Blog, cliamte change, coast, coastal flooding, science, sea level, sea level rise, storm surge

New paper: Act now and we can save 3.5 m of sea level rise by the year 2300.

If we act now and cut carbon emissions, we will save up to 40 cm of sea level rise by 2100, but up to 3.5 m by 2300. That is the conclusion of our new study which developed a new approach to predicting future changes in temperature and sea level rise over the next three centuries. It’s not easy to think 300 hundred years ahead. And yet we must remember our actions today, will impact our great, great, great, great, ….., great grandchildren.

 

The Paris Agreement

The 2015 Paris Agreement to strengthen the global response to the threat of climate change committed the 175 countries that ratified the treaty to aim to hold ‘the increase in the global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels recognizing that this would significantly reduce the risks and impacts of climate change’. However, while reducing human emissions of greenhouse gases will stabilise temperature, it is often forgotten that sea-level rise will continue for many centuries irrespectively. This is because it takes many hundreds of years for the cryosphere (the area of our planet covered by ice) and the deepest parts of the ocean to adjust to increased air temperatures. This brings long-term challenges to those living in the coastal zone. These challenges may be immense unless we take action today to mitigate for climate change and where possible, appropriately adapt.

 

WASP model

Most projections of climate change stop at the year 2100, for two good reasons: (1) it is hard to think and plan beyond this time-frame; and (2) global climate models are computationally expensive to run. To make it easier to investigate changes in temperature and sea-level rise beyond 2100 we developed a simple but clever global climate model that we call WASP. This stands for the Warming Acidification and Sea-level Projector. It represents the whole earth using 8 boxes (Figure 1) that capture the interaction between the atmosphere, vegetation, soil and the different layers of the ocean. Using WASP, we can accurately predict future changes in climate, for different carbon emissions, on a global scale. Because of the simple way we represent the earth, we can relatively quickly run the model out to the year 2300, many millions of times; something that is simply not possible using regular climate models. WASP is so efficient that you can run it on a smart phone in seconds (give it a go – click here).

 

Figure 1: The WASP modelling framework.

 

To avoid the most dangerous consequences of anthropogenic climate change, the Paris Agreement provides a clear and agreed target of stabilizing global surface temperatures to under 2.0 °C and preferably closer to 1.5 °C. However, policy makers do not currently know exactly what carbon emissions pathways to follow to stabilize warming below these agreed targets, because there is large uncertainty in future temperature rise for any given pathway. This large uncertainty makes it difficult for a cautious policy maker to avoid either: (1) allowing warming to exceed the agreed target; or (2) cutting global emissions more than is required to satisfy the agreed target, and their associated societal costs. We therefore, develop a new approach to restrict future warming to policy-driven targets. These uses what we call ‘Adjusting Mitigation Pathways’, in which future emissions reductions are not fully determined now but respond to future surface warming each decade in a self-adjusting manner.

 

Sea level rise predictions

Using our self-Adjustment Mitigation Pathways, we ran the WASP model to the year 2300, comparing climate stabilization targets ranging from 1.5 to 4.5°C with a high emissions scenario assuming no climate change mitigation. For our scenario, in which global surface temperatures were limited to 1.5°C we predicted an average sea-level rise of 40 cm above present levels by 2100 and 1 m by 2300. For the 2.0°C we predicted an average sea-level rise of about 50 cm above present levels by 2100 and around 1.30 m by 2300. In comparison, for the high emission scenario, where carbon emissions continue as normal and temperatures each 9.5°C in 2300, we predicted an average sea-level rise of 80 cm above present levels by 2100 and 4.5 m by 2300. We show therefore, that if we can meet the Paris Agreement, we can save up to 40 cm by 2100, but around 3.5 m by 2300. The latter is a big difference and will affect millions of people and important ecosystems world-wide.

 

Figure 2: Our sea level projections.

 

How will this impact people?

