Sea Level Rise: Global Warming’s Coastal Threat and Future Projections

On a practical level, rising seas provoked by melting ice and the thermal expansion of seawater will become the most notable challenge related to global warming (ranging from inconvenience to disaster) for many millions of people around the world. Human beings have an affinity for the open sea. Thus, many major population centers have been built within a mere meter or two of mean sea level. From Mumbai (Bombay) to London to New York City, many millions of people will find warming seawater lapping at their heels in the coming years. Sea levels have been rising very slowly for a century or more, and the pace will increase in the coming years.

The last time that temperatures were 3°C higher than today worldwide was during periods of the Pliocene, two to three million years ago. The seas at that time were about 50 to 115 feet higher than today. The level of carbon dioxide in the atmosphere at that time peaked at about 425 parts per million (ppm). The level today (2016) has breached 400 ppm and has been increasing 2 to 3 ppm per year. Thus, by the end of this century, if not before, we probably will have enough warming “in the pipeline” to raise sea levels by as much as 80 feet within 150 to 200 years. One billion people today live within 25 meters (80 feet) of sea level. Because of thermal inertia, a century or two will be required for our Pliocene-level carbon dioxide to melt enough ice to raise sea levels several dozen feet.

The oceans are reaching their limits as “sinks,” or absorbers, of carbon dioxide and methane. Ken Caldeira and Philip B. Duffy assert in Science that “uptake,” or removal, of human-produced carbon dioxide by the oceans is less than many earlier investigators have assumed, and that, as temperatures warm, carbon uptake will diminish further. Additionally, Caldeira and Duffy contend that absorption of carbon into the oceans of the Southern Hemisphere (the focus of their study) has been diminishing since fossil fuel effluvia became a factor in the composition of the atmosphere around 1880. “Ventilation of the deep Southern Ocean was much more vigorous in the period from about 1350 to 1880 than in the recent past” (Caldeira and Duffy 2000: 620).

A large number of the people who could be displaced by modest sea-level rise are residents of broad river deltas, such as the Mississippi near New Orleans, the Nile in Egypt, and the Ganges, which joins with several other rivers in Bangladesh. Half of Bangladesh itself is less than five meters above sea level. A 1-meter sea-level rise in Bangladesh would displace about 10 to 20 percent of the human population and cost at least 6 percent of the gross national product (GNP). An ocean-level rise of three meters would affect 27 percent of the people there and cost roughly 15 percent of GNP

The projected rise of sea levels as the global climate warms may be the major controversy of global warming science. Many models project sea-level rise to the end of the twenty-first century, not because it will stop at that time, but because various projections become vague after that. Even modest estimates spell trouble for hundreds of millions of people who live close to Earth’s coastlines. “Sea level rise isn't going to stop in 2100,” one observer said. “I think that’s something that people don't really take on board.” Eventually, they will. “Projections of sea level rise far into the future jump from tens of centimeters to tens of meters” ( Jones 2013).

By 2016, several teams of scientists using paleoclimatic models were forecasting sea- level rise beyond the year 2100, anticipating an acceleration that places many of the world’s coastal cities on a carbon dioxide clock. Scientific debate no longer dwells on whether present levels of greenhouse gas in the atmosphere would drown the coasts. Given greenhouse gas levels already present, the question becomes when and by how much.

A scientific team in February 2016 released a projection that sea levels would rise three to four feet by century’s end. That was not startling. By that time, such projections were common (one was contained in a 2013 report by the Intergovernmental Panel on Climate Change), and “sunny-day flooding” already was afflicting coastal cities from Miami to Norfolk, Virginia. What was new in this report was a projection that sea- level rise will accelerate after the year 2100, to the point that, as an account in The New York Times paraphrased it, “Experts say the situation would then grow far worse in the 22nd century and beyond, likely requiring the abandonment of many coastal cities” (Gillis 2016). “I think we need a new way to think about most coastal flooding,” said Benjamin H. Strauss, the primary author of one of two related studies. “It’s not the tide. It’s not the wind. It’s us. That’s true for most of the coastal floods we now experience” (Gillis 2016).

Entering the third decade of the twenty-first century, more accurate pictures of sea- level rise emerge. This is partially due to better satellite images and a larger comparative timeframe regarding such photos. The data illustrates that the sea level will increase, but reports still disagree on the exact amount. More conservative estimates predict a rise of one foot by 2050. Another assessment considers the problematic reduction of the Antarctic and Greenland ice sheets, which could add as much as ten feet to the world’s oceans. Added to the problem of rising sea levels are also reported sinking cities located along the American seacoasts. The large-scale groundwater extraction causes the ground upon which the towns are built to sink, thus exacerbating any sea-level rise over the next twenty-five years.

According to NASA, between 1993 and 2024, sea levels have risen about 10 centimeters. While this by itself may not sound that alarming, scientists are closely monitoring the potential collapse of ice sheets located along the Arctic and Southern oceans. The potential collapse of the Greenland ice sheet by as early as 2025 has given rise to alarm. The melting of these sheets could very soon catapult the sea-level rise to up to two meters. Besides coral atoll nations located in the Indian and Pacific oceans, such a drastic upsurge could adversely impact larger nations such as Bangladesh, China, India, and the Netherlands. It could trigger an exodus of as many as one billion people.

FURTHER READING: Archer, David. 2009. The Long Thaw: How Humans are Changing the Next 100,000 Years of Earth’s Climate. Princeton, NJ: Princeton University Press.

Caldeira, Ken and Philip B. Duffy. 2000. “The Role of the Southern Ocean in the Uptake and Storage of Anthropogenic Carbon Dioxide.” Science 287: 620-2.

Edgerton, Lynne T. and the Natural Resources Defense Council. 1991. The Rising Tide: Global Warming and World Sea Levels. Washington, D.C.: Island Press.

Gillis, Justin. 2016. “Seas Are Rising at Fastest Rate in Last 28 Centuries.” The New York Times, February 22. http://www.nytimes.com/2016/02/23/science/sea-level-rise-global-warming-climate -change.html. Accessed March 14, 2018.

Holland, Jennifer S. 2011. “‘The Acid Threat’: As CO₂ Rises, Shelled Animals May Perish.” National Geographic 110-11.

Jones, Nicola. 2013. “Rising Tide: Researchers Struggle to Project How Fast, How High and How Far the Oceans will Rise.” Nature 501. http://www.nature.com/news/climate-science-rising-tide -1.13749