According to recent estimates, even the more modest sea-level rise predicted by the IPCC, a rise of up to twenty-four inches by 2100, could displace hundreds of millions of people and inundate four hundred thousand square miles of the world’s agricultural land and coastal cities. Ten percent of the world population lives near coasts on land less than thirty-three feet above the present sea level. Eleven of the world’s sixteen megacities, each home to more than fifteen million people, are built on coasts or estuaries: Tokyo, Guangzhou, Shanghai, Mumbai, New York, Manila, Jakarta, Los Angeles, Karachi, Osaka, and Kolkata. That number will grow as the world’s population increases, with a further four coastal megacities predicted by 2025: Buenos Aires, Dhaka, Istanbul, and Rio de Janeiro.15 There are three possible ways we can respond when rising tides threaten: retreat, adapt, or defend. Defense is our usual reaction to threats from the sea.
For centuries Holland has been one of the most densely populated countries in the world. Its flat lands and low elevation mean the Dutch have always been intimate with the North Sea. Storm surges would periodically push the sea far inland, drowning farmland and ruining crops. It is hardly surprising then that the Dutch became masters long ago of the science of coastal defense; they constructed elaborate dikes as early as the fifteenth century to both hold back the sea and increase fertile land. Contemporary illustrations show complex protection schemes of seawalls and groins (walls or fences built at right angles to the shore to intercept the movement of sand) that reveal a detailed understanding of the forces that build and destroy coasts. Dutch engineers were in demand across Europe by the seventeenth century, and they exported the technologies of coastal protection, or “hardening,” that remain in use across the world.
In 1953, the Dutch learned through a harsh lesson that sea level was only half of their worries. An intense storm surge from the north combined with an extreme high tide to devastate low-lying areas, killing eighteen hundred people and destroying ten thousand houses. To prevent a repeat they embarked on the world’s most ambitious program of coastal defense, raising dikes and levees to close off the North Sea from the Rhine/Meuse delta. Today a quarter of Holland lies below sea level and owes its existence to these defenses. But recent sea-level rises threatens once again, and their coastal defenses have come under renewed scrutiny.16 The new plans foresee a sea-level rise of up to fifty-one inches by century’s end, and work is underway to upgrade their coastal defenses to meet the threat.
The Dutch are better equipped than just about anyone to protect their coast. The problem is, sea-level rises pose urgent threats to many developing countries that don’t have half the resources. Nowhere is the battle to protect land more pressing than on low-lying oceanic islands such as the Maldives, Phoenix Islands, and Tuvalu.
Flying into the Maldives’s international airport is like landing on the ocean itself. The runway seems such an insubstantial sliver of tarmac amid the endless sea. The capital city of Malé is made up of a clutter of concrete buildings that completely conceal the coral island upon which it perches. A rampart of bleak concrete tetrapods defends the city from the waves of the Indian Ocean. Like Malé, the airport is edged in concrete, a stark reminder that the sea that gave birth to these islands could soon claim them back.
The Maldives is a waterworld. None of its twelve hundred islands rises more than a yard or two above the sea, and each sits atop one of the countless reefs that make up this nation of fish and coral. Seen from the window of a seaplane the islands look like a scatter of emeralds set within white bands of coral sand. Collectively they cover less than one thirtieth of 1 percent of the country. The rest is ocean.
Not surprisingly, Maldivians seem as much at home on their boats as on land. The captain who ferried me from the resort to the dive site on my last visit looked every bit the sea gypsy as he coaxed his boat through the hazard-filled coral labyrinth, foot on the tiller and a cell phone pressed hard to one ear in an effort to hear above the roar of the engine.
