The Attacking Ocean (4 page)

By 4000 to 3000 B.C.E., global sea level rise had virtually ceased, except for local crustal adjustments as a result of melting ice sheets. Many islands and coastal areas far from glaciated areas experienced sea levels several meters higher than they are today. The earth’s crust responded locally to changes in ice coverage and water loading by siphoning water away from equatorial ocean basins into depressed areas close to vanished ice sheets. At the same time, the weight of increased amounts of meltwater affected continental shelves by tilting the shoreline upward and lowering local sea levels. Despite these changes, the rate of global sea level rise remained very low until the mid-nineteenth century C.E.

Now the situation has changed significantly. Data from coastal sediments, tidal gauges, and satellites tell us that sea levels have been rising once more since the late nineteenth and early twentieth centuries. Readings from the altimeter aboard the TOPEX/Poseidon satellite record an even higher rate of about 2.8 millimeters annually in recent years, hinting at a long-term acceleration of sea level rise.
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Satellites also tell us that the Greenland and Antarctic ice sheets are discharging ice into the oceans more rapidly. But with the kinds of temperature-rise projections of 2 to 5 degrees Celsius projected for the twenty-first century, the resulting virtually total meltdown would take several hundred years. Nevertheless, even with possible accelerated discharge from the West Antarctic ice sheet, it seems unlikely that the rate of sea level rise would exceed those of major post–Ice Age meltwater pulses.

Comforting words, perhaps, but the sea level rises of thousands of years ago came at a time when global populations were a fraction of those of today. Even as recently as five thousand years ago, the most concentrated urban populations in nascent cities like Uruk in Mesopotamia would have numbered less than ten thousand people. Elsewhere, in a world still peopled by small farming communities and hunting bands that were normally on the move, population densities never would have numbered more than a few people per square mile. The world was far from congested, so adjusting to rapid sea level rise was either a matter of shifting camp or of clearing new land for farming villages that were, in any case, often rebuilt or moved every few generations. Today, with cities in the millions and billions of coastal dwellers living at sea level or
close to it, the long-term challenges of accelerated sea level rise are already making themselves felt in a much more vulnerable world. As many experts have testified, increased warming brings a higher incidence of extreme hurricanes and severe gales, and also of tropical cyclones and their sea surges, quite apart from the terrible consequences of tsunamis triggered by earth movements on the ocean floor that devastate coastal settlements. These may be short-term events, but they are deadly. Witness the notorious Lisbon earthquake and tsunami of 1755.

LISBON, PORTUGAL, NOVEMBER 1, 1755. All Saints’ Day was the most important religious holiday in the calendar. Everyone—rich and poor, young and old—flocked to cathedrals and churches, for not to attend was to open oneself to accusations of heresy, no light matter in eighteenth-century Portugal.
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People jostled for space; overflow crowds spilled into the streets. Most of the foreigners in the cosmopolitan city stayed at home. As the hymns and prayers were at their height, a loud rumbling and trembling from deep beneath the earth drowned out the sounds of worship, for all the world like distant peals of thunder or the wheels of a passing heavy carriage. Then came the shaking, so violent that houses collapsed in seconds. Towers and spires swayed uncontrollably; church bells pealed in terrible cacophony, then tumbled to the ground. Entire congregations met their deaths under collapsing roofs and church walls. Huge chunks of masonry and jagged rubble filled alleyways and streets, burying helpless, screaming victims. Dense clouds of dust obscured the blue sky of what had been a lovely sunny day. Within minutes, Lisbon was a rubble field, ravaged by fires started by church candles and scattered hearths.

The survivors fled to open ground, away from tottering buildings and raging fires. Many headed for the open banks of the Tagus River, where they stood in shock, convinced that the Day of Judgment had come. They loudly begged for divine mercy. Priests moved through the crowds urging them to repent for their sins in the face of God’s wrath. How else could such a catastrophe have descended on them on the holiest of days? There were, at the time, no plausible scientific explanations.

Figure 1.3
The Lisbon earthquake and tsunami as depicted in Georg Ludwig Hartwig’s
Volcanoes and Earthquakes: A Popular Description,
published in 1887. Author’s collection.

Ninety minutes after the earthquake, the crowds at water’s edge saw the Tagus rock and roll. Ships at anchor offshore gyrated wildly. Then “there appeared at some distance a large body of water, rising as it were like a mountain; it came foaming and roaring, and rushed towards the shore with such impetuosity, that we all immediately ran for our lives.”
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The roaring came from a huge tsunami wave, said to have been at least twelve meters high, which swept up the river and surged ashore as spectators fled. The three thousand or so people standing on a new stone wharf in hopes of finding a boat drowned when the quay tipped over. The wall of water rushed as far as 2.4 kilometers inland, inundating buildings, overthrowing bridges, and dashing thousands of helpless onlookers against buildings and walls. Then the wave receded precipitously, carrying hundreds more victims to their deaths and uncovering areas of the riverbed that were normally twelve meters underwater. Oceangoing ships lay helplessly aground. The surviving onlookers moved to the bank, gasping at the sight of fish flapping helplessly. Ten minutes later a second even more violent fifteen-meter wave advanced up
the estuary and also cascaded ashore, promptly followed by a third, both traveling “like a torrent, tho’ against the wind and tide.”
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The waves moved so fast that galloping horses could barely escape them.

As many as twenty thousand to sixty thousand people died in the Lisbon earthquake and tsunami. Entire cities in southern Portugal and Spain fell victim to the waves. Tsunami waves swept ashore as far away as Morocco and the island of Madeira (520 kilometers) west of Africa in the Atlantic. Nine hours later, 3.6-meter breakers flooded lowlands on Caribbean islands. Northern Europe felt the effects of the tsunami, the first global natural disaster that prompted the first serious research into such events.