We have investigated the impact this has on the area of land that would be inundated by the year 2300 and the number of people that will likely be exposed, in another study. We estimate that if we restricted warming to 1.5°C, 1.5% of the global population will be exposed to sea-level rise by 2300. In contrast, for the high emission scenario, more than 5% of people on the earth will be exposed to coastal flooding. If flood defences are not taken into account, the area of land that may be inundated due to sea-level rise in 2300 will almost double for the aggressive mitigation and the non-mitigation scenario, respectively.

 

We find in another study that many highly populated low-lying delta areas could be totally flooded unless adaptation is considered and sediments are allowed to settle on adjacent land during times when rivers flood. However, adaptation has to be undertaken wisely, as decisions we make today may have long-term consequences. For example, many of the millions of people living in delta regions world-wide rely on regulated water supply or electricity via dams up-river. Dams not only control water, but also hold-back sediment from moving downstream. When rivers periodically flood, it means there is less sediment to deposit on the adjacent land. In coastal regions, this potentially means that the land will subside relative to the river and become more flood prone. Further defences, such as dikes may be required, but this exacerbates the problem. Without sedimentation, deltas areas are at high risk from sea-level rise. Subsequently deltas and other vulnerable coastal regions need to look to the future and consider plausible long-term international strategies protect millions of people against flood risk. This will not be easy as national and international collaboration may is needed, so that adaptation to sea-level rise is integrated into existing policies and development.

 

Conclusion

Sea-level rise poses an international threat, not just to coastal communities, but those inland who rely on the coast for livelihoods. Mitigating for climate change will reduce the rate of sea-level rise, but it will continue to rise for centuries. This is inevitable and must be something we plan for.

 

Authors

Ivan Haigh  – University of Southampton

Sally Brown – University of Southampton

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Barriers, canals and fake islands: how we can save cities from rising sea levels

The Conversation

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

Sally Brown, University of Southampton; Ivan Haigh, University of Southampton, and Robert Nicholls, University of Southampton

Extreme storms and rising sea levels will threaten the existence of coastal cities worldwide, unless preventative action is undertaken. With population growth and sea-level rise set to continue, research has estimated that by 2050, we can expect more than US$1 trillion worth of damages per year to be incurred by 136 of the world’s largest cities, if there is no attempt to adapt.

The game changer came in 2005, when we saw one of the most active hurricane seasons in US history. Hurricane Katrina, the fifth hurricane of that season, resulted in nearly 1,600 deaths. Almost half of these fatalities occurred in New Orleans: 80% of the city was flooded, at a cost of US$40 billion. When the water subsided, so did the population: ten years on, the city that used to house 500,000 is now home to only 300,000 people.

There are a number of ways to go about changing cities to account for rising sea levels: we can raise coastal defences, build houses on stilts, or simply move cities and their populations away from the coast. Which of these strategies works best was one of many questions set out in Climate Change: A Risk Assessment – a new report led by Sir David King and the Foreign and Commonwealth Office.

Constant threat

Globally, sea levels have been remarkably stable since civilisation started to develop several thousand years ago. During the 20th century, sea levels rose about 17cm, at an average rate of 1.8mm per year. Over the past few decades, that rate has doubled to more than 3mm per year. This trend is expected to continue and accelerate. According to the latest Intergovernmental Panel on Climate Change report, the sea level is projected to rise up to 1m by 2100. If the large ice sheets of Greenland and Antarctica melted, even higher rises are considered possible, albeit highly uncertain.

Importantly, if carbon emissions are stabilised, or even decrease, the sea level will continue to rise for many centuries, as the deep ocean slowly warms and the large ice sheets reach a new equilibrium. Simply put, sea-level rise is here to stay. It is likely to lead to greater flooding, salinisation (the build up of salt in surface and groundwater) and erosion in coastal areas, affecting millions of people worldwide and costing billions of dollars of damage.

The devastation of Typhoon Haiyan.
EU Humanitarian Aid and Civil Protection/Flickr, CC BY-ND

The high costs of economic damage and loss of life are becoming less acceptable in a world where extreme weather events can be accurately forecast and coastal protection is possible. In many parts of the world, damages and loss of life remain high, as seen during Typhoon Haiyan, which hit the Philippines in 2013. Preparing coastal cities for extreme events and adapting them to cope with sea-level rise remains challenging: King’s report highlights the engineering, financial and socio-political limits of the adaptation challenge.