The reefs in the Maldives have grown over the course of thousands of years on top of platforms planed flat by the sea during the last ice age. These platforms, now 130 feet to 160 feet below sea level, stretch north to south for five hundred miles in a chain of atolls that follows an ancient wrinkle in the Indian Ocean seabed. Corals grow by absorbing carbonate, the main ingredient of chalk, from seawater. They crystallize carbonate into hard skeletons that form the backbone of reef growth. Swimming across a Maldivian reef crest in 2010 I came upon a coral head freshly smashed open by waves from a passing squall. From the outside, stony corals like this one give an impression of immovable solidity, but inside the skeleton was riddled with tubes and cavities that had been bored and dissolved by a host of creatures that sought refuge from the predatory world outside. This hidden community represents the other side of the reef balance: the erosional yin to the depositional yang. Reefs only grow where deposition outpaces erosion.
The Maldives and thousands of other oceanic islands scattered throughout the tropics exist only because corals have flourished sufficiently to win the race against erosion. Climate change is tipping the balance the other way. As I mentioned earlier, Maldivian reefs bleached badly when temperatures soared in 1998. Corals were on the ascendancy again when I visited twelve years later, but like the regrowth of a forest after a wildfire, there were fewer species than before, and most were shrubby branching forms that grew quickly. Most of the giants of old, corals that might have begun life when Europeans were just setting forth on voyages of global discovery, were dead.
Healthy coral reefs are naturally self-repairing breakwaters, better at protecting coasts than any concrete wall we can throw up to defend cities, resorts, or agricultural land, and far cheaper. The combination of faltering coral growth and rising seas means that nations whose fortunes are founded on coral have begun to contemplate the unthinkable. Tuvalu in the South Pacific has made an arrangement to relocate its people to New Zealand when sea levels rise beyond their capacity to adapt. Already the highest tides wash over islands in Tuvalu that were dry a generation ago.
At least Maldivians can take comfort from the fact that their nation rests on a solid foundation.17 Rapid relative sea-level rise in places where coasts are subsiding has been a reality for decades, even centuries. For them it is not the eight inches by which sea levels have risen in the last century that matter; it is the amount by which land has sunk. There are several reasons why coasts sink. During the last ice age, vast ice sheets up to ten thousand feet thick lay across much of North America, Europe, and Asia. They pressed the Earth’s crust deeper into the mantle, creating depressions. A bit like the hollow on a cushion that is left when you stand up, these indents have gradually disappeared as the Earth has returned to its former shape. Since it is a geological process it is monumentally slow, and it is still underway today, eighteen thousand years after the peak of the last ice age. Northern Scotland, for example, is still rising, while southern England, which was not covered by ice then, is sinking. This helps explain the difference in the amount of hard coastal defenses between these countries. While 8 percent of Scotland’s coast is protected by seawalls, rock armor, and the like, half of England’s is defended.18
Another major contributor to subsidence or sinking is sediment supply. Where supplies are cut off behind dams or levees, coastal marshes and mudflats are starved of the material they need to build upward as sediments settle beneath. In the southern United States, for instance, the Mississippi Delta now loses twenty square miles of land each year as a result of subsidence and sea-level rise. In years of heavy snowfall the Colorado River used to disgorge upward of 220 million tons of mud into its delta at the northern end of the Gulf of California. After the Hoover Dam was built in the 1930s, supplies of mud were choked off, and the river today carries a few million tons at most to replenish delta wetlands. There are now more than forty-five thousand large dams around the world, which together trap a quarter to a third of all the suspended sediment carried by rivers.19
Sediment supplies can also be cut off by structures that interfere with coastal currents. Some coasts consist of soft rocks or soil and naturally shed sediments to the sea. That sand and mud is carried by waves and currents to other places, where it settles into barrier islands, lagoons, sandy beaches, and dunes. Such areas are often stabilized by plants such as mangroves, sea grass, and salt-marsh grasses that bind mud together. So coastal development can have far-reaching and unintended consequences, as it disrupts the natural flow of tides and currents, leading to erosion and loss of sandbanks out at sea.
