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6. Sea Level Rise and Environmental Refugees

Published onApr 10, 2020
6. Sea Level Rise and Environmental Refugees

Our people will have to move as the tides have reached our homes and villages.

—Anote Tong, President of Kiribati, in talks with Fiji about moving his entire nation to a new site1

In March 2012, President Anote Tong of Kiribati began negotiations with the government of Fiji to purchase land on Vanau Levu. His island nation will need a new a home after it is inundated by the rising seas caused by climate change. The leaders of Fiji recognize that such a move will be difficult, because “they are going to leave behind their culture, their way of life, and lifestyle.” Technically, this will be a planned migration, but make no mistake: the people of Kiribati will be refugees, among the many who will be displaced by a global increase in the sea level.

The vulnerability of Kiribati, Vanuatu, and other islands in the Gilbert chain, located north and east of Australia, was shown clearly by the arrival of Cyclone Pam in March 2015. This intense storm packed winds of 165 mph (270 kph) and devastated the islands, and the residents of these islands are not alone: this is but one example of the problems that face people who live close to the ocean.

A 2007 report on the risks of climate change to those living in low-elevation costal zones provided insight into the scope of the problem.2 The report’s authors found that 2 percent of the earth’s surface is ten meters or less above sea level. However, because people tend to live in coastal areas, this area is home to 10 percent of the world’s population and 13 percent of all urban dwellers. Although many of the countries in the survey are island nations like Kiribati, most countries with large populations have a high concentration of individuals living on river deltas that are at or near sea level. The IPCC Fourth Assessment Report singled out the Nile, Ganges-Brahmaputra, and Mekong deltas because of their extreme vulnerability to flooding. They classified the Mississippi Delta as a region of high vulnerability. These delta regions have become vulnerable because of the combined effects of (1) subsidence due to pumping out water and petroleum located beneath them, (2) reductions in delta maintenance due to dam blockage of sediments and other factors, and (3) increases in sea level due to climate change.

Sea Level

Initially, measuring the level of the ocean seems like a simple task. One only needs to place a measuring device on a post in the ocean and record the change in water level at any given moment. For centuries, this is how it was done. The average result was recorded as the mean sea level. This is in accord with the IPCC glossary, which defines mean sea level as “the surface level of the ocean at a particular point measured over an extended period of time.”3 These measurements were made with tide gauges placed at various points around the earth until 1993, when these tide gauge measurements were augmented by devices carried on a series of satellites.

Satellite-derived measurements are based on the round-trip time of radar or laser energy emitted by the satellite as it is reflected back to the satellite and a knowledge of the satellite’s position, and these measurements depend on GPS technology. The change from tide gauges to satellites results in a difference in the reference point for the measurements. Tide gauges use the land surface as the point of reference, whereas satellites use the immobile center of the earth as the point of reference.

Satellite measurements produce what is known as the geocentric mean sea level. Satellites are also used to measure the elevation of land, sea ice, the snow over sea ice, seawater, and clouds. Tide gauge measurements are affected by a rise or fall in the water, a rise or fall in the land, or both. Thus, tide gauge and satellite-based measurements are not always the same. For example, tide gauge measurements made in Stockholm, Sweden, show a fall in mean sea level. This is due to an uplift of the land that is occurring due to the absence of the pressure of ice on the earth’s surface during the last ice age, some twenty thousand years ago. In other areas, such as Manila in the Philippines, the land is sinking because large amounts of water have been pumped out of the ground. In a literature review, a 60 cm increase in geocentric mean sea level would result in sea level increases measured with tide gauges of 70 cm in New York City, 88 cm at Hampton Roads, Virginia, and 107 cm at Galveston, Texas, where particularly large amounts of water and petroleum products have been removed.4

A closer look at the issue reveals some additional complexities that may seem trivial until you consider the vastness of the oceans. Sea level will change if the shape of the ocean floor changes, if the amount of water in the ocean changes, if the density of the water in the ocean changes, or due to some combination of these factors. These changes are referred to as steric changes, which may be due to changes in the temperature of the water (thermosteric) or due to changes in salinity (halosteric). Halosteric or thermosteric changes can be local or global or both. Melting ice sheets reduce the salinity and hence the density of water, as does heating. To this fact, add the effects of ocean currents, winds, periodic oscillations such as El Niño, deformations of the earth’s crust due to tectonics, and other elements. In most cases, when discussing sea level changes, authors refer to mean sea level or global mean sea level.

