7.2: Sea Level Rise

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We begin by making a simple observation about past sea level rise and human civilization. Look again at the red curve at the bottom of Figure $\PageIndex{1}$, showing changes in the volume of ice sheets on land. All that water locked in the ice came from the ocean, and so when there are extensive ice sheets there is less water in the ocean. Sea level must have been lower. How much lower? The answer is, roughly 130 meters (400 feet). We know this because we know the volume of land ice and also have direct geologic evidence of ancient shorelines.

Figure $\PageIndex{1}$: Temperature inferred from the deuterium ratios in two Antarctic ice cores (green and blue curves), and ice volume inferred from the oxygen isotope ratios of marine microfossils in ocean floor sediments (red curve). Note that the ice volume curve is flipped, so that high is on the bottom and low on the top, to make it easier to compare with temperature. Source: https://commons.wikimedia.org/wiki/F...ge_Temperature. png?uselang=en-gb.

Figure $\PageIndex{2}$ illustrates sea level rise to modern values from its low point of about 130 meters (roughly 400 feet) below today’s level, about 22,000 years ago. Notice that sea level has been remarkably stable for the last 7,000–8,000 years—coincident with the time that human civilization developed.

And that is just the point. Because our prehistoric ancestors were nomadic, they did not build permanent cities. They probably did not even notice the 400 foot rise in sea level over 10,000 years (about 0.5 inch per year). Civilization developed during a time of unusual climatic stability and is exquisitely tuned to the climate of the past 7,000 years. Much damage would be done by a change in sea level of a few feet, let alone 400 feet. It is entirely academic whether the present climate is ideal for human society, as any modest climate shift in either direction will be highly problematic.

Sea level rose through the 20th century and has continued to rise in the present one; its rate has increased to a little more than 0.1 inch per year, mostly owing to thermal expansion as ocean waters warm. Runoff from melting ice in Greenland and West Antarctica is expected to further increase the rate of sea level rise over coming decades, and projections range upward to an increase of around 1 meter (3 feet) by 2100. Elevated sea levels make coastal regions more susceptible to storm induced flooding, as evidenced by the aftermath of Hurricane Sandy, for example. Rising seas also infiltrate aquifers, putting freshwater supplies at risk. Many cities, such as New York, are weighing the costs and benefits of adaptation strategies such as building massive storm barriers versus hardening individual buildings and reducing exposure over time.

Figure $\PageIndex{2}$: Sea level increase (in meters) since the peak of the last ice age, based on detailed geologic evidence from a number of coastal and island locations. Source: https://commons.wikimedia.org/wiki/F..._Sea_Level.png

Figure $\PageIndex{3}$ shows, for three periods during this century, projected probability distributions of annual U.S. property losses owing to sea level rise, assuming that hurricane activity does not change. By the end of the century, annual losses from sea level rise alone may exceed $20 billion in 2011 dollars.

But sea level will not stop rising in 2100 even if by then we manage to eliminate emissions. The last time Earth’s atmosphere had a concentration of 400 ppm of CO₂ was during the Pliocene period, about 3 million years ago, during which time sea level was about 25 meters (80 feet) higher than it is today. It may take thousands of years, but that is where sea level is headed, and scientists are not confident about forecasting how fast land ice will melt. There is no way that coastal cities can adapt to that level of change; they will simply have to relocate.

Figure $\PageIndex{3}$: Projected probability distributions of increases (billion 2011 USD) in U.S. property losses as a result of sea level rise in the absence of increased hurricane activity. From Economic Risks of Climate Change: An American Prospectus.