Global sea levels are unquestionably rising. Low-lying areas are at risk of being completely flooded, affecting millions of people in coastal regions around the world. But while scientists agree about the significance of the oceans' encroachment on land, it's unclear how high and how quickly the waters will rise--and whether anything can be done to prevent it.
Schoolchildren in the Seychelles have noticed it. They have less space on which to play on the beach at home time. Day-tripping tourists to St Michael's Mount in Cornwall have noticed it, too, because they have less time to travel back and forth across the tidal causeway that links the castle to the mainland. Sea levels are rising, and for once, in the field of great environmental themes, issues and debate, there's that rare thing: consensus.
Inexorably, the sea is taking over the coastal fringes of our land masses and swelling up against mid-ocean islands. Highly respected US environmental charity the Nature Conservancy memorably described the phenomenon as akin to being slowly attacked and overwhelmed by a giant snail: in many ways, the changes are so incremental they may seem innocuous, but added together, they may well amount to epoch-defining significance.
RATE OF RISE
So what does the science say? As ever, the Intergovernmental Panel on Climate Change (IPCC) has correlated the data from scientific research around the world, and for once, its findings appear to be challenged only for their apparent conservatism. IPCC figures in 2007--gathered from its fourth assessment report from observations that include tidal gauges and measurements of interior ocean temperatures and water flows from glacial ice sheets--show that during the second half of the 20th century, global sea levels rose by 1.8 millimetres a year. Between 1993 and 2003, they rose by three millimetres per year, and are believed to currently be rising at the same rate.
By 2100, the IPCC concludes, sea levels will have risen by between 18 centimetres and 59 centimetres. This is more conservative than the figure of a four-metre rise put forward by the American Association for the Advancement of Science, which placed the world firmly in the spectrum of Hollywood disaster movies, with cities such as Shanghai, London and New York being overwhelmed.
'We know that sea levels are rising because we've had instrumental records for more than 300 years,' says Dr Svetlana Jevrejeva, research scientist at the Proudman Oceanographic Laboratory in Liverpool. 'No-one questions that any more. The records show that sea levels were stable during the 18th century and rose slowly by six centimetres during the 19th century. We also know from historical data that sea levels were stable--within a range of plus or minus 20 centimetres--2,000 years ago.'
But the IPCC projections came with a heavy caveat: they didn't factor in the impact of melting ice sheets in Greenland and Antarctica, or the potential for this melting to accelerate. Away from the IPCC, many scientists believe that sea levels may well increase by more than a metre by the end of the 21st century. 'If we make the assumption that rapidly moving ice sheets increase their rate of melting in line with temperature rises, then the IPCC adds another 17 centimetres on top of its scenario,' says Dr Jason Lowe, head of mitigation at the UK Met Office's Hadley Centre for Climate Change. 'But there is big uncertainty. It could actually be beyond that range. The IPCC can't rule that out. We can't rule out an increase above 90 centimetres for the UK, but we have no evidence for a rise of two metres.'
The IPCC's projections are modelled on physics, but other models, looking at the correlation between past sea levels and previous temperatures, suggest that the rise by 2100 could be as much as two metres. Jevrejeva has suggested that scenarios for sea-level rises might be between three and four times as high as those produced by the IPCC--this suggests a rise of 1.6 metres if the temperature rises by 4[degrees]C from 2000 to 2100; or a rise of around 80 centimetres if the world warms by 2[degrees]C.
Other models have looked at sea-level rises during the last inter-glacial period--about 120,000 years ago--and taken the extreme scenario of all the glaciers melting more rapidly than has ever been observed. Under such a scenario, the estimate also comes in at 1.6 metres.
However, the more generally accepted estimate is for a rise of 60-80 centimetres. 'A rise of 60 centimetres doesn't sound like much, but the IPCC range of 20-60 centimetres is enough to see an additional 10-100 metres [inland] flooded every year,' says Lowe. 'For small island states, nations that have significant parts of land close to sea level, that suggests a potential hazard.'
