Will London soon be underwater?

Ivan D. Haigh1 and Robert J. Nicholls2

  1. School of Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton and Fathom
  2. Tyndall Centre for Climate Change Research, University of East Anglia

Will London soon be underwater? We can’t count the number of times over the last year that we have both been asked this question, or something similar. It has most often come from either the media or general members of the public after they have seen the flood maps produced by non-profit news organisation Climate Central ( 

Climate Central’s coastal risk screening tool — which you can access here ( — allows users to zoom into a map anywhere in the world and see areas of land that lie below the annual flood level, from the year 2030 all the way until the distant year 2150, accounting for sea-level rise caused by climate change.

It is very evident from these maps, that large areas of London already lie below the height of a high sea level event that is likely to occur at least once a year (area shown in red shading); and that even larger areas will be at risk of flooding as sea-level rise accelerates into the future. These maps of London have been widely shared across social media, raising many questions and fears.

Screen shot of Climate Central’s flood maps. 

Taken at face value, these maps suggest that those living on low-lying areas around the Thames Estuary already face a serious and growing problem. Numbers estimated by the Environment Agency ( highlight the scale of the issue: up to a staggering 1.42 million people, £321 billion worth of residential property, 496 education facilities, 711 healthcare facilities and four world heritage sites currently lie within or near to the tidal flood plain in London, Kent and Essex. 

The flood zone also contains critical energy, transport and water infrastructure, including the Thames Freeport, the Blackwall Tunnel and the Dartford Crossing, 167 kilometres of rail routes, 116 train or tube stations, over 2400 kilometres of roads and nine power stations.

The questions we have been asked from those that have seen these flood maps, and the fears and concerns that they have voiced to us, include: “Should I sell my home?”; “Will I be able to get insurance that covers flooding?”; “Will I be forced to move out of London?”“How much will my businesses suffer if we are flooded”?; “How will this affect tourism in the city”; “Will many of our important historic buildings be lost to the sea?”“Will the houses of parliament have to be moved?”“Can London continue to be our capital city, if the flooding is going to be this bad in the future?”.

The silent protector

There is one important thing missing here. Climate Central’s maps don’t consider the existing flood defences that protect these areas along the Thames Estuary. London is in fact one of the best defended coastal cities in the world, with excellent existing defences, and a detailed flood management plan  – the Thames Estuary 2100 Plan ( – that aims to protect areas along the Thames Estuary from flooding through to the end of the 21st century and beyond. 

As long as these defences continue to be maintained, and the Thames Estuary 2100 Plan is implemented, it is very unlikely that the areas indicated on Climate Central’s maps will actually be flooded. 

The Thames Estuary is currently protected by a world-class system of flood defences that includes the iconic Thames Barrier, eight other smaller flood barriers, over 330 kilometres of walls and embankments.

Flood defences of the Thames Estuary; Source – Environment Agency

The Thames Barrier is the jewel in the crown and is one of the largest movable storm surge barriers in the world. The barrier spans 520 metres across the River Thames near Woolwich. It has ten steel gates that can be moved into position across the River Thames. Each main gate weighs 3,300 tonnes; the equivalent of 275 double decker buses. The main gates, when raised, stand as high as a five-storey building. The barrier is closed under storm surge conditions to protect London from flooding from the sea and also during periods of high flow over Teddington Weir to reduce the risk of compound flooding from the combined river and tide in some areas of west London.

The Thames barrier; Photo taken by Ivan Haigh taken during a test closure on October 25 2021. 

Plans for the Thames Barrier began after the disastrous “Big North Sea Flood (” of January 31 and February 1 1953; which killed 307 people in England and flooded 160,000 acres of land along the East Coast, including about 60 deaths in Canvey Island on the Thames. Construction of the barrier began in 1974 and was completed in 1982 at a cost of £1.6 billion (in today’s prices). The barrier was officially opened by Queen Elizabeth II on May 8 1984. The barrier was first closed to protect London from Flooding on 1 February 1983. On October 21 2021 it closed for a remarkable 200th time.

In addition to the Thames Barrier, eight other small barriers protect low lying land on tidal rivers that are connected to the Estuary. These include the smaller Barking and Dartford Creek Barriers, which have been closed many more times than the Thames Barrier. There are also 330 kilometres of walls and embankments around the Thames Estuary and 400 other flood gates, outfalls and pumps. All work together in an interconnected way to protect the tidal flood plain of the Thames Estuary from flooding. 