But cities are starting to embrace these challenges. For example, last year, Boston put forward the bold, novel idea of becoming an American Venice – a city full of canals to hold water as sea levels rise. New York has considered building a barrier to keep water out, in light of the fact that, with a 1m rise in the sea level, a 1-in-100 year event (that is, a severe storm one would expect to occur once every 100 years) could become 200 times more likely to occur.

London has also developed a range of flexible options that would protect the Thames Estuary against up to 5m of sea-level rise. These include raising defences, implementing flood storage and constructing a new and bigger Thames Barrier further downstream.

Developing better cities

In developing countries, few cities are preparing for sea-level rise, despite the awareness that this is a long-term hazard. Developing cities also frequently have rapid population growth. In Shanghai and Kolkata more than 400,000 people live less than 2m above the present-day sea level. A rise of 1m will increase the frequency of a current 1-in-100 year event by 40 times in Shanghai, and about 1,000 times in Kolkata.

Local ground subsidence is another factor to worry about. This involves the sinking of the land relative to the sea due to natural and sometimes human processes (such as groundwater withdrawal). Local ground subsidence will worsen conditions in about a quarter of coastal cities – namely, those built on susceptible deltaic soils (those at the mouth of a river).

Malé’s protective tetrapods.
Sally Brown, Author provided

Small islands and their cities are also under serious threat from sea-level rise as they are low-lying, remote and dispersed in their territories, and often have limited financial resources. Far from being a green, spacious island, Malé – the capital of the Maldives – is one of the world’s most densely populated cities. Building protective structures is one way of reducing the impacts of extreme events: Malé is surrounded by a sea wall and giant tetrapods (a four-pronged concentrate structure about 2m high). But a lack of space limits future coastal protection.

To overcome this, a new island has been constructed, Hulhumalé, with sea-level rise also in mind. The solution to sea-level rise is simply to build upwards: The island was raised to 2m above present day sea level to protect against storms. This buys time, but moving into the late 21st or early 22nd century this may not be enough. Other Maldivian islands are following suit, with the Safer Islands programme selectively raising parts of islands. This may help the parts of the country, but clearly much more work is required to ensure the long-term prospects of this fragile island nation.

Ultimately, these case studies show us that there’s no one-size-fits-all approach to adapting cities to rising sea levels. Rather, the best bet for cities to adapt against rising sea levels is to dare to be different. Both engineering design, government authorities and social attitudes must acknowledge that change needs to occur, if we’re to avoid disaster.

Sally Brown, Research Fellow, University of Southampton; Ivan Haigh, Lecturer in Coastal Oceanography, University of Southampton, and Robert Nicholls, Professor of Coastal Engineering, University of Southampton

Blog, coast, coastal flooding, extreme events, flooding, science, sea level, storm surge

New paper just published – Variability in coastal flood risk

We have just had a new paper (Assessing the variability in extreme high water levels for coastal flood risk assessment) published in the Journal of Geophysical Research-Oceans – see here.

The probability of extreme storm-tide events has been extensively studied, however the variability within the duration of such events, and implications to flood risk, is less well understood. This research quantifies such variability during extreme storm-tide events (the combined elevation of the tide, surge, and their interactions) at 44 national tide gauges around the UK. Extreme storm-tide events were sampled from water level measurements taken every 15 minutes between 1993 and 2012. At each site, the variability in elevation at each time step, relative to a given event peak, was quantified. The magnitude of this time-series variability was influenced both by gauge location (and hence the tidal, and non-tidal residual characteristics) and the time relative to high water. The potential influence of this variability on coastal inundation was assessed across all UK gauge sites, followed by a detailed case study of Portsmouth. A two-dimensional hydrodynamic model of the Portsmouth region was used to demonstrate that given a current 1 in 200 year storm-tide event, the predicted number of buildings inundated differed by more than 30% when contrasting simulations forced with the upper and lower bounds of the observed time-series variability. The results indicate that variability in the time-series of the storm-tide event can have considerable influence upon overflow volumes, hence with implications for coastal flood risk assessments. Therefore, further evaluating and representing this uncertainty in future flood risk assessments is vital, while the envelopes of variability defined in this research provides a valuable tool for coastal flood modellers.