Perhaps unexpectedly, sea-level rises aren't uniformly spread out across the globe. 'There are often huge regional differences in variability,' explains Jevrejeva. A combination of the Earth's rotation, ocean currents, wind stresses and atmospheric changes in pressure all mean that some places see greater rises than others. 'If the ice caps in Greenland melt, then the gravitational effect means you see higher sea levels in the Southern Hemisphere. You get the opposite result if Antarctica melts. We're still struggling to understand the physics of it all.'
The world's great ice sheets--Greenland and West Antarctica--hold the key to the more dramatic scenarios. If the Greenland ice sheet melted, it would raise sea levels by six metres on average across the globe. A paper in Science reported that if the bulk of the West Antarctic ice sheet melted, then global sea levels would rise by 3-3.5 metres. The two are additive; amounting to a total potential sea-level rise of between nine and ten metres.
In 2006, British Antarctic Survey research found that 90 cubic kilometres of the West Antarctic ice sheet were being lost to the ocean each year, amounting to a quarter of a millimetre of the global average sea-level rise. 'It doesn't sound a lot,' says Dr Adrian Jenkins, science programme co-ordinator at the British Antarctic Survey. 'But that makes for a notable part of the worldwide rise coming from this sector. The concern is that the melting is accelerating.'
Such statistics suggest some pretty unsettling scenarios. The Hollywood disaster movie potential, along with artist's impressions of London Bridge being overwhelmed, lie in the realms of extremes, but more probable events are no less cause for concern. According to the IPCC's 2007 report Impacts, Adaptation and Vulnerability: 'Very large sea-level rises that would result from widespread deglaciation of Greenland and West Antarctic ice sheets imply major changes in coastlines and ecosystems, and inundation of low-lying areas, with greatest effects in river deltas.'
A one-metre rise would devastate small island states and low-lying countries such as Bangladesh, which, with 710 kilometres of coastline, would lose a third of its landmass. According to the Nature Conservancy, about 100 million people live within roughly a metre of sea level. River valleys, such as that of the Nile, that depend on regular high flood tides to make crops grow would be permanently underwater.
'I was struck by children in Senegal who talked of their grandparents' houses, now sunken below the waves, and doctors in Ghana reporting higher levels of heart disease among coastal people, which they attributed to salt water infiltrating aquifers,' says Tim Bromfield, who was part of an expedition that circumnavigated the Atlantic on land along the one-metre-sea-level-rise contour line last year.
A two-metre rise would effectively lead to the Maldives (average height 2.1 metres above sea level) and Tuvalu being washed over. The Maldives government underlined this point in 2009 by staging a cabinet meeting--in which it called for global cuts in carbon emissions--five metres underwater.
And this is no story about faraway lands: Britain, with a coastline of about 31,000 kilometres (if you include its principal islands), has as much interest in sea-level rises as any country. The value of Britain's coastal assets at risk from coastal flooding has been put at about 150 billion [pounds sterling], and London alone has 75 billion [pounds sterling] of assets at risk of flooding.
WHY THE SEA CHANGE?
Two main mechanisms are prompting sea levels to rise: warming water and melting ice sheets and glaciers. Thermal expansion is currently thought to account for about three quarters of sea-level rises. According to the IPCC, between 1993 and 2003, thermal expansion of the sea accounted for 1.6 millimetres of the annual rise, meltwater from glaciers and small ice caps 0.77 millimetres; Greenland 0.21 millimetres and Antarctica 0.21 millimetres.
'We have most confidence that as water expands, sea levels rise,' says Lowe. 'We also believe that there's a net source of gain from glaciers--that melting ice sheets pour more water into the sea than is locked up in ice by any climate-change-related increase in snowfall.
'The contribution of the ice sheet is increasing particularly rapidly,' he continues. 'There are observations from various large glaciers in Greenland that are increasing their speed of flow. We don't know if that will continue or if it will slow down.'
The phenomenon of sea-level rise is clear, and the mechanisms--thermal expansion and glacial meltwater--agreed upon, but the science is far from unanimous on what is ultimately triggering this process.
'We just don't have the length of record to pull out whether [the melting of glaciers] is natural behaviour or whether it might be unusual,' says Jenkins. 'Glaciers are thinning, but we don't know if that's a result of something that happened a long time ago or a current change in the delivery of ocean heat. It's difficult to tell, because we know very little about the average state. We don't really know what the equilibrium looked like.' And despite the powerful evidence for sea-level rises, even the Hadley Centre's Lowe admits that 'people are debating at some length whether this is permanent or cyclical'.
