Cities in the Sea
December 2009
Building in the ocean.Abstract:
There are two distinct aspects to the "climate change" problem. The one getting the most attention is how to reduce warming. Let's set that aside for a moment and think about the other aspect: what are the expected consequences of the climate trend in this century, and how can we prepare for them? In this note, we'll look at the impacts of the rising sea level on coastal cities.
You know how you sometimes receive a thought from someone – perhaps a teacher, a writer, or a friend – that opens the floodgates of new thought within your own mind? It was like that when I took a course, many years ago, with the intriguing title, "Fluvial Geomorphology." Meaning, "the physical movement and effects of rivers." More than anything else in the course, I remember the instructor showing a diagram of a river running through a valley: "Here are the banks of the river; here is the '100-year floodplain', and here's the high ground where flooding from the river should not be a problem." He asked: What would you define as the river? And here came the awakening thought: The answer is, "the flood plain". The flood plain is the river. The river owns its historic course and extent; if it has been there before, it will be there again. The thousands of houses that have been built in the flood plain have been built in the river! And building houses in the river is fairly dumb.
Let's ask the same question of the ocean: What are its boundaries? And just as with the river, we can answer: the boundaries of the ocean are where the ocean has been. The ocean, like the river, says, "I'll be bahk!" For the principle is the same: The ocean also seeks its old haunts. It also rises and falls. Like the river, the ocean comes home. Over the past two million years, the ocean has risen and fallen in opposition to the building and melting of glaciers – it has been higher than at present, and it will undoubtedly continue to rise and fall in the future. As many as thirty times during that period, the transfer of ocean water into ice in huge continental glaciers has lowered the ocean. Then, each time the climate warmed and a glacial period ended, the sea filled and rose, sometimes hundreds of meters higher than the present sea level.
The figure to the right, from the University of Florida, shows that during the height of glacial build-up, when the sea level was lower, Florida's dry land reached far out into the present ocean (the dotted line). Conversely, when the sea was at its highest, as much as 150 feet above the current sea level during the interglacial periods, Florida existed only as a group of islands (solid green). So we see that Florida's present coastline (solid line) is nothing but a temporary feature.
And now the sea is rising again, as the remnants of the last great glaciers, covering Greenland and Antarctica, are melting. We didn't know all that some hundreds of years ago, when we built our great cities on our coasts. We didn't know what we now know:
That we have built our cities in the ocean.
What drives the sea level?
The cause of the rise and fall of sea levels is the Earth's near-surface temperature. As scientists have gained skills and tools that have allowed them to detect sea levels and temperatures from as far back as there has been complex life forms on Earth - some 550 million years - they have found that by and large, just like today, the level of the ocean depends on the amount of water tied up as ice: Warm temperatures equal less ice and a higher sea level.
Warmer temperatures affect the sea level in two ways: First, contact with warm air heats the water, which expands. Second, warm air hastens melting of continental ice. The Antarctic ice cap, where the average annual temperature is around -35° F, is not in danger of melting any time soon, though some of it will. The Greenland ice cap, on the other hand, is melting at a significant rate. If it all melts, it could contribute about 23 feet to the sea level. The rate of melting of the Greenland ice cap is of great interest to city managers trying to adapt to changes in the sea level.
For perspective, we should keep in mind that we are still in an ice age. The graph to the right, from NASA, shows the long-period fluctuations (the red line) in the Earth's temperature during the past 500+ million years (the "Phanerozoic Era") that have brought on glacial periods roughly every 150 million years. There was an earlier ice age 700-600 million years ago that also fits this pattern. (The letters along the top and bottom are geologic periods, from the current Neogene to the ancient Cambrian.) The highlighted Glacial Periods are times when Antarctica has been icy. As we see, it's equally "normal" that it's ice free.
We see that the past million years has been the coldest period in the last 450 million years. The world's temperatures today are well below historical average temperatures. The sea level tracks temperature, so the current sea level is also historically low. During the past 30-40 million years, we've been in a glacial period, and over the past several million years the glaciers have pulsated with an uneven rhythm of some 100,000 years. Right now we're between major glaciations – the most recent major glaciers began to withdraw some 20,000 years ago. What we don't know is whether the natural cycle will now take us back into a major glaciation, covering North America and northern Europe again with glaciers, or whether we're on our way out of the glacial period, in which case the remaining glaciers (mainly Antarctica and Greenland) will melt off and the sea will undergo a major rise. The third, and most comfortable, possibility is that our current relative warm spell in this glacial period will continue for a few thousand years more. But, of course, we don't know that either. What we do know is that we are currently in a warming spell of unknown duration, and the sea is rising.
The real problem.
It's fair to ask, "If the temperature and the sea level today are well below the averages for hundreds of millions of years of animal history, what's the problem? Can't we accommodate?" Well, it turns out that that is just the problem: Modern humans accommodating to changing climate. We were once good at it, but as Brian Fagan tells in The Long Summer, the more we humans complicated our societies, the less adaptable we became. If there's a crisis in the coming climate change, it's not an environmental crisis: the Earth has been through this countless times before. Nor is it a biological crisis: polar bears, for example, will regain fur pigment and become brown bears, as they once were, when the ice disappears. No, the climate crisis is a crisis of human adaptability. It's a crisis for us because we've concentrated our resources in vulnerable places, and nailed them down. We've dared the sea to rise, and it has taken us up on it. This century may finally see the long-awaited showdown between nature and human arrogance. In the first act, the showdown will be between the sea and our cities.