Changes in Sea Level

During the past 120 million years, there have been huge changes in the mean sea level. Data from the analysis of oxygen isotopes in fossils and other sources have shown that sea levels in the past were as much as 150 meters higher or lower than at the present time, depending on how much ice covered the earth.5 These data are an indication of how much potential there is for future sea level changes.

More pertinent to modern life, measurements of sea level made since the late nineteenth century demonstrate an increase of almost nine inches, as shown in figure 6.1. An analysis of the tide gauge data shows that the rate of sea level rise was 1.7 ± 0.3 mm per year between January 1870 and December 2004. The scientists who recorded and analyzed these data restricted their inclusion of measurements to those made at geologically stable sites—that is, sites where neither the subsidence nor the elevation of the land was significant. In that interval, there was a significant acceleration of the rise in sea level of 0.013 ± 0.006 mm per year.6 The validity of this measured acceleration of the rate of sea level rise has been confirmed by satellite altimetry data collected between 1993 and 2003 that show a 3.1 mm per year rate of increase.7

Figure 6.1 Global average absolute sea level changes, 1880–2013. Cumulative sea level changes based on tide gauge measurements, and likely range of values based on number of measurements and methodological precision (grey) and satellite altimetry (black). ¶ Source: EPA, “Climate Change Indicators in the United States,” accessed October 24, 2014,

The most recent IPCC report concludes, with high confidence, that 75 percent of the rise in sea level in the past decades is due to a combination of an increase in the volume of existing sea water due to heating (thermal expansion) and the addition of water to the oceans due to melting glaciers, as shown in figure 6.2. On average, glaciers were about twelve meters thinner in 2005 than in 1960. The melting of glaciers and thermal expansion of the oceans’ waters have both been caused by human activities that have caused climate change. If the climate warms sufficiently, glaciers will disappear completely and will no longer contribute to increases in sea level. However, thermal expansion of the oceans will continue for very long periods, even if temperatures stabilize.8 The inertia in this system is due to the fact that the land, sea-surface, and air temperatures are not in a state of complete equilibrium with the temperature of water in the deepest parts of the oceans. Figure 6.3 depicts sea levels predicted to occur between 2081 and 2100 compared to a 1986 to 2005 baseline broken down by the source of the change and four different scenarios that predict the possible climates of the future.

Figure 6.2 Average yearly and cumulative thickness of mountain glaciers. In most parts of the world, glaciers are shrinking in mass. Between 1961 and 2005, the thickness decreased by approximately twelve meters. Source: National Snow and Ice Data Center; image in public domain, modified from Wikimedia Commons (Wikipedia entry: Future Sea Level).

Figure 6.3 Future sea levels predicted by the four RCP scenarios. Data extracted from J. A. Church, P. U. Clark, A. Cazenave, et al., “Sea Level Change,” in Climate Change 2013: The Physical Science Basis; Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. T. F. Stocker, D. Qin, G-K. Plattner, et al., 1137–1216 (New York: Cambridge University Press, 2014).

Water can store much more heat energy than air. This fact underlies observations indicating that upward of 90 percent of the earth’s energy gain due to climate change is stored in the oceans.9 Most of this energy gain is found in the 700 meters of water closest to the surface, where the increase in surface water temperature parallels that of the atmosphere. This warmed water becomes less dense and expands, causing it to remain near the surface in spite of upwellings and other factors that promote mixing of deep ocean water with water near the surface. These physical factors have made thermal expansion of the oceans the leading cause of rising sea level.

Because of the physics of heat transfer into the oceans, altering the time course of thermal expansion is extremely difficult.10 As a consequence, models that predict future sea level increases yield similar results in the near term. This means that even in the extremely unlikely event of a near-instantaneous and sharp reduction in greenhouse gas emissions, sea levels will continue to rise. What’s worse, models predict that sea levels will continue to rise due to thermal expansion long after stabilization of the climate. In a rather pessimistic statement, the authors of a recent paper on the reversibility of sea level rise write, “Therefore, despite any aggressive CO2 mitigation, regional sea level change is inevitable.”11

Huge amounts of water are trapped in the ice that covers Greenland and the Antarctic.12 The 150-meter deviations from present sea levels that are found in the paleoclimate record are a reflection of changes in these two repositories. The effects of climate change on Greenland ice are becoming increasingly clear: the ice is melting at an accelerating rate and will cause a rise in mean sea level. Antarctic ice is proving to be more difficult to understand and model. As the atmosphere warms, it is capable of holding more water. Some of this water will be deposited as snow in Antarctica, which, in spite of all of the ice that covers it, has near-desert-like conditions because of the relatively small amount of snow that falls each year. Thus, because of warming and the associated increase in Antarctic snowfall, it is likely that there will be an increase in ice in Antarctica. This increase is expected to have major effects during the present century and cause a fall in sea level between 0 and 70 mm.

