Section 1.2: Sea Level Rise and Land Loss Along the Louisiana Coast
A major factor causing coastal land loss in Southeastern Louisiana (and beyond) is sea level rise. We could imagine dumping buckets of dirt into a bathtub and filling the tub with water until the dirt was partially covered with water. As the level of the water rises, more and more of the dirt will be covered by water. Similarly, as global sea level rises, more and more of the world’s land surfaces are covered by sea water.
Sea level rise has happened at many scales over the earth’s history. Major periods of glacial melt occurred in the transition from Ice Ages, which are technically called glacial periods, to relatively warm and ice-free periods, such as the Holocene, which are referred to as interglacial periods. The nearly 120 meter sea level rise that occurred at the end of the Pleistocene is among the most extreme instances of sea level rise in the earth’s history, lasting for thousands of years and dramatically reshaping earth’s coastlines.
Once sea level stabilized following the melting of glacial ice at the end of the Pleistocene, sea level continued to rise at a much slower rate. Over the last 7,000 years, sea level has risen about another 5 meters, with some significant “ups” and “downs” within that trend. This 5-meter sea level rise isn’t much when compared with the huge changes that occurred at the end of the Pleistocene. Yet, it has had profoundly important consequences for past human groups living along coastlines and for global climate in general.
For coastal Louisiana, the prospect of sea level rise is particularly troubling. The very low incline of the region—in other words, the flatness of the Mississippi Delta—means that even very small sea level rises can have enormous effects on the location of the coastline and the loss of land. As global sea level has risen over the past several millennia, vast tracts of land along the Gulf Coast have been inundated by sea water. Furthermore, there is significant evidence that this trend has accelerated sharply over the course of the last century—once again, a troubling prospect for coastal populations.
The cause of the ongoing sea level rise during the Holocene has been the melting of glacial ice in two main regions: (1) Greenland and (2) Antarctica. The frozen water on these two land masses contain around 2/3 of all of the fresh water on earth and, if these continental glaciers were to melt completely, global sea level would rise by about another 65 meters (Rahmstorf, 2010). During the Holocene, the continued melting of these continental glaciers has accounted for the bulk of sea level rise (Nicholls & Cazenave, 2010).
Given this situation, sea level rise is obviously tied to increasing global temperatures, since warmer temperatures accelerate the melting of terrestrial glaciers. Warmer ocean temperatures also cause thermal expansion—in other words, the expansion of water as it warms— which increases the total volume of ocean water. In general, the rate of glacial melt and sea level rise correlates closely with our estimates of global temperature change over time. As global temperatures continue to rise in 21st century, this is a major existential concern for coastal populations dealing with sea level rise.
Over the course of the 20th century, average global temperature rose by about 0.7 degrees Celsius. Correspondingly, global sea level rose by about 20 centimeters during that time. These rates of both global temperature increase and global sea level rise are much higher than what was going on in the previous several millennia. Disturbingly for the world’s coastlines, this means that sea levels are now going up much faster than they have at any time since the last Ice Age’s end.
The issue of global warming, which is at the root of the sea level rise problem, has become a major political controversy. On the one hand, the world’s climate scientists are virtually unanimous that global temperature increase is the result of human activities, in particular the release of greenhouse gases into the atmosphere through the burning of fossil fuels. On the other hand, many politicians have continued to question the human contribution to global temperature rise, sometimes pointing to prehistoric periods of warming as evidence supporting the idea that this current warming period is somehow “natural.” As we will discuss later in this primer, such a view of global warming and sea level rise holds great dangers for the future of our coastlines.
In summary, higher average global temperatures cause the melting of glacial ice and other dynamics leading to sea level rise. Global temperatures are very high today—as high as they have been in more than a thousand years—and they are still going up extremely fast. Modern global warming, and the sea level rise that goes along with it, clearly spell trouble for the Mississippi River Delta Gulf Coast of Louisiana and beyond.