Cities at risk.
All coastal cities face the hazards of storm surges, extraordinary spring tides, and tsunamis. As the sea rises, these hazards will become an even greater problem for city planners and managers, as well as for citizens. But
some coastal cities face a greater risk than others from such hazards, for two reasons: First, they may be unsheltered from the sea by natural features, with flat terrain already prone to flooding. Second, cities differ greatly in their capacity to prepare for and deal with natural disaster.
In many parts of the world, not only is the sea rising, the land is sinking to meet it, which increases the rate of sea level rise relative to the land, and thereby increases the hazard. In the northern hemisphere, and in southern South America, the great weight of the deep glaciers of the last glaciation period depressed the land. Imagine a bowl of bread dough, where if you press down on one side of the dough, the other side bulges up. The principle is the same with continental crust, and is called "isostasy." When the glaciers depressed the land in one place, the nearby ice-free land rose, like the dough in the bowl.
And now that the glaciers have largely receded, their great weight is removed from the once-glaciated lands, so these are rising to their earlier level, while the neighboring lands that rose are now sinking to their original level, under the isostatic principle. So in the US, the south-east Atlantic coast, Florida, and the Gulf coast – which rose as glaciers depressed the northern lands – are now sinking. In Europe, most of southern Europe, with France and southern England are subsiding. In South America, the south coast of Brazil, including Rio de Janeiro, is settling. Since these places are sinking, meeting the rising sea, they have to deal with the extra difficulty of more rapid relative sea level change.
One recent study of the susceptibility of coastal US cities to flooding from the rising sea has been done by "Architecture 2030." Their study constructed flooding maps of 31 coastal cities, towns, and areas in the US at various sea levels. Go here to see the dramatic results: "Nation Under Siege." In short, they found that dozens of US coastal cities, and millions of inhabitants, could be seriously impacted by rising sea level in this century. The most serious impacts would be in Florida and the Gulf states, but no coastal city is immune.
Cape Coral, FL, pop. 102,286. This will nearly all be under water after a 1.25m (4 feet) S/L rise.
The sea level rise will not be uniform throughout the world. New York and adjoining New Jersey, for example, may be especially at risk: a rate of sea level rise up to twice the world average is expected there, due to a local combination of ocean currents, tides, and gravitation. The area may also find Newark Airport unusable at as little as one meter of sea level rise. We need to realize that flooding will occur long before the sea level actually reaches the streets of the city. A rise of one meter will greatly increase the number of flooding events from tides and storm surges in coastal cities.
Most of the major world cities hit worst by a rising ocean will not be in the US. In Europe, cities of the Netherlands, such as Amsterdam and Rotterdam, lying below sea level, are already struggling against the recent few inches of rising sea. London is concerned about the danger from storm surges advancing up the Thames from violent North Sea storms. And Venice is sinking ("isostasy" - the bread dough effect again) to meet the rising waters of the Mediterranean.
But the greatest crises from the rising sea will come in the coastal cities of southern Asia. There, at the top of everyone's list of disasters-waiting-to-happen is Bangladesh. Dhaka, the capital, is the world's most densely populated city and also one of the largest and fastest growing, with a current metropolitan area population of more than 12 million. Dhaka has a long history of flooding and damage both from two major rivers (Ganges and Brahmaputra) and from cyclonic storms. In 1970, the "Bohla" cyclone killed upwards of 500,000 people in the region.
As the sea level rises and more frequent and violent storms occur, Bangladesh can expect to be battered even more often and more violently. The UN Environmental Programme (UNEP) has calculated that a sea level rise of 1.5m (5 feet) would claim 16% of Bangladesh's most productive land and displace 15% of the population. The effect on this poverty-stricken country of 162 million (2009 estimate) may be disastrous. To the right is a diagram of the expected invasion of the sea, prepared for UNEP. (1989 population shown.)
Very many other Asian countries rightly fear the rising tide. Many have cities that lie less than two meters above the sea. And many, like Bangladesh, are poorly equipped financially and technically to take meaningful measures against flooding.
And not only flooding . . .
While the most obvious effect of the rising sea level will be flooding of the land, more subtle effects can also be very serious and may be felt well before flooding occurs: Salt water intrusion into coastal aquifers may make the groundwater unsuitable for drinking or irrigation. Many cities draw much of their water supply from groundwater zones that could be made useless by rising seas. Sea water movement under ground can also raise the water table under buildings and undermine structures. Saturation of the soil may make these less earthquake resistant. There may be seepage into foundations and basements, and into the many tunnels under our cities: subways, utility and access tunnels, buried cables, sewers, etc. Auto and rail tunnels under harbors or rivers, as in New York City, would be exposed to increased external water pressure and may suffer leaks or damage. (I will note without much comment that the vast shift in weight that will attend the transfer of water from ice to sea could also result in great geologic impacts, such as on the occurrence of earthquake and volcanic activity, shifts in gravitation and the Earth's magnetic field, and changes in ocean current patterns. The combined effects of these could cause major shifts in the Earth's climatic zones, with the development of desert conditions in currently well-watered areas and vice versa, resulting massive population shifts – with the armed conflicts that must ensue from that since dealing with it rationally will be beyond our ability. Sorry about the editorializing – it just slipped out.)