Coasts also subside where we pump water or oil from beneath the ground. Much of Houston in Texas lies barely three feet above sea level today, having sunk from the removal of both groundwater and oil. Some districts have sunk ten feet since the 1930s, and in parts of the city recent sinking rates exceed two inches a year.20 It is on the front line of cities threatened by sea-level rise, although it doesn’t make the top ten global list of cities with uncertain futures. That list is topped by Miami, while New York/Newark, and New Orleans are second and third. Miami’s art deco district survives today only because of regular applications of dredged offshore sand to hold back the sea. The lower floors of high-rise buildings to the north are regularly splashed by salt spray from Atlantic storms.
It is inconceivable that we would abandon to the ocean some of the world’s most populous cities—New York or Nagoya, Japan, for example. Doubtless we will girdle them with walls to hold back the ocean. London has been defended since 1984 by a barrier across the River Thames whose sail-like gates swing shut when high tides combine with strong onshore winds to lift the sea to dangerous levels. The record of how many times the gates have been closed tells its own story. They were shut once a year for their first three years of operation, but now typically close more than twenty times a year.
New Orleans gave us a striking example of the risks faced when we choose defense over retreat. If defenses are breached, as New Orleans’ levees were during Hurricane Katrina, the ensuing floods are catastrophic. Bangkok has sunk several meters in the last century due to groundwater pumping in the Chao Phraya River delta. Much of the city had to be evacuated for the first time in late 2011 as floodwaters combined with high tides to overwhelm the ring of dikes thrown up around the city.
Defense becomes increasingly expensive and risky as sea levels rise. If the thaw of ice sheets adds more than three feet to sea levels in the coming century, we will have to abandon parts of many cities and coasts to the sea. Three feet doesn’t sound like a big deal, but any seawall would have to fend off the worst the seas could throw at them, and the extremes are expected to rise faster than the average.21 New York, for example, may soon find itself within the Atlantic hurricane belt as warmer seas sustain hurricanes into more northerly latitudes. New Yorkers got a shock in 2011 when lower Manhattan had to be evacuated due to flooding from Tropical Storm Irene. This storm may be a harbinger of things to come, as there is now broad agreement that hurricanes will intensify over the coming decades.22
The great river deltas of the world are humanity’s breadbasket. They support production of staple crops, such as wheat, rice, and corn, and are highly vulnerable even to modest rises in sea level. A survey of forty of the planet’s largest deltas—areas that collectively receive over 40 percent of the runoff from the world’s landmass—showed rates of effective rises in their sea levels of up to half an inch per year, nearly four times the rate at which the sea is rising.23 The higher rate is due to land subsidence in places where a supply of sustaining sediments has been blocked by dams or there has been groundwater or oil abstraction—or a combination of the two. This has led to a rapid loss of land around the world’s great deltas, like those of the Nile, Ganges, Mekong, and Mississippi, and so ends a long period of delta growth that followed the stabilization of the sea level seven thousand years ago.
There is little doubt that sea-level rises will provoke mass migration in the coming century as tens of thousands of square miles of delta lands are at risk of flooding and submergence.24 Relative sea-level rise, accelerated by subsidence, could displace tens of millions of people from these lands and would cut off agricultural production from some of the world’s most fertile soils just at a time when we will have the greatest need for it because of population growth.25
The human cost of climate change in deltas is becoming more obvious with each passing year. Take Bangladesh, for example, a land of floodplains and swamps better known to the world as a land of natural disasters. Formed at the confluence of two immense rivers, the Ganges and Brahmaputra, more than a third of the country barely rises above water. The people who inhabit this aquatic landscape live and often die by water. Monsoon rains and Himalayan snowmelt nourish their fields and rice paddies, but water the giver also takes away, as typhoons or deluges periodically bring catastrophe. It is hard to argue with the logic of dikes, levees, and seawalls when faced with images of desperate people stranded on the roofs of their houses in the midst of lakes that stretch from horizon to horizon. Yet, ironically, Bangladeshis would become more vulnerable to inundation from the sea if they build embankments to defend themselves from river-borne floods, as their now sediment-starved lands would sink faster. We cannot sustain life and society for long on coastal floodplains where those life-giving floods have been quenched.