A vital element are accurate water level and river discharge predictions for the southern North Sea and Thames which predict flooding threats and allow time for the barriers to be closed. Staff at the Flood Forecasting Centre in Exeter and the Thames Barrier work, and super-computers run, 24-hours a day to produce and monitor these weather, sea-level and river flow forecasts. 

We are always surprised at how few people living and working in London actually know that the Thames Barrier and other defences exists, or if they do how it works. Andy Batchelor, the manager of the barrier, refers to the Thames Barrier as “the silent protector”. This is very appropriate as every day, while millions of people work, live and visit London, the Thames Barrier and other defences are quietly protecting them, their homes, restaurants, shops, cinemas and businesses from flooding.

Returning then to the Climate Central Flood maps, we can assure people living along the Thames that they are protected by a world-class system of flood defences; and that the areas indicated on the map are extremely unlikely to be flooded currently or in the near future. The maps do a good job of showing what could flood, but by not considering the extensive and effective flood defence system, they do not show realistic levels of risk, contrary to the name of the tool.

However, these defences come at a significant cost, both to build and maintain, and there is always a residual risk that remains. If the defences were to fail during a major storm there could be significant social and economic impacts, which is why very large investment is being made to ensure reliable defences. The Thames Barrier has multiple backup systems in place and has reliably protected London from flooding on more than 200 occasions since it became operational. This justifies the costs of the barrier and its maintenance.

The future

It is also important to consider climate change and how effective these defences will be in protecting London in the future. Sea-level rise, caused by thermal expansion as the oceans warm and melting of land-based ice from glaciers and ice sheets, is one of the most certain consequences of climate change. 

It will increasingly put pressure on flood defences around the Thames Estuary. Over the last 150 years, average sea-levels within the Thames Estuary have risen by around 20cm (, measured accurately with tide gauges. The latest report ( from the Intergovernmental Panel on Climate Change (IPCC) suggests that sea levels could rise a further 0.5 to one metre by the year 2100 within the Thames Estuary (and possibly even as high as two metres if a major ice sheet collapse occurs, although this is currently considered very unlikely). 

This rise in the mean also raises extreme sea levels during storms, even if the storms are not more severe. Climate change however, may also result in more frequent and bigger storm surges in the southern North Sea and is likely to lead to increased peak flows of the rivers discharging into the Thames Estuary.

A vital and appropriate question – one that we have been asked multiple times by concerned people, including politicians, that needs answering is – “How well can the Thames Barrier, which is almost 40 years old, and the other defences protect London throughout the remainder of the 21st century and beyond in the face of rising sea levels, changes in storminess and increased river flow? 

This has and continues to be been carefully considered by the Environment Agency, who developed the Thames Estuary 2100 Plan ( Released in 2012, this plan sets out how the Environment Agency and its partners will work together to manage the increasing tidal flood risk in the Thames Estuary that will result from climate change, ageing flood defences and population growth and development. We have both been involved in helping to shape this Plan and have played a lead role in the 10-Year review of it, the first output of which was published in 2021 ( 

The Thames Estuary 2100 (TE2100) Plan is one of the most sophisticated flood management plans in the world and was the first to propose an adaptive flood risk management strategy. This considers a wide range of potential measures that could be selected. Further it recognises that adaptation is a process rather than a one-off action — London will be adapting to flooding far into the future and the choices will involve a number of steps. The TE2100 Plan puts these options together into a series of adaptation pathways or steps. While the precise timing and magnitude of sea-level rise is uncertain, the preferred pathways to adapt to the growing threat of flooding can be decided. The timing of implementation can depend on the observed sea-level rise and improving projections. Hence this approach allows decisions to be made under the high uncertainty that exists. Both of us are strong advocates of this approach which is termed adaptive management in that what is done is adjusted as we learn about the future.  

We don’t know how much sea-level will rise in the future, as this is very much dependent on our greenhouse gas emissions and the uncertain response of the ocean and cryosphere (the frozen parts of the planet) to that warming. The Plan, as James Brand from the Environment Agency explained to us, accounts for this uncertainty and has two main adaptive elements.

First, it maps out a range of approaches to manage flood risk in the Estuary, based on different possible magnitudes of sea-level rise. One route for example involves raising defences and improving the existing Thames Barrier. Another more expensive option would be to build a new and bigger storm surge barrier further downstream. Currently, the plan is following a path of ‘no regrets’ upgrades to current defences and has specified practical steps of work to achieve this over the coming decades. With monitoring of actual sea-level rise and more certain projections, the plan could be adjusted including more costly options if required.

Illustration of the adaptive nature of the range of possible flood defence upgrades in the Thames Estuary 2100 Plan. Source – Environment Agency. 

