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Coastal deltas occupy only 1% of Earth’s land surface, yet they are home to 500 million people; this translates to a population density that is 10 times the world average. More amazing, these people all live within only 5 m of sea level! The importance of deltas for global agriculture cannot be overstated, as they are the “rice bowls” of the world, providing one of the major food staples for human populations. Unfortunately, many of these systems are threatened by sea-level rise and flooding in the future. In fact, most coastal plains around the world less than 1 m above sea level are under the threat of being drowned within the next century, something that has not happened at this rate over the past 7,000 years. This means that major cities like Shanghai, Dhaka, and Bangkok are currently threatened and, in most cases, have no viable plan to deal with this threat. A concrete scientific plan is needed to manage and sustain these dynamic systems, or many will be lost. Although the effects of climate change on coastal regions and issues of management response have been topics of concern for at least the last 20 years, other human drivers of environmental change that are specific to certain regions, primarily linked with population expansion, have made it difficult to develop comprehensive plans for coastlines. The northern portion of the Nile River Delta, whose fertile soil allowed Egypt to become one of the cradles of civilization, is tilting and sinking toward the Mediterranean Sea at an alarming rate. In the northeastern part of the delta, near the Suez Canal, the land is sinking by as much as 0.5 cm/yr. Recent studies show that the weight of river sediments accumulated over the centuries has resulted in enhanced subsidence of deltaic sediments. Subsidence is an inherent problem in all delta systems because of the high accumulation of sediments, which over time continue to settle, resulting in compaction and dewatering of these thick mud layers.

As I briefly mentioned in Chapter 3, the global mean sea level, as deduced from the accumulation of paleo-sea level, tide gauge, and satellite-altimeter data, rose by 0.19 m (range, 0.17–0.21 m) between 1901 and 2010 (see Figure 3.3). Global mean sea level represents the longer-term global changes in sea level, without the short-term variability, and is also commonly called eustatic sea-level change. On an annual basis, global mean sea-level change translates to around 1.5 to 2 mm. During the last century, global sea level rose by 10 to 25 cm. Projections of sea-level rise for the period from 2000 to 2081 indicate that global mean sea-level rise will likely be as high as 0.52 to 0.98 m, or 8 to 16 mm/ yr, depending on the greenhouse gas emission scenarios used in the models. Mean sea-level rise is primarily controlled by ocean thermal expansion. But there is also transfer of water from land to ocean via melting of land ice, primarily in Greenland and Antarctica. Model predictions indicate that thermal expansion will increase with global warming because the contribution from glaciers will decrease as their volume is lost over time. (Take a look at Figure 5.1 if you have doubts about glaciers melting.) And remember our discussion in Chapter 2 about the role of the oceans in absorbing carbon dioxide (CO2) and the resultant ocean acidification in recent years. The global ocean also absorbs about 90% of all the net energy increase from global warming as well, which is why the ocean temperature is increasing, which in turn results in thermal expansion and sea-level rise. To make things even more complicated, the expansion of water will vary with latitude because expansion of seawater is greater with increasing temperature. In any event, sea level is expected to rise by 1 to 3 m per degree of warming over the next few millennia.

As human populations have increased on the planet, so have their effects on the natural landscape. When human-engineered changes in the movement of soils and rocks occur in the vast watersheds of major rivers, they can have dramatic consequences with respect to the amount of sediment needed to “feed” and support large river deltas at the coast. Many of the largest effects of human activity on the surface of the earth have occurred recently—in the past 200 or so years—and they have been so dramatic it has been argued it is time to create a new epoch in the Geologic Time Scale, one called the Anthropocene. That suggestion is being considered seriously. Nevertheless, the first alterations of the landscape began as early as the Paleolithic, approximately 400,000 to 500,000 years ago, when our human-like ancestors Homo erectus are believed to have begun altering the natural landscape with simple dwelling structures. As humans evolved, so did the tools they used, from sticks and animal antlers to wood and iron plows. Although modern humans (Homo sapiens sapiens) had developed in East Africa by about 200,000 years ago, their ability to extensively modify the landscape through agricultural activities did not likely happen for another 120,000 years. Incredibly, there was a rise in agricultural communities about five millennia ago that seems to have occurred simultaneously, yet independently, in six different regions of world (see Chapters 1 and 2 for linkages among human civilizations, deltas, and stabilization of climate in the Holocene). After the invention of the wheel in the middle Holocene, it became much easier to perform earth-moving activities. This was followed by the Iron Age, around 2,500 years ago, during which iron replaced earlier, less efficient copper and bronze tools for moving earth. Amazingly, the first man-made canal, connecting the Mediterranean and Red seas, was constructed before the Iron Age, around 3,600 years ago. Today, humans are the most effective animals on the planet with respect to altering Earth’s surface, and the use of machinery enables earth-moving activities, such as strip- mining, for extraction of valuable mineral resources like copper and silver.