When we harden coasts with engineered defenses, rises in sea level will narrow and steepen shorelines and increase the erosive force of waves. Some of the most productive marine habitats will therefore suffer. The shifting matrix of wetlands that separates land and sea on low-lying coasts will dwindle as it is squeezed between elevated sea and seawalls. Estuarine mudflats, mangroves, and salt marshes will find themselves in retreat with nowhere to go, their natural adaptability blocked by our handiwork. The fates of thousands of species that depend on them, such as those of the wading birds which use coastal wetlands as migration refueling sites and winter habitat, will be sealed depending on what happens to them. The Humber Estuary, not far from my home, supports over 150,000 such waders in winter. They enliven the mudflats and lift the spirits as they twist and turn in shape-shifting flocks across the evening sky. A hundred or so species of wetland birds are already at risk of extinction worldwide as a result of wetland loss. Sea-level rises do not bode well for them.
Like coral reefs, wetlands and dunes are living barriers between land and sea. They can be battered and breached by tempests, but the living matrix renews itself with time. I once sat out a terrible hurricane in the U.S. Virgin Islands (in the toilet, whose block-built walls had a comforting solidity!). The wind raged all night with a dreadful roar that left my ears ringing. The ground shook, the house trembled, and through the darkness came muffled crashes as our neighbors’ homes disintegrated. When dawn finally came, I emerged to an island whose verdant green had been stripped back to the grays and browns of shattered branches and twisted trunks. Along the shore, mangrove trees stood like skeletons, their crowns smashed by wind and wave. But still their gnarled and knotted trunks, rooted deep in the mud, held the line against the sea.
Mangroves in Indonesia and Sri Lanka likewise seem to have helped dissipate the destructive force of the terrible tsunami of 2004.26 (The correlation between reduced wave impacts and the presence of mangroves has been challenged, because mangroves colonize sheltered shores that were already likely to be less affected by the tsunami. But where would you prefer to be if a tsunami struck: on a shore fronted by open sea or behind a dense natural stand of mangrove trees?) The incredible debris flows pushed by the Japanese tsunami of 2011 showed that even mangroves will do nothing to stop the worst of the waves. But they could save many lives in places at a greater distance from the epicenter of destruction.
The protective function of wetlands has not been properly appreciated, nor have their many other values, such as filtration and purification of runoff or their role as nurseries for commercially important fishery animals. Instead, they have been, and still often are, p
erceived as mosquito- and disease-ridden swamp or wastelands. As a result wetlands have been drained, filled, and cleared for agricultural land, marinas, homes, shrimp ponds, airports, malls—you name it. Pretty much any use that can be made of flat coastal land has been.27 Developers in search of prime seafront see wetlands as easy wins in planning applications. So the wetland expansion that has prevailed since sea levels stabilized has in most places been reversed. Around 30 percent of the world’s mangrove forests have been cleared since the middle of the twentieth century, and some countries, such as the Philippines, have destroyed half to three quarters of their mangroves.28 Nearly a third of the global tally of sea grass beds has gone since the late nineteenth century, and the rate of decline has increased to 7 percent per year since 1990.29 A similar tale of loss can be told for salt marshes.
There is a physical relationship between sea level and erosion on open sandy coasts that says the shore will retreat two orders of magnitude more than any rise in sea level. A barrier island might lose six hundred feet to the eight-inch rise in sea level we have experienced in the last century, but it could lose three thousand feet to the thirty-nine-inch rise that is predicted in the coming century. This will not bring comfort to those who live on the coastal barrier islands that fringe the eastern and southern coasts of the United States. The American geologists Orrin Pilkey and Rob Young predict that it will become impossible to sustain communities on these barrier islands into the twenty-second century if sea levels rise as predicted.30 Seafront streets and marinas beloved of vacationers to Nantucket, Long Island, or Martha’s Vineyard could be washed away. Seafront towns like Ocean City and Atlantic Beach will have to defend themselves or disappear, while to the south, the sea will probably erase all homes and infrastructure from Cape Hatteras and Sea Island, Georgia.
The Ocean of Life Page 11