Second, the plan is adaptive in terms of the timing of key interventions. For example, currently the first deadline in the Plan is for defences to be raised downstream of the Thames Barrier. This is currently planned to be completed by 2040, however, this deadline can be brought forward if sea levels are found to be increasing faster than predicted or delayed if changes are slower than expected. Later in the century defences upstream of the barrier will need to be raised. 

We both love walking along Thames and seeing the river when we visit London, and we are sure this is a highlight of a visit to London for many people. Raising defences can separate London from the river, by restricting the view and access to the river. Therefore, careful thought and design is needed to minimise this negative side effect.

Illustration of the adaptive nature of the timings of key interventions in the Thames Estuary 2100 Plan. Source – Environment Agency. 

For an adaptive approach to be effective, key indicators of change, such as rate of sea-level rise, must be monitored and reviewed regularly. Every five years, the Environment Agency assess whether the Thames Estuary has changed in line with the projected changes in the Thames Estuary 2100 Plan. As well as physical changes in the estuary, they consider the latest scientific guidance and policy changes. Every ten years, they conduct a full review and update of the recommendations in the plan. 

We have spent the last two years working with the Environment Agency on the monitoring report which was published in 2021 ( and is the first output from the 10-Year review of the Thames Estuary 2100 Plan. 

The success of the Plan will depend upon the Environment Agency working in partnership with other risk management authorities, academics and other experts, businesses, and communities to ensure it is periodically reviewed in line with the latest science and its actions are implemented.   

A criticism we have often heard raised is the considerable cost to the tax-payer to implement the Plan over the remainder of the 21st century. The cost is indeed very large. However, it is justified by the huge benefit the protection the flood defences offer to millions of people living and working around the Thames Estuary and the overall benefit to London and the UK’s economy. 

We have also been often asked specifically about the Thames Barrier, and how effective it will continue to operate give it is almost 40 years old. The Thames Barrier was designed with a 50-centimetre allowance for sea-level rise included. It was expected that the Barrier would close more frequently over time and while closures vary from year-to-year, in the long-term, this is exactly what is being observed. 50 closures per year would be challenging for the Barrier as maintenance would start to be compromised. In the extreme and unusual autumn and winter season of 2013/14 there were 50 closures for the first time. However, the average number of closures is currently 5 per year and increasing at a rate of approximately 2 more extra closures per decade. Hence, over the next few decades increasing closures are not considered a threat to the protection the Barrier provides and closures are also carefully monitored and considered in the TE2100 Plan. 

All this means that an innovative adaptable plan is in place to protect communities and infrastructure in the Thames Estuary flood plain through to the end of the 21st century and beyond that can cope with as much as 5 metres of sea-level rise and maybe even more. While another mobile storm surge barrier might be built first, ultimately to keep London where it is in the long-term with a large sea-level rise (roughly 3 metres or more) would involve a fixed barrage (i.e., a dam with pumping of the Thames flow over it). So the relationship of London to the sea would be fundamentally changed by a large rise in sea level and how we adapt to this change. 

The Thames Estuary Plan is internationally recognised ( as a leading example of a climate adaptation strategy, one that enables policy makers and practitioners to plan, monitor and review how to adapt to flood risk over time. Other cities around the world, such as New York, are starting to consider ( this adaptive approach.

Other coastal cities

Of course, London is only one of many large cities located on the coast. We have mapped and analysed the growing flood exposure and risk in other coastal cities around the world. We identified ( at least 136 cities on the coast with more than a million inhabitants and numerous smaller towns and villages.

This analysis shows the huge growth in coastal cities in Asia and adaptation to sea-level rise will be particularly important in this region – especially as some of these cities are subsiding (, in addition to experiencing climate change. 

Singapore is one of the leaders in city adaptation and they are learning from what is happening in places such as London and also the Netherlands. Cities such as Shanghai and Tokyo have also addressed increasing coastal flooding, via major investment in defences and pumps. 

Jakarta, the capital of Indonesia, is a coastal mega-city with a population of more than 10 million people. As a result of land subsidence associated with groundwater extraction (rates of subsidence are 10 centimetres each year ( and to a lesser degree climate-induced sea-level rise, the frequency of coastal flooding in the city has increased enormously ( in recent decades. In 2019, President Joko Widodoto announced ( the radical decision to move the national capital from Jakarta to the province of East Kalimantan, on Borneo. Hence, in some cases, like Jakarta, moving a whole city and all that that entails, may be the preferred option. 

The UK has the money and know how to finance the billions of pounds required to maintain and update the flood defence systems that protect London and the other regions at risk along the Thames Estuary. In contrast, many coastal cities presently lack the money and expertise to build such complex flood defence systems. 