STUDYING PAST RISES
In an attempt to determine where responsibility lay, Jevrejeva and colleagues sought to establish the role of natural events by running models that concluded that solar activity and volcanic eruptions accounted for four centimetres of the sea-level rises--about a quarter of the total--during the 20th century. 'That leaves humans responsible for a little more than three quarters,' she says.
Further light may be thrown on this question by a new 3.3million [pounds sterling] project to study the response of ice volume and sea level to different climate states during the past five inter-glacials (the periods between ice ages), which include periods with significantly higher sea levels than the present. This palaeoclimatic research should also identify the potential impact of reduced ice cover over Greenland and West Antarctica.
'By studying how sea level responded during the warmer periods between ice ages, we hope to obtain a better picture of how much and how quickly sea level may rise as a result of global warming,' said Professor Eelco Rohling, the project co-ordinator from the University of Southampton's School of Ocean and Earth Science. 'A key outcome will be to place sound limits on the likely ice-volume contribution to maximum sea-level-rise scenarios for the end of the next century and beyond.'
As Rohling points out, the ice sheets have a large part to play in how humanity understands and addresses the issue of sea-level rise. And recent research by the British Antarctic Survey appears to have pinpointed the location where one of the 'engines' of this phenomenon begins.
Focusing on a part of the West Antarctic ice sheet that stretches from 90[degrees]W to 120[degrees]W, in 2009, Jenkins and colleagues dispatched a submersible autonomous vehicle underneath the Pine Island glacier, one of the three largest outlet glaciers that drain the West Antarctic ice sheet. What they found suggested an intimate relationship between rising sea levels, warmer sea temperatures and melting ice sheets. 'The warm water that causes most of the melting is very common in the deep ocean. A lot of the currents that transport warm water go around the continent rather than take it inwards,' says Jenkins.
But the research revealed that warm water was finding its way under the front of the glacier, suggesting an internal natural feedback, with the warm water contributing to the natural thinning of the glacier. This warmer deep-ocean current, argues Jenkins, has been pushed in by wind--wind that has been made variable by climate change and warmer temperatures.
'The jury is still out on whether climate change is causing the melting,' he acknowledges. 'But the issue of the wind's involvement does open the door for climate change. If it was about warming of the waters only, then we could rule out recent climate change having anything to do with it. These warmer waters from the deep ocean happened hundreds of years ago. Recent surface-temperature rises couldn't do that.
'But climate change makes subtle changes to wind patterns, and if you change the wind patterns, then it enables the warm water to get into the ice sheet, and that would account for the melting, rather than pure natural variability.'
HOLDING BACK THE TIDE
Rather than waiting for the science to definitively determine what's behind rising sea levels, mitigation measures are already being implemented by nation states, organisations and individuals. Small-scale projects that may point the way to adaptation and mitigation on a local level, such as the planting of mangroves by grassroots communities, are increasing along the west coast of Africa, a measure that is endorsed by Carl Gustaf Lundin, director of the International Union for Conservation of Nature's (IUCN) global marine and polar programmes. 'Mangroves are very productive and can move up and down coasts,' he says. 'They are dependent on low-lying mud flats, and soil washed down from the interior, and my sense is that mangroves will be able to cope.
'There's probably a lot we can do that we don't yet know about,' he continues. 'We have a lot to learn about resilience of reefs and fishing pressures, and carbon sequestration.' One third of the carbon we burn ends up in the oceans, and Lundin believes that by encouraging fish stocks to eat the plankton that absorb this carbon, sequestration will be enhanced--because the carbon waste matter excreted by fish will generally sink to the seabed, rather than re-enter the climate cycle.
Closer to home, the concept of simply letting some land go rather than strengthening sea walls is slowly being embraced along the coastline. In 2008, in the first move of its kind in the UK, the Environment Agency decided to turn the Cuckmere Valley in East Sussex into a salt marsh nature reserve rather than continually building ever bigger flood defences.
