends upon the ability of individual reefs in the system to grow vertically to “keep up” with rising sea level.

The jury is still out on whether the outer reefs will be able to keep up with predicted rises. This is an active area of research.

However, it is clear reefs that are extensively affected by coral bleaching will struggle to maintain the essential processes required for productive reef-building. Many reefs are now experiencing net erosion.

Predictions of ocean warming suggest that bleaching events will become even more common in coming decades. Increasing levels of atmospheric carbon dioxide are also making the oceans more acidic, which makes it more difficult for organisms such as corals to maintain their skeletons, which are made of calcium carbonate. This mineral dissolves more rapidly with increasing acidification, reducing the reef’s capacity to recover from storm damage and coral bleaching.

Therefore, as bleaching events and acidification continue, the outer reefs that protect the Queensland coast from ocean waves will increasingly struggle to perform this function.

The ConversationIn turn, over time the Queensland coast will potentially suffer from more coastal erosion, which may increase the vulnerability of coastal infrastructure. This effect, combined with rising sea levels leading to more coastal inundation events, multiples the risks to coastal settlements and infrastructure.

Mark Gibbs, Director, Knowledge to Innovation; Chair, Green Cross Australia, Queensland University of Technology

This article was originally published on The Conversation. Read the original article.