On the other hand, warming oceans and accelerating movement of Antarctic ice are working in the opposite direction. Research indicates that this trend is most characteristic of West Antarctica. The theoretical concept referred to as marine ice instability suggests the possibility of a positive feedback cycle that could result in relatively rapid movement of Antarctic ice from the land surface into the ocean, which would cause a correspondingly large increase in sea level. Although there are two recent independent reports of “unstoppable” movements of Antarctic ice, with one glacier retreating 35 km between 1992 and 2011, a massive collapse of the Antarctic ice sheet does not seem to be likely during the next century or two.13 However, very long-range projections favor a loss of Antarctic ice resulting in a rise in sea level. A 2015 report with the self-explanatory title “Combustion of Available Fossil Fuel Resources Sufficient to Eliminate the Antarctic Ice Sheet” makes the point that a fifty-eight-meter rise in sea level would follow burning all of the earth’s fossil fuels.14 The good news is that this would take centuries. The bad news is that the most dramatic rise in sea level would occur early on.

Not all human activity has caused sea levels to rise; some has had the opposite effect. The authors of an analysis of the effect of water storage in reservoirs estimated that about 10,800 cubic kilometers of water have been stored on land.15 This reduction in the amount of water entering the oceans has prevented the oceans from rising by about three centimeters. The number of reservoirs has dropped since around 1980, and the total amount of water being impounded has started to level off, so this effect is likely to be transient.

Storm Surges and Severe Storms

Storms commonly create a temporary bulge in the water, known as a storm surge. Surges, combined with increases in sea level, will make coastal areas more vulnerable to storm-associated flooding. Storms are predicted to increase in intensity as the climate warms.

About 95 percent of the height of a surge is due to the wind as it pushes water ahead of the core of the storm. This effect forms a bulge on the surface of the ocean—seen as a temporary increase in sea level. The low barometric pressure associated with severe storms makes a relatively minor contribution to the developing surge. As the bulge encounters the shallower water near the shore, it becomes more pronounced. Certain shoreline configurations may act as a funnel, leading to further increases. These surges are superimposed on the more easily predicted astronomical tides caused by the moon. The storm surge plus the astronomical tide is referred to as a storm tide. Storm tides are most damaging when the landfall of the storm surge coincides with an astronomical high tide. Waves that are superimposed on the storm tide add an extra measure to the increase in sea level.

Superstorm Sandy, which made US landfall on October 29, 2012, was one of most damaging storms ever to hit the United States. It was an atypical event. It occurred relatively late in the hurricane season and had a highly unusual path. Sandy made an abrupt turn toward the west, striking the mid-Atlantic shore, rather than following a more typical path to the northeast. As it evolved from a tropical hurricane deriving its energy from the ocean to a post-tropical storm that derived its energy from the atmosphere, Sandy grew to an immense size. As a result, even though it was only a category 2 storm when it made landfall, the associated storm surge of 8.99 feet at Battery Park in lower Manhattan was more than twice as high as its nearest rival.16 The eighteen-inch rise of sea level that has occurred since 1850 combined with the fact that the surge coincided with a typical astronomical high tide resulted in a water level that was over thirteen feet above the mean low-level water mark. The results were catastrophic.