But the adaptive approach outlined here is flexible and allows progressive action as we learn. Each city has a different setting so the options and their timing will be different and analysis and formulation of a plan is needed in each case. The starting point is the political will and institutional capacity to think in this way and start the process of adaptation now. This will take each city in its own individual direction and allow a plurality of responses.

For a long time, we have used the term coastal defence and this is still widely used. The word defence has a clear military connotation; it reflects our thinking that we are in a constant battle to protect coastal communities from the sea. But increasingly there is a recognition that we need to move away from a fixed coastal defence mindset to coastal management, where we consider a wider range of options and how they might work together in a more flexible way, including land use planning, raising land, flood-proofing buildings and more effective warning systems to give a few examples. In some specific cases, this could include the radical decision to move away from the coast. 

This could also include consideration of nature-based solutions and making more space for water, which are attracting strong interest. For example, we could create parks and green spaces, which people could enjoy during normal conditions. But those spaces could be designed so that they flood first during storms, diverting water away from residential areas. We could also improve habitats, such as salt marshes near to the mouth of the Thames; as these can slow down water and act as a natural flood defence. A key challenge is finding the space for such measures as space is at a premium in cities.

The Thames Estuary Plan takes some account of this shift already, and as the plan evolves more account can be taken of these approaches. At the same time, lock-in is also apparent due to large investments in fixed infrastructure such as the Thames Barrier. Maybe this is unavoidable, but it is certainly worth recognising these issues and thinking about them as we move forward.

The two of us are privileged to live in a country where we have the resources and expertise to protect the low-lying land around the Thames both now and into the future and are therefore unlikely to see flooding in the areas identified. But even if we are able to significantly reduce human emissions of greenhouse gases and stabilise temperature, sea-level rise will continue for many centuries. This is because it takes many hundreds of years or longer for the cryosphere and the deepest parts of the ocean to adjust to increased air temperatures. 

So there is no question we are going to have to live with sea-level rise for many centuries. All our coastal cities, such as London, will have to adapt to these changes in innovative ways as they learn more about the problem and the possible solutions.

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.



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.



Ivan Haigh  – University of Southampton

Sally Brown – University of Southampton


New Paper: New understanding of rip currents could help to save lives

This is a press-release for our new paper – Wave breaking patterns control rip current flow regimes and surf zone retention, which can be accessed here.


Research by the Universities of Southampton and Plymouth has found a new link between breaking waves and the hazard posed by rip currents.

The research provides a better understanding why some surf zone conditions are more hazardous than others and could result in more lives being saved.

Researchers deploying GPS drifters

Researchers Dr Ivan Haigh (left) and Dr Cristos Mitsis deploying GPS drifters.

Hazardous rip currents are features on many beaches worldwide, and are thought to account for 68 per cent of rescue events involving the Royal National Lifeboat Institution’s beach lifeguards in the UK.

The study, which also involved researchers from Macquarie University (Sydney, AUS), and Deltares (Netherlands), used a combination of video imagery and in-situ rip current measurements at Perranporth Beach in Cornwall, which is well known for experiencing dangerous rips.

The researchers found that when waves break across the end of a rip channel, it in effect closes the channel and stops the currents from travelling far offshore. Crucially, however, they found that the absence of breaking waves across the channel promotes the formation of a much more hazardous rip current that can extend far offshore.

Sebastian Pitman, a PhD student in Ocean and Earth Science at the University of Southampton, who led the study, said: “For the first time, we combined images captured by cameras at the beach to detect wave breaking and GPS drifters to track the rip currents to better understand what drives rip dynamics. We used the images to identify whether the waves were breaking across the end of the rip channel, or not, and worked out what behaviour the GPS drifters in the rip current were exhibiting at those times.”

Perranporth Beach
Perranporth Beach – arrows show the location of rip current channels

Co-author Associate Professor Ivan Haigh, also of Ocean and Earth Science at the University of Southampton, said: “The combination of video imagery and GPS allowed us to identify that when wave breaking occurred across the rip channel, the rip current was often prevented from flowing far offshore. This would mean that anyone trapped in the current would be kept relatively close to the beach. However, when the waves ceased to break across the channel, we noticed that the rip currents would instead flow far offshore, presenting a much greater hazard to swimmers.”

This is the latest research into rip currents involving the University of Plymouth, with previous work having focussed on combining GPS drifter data with information recorded using current meters and water level sensors. This study builds on existing research between Plymouth and the RNLI and, for the first time, uses images captured at the beach to provide a comprehensive picture of the threats posed by rip currents.

