This has been a bad week in a bad year across Britain for flooding. Despite an early drought in England, the volume of rain has more than caught up with the average and flood defences across England have been overwhelmed. Despite rivers up at critical levels (e.g. the Nith at Dumfries), Scotland has got off lightly, despite heavy rains here too.

Whether you believe in climate change being behind this or not, all of this has distracted from what may wind up being a far more serious issue because it will become permanent and, in most places, unstoppable: coastal flooding. Because much of Scotland’s coast is rugged, sea level rises of a few centimetres make little difference so residents of Ardnamurchan or Arran can sleep safe in their beds.

Even low-lying areas like the Angus coast or much of the Forth have an ace up their sleeve because a recent study at Durham University examined samples of peat, sand and clay sediments from 80 sites around the British and Irish coasts to see how the land has changed over time. Basically Scotland and Northern Ireland appear to be still rising after the last Ice Age, while Southern England and Ireland appear to be sinking. Their resulting Coastland Map is shown below in terms of mm/year

The Coastland Map: Changes in Land Levels (source: University of Durham)

Given that tides go up and down anywhere between 3 and 13m, depending on location and lunar cycle, this doesn’t seem much—Skegness will subside by 5cm/2″ over the next century. But for low-lying areas—especially Essex and the Lincolnshire coast where Skeggie is, it spells disaster.

Much was made of Antactica’s Larsen Ice Shelf fragmenting over the last decade and over the last two summers, the elusive NorthWest Passage—graveyard of many 19th century explorers—has opened up to ships. Indeed, seen from a distance, the scale of either becomes mind-bogglingly apparent

Recent History of the Disintegrating Larsen Ice Sheet: Red Line is the New Coast

Satellite View of Canada’s Northwest Territories, Sept. 2012. Dark Blue is Open Sea

Although both examples are startlingly graphic, neither has contributed appreciably to raising sea levels. This is because both areas were sea ice, which already displaces 90% of the volume of water it produces as it melts. Whether the world is warming or not, sea levels are rising; the main contributors are:

• expansion of water itself as it warms—current global warming of the oceans results in a sea level rise of 1.6±0.5mm per year.
• ice shelves like Larsen or other sea ice.
• mountain glaciers
• ice sheets, the two biggest being Greenland and Antarctica (so far have shown little erosion—but if they do go, much of the world will be flooded)

Evidence from retreating glaciers from Austria to Alaska show they are pouring ever more meltwater into the ocean. Every cubic km of iceberg calved displaces 0.9 cu km of ocean; every cubic km of glacier melted displaces the same cubic km of the sea. So, ice loss from glaciers is ten times worse than melting sea ice.

The Intergovernmental Panel on Climate Change (IPCC), in its most recent review of climate science, estimated that the net effect of all of the above sea levels was 2mm annually. From recent evidence, however, the figure looks more like 3.2mm annually. Leaving aside any acceleration of melting (ice reflects far more energy back into space than open sea where ice once was) the onset of serious coastal flooding is to be measured in decades, not, as we once thought, centuries.

Evidence for this came with Hurricane Sandy last month. The 80+mph winds in one of the biggest storms ever seen created low pressure, which piled a 3m (9ft) storm surge on top of an unusually high spring tide, then topped the mix with 4m waves. The low-lying undefended coastal communities on the barrier islands of Delaware, New Jersey and New York had never seen anything like this.

Walls of water surged through the communities of Atlantic City, Ocean Grove, Staten Island, Queens and others. All had houses washed off foundations, boats parked on top of cars and major utilities lost for weeks on end. In Manhattan, all subway service was suspended as tube tunnels flooded, all tunnels off the island also flooded and much of lower Manhattan was plunged in darkness as substation transformers blew up when the water reached them. The damage was in billions.

Because it is not permanent flooding that makes land untenable; inundations by salt water every few years is enough to poison farmland, render properties uninsurable and make the whole effort of staying and fighting unprofitable. This is especially true in rural coastal areas where it may be easier/cheaper to simply evacuate. This is why the Essex, Suffolk and Lincolnshire coasts are top of the UK vulnerability list.

As far back as 1953, a combination of a high spring tide and a severe northerly gale caused a storm surge. In combination with a tidal surge the water was funneled south into the ever-narrowing North Sea to overwhelm flood defences, especially in Essex where water levels exceeded 5.6 metres (18.4 ft) above mean sea level. Even further back, the Somerset levels were similarly inundated in 1607, drowning an estimated 2,000 or more people, sweeping houses, livestock and villages away and flooding 200 square miles (518 km²) of farmland inundated, and destroyed.

Both events were caused by circumstances similar to Sandy described above: large waves on top of a large onshore storm surge coinciding with high spring tides. Since this rarely happens together, all three are considered 1-in-100 year events. But if we take the projections for Essex (3.2mm sea level rise added to the o.5mm land drop), by the end of this century, average sea level there will by 0.33m higher, making it more like a 1-in-25 year event.

One glance at a coastal map of Essex and  the long muddy inlets of the Ore, Deben, Orwell and Stour between Orford Ness and Southend seem indefensible. The major container port of Felixstowe is officially 0.5m above sea level. Manningtree is 15 miles ‘inland’ but still only 1m above sea level and a stop on the main line to Ipswich and Norwich.

Since 1945, extensive drainage and fertilisation of the Essex marshes for arable cropping and improved pasture has led to widespread fragmentation and loss (64 per cent) of the traditional wetland character of the marsh. Drainage leads to a similar situation to the Fens where much rich agricultural land lies below sea level. Partly because of centuries of reclamation and partly because the Wash acts less of a funnel for storm surges, the Fens between Kings Lynn (4m/on the sea), Cambridge (6m/40 miles) and Peterborough (3m/24 miles) seem less likely to flood. They are nonetheless under threat, along with the Somerset Levels and the Humber estuary.

In Scotland, the situation is not so urgent. Not only does the continuing rise in land mean that sea levels here will rise only by around 0.24m by the end of the century, but there are no equivalents to the Fens, fewer low-lying coastal marshlands and fewer ‘funnel’ effects for storm surges. But Alloa and Perth, although miles from open sea, are both 1m above sea level; both Forth and Tay do get storm surges up to 1m, so we have no reason to be smug. Our comeuppance is due a century later.

Next time you jump in your Chelsea Tractor to run the kids to school, think about its effect and whether you should leave a bequest for their children to get swimming lessons and a boat.

What We’ve Already Lost—Map of the Early Holocene ~8,000BC (Source: University of Exeter)