The New York subway system floods when sea level rises to 10.5 feet above the mean low-level water-reference point, and so the storm surge of just over thirteen feet flooded the system.17 Many communication centers were also engulfed. Other critical infrastructure elements, such as Bellevue Hospital, also flooded. When the final toll was counted, at least 233 people were believed to have died and property damage was estimated to be about $68 billion.18

This disaster pales by comparison to Cyclone Bhola, which struck what is now Bangladesh on November 12, 1970.19 This cyclone (i.e., a hurricane occurring in the South Pacific or Indian Ocean) grew rapidly in the Bay of Bengal. It slowed as it approached the Ganges-Brahmaputra Delta, which allowed the storm surge to grow. Because the northern part of the bay narrows, acting as a funnel, the surge gained still more height. Silt from the rivers blocked backflow from the surge, compounding the problem. The surge eventually grew to an estimated twenty-five feet, converting the low-lying coastal region into what was described as a “death trap.” Sustained winds of 130 mph created waves that added to the death and destruction. The death toll was never determined with accuracy, but estimates ranged up to five hundred thousand, making it one of the worst natural disasters of the century.

As discussed in chapter 1, political stability and stakeholder involvement are among the ideal prerequisites for effective management of the elements of climate change. The area struck by Cyclone Bhola, which was then part of East Pakistan, was a country where a disaster was waiting to happen. The cyclone triggered a substantial amount of political unrest in a region that already faced immense problems. The ensuing Bangladesh Liberation War led to a change of government and the eventual establishment of Bangladesh as an independent nation in 1971.

Thus, it should not have been too surprising that there was virtually no meaningful response to this cyclone: shallow water made the area inaccessible to ships. There were virtually no helicopters available to come to the aid of the stricken population. Although the United States had many helicopters in Vietnam, those that eventually arrived came from the continental United States.

The 1970 Bhola disaster was, if nothing else, a powerful learning experience. Analyses of the catastrophe led to substantial improvements in disaster response planning and public health preparedness in Bangladesh.20 High-tech improvements included the creation of early warning systems. Low-tech measures allowed these warnings to be distributed by volunteers riding bicycles. As a result, when a cyclone of similar intensity made landfall in the same area in 1991, there were about 140,000 deaths: still a lot, but nowhere near the total from the 1970 storm and floods. Additional improvements reduced the death toll to 4,234 when Cyclone Sidr, a category 5 storm with winds of 160 mph, ravaged the area in 2007.21 This decline in the death toll shows that adaptive disaster preparedness measures can make an enormous difference.

Predicted Flooding

In a New York Times feature titled “What Could Disappear,” journalists published a series of maps showing the extent of flooding that would be expected given various scenarios.22 The authors used elevation maps from the US Geological Survey and tidal data from the National Oceanographic and Atmospheric Administration to calculate the amount and location of flooding expected if sea level rose five, twelve, or twenty-five feet, as envisioned by the authors of the IPCC Fourth Assessment Report. With a rise of five feet, flooding would engulf 26 percent of Cambridge, Massachusetts, 19 percent of Charleston, South Carolina, and 20 percent of Miami and 94 percent of Miami Beach, Florida. Sacramento, California, thought of as being “inland,” would experience 4 percent flooding. These estimates do not include any of the additional increases in sea level from storm surges or just plain bad luck associated with storm surges that would coincide with unusual but predictable high tides. Additional details from that report are shown in table 6.1.

Table 6.1 Sea level rise and percent of expected flooding in major US cities

The data in the table show the extent of flooding without any attempt to monetize the result or to evaluate the impact on affected populations. However, it is not difficult to imagine that there would be an enormous impact from a five-foot increase in sea level on cities like Miami, Miami Beach, and New Orleans. An increase in sea level of that magnitude would engulf major portions of these important metropolitan areas. One prediction for Miami, Florida, suggests that a large storm surge could cause damages measured in the tens of billions of dollars.23

Port cities around the world are highly vulnerable to the effects of increasing sea level. A 2005 analysis of ports with more than one million inhabitants concluded that approximately forty million people are currently vulnerable to coastal flooding due to the combined effects of sea level increases and storm surges that would be expected during a one-hundred-year event, one that has a 1 percent probability of occurring in any given year.).24 The value of exposed assets was estimated to be around $3 trillion, or 5 percent of the 2005 global gross domestic product (GDP). The United States, Japan, and the Netherlands have the greatest financial exposure. By 2070, the worldwide population that would be ravaged by a one-hundred-year event could grow by a factor of three due to increases in sea level, coastal subsidence, population growth, and urbanization. At that time, total asset exposure could rise to as much as 9 percent of the global GDP. The study’s authors conclude that there are significant potential benefits associated with protecting cities to reduce risk.