Gerd Masselink, Professor of Coastal Geomorphology at the University of Plymouth, said: “It is possible to use the visually-observed wave breaking patterns to better understand why some surf zone conditions are more hazardous to bathers than others. This new information provides a useful means by which lifeguards on the beach can assess the hazard posed by a beach at a given time, which could result in more lives being saved.”

The findings are published in the Marine Geology journal, and are available here:

Blog, coast, coastal flooding, tides

After the supermoon, comes the supertide

The Conversation

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

Ivan Haigh, University of Southampton and Kevin Horsburgh, National Oceanography Centre

The city of Plymouth, on England’s south coast, normally has fairly moderate tides. However this week it will have a 6m “supertide” – the highest tide in 18 years. This comes just days after the celebrated “supermoon”.

In fact, many locations along the UK, US and Australian coasts will experience their highest tides for tens of years around September 29 or 30. Coastal roads in Miami, for instance, have already been closed in anticipation of exceptional tides.

These high tides may bring water levels uncomfortably close to the tops of harbour walks and flood defences, emphasising the threat of rising sea levels. In the UK they are unlikely to be a major problem on their own unless they coincide with storms (a strong storm surge has a greater impact than even the most exotic of tides). However in other areas, like in parts of America and the Pacific, no storms are necessary: these high tides on their own can lead to nuisance flooding.

Why do we expect such extreme tides?

Tides are controlled by changes in the position and alignment of the moon and sun relative to Earth. Every fortnight – at new moon or full moon – the Earth, sun and moon are in an approximately straight line as seen from space and the additional gravitational pull of the sun causes stronger tides, known as spring tides.

The Bay of Fundy on Canada’s Atlantic coast has the world’s highest tides.

Yet each month one set of spring tides is higher than the other. This is because tidal forces are strengthened when the moon is at “perigee” and its elliptical orbit takes it closest to Earth. Tide-generating forces are also enhanced when the moon is directly overhead at the equator, part of a cycle lasting 27.2 days – a so-called “draconic month”.

Elliptical orbits of: (A) the moon around the Earth; and (B) the Earth around the sun.
Author provided

Tides can differ over the course of a year, as the Earth moves from its closest (perihelion) to furthest (aphelion) point from the sun and back. More important is the variation in the sun’s position north or south of the equator, which causes the seasons. The tide-generating forces are greatest at the equinoxes in March and September when the sun is directly overhead at the equator. Spring tides are always higher at these times of year.

A perfect tide?

Over periods longer than a year, very large spring tides occur when all the astronomical factors we have mentioned earlier coincide.

Two longer-term motions of the moon’s orbit around the Earth are important. These motions (astronomers call them precessions) are the reason we are seeing unusually large spring tides this year.

The first precession is known as the cycle of lunar perigee, and influences tides about every four to five years. The elliptical orbit of the moon around the Earth slowly moves in relation to the sun, completing a full circuit every 8.8 years. This means at either the March or September equinox approximately every 4.5 years the moon is both at its closest point to the Earth, and is also overhead at the equator.

The second precession is known as the lunar nodal cycle and is due to a very slow change in the moon’s orbit. Imagine the Earth’s orbit around the sun took place on an enormous sheet of glass – what astronomers call the ecliptic plane. The moon’s orbit cuts this surface at an angle of approximately 5 degrees. Over 18.6 years the moon’s orbit slowly rotates around so it cuts through the ecliptic plane in a different place.

One effect of this is to change how far above or below the equator the moon can reach in its orbit. In 2015 the moon is at the point where it deviates the least from the equator. This slightly increases the chances of the moon being directly overhead at the equator at any given point, and thus coinciding with the other factors that contribute to extreme tidal forces.

A lot of things have to fall in place at once to generate record-breaking tides and this year the cycle of lunar perigee and the lunar nodal cycle nearly perfectly coincide, resulting in some of the highest spring tides for decades.

The authors help run the SurgeWatch website and would welcome any photos of high tides during this period.

Ivan Haigh, Lecturer in Coastal Oceanography, University of Southampton and Kevin Horsburgh, Head of Marine Physics and Ocean Climate, National Oceanography Centre


20th Birthday for National Oceanography Centre

This year we celebrate our 20th birthday at the National Oceanography Centre (NOC). I still remember very clearly the first time I visited NOC in 1997 on a open day. I was captivated walking into the building and knew that one day I would have to work here.

Some statistics in the 20 year history:

  • We have graduated 481 PhD students since moving to the Centre in 1995;
  • We have published 9,801 papers with the SO14 3ZH post code since 1995 and these have attracted 107,573 citations.