Rising Seas

Past, Present, Future

Vivien Gornitz

• Publisher: Columbia University Press
• Year: 2013
• ISBN:
  9780231519205
• Topic: Naturwissenschaften
• Subtopic: Meeresforschung

The Earth's climate is already warming due to increased concentrations of human-produced greenhouse gases in the atmosphere, and the specter of rising sea level is one of global warming's most far-reaching threats. Sea level will keep rising long after greenhouse gas emissions have ceased, because of the delay in penetration of surface warming to the ocean depths and because of the slow dissipation of excess atmospheric carbon dioxide. Adopting a long perspective that interprets sea level changes both underway and expected in the near future, Vivien Gornitz completes a highly relevant and necessary study of an unprecedented age in Earth's history.

Gornitz consults past climate archives to help better anticipate future developments and prepare for them more effectively. She focuses on several understudied historical events, including the Paleocene-Eocene Thermal Anomaly, the Messinian salinity crisis, the rapid filling of the Black Sea (which may have inspired the story of Noah's flood), and the Storrega submarine slide, an incident possibly connected to a sea level occurrence roughly 8,000 years old. By examining dramatic variations in past sea level and climate, Gornitz concretizes the potential consequences of rapid, human-induced warming. She builds historical precedent for coastal hazards associated with a higher ocean level, such as increased damage from storm surge flooding, even if storm characteristics remain unchanged. Citing the examples of Rotterdam, London, New York City, and other forward-looking urban centers that are effectively preparing for higher sea level, Gornitz also delineates the difficult economic and political choices of curbing carbon emissions while underscoring, through past geological analysis, the urgent need to do so.

1

The Ever-Changing Ocean

Not only do the tides advance and retreat in their eternal rhythms, but the level of the sea itself is never at rest. It rises or falls as the glaciers melt or grow, as the floor of the deep ocean basin shifts under its increasing load of sediments, or as the earth’s crust along the continental margins warps up or down in adjustment to strain and tension. Today a little more land belongs to the sea, tomorrow a little less. Always the edge of the sea remains an elusive and indefinable boundary.

—RACHEL CARSON, THE EDGE OF THE SEA

In the seemingly limitless extent of the ocean, the ancient sages envisioned a state of primeval formlessness out of which all life emerged. While the ancients had no concrete evidence for their intuitive insights, we now know from the geological record of sedimentary rocks that oceans have existed on Earth for at least 3.8 billion years, possibly more. Life has been present on Earth for almost as long. The first living cells most probably did originate in the ocean, although it has been debated whether this occurred in a shallow tidal pool or in the deep ocean at hydrothermal vents. Yet, in spite of the ocean’s antiquity on this planet, tiny changes in its shape, depth, and volume from one moment to another have led to major reconfigurations over geological eons. As the mean water level of the ocean varies over time, the boundary between land and sea shifts back and forth in an endless battle between rock, sand, and waves.

Standing on the beach on a balmy, sunny day, as the tides gently roll in and out on their daily round with the Moon, today’s beachgoer is blissfully unaware of the subtle, almost imperceptible changes in the oceans’ average elevation that are currently taking place. But over longer periods of time, perhaps even within a person’s lifetime and even more evident over the course of several generations, the rising sea leaves a clearer mark. Small low-lying islands and some tidal marshes may become submerged. Barrier beaches may gradually migrate landward as sand is eroded from the seaward margin and redeposited on the bay side. Grassy tidal wetlands may also migrate farther inland, occupying higher ground. Inhabitants of coastal cities may notice that port facilities and shorefront neighborhoods are flooded more frequently during storms. Coastal aquifers may become more saline. Yet the sea displays a host of contrasting moods ranging from mirror-calm stillness to stormy towering waves that can sink large oceangoing vessels and pound fiercely against the shore, undercutting cliffs and washing away expensive ocean-view homes.

The oceans are in constant motion on timescales ranging from minutes to millennia and eons, their heights rising and falling daily with the tides, with the waves and surge of a passing storm, more slowly with the changing seasons, and almost imperceptibly with gradual alterations of the Earth’s climate and configuration of the ocean basins. Currents, waves, and wind constantly reshape the shoreline, at the dynamic interface between land, sea, and air. The ephemeral changes of the ocean surface ride upon the longer-term trends in sea level, which are the main focus of this book. Although barely noticeable from day to day, even from one year to the next, these tiny shifts grow into major differences over extended periods of time. What drives these changes and how will they affect those who live near the sea, now and in the future? We need to place the 20th-to-21st-century transition that is already under way into a longer-term context extending well beyond the period of instrumental records. By reconstructing ancient sea levels using natural “archives” of environmental change recorded in ocean sediments, corals, and ice cores, and running computer models, scientists can place bounds on past sea level variability. Thus, a glance backward may teach us vital lessons that will help us anticipate coming trends and prepare better for the future. Placing today’s changes in sea level and climate into a much longer, geologic-scale time frame enables us to draw parallels between sea level during previous warm periods and possible future behavior of the ice sheets and oceans.

As greenhouse gases accumulate in the Earth’s atmosphere and signs of global warming become increasingly apparent, hundreds of millions of people living near the coasts of the world and on small, low-lying islands face the prospect of rising sea level. Dramatic changes in sea level are nothing new in this planet’s history, however. Sea level fluctuated roughly 120 meters between glacial and interglacial periods, witheven greater differences further back in geologic time. Of great concern is the possibility that human-induced global warming could trigger a major meltdown of the polar ice sheets, submerging major coastal cities and low-lying islands.

Our task will be to gain a closer understanding of the natural processes that govern variations in the seas’ average elevation and of why anthropogenic climate change may cause the elevation to increase in the future. Before embarking on this exploration of past sea levels and future prospects, we briefly review processes underlining shorter-term ocean variations, because they can locally magnify the effects of global sea level rise, affecting the safety and well-being of coastal residents.