In 2011, researchers published an extremely detailed analysis of the effects of sea level rise on the city of Copenhagen, Denmark.25 The researchers report that with a relatively modest increase in sea level of 0.5 to 1.0 meters the total insured value for property at risk would be nearly €2.3 billion, including around €1 billion for residential property, €900 million for commercial property, and €400 million for industrial property. After a hypothetical 0.5-meter rise in sea level and a 1.5-meter storm surge, the researchers predict that around 3,500 jobs would be lost in the personal services sector three months after the event. They estimate that this would fall to about 1,300 missing jobs one year after the flooding. Construction jobs would increase by about two hundred at the three-month interval and by about 1,800 after one year. Full recovery would take many years. Greater increases in sea level would lead to even larger losses, disrupting the economy of the city and the nation. Surrounding the city with a coastal flood-protection system consisting of dikes and sea walls was suggested to be a relatively simple task, with an estimated cost of several hundred million euros. This system would require a budget to fund maintenance, pumps, drainage systems, and other infrastructure improvements. The researchers concluded that such a system would be a rational investment.

Storms of the Future

Predicting future tropical hurricane and cyclone activity is one of the most difficult tasks facing meteorological researchers. One might expect that the anthropogenic increases in land surface and sea surface temperatures and associated increases in atmospheric water content would set the stage for increases in the number and intensity of destructive storms. However compelling this hypothesis might be, support is elusive. Some of the difficulties in this realm center on a lack of highly accurate information concerning past storms. In addition, hurricanes and cyclones are somewhat rare events. Although these storms are most likely to occur at specific times of the year, the variance in annual occurrence data makes it difficult to define trends. This fact is illustrated by an analysis of damage due to hurricanes between 1900 and 2005.26 In many years, there was virtually no damage, whereas in others—exemplified by the 1926 data—normalized damages were over $150 billion. This degree of variance makes it difficult, if not impossible, to detect trends.

However, there are some useful long-term data—such as an example derived from an examination of sediments retrieved from a low-lying lake near Boston, Massachussetts.27 Core samples from the lake were correlated with known hurricanes documented in the written records from the era. An examination of ten out of eleven candidate layers of sediment showed a concordance between their content and known category 2 and 3 storms. This established a basis for examining older, deeper layers that extended back one thousand years. From this examination, study authors concluded that hurricane activity was high between the twelfth and sixteenth centuries, and lower during the eleventh century and between the seventeenth and nineteenth centuries. Although they are important, these data are insufficient for forecasting future storm activity.

In the current era of satellite-aided meteorological research, a much more accurate inventory of storms is possible. However, satellite observations do not cover all of the earth evenly, and some deficiencies in the data gathered in the present still exist.

In spite of these limitations, remarkable progress toward accurate prediction of future hurricane activity has been made in the past several years. Converging results predict an increase in the number of category 4 and 5 storms. One such study predicts a reduction in the total number of hurricanes but also that the number of high-intensity storms will double by the end of the twenty-first century.28 They forecast that the largest increase will take place in the western Atlantic Ocean north of the line at twenty degrees north latitude. The authors of another review, published at about the same time, came to similar but somewhat more specific conclusions in their analysis.29 They predict that anthropogenic increases in greenhouse gases will lead to a shift toward more intense storms of between 2 and 11 percent by the end of this century. They further predict that the intensity of these stronger storms, as measured by precipitation rates within 100 km of the storm’s center, will increase by about 20 percent. Finally, they report that multiple models predict a reduction in the frequency of tropical storms of between 6 and 34 percent by 2100.

The Trajectory toward the Future

A large portion of the earth’s population is at risk as sea levels rise, storms increase in their intensity, and people flock to already crowded, urbanized coastal areas. In the summer of 2015, the news was filled with descriptions of hundreds of thousands of refugees struggling to leave Africa and the Middle East, hoping to find a better life in the nations of the European Union. There is real potential that these numbers and the suffering they represent could be dwarfed by the number of refugees created by climate change–associated coastal flooding. As always, the most vulnerable will bear the heaviest burden.

Predicting the future is always difficult. Here, the task is made difficult because of incomplete data from the past and the complexities associated with looking toward the future. However, it is clear that the climate is warming and too little is being done to mitigate and adapt to the changes that seem too likely to occur. Even if a miracle were to occur that halted emissions of greenhouse gases overnight, the sea would continue to rise. Rising temperatures will evaporate more water from the warmer oceans, fueling more powerful storms. We may be in for a rough ride.

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