One of the most serious problems that must be addressed by coastal cities is the higher and more severe wave action that will be expected as the sea rises. First, the higher base sea level will elevate the wave tops, and may overtop protections built for a lower sea, and second, the sea's warmer temperature is expected to bring more frequent and more severe storms. Harbor structures will take an increased beating with rising seas, breakwaters will be less effective, and coastal erosion will increase. A special hazard is the storm surge, a large-area mound of higher water driven by storm force winds and exacerbated by the low barometric pressure of the storm. When a storm surge coincides with high tide, unexpectedly high and violent water can result. For cities located in a narrow inlet or on a river, such as Dhaka, London, or New York, a bore reminiscent of a tsunami may form in the inlet or river as the surge moves inshore. A more prosaic, yet critical, problem is the city's storm sewers. These drain by gravity, and typically exit near the high water mark. A relatively small rise in the sea level may be enough to cause backups.
How should cities respond?
Among the first difficulties that face city leaders are a) building public recognition that the rising sea constitutes a serious problem, and b) getting credible prognoses about the rate of sea level rise and its probable effects in their area. Each of the issues discussed above requires its own solution, and they are all expensive. The measures that can be taken against the rising sea fall in two categories: Yield, or Resist. At lightly populated shores many governments will choose to yield, to let the sea advance where it will. The local population would need to resettle. But the situation is different in the city. Here properties are not so cavalierly abandoned, and the path of resistance will be chosen as long as possible. To mitigate storm wave and surge damage, many coastal cities already have breakwaters or barriers. These may need to be strengthened and/or raised, and new ones built. This alone can stretch a government's financial resources. In addition, city shorelines may need to be raised, and hardened with rip-rap or revetment. These measures have the drawback that beaches and wetlands are lost, but within the city that may be necessary. (Beaches and wetlands can only be maintained where the rising sea is allowed to encroach, gradually moving these inland.) Port structures will be at risk, and many will need to be raised, also a major expense. Low-lying airports, such as Newark, will eventually need to be diked, raised, or relocated. Where a city's ground water supply is affected by sea water intrusion, a new supply must be found unless the city can build and operate a water desalination system. The solution to impeded storm sewers may involve redesign and installation of pumps.
How are cities responding?
Leaders of major cities realize that they have some time to prepare for the coming rise of the sea. New York City is not atypical. The city will take low-cost measures first. Meanwhile studies are being carried out on the need for storm barriers at Verrazano Narrows and the upper East River, and the need to protect the vast underground utility and tunnel network is well understood.
London, England, has for some time been visited by winter storm surges, an average of 20 per year lately. The surge travels down the North Sea, and gains height as it is forced into the shallow Thames estuary. The storm surges have several times reached more than ten feet in height. After several flooding events in London, a moveable barrier was built across the Thames River in 1984. Over the first 19 years of operation, the gates were closed 64 times for protection of London, averaging 2-4 times per winter season. The Thames Barrier was built to be adequate until 2030, assuming the rate of sea level rise from the last century (8 ½ inches). There's now concern that it will soon become inadequate and should be upgraded. Nevertheless, London has a good start on protection.
The future of Dhaka, on the other hand, looks bleak. Without a stout infrastructure or resources, the only choice for meeting the rising tide will be relocation of the coastal population. The city itself is a worse problem, but to date no serious planning seems to be underway for what may come. A river barrier is a theoretical mitigation, but the cost appears prohibitive without massive international aid.
According to UN's "State of the World's Cities 2008/9," more than 3000 coastal cities are in danger of serious or catastrophic damage and loss of life from the rising sea level. In developed states, planning is well underway for at least the initial stages of sea level rise, though the resources may be lacking for the most ambitious construction projects. In less developed states in Asia, Africa, and South America, as a rule they will not be able to find resources on their own to save their cities into the next century. They will need financial assistance in the form of grants, and this must be internationally coordinated, perhaps by the UN. In any case, if warming continues through this and the next centuries, no coastal city can be maintained where it is. They are in the sea.
A personal conclusion: The world-wide cooperation needed to secure even a temporary respite from the rising sea will require resources that are not currently available. At the same time, massive economic resources are used by nearly all countries in arming themselves against their neighbors. I see our only reasonable hope for success in a "consilience" of the environmental battle and demilitarization. Shifting the resources now uselessly spent on preparing to fight each other to the cooperative venture of living successfully with our planet will give humanity a better future on all counts.
So: Climate change is not an environmental crisis; it's just environmental change. But it is a human crisis.
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