 

MATLAB Handle Graphics

 

Australia, Blog, coast, coastal flooding, extreme events, flooding, Journal paper, science, sea level, storm surge, tides

New paper, just published: Australian Sea Levels – Trends, Regional Variability and Influencing Factors

While there has been significant progress in describing and understanding global-mean sea-level rise, the regional departures from this global-mean rise are more poorly described and understood. In this new paper, which you can view here, we present a comprehensive analysis of Australian sea-level data from the 1880s to the present, including an assessment of satellite-altimeter data since 1993.

We find that After the influence of El Niño Southern Oscillation is removed and allowing for the impact of Glacial Isostatic Adjustment and atmospheric pressure effects, Australian mean sea-level trends are close to global-mean trends from 1966 to 2010, including an increase in the rate of rise in the early 1990s. Given that past changes in Australian sea level are similar to global-mean changes over the last 45 years, it is likely that future changes over the 21st century will be consistent with global changes.

 

Blog, coast, science, sea level

Back to the future to determine if sea level rise is accelerating

Here is the press release from my recent paper published in Nature Communications – ‘Time-scales for detecting a significant acceleration in sea level rise’.

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Scientists from Ocean and Earth Southampton and National Oceanography Centre Southampton have developed a new method for revealing how sea levels might rise around the world throughout the 21st century to address the controversial topic of whether the rate of sea level rise is currently increasing.

The international team of researchers, led by the University of Southampton and including scientists from the National Oceanography Centre, the University of Western Australia, the University of South Florida, the Australian National University and the University of Seigen in Germany, analysed data from 10 long-term sea level monitoring stations located around the world. They looked into the future to identify the timing at which sea level accelerations might first be recognised in a significant manner.

Lead author Dr Ivan Haigh, Lecturer in Coastal Oceanography at the University of Southampton, says: “Our results show that by 2020 to 2030, we could have some statistical certainty of what the sea level rise situation will look like for the end of the century. That means we’ll know what to expect and have 70 years to plan. In a subject that has so much uncertainty, this gives us the gift of long-term planning.

“As cities, including London, continue to plan for long-term solutions to sea level rise, we will be in a position to better predict the long-term situation for the UK capital and other coastal areas across the planet. Scientists should continue to update the analysis every 5 to 10 years, creating more certainty in long-term planning – and helping develop solutions for a changing planet.”

The study found that the most important approach to the earliest possible detection of a significant sea level acceleration lies in improved understanding (and subsequent removal) of interannual (occurring between years, or from one year to the next) to multidecadal (involving multiple decades) variability in sea level records.

“The measured sea levels reflect a variety of processes operating at different time scales,” says co-author Dr Francisco Calafat, from the National Oceanography Centre. He adds, “One of the main difficulties in detecting sea level accelerations is the presence of decadal and multi-decadal variations. For example, processes associated with the North Atlantic Oscillation have a strong influence on the sea levels around the UK over multi-decadal periods.  Such processes introduce a large amount of ‘noise’ into the record, masking any underlying acceleration in the rate of rise. Our study shows, that by adequately understanding these processes and removing their influence, we can detect accelerations much earlier.”

Co-author Professor Eelco Rohling, from the Australian National University and formerly of the University of Southampton, adds: “By developing a novel method that realistically approximates future sea level rise, we have been able to add new insight to the debate and show that there is substantial evidence for a significant recent acceleration in the sea level rise on a global and regional level. However, due to the large ‘noise’ signals at some local coastal sites, it won’t be until later this decade or early next decade before the accelerations in sea level are detection at these individual tide gauge sites.”

The findings of the study, funded by the Natural Environmental Research Council (iGlass consortium), are published in this months issue of the journal Nature Communications.