WATERWORLD

The oceans cover 71 percent of the Earth’s surface, or 362 million square kilometers (140 million square miles). Thus, our planet rightfully deserves to be called “waterworld.” Earth is the only planet in the solar system on which water can exist at the surface simultaneously as a liquid (water), a solid (ice), and a gas (atmospheric water vapor). The oceans contain an enormous quantity of water—1.34 billion cubic kilometers (0.32 billion cubic miles), or 96.5 percent of the total at or near the Earth’s surface (table 1.1).¹ The polar ice caps and glaciers constitute the second-largest reservoir (24 million cubic kilometers or 5.8 million cubic miles)—a mere 1.74 percent of the total. The transfer of large quantities of water from oceans to ice sheets and vice versa between past glacial and interglacial periods has led to major changes in global sea level. Chapter 7 explores the possibility that future warming could melt a significant volume of the Greenland and/or Antarctic Ice Sheets. Groundwater stores an amount of water roughly comparable to that in ice—23.4 million cubic kilometers or 5.6 million cubic miles. Remaining freshwater sources hold a vital (in terms of our needs) yet tiny residual fraction of the total (table 1.1). Among the freshwater reservoirs, 68.7 percent are locked up in ice sheets and glaciers, 30.1 percent occur in groundwater, and the remaining 1.2 percent are distributed in lakes, rivers, soils, and swamps.²

Table 1.1 Distribution of the Earth’s Water

Sources: Gleick, 1996; Shiklomanov, 1997.

Figure 1.1 The water cycle, illustrating major water reservoirs and movement of water between reservoirs. (After USGS.)

Figure 1.1 schematically illustrates the major reservoirs of water and their movements around the Earth. These movements constitute the hydrological cycle. Water evaporates from the oceans and land and rises into the atmosphere. The moisture condenses into tiny water droplets or ice crystals as it cools. These coalesce into clouds. The atmosphere becomes saturated, holding all the water vapor it can at a given temperature. Eventually, when water droplets or ice crystals grow large and heavy enough, they will fall back to the ground as precipitation. Of the total moisture evaporated, close to 80 percent is precipitated over the oceans while the balance falls over land. While most water evaporated at sea precipitates over the ocean, around 10 percent is transported landward by wind. Over land, water is also evaporated from the soils, lakes, and reservoirs or evapotranspired by plants. This water falls back on land as rain, snow, and hail, and also runs off in rivers, returning to the ocean. A substantial fraction percolates downward beneath the surface, forming groundwater that is often confined for long periods in underground aquifers but eventually flows back to the ocean, thereby completing the cycle.

The world’s oceans are separated by continents and physiographic basins and are conventionally divided into the Atlantic, Pacific, Indian, Arctic, and Southern. The Pacific Ocean—the largest—covers an area of 181 million square kilometers, followed by the Atlantic Ocean (94 million square kilometers), the Indian Ocean (74 million square kilometers), and the Arctic Ocean (12 million square kilometers). The average depth of the oceans is 3,700 meters (12,100 feet). The continental shelves surrounding the continents are the shallowest portions of the oceans, with a depth of less than 130–135 meters (430–443 feet)³ (fig. 1.2). These actually represent submerged extensions of the continents. Typically, continental shelves vary in width from hundreds of meters to more than 1,000 kilometers. The continental shelf is bounded by a much steeper continental slope that grades seaward into a more gently sloping continental rise, and ultimately the smooth, relatively featureless abyssal plains (fig. 1.2).

The deep ocean is intersected by a nearly continuous submerged mountain chain that is 65,000 kilometers (40,000 miles) long, known as the mid-ocean ridge, rising 2–3 kilometers (1.2–1.9 miles) above the adjacent ocean floor and encircling the seafloor like the seams on a baseball. A narrow rift valley at its crest is the locus of active volcanism and seismicity. Fresh lavas emerge from the mid-ocean ridges along fissures, as the ocean gradually separates over millions of years, in a process called seafloor spreading. North America pulls away from Europe at an average rate of around 2.5 centimeters (1 inch) per year, roughly the rate at which fingernails grow.⁴ Other mid-ocean ridges spread faster, such as the East Pacific Rise, which is moving at 8–13 centimeters (3–5 inches) per year.

Figure 1.2 Major physical features of the ocean. Also illustrated are the processes of sea-floor spreading and subduction of oceanic lithosphere. (Adapted from Thurman, 1997.)

The deepest parts of the ocean lie in the deep-sea trenches that encircle the Pacific Ocean, parts of the Caribbean Sea, and the eastern Indian Ocean. The Mariana Trench in the Pacific is the deepest, reaching 11,000 meters (36,000 feet)⁵ beneath the ocean surface. Oceanic crust is dragged down into the Earth’s mantle at the deep-sea trenches, in a process known as subduction. Subduction zones are sites of active volcanism and earthquakes (see chapter 2).

Seafloor spreading and subduction lead to fragmentation and movement of large segments of the lithosphere (or plates), according to the unifying concept of plate tectonics.⁶ Gradually over tens of millions of years or more, these geologic forces ultimately reconfigure the ocean basins and continents, change the mean ocean depth, uplift mountains, and even alter the Earth’s climate. The relationship between these internal geological processes and changes in climate and sea level will be examined further in subsequent chapters.

CURRENTS AND COUNTERCURRENTS

As the dominant reservoir of water on the Earth’s surface, the oceans play an important role in influencing the climate. They accomplish this both by physically transporting warm water toward the poles and by transferring vast quantities of heat energy via large-scale ocean currents. Winds blowing...