The National Trust also invited controversy when it raised the idea of allowing part of the Cley Marshes in Norfolk--drained and turned over to fresh water during the 18th century--to be reclaimed by the sea over the next century. The trust intends to allow the water and tides at Cley to operate in a 'more dynamic' way. 'A lot of what we see around the coast is anecdotal, but the evidence is there that we are seeing completely different patterns of storm surges, and that these are likely to happen in the future,' says John Sizer, the National Trust's property manager for the Norfolk coast. 'There's a greater risk in not doing anything. Ultimately, it's unsustainable to resort to engineering solutions. They are achievable, and the science is there to do it, but it's not financially viable.'
Even so, any mitigation measures will take an awfully long time to take effect; certainly, there are few people alive today who are likely to see the benefits. This is because of the painstakingly slow pace at which oceanic change occurs. 'The striking thing about expanding water is that it's very slow to respond,' says Lowe. 'If you make a change to the amount of C[O.sub.2] you put into the atmosphere, the temperature will respond much more quickly. But you'll still be potentially experiencing the impact on sea levels 1,000 years later. It's important to remind people of the length of time that the sea will take to straighten itself out.'
These long thermal lags of the ocean system also make a compelling case against resorting to geoengineering options, such as pumping sulphates into the atmosphere, to slow climate change. Research by Jevrejeva and other scientists found that injecting the same amount of sulphur dioxide into the atmosphere as a volcano such as Mount Pinatubo in the Philippines spews out every four years would delay sea-level rise by just 12-20 years. This means that sea levels would still rise by between 0.6 and 1.6 metres in around 2112 or 2120.
'The ocean is a great accumulator of heat,' says Jevrejeva. 'Even if we cut all emissions right now, it wouldn't stop sea-level rises--they would carry on for centuries. Geoengineering would affect the oceans--what response would we get from sea levels? No-one knows what would happen to that ecosystem. The effect of geoengineering would be very small, and if you stopped it, then the sea-level rises would increase at a much faster rate.'
NO EASY CURE
All this leaves a pretty uncertain picture, in which humanity appears stuck with a phenomenon it has triggered but has little power to alter. As Lowe points out: 'There's a storyline behind sea-level rises--what the world will look like, our use of fossil fuels, wealth, how many people there are on the planet.'
One option is clear enough, according to Jevrejeva. 'I don't want to be involved in politics, but I'm sure that 95 per cent of the science says we have got to cut our emissions. So we have to reduce C[O.sub.2]. If you think we have some magical, easy solution--we don't.'
Seeking to be positive, Lundin notes that species can find ways of fighting back in daunting circumstances. 'Many of the small-island species are remarkably resilient. Many of them have gone through near extermination before--it's not uncommon for catastrophic events to hit small islands. It will be a mixed picture. Some parts of the planet will show the way forward, but in others, there'll be a significant decline.'
Nevertheless, he admits the world faces an uphill challenge. 'There are a lot of things we can do, but we shouldn't be too optimistic that we will do them. I've visited more than 100 countries with my job, and the rate of destruction from tides is increasing. There's no doubt that we are going to see the degradation and loss of lots of habitat and species.'
While until now, attention has mostly focused on the actual and potential impact of sea-level rise on human populations, natural ecosystems are also feeling the effects. 'Terrestrial species are vulnerable to saltwater intrusion as water tables that are consistently inundated with sea water are likely to take centuries to re-establish themselves, making such places uninhabitable for humans and other species,' says the IUCN's Carl Gustaf Lundin. 'Seagrass beds, which exist in a relatively narrow range, may also be vulnerable, particularly to changes in currents that may be associated with rising sea levels.'
Lundin worries that coastal regions, reefs and beaches are in a bad condition even before the biggest impacts of sea-level rise kick in. 'We've already seen a die-off of corals that is unprecedented in our history; he says. 'Ecosystems that are robust can bounce back quite quickly, but not if they are in a poor state of health. Even a modest sea-level rise often centimetres can lead to large morphological change of a coastline, completely change currents and lead to pollution from fertilisers and pesticides washed down from farmland.'
Glorious tree species that are characteristic of many coastal areas--such as the Alexandrian laurel, also known as Borneo mahogany, which is widespread across the sandy coastal fringes of the Indian and Pacific oceans--may also be unable to adapt. With them, warns Lundin, could go the bird colonies they attract, such as those of the fairy tern, the world's only all-white tern, which is found on many tropical islands.