Tsunamis are unpredictable events generated by invisible earth movements, such as earthquakes caused by the collision of tectonic plates far below the sea surface. The word “tsunami” itself is a Japanese term meaning “harbor wave,” which dates back at least four centuries and is said to have been coined by fishermen when they returned to devastated harbors after not even having noticed tsunami waves in deep water. Great tsunamis result from extensive displacements of the seafloor, perhaps over hundreds of kilometers, over distances longer than the depth of the water. The waves generated by these enormous earthquakes are extremely long and travel great distances at speeds up to 640 kilometers an hour.
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They are far more damaging than smaller scale tsunamis caused by more local events such as a underwater landslide. In such cases, big waves may result, but they soon dissipate. Large tsunami waves are immensely powerful and quite different from the conventional breakers beloved by surfers. They are solid walls of water that sweep everything before them and rush ashore until friction or gravity cause them to slow and recede.

The Lisbon tsunami of 1755 was not unprecedented. We know, for example, that at least eight large tsunamis have struck the coasts of Spain, Portugal, and Morocco over the past twelve thousand years, at intervals of about fifteen hundred years. In 6100 B.C.E., the Storegga underwater landslide displaced vast amounts of seawater off western Norway and caused a tsunami as far away as the Orkney Islands off northern Scotland.
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Then there is what one might call the mother of
all historical natural disasters, the great eruption and resulting tsunami that blew much of Santorini Island in the Aegean into space in about 1627 B.C.E. An entire town, now known as Akrotiri, vanished under a cloud of ash and pumice.
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The inhabitants must have had some warning, for no skeletons lie among its ash-smothered dwellings, which stand up to three stories high. Wine jars, storage pots, the remains of a bed, and bright friezes are all that remains of a once-vibrant community. On the walls, a fisherman returns home with his catch; two boys exchange fisticuffs. Fast ships with serried oarsmen pass by a town amid a pod of dolphins.

A visit to Akrotiri is a stroll through a moment frozen in time. One can imagine the inhabitants grabbing their possessions, driving bleating goats into boats, and rowing hastily away as lumps of pumice drop into the seething water. Then a sudden explosion and oblivion, and a once-prosperous town was forgotten until Greek archaeologist Spyridon Marinatos unearthed some of its houses and alleyways in 1967. The scale of the explosion boggles the mind. What had once been one island measuring about nine by six kilometers became four small ones. Ash from the eruption fell over a large area, some of it on Crete, 177 kilometers to the south, at the time the center of Minoan civilization with its far-flung trade networks, extensive olive groves, and wealthy palaces. A tsunami after the eruption lashed the Cretan shoreline with huge waves, which must have caused considerable damage and disrupted mercantile activity over a vast area. Many experts believe the surging ocean permanently weakened Minoan civilization.

When visiting the deep Santorini crater, one’s mind turns to Plato’s account of the lost continent of Atlantis immortalized by the Greek philosopher with his tale of kings “of great and marvelous power,” overthrown by “portentous earthquakes and floods.”
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Despite enduring searches by the obsessed, Atlantis is almost certainly a figment of classical imagination and never existed, perhaps a folk memory of the Santorini cataclysm or some other tsunami. The Greek historian Thucydides witnessed an earthquake at Orobiae in the Euboian Gulf off eastern Greece in 429 B.C.E. He recorded how the sea, “retiring from the then line of coast, returned in a huge wave and invaded a great part of the town, and
retreated leaving some of it still under water, so what was once land is now sea; such of the inhabitants perishing as could not run up to the higher ground in time.”
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Really major tsunamis like the Lisbon event have global consequences. The earthquake and tsunami that hit Shimoda, south of Tokyo in Japan on December 1, 1854, brought small waves to San Diego and San Francisco. When the island of Krakatoa in Southeast Asia blew up in 1883, a tsunami with fifteen-meter waves destroyed 165 villages along the Java and Sumatra coasts.
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Thirty-five thousand people perished along the Sunda Strait coastline alone. As the Lisbon disaster, the Indian Ocean tsunami of 2008, and the Japanese earthquake and tsunami of 2011 remind us (see
chapter 10
), great tsunamis ravage low-lying shores and raze entire communities with devastating force. The hazard is far greater today than it was in 1755, when Lisbon had 200,000 inhabitants, and the largest city in the world, Beijing, about a million. Today, tens of millions of us are crowded in cities and towns a few meters above sea level. Lisbon alone has 547,000 inhabitants—and rising.

EXTREME WEATHER EVENTS come in many forms—blanketing snowstorms, tornadoes, torrential rainfall, and long-enduring droughts, to mention only a few. However, the most dangerous are hurricanes and tropical cyclones, which generate not only powerful winds and sheets of rain, but also violent sea surges. The infamous Hurricane Katrina, which devastated New Orleans in 2005, alerted us forcibly to the dangers of exceptional storms along low coasts besieged by subsidence and rising sea levels. As we describe in
chapter 13
, much of the damage and loss of life came not from the hurricane-force winds and rain, but from the sea surge and high tides that followed on the storm. Raging waters swept ashore and carried away entire parishes and massive artificial levees that protected low-lying parts of New Orleans.

Hurricanes like Katrina generate sea surges by the wind blowing directly toward shore and pushing water up onto the land. This is what devastated the Mississippi delta in 2005 and Galveston, Texas, in September 1900, when a hurricane-generated surge flooded the city streets
to a depth of at least six meters, destroying thirty-five hundred buildings and killing over six thousand people. Since the Galveston disaster, improved early warning systems, seawalls, and stronger buildings have reduced casualties in better-developed parts of the world, but rising urban populations and the complex and expensive logistics of warning, evacuation, and recovery make it increasingly difficult to avoid truly catastrophic human and material destruction.