So far, the IUCN is unaware of any species that has been lost to rising sea levels. 'A number of them have problems, but extinction is still down to human or invasive cause,' says Lundin. One group that may be at particular risk, however, is the sea turtles. 'Many beaches are vulnerable. If we lose the sand, then there's no possibility for the turtles to reproduce. It's unlikely the hinterland would have the sand dunes they require.'
EFFECTS OF A ONE-METRE SEA-LEVEL RISE
These graphs show the area, population and economy on each continent that would be affected by a rise in sea level of one metre. By far the hardest hit would be Asia, which has several heavily populated, low-lying river deltas
Tides of change
From isolated low-lying islands sitting in the middle of the ocean to long chains located at its edges, the Pacific is where the impacts of sea-level rises are being felt most acutely..
Average annual sea-level rise in the Pacific is reckoned to be five millimetres--slightly above the global average. But even such seemingly modest sea-level rises, coupled with high tides--known to Pacific islanders as king tides--can cause major damage, according to Barry Coates, Oxfam's executive director in New Zealand. 'It makes freshwater aquifers prone to infiltration from salt water, and we're seeing greater sea swells that can overwhelm coastal defences' Root crops are particularly sensitive to salt water, and Oxfam has recorded instances where higher tides have now destroyed harvests for years to come.
'The hinterland may not be suitable to relocate to--or there may not even be a hinterland,' says Coates. On some island groups, such as the Carteret Islands near Papua New Guinea, islanders are already leaving--although in the case of Carterets, sea-level rises are attributed in part to the shifting of tectonic plates beneath them. Elsewhere, coastal erosion is triggering acrimonious land disputes over tenures held for centuries by family clans.
'The most severe impacts aren't necessarily economic,' says Coates. 'They're more about loss of identity, culture, loss of your homeland and even your citizenship. The most subtle impact is on children. Their future is one where they know their land is likely to be devastated and they aren't going to be able to live in the land in which they were born.'
Delta forces: Vietnam's vulnerabilities
According to the World Bank, Vietnam holds the unwelcome distinction of being one of the world's two most vulnerable countries to a one-metre rise in sea levels (the other is the Bahamas).
This is almost entirely down to the deeply populated and highly economic low-lying Mekong and Red River deltas, where nearly 11 per cent of its population nine million people--would be affected. A one-metre sea level rise would leave 90 per cent of the Mekong Delta inundated every year.
The Mekong Delta province of Ben Tre would be worst hit, with more than half of the land area--around 1,130 square kilometres--and 750,000 people affected, but that number rises steeply if storm surges are also taken into account.
An Oxfam report established that in a few of the province's coastal areas, the concentration of salt in the water has already reached 30 parts per thousand (ppt), which makes growing most agricultural products virtually impossible (salinity in the Pacific Ocean averages between 32ppt and 35ppt). Rice can't grow in concentrations of more than 4ppt, while even levels of 1 ppt can affect orchards and vegetable nurseries.
Officials from Vietnam's Ministry of Agriculture and Rural Development say that the end of the dry season in May now typically sees salty water covering two thirds of the province and travelling 60 kilometres up the rivers from the sea--an increase often kilometres in the past five years.
BAD NEWS FOR BANGLADESH
The densely populated South Asian country is susceptible to rising sea levels, with 11 per cent of its 164 million people at risk if levels rise by 1.5 metres
ESTIMATED CONTRIBUTIONS TO SEA-LEVEL RISE Thermal expansion of the world's oceans--as water's temperature increases, so does its volume--is currently the biggest contributor to rising sea levels. But the contributions from melting glaciers and ice sheets are rapidly increasing TOTAL 2.83 [+ or -] 0.7 mm/yr Greenland ice sheet 0.2 [+ or -] 0.1 mm/yr Antarctic ice sheet 0.2 [+ or -] 0.4 mm/yr Glaciers and ice caps 0.8 [+ or -] 0.2 mm/yr Ocean thermal expansion 1.6 [+ or -] 0.5 mm/yr SOURCE: Intergovernmental Panel on Climate Change Note: Table made from pie chart.