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Sea level rise: Warming seas, melting glaciers

By DR. SURI SEHGAL - | Nov 3, 2020


As the ocean water warms, the sea level rises, and the water from melted glaciers leads to further rising. These global phenomena are occurring here in the Gulf, in our own backyard.

Sea levels around the globe have fallen and risen dramatically over millions of years, driven primarily by glacial advance and retreat. Sea level changes in the distant past were often substantial and occurred faster than the incremental increases we see now. The sea level rise (SLR) has been modest historically (1.2 mm per year) whereas now it is 3.4 mm/year. The rate of increase is likely to go higher due to the steady rise in average global temperature. The pace of SLR has recently accelerated primarily due to ice caps melting into the ocean, warming seawaters, which expand when warm. The slowing Gulf Stream and sinking land contribute to SLR in some areas. Global warming is the culprit.

The average global temperatures have already risen by 1 to 1.2 Celsius since the preindustrial era (PIE) and continue to rise, driven by increasing greenhouse gas (ghg) emissions and atmospheric concentrations of ghg. As the globe warms, so does the ocean water. The decade ending in 2020 was a time of extremes — the warmest decade on record. The year 2016 was the hottest year, but per the latest data compiled by Zeke Hausfather for CarbonBrief, 2020 is more likely even hotter. Greenland lost a record amount of ice in 2019 — more than twice the annual average since 2003 according to Alfred Wegener of Germany. Several publications on climate change report that the South Pole warmed at three times the global rate over the past 30 years. As a result, parts of coastal Antarctica are losing ice, which contributes to SLR. The rising seawater is affecting the tidal system, raising the height of the tides higher than before and extending further inland. One can see this happening over time as the beach becomes narrow, and a modest surge or tidal waves take water right into coastal vegetation.

Beaches need periodic nourishment to mitigate erosion and maintain their character. This is being done periodically by the Captiva Erosion Prevention District, fully recognizing that it is not a long-term solution to coastal erosion. But it buys time before renourishment or replenishment is needed again to increase beach width and mitigate erosion.

How high the sea will rise is anybody’s guess and is a subject of debate by scientists. Whether it will be a few inches or a few feet depends on the assumptions made on temperature rise. A rise in the global mean temperature by 3 degrees Celsius (or more since PIE) could trigger a rise of a few feet by the end of this century. One can’t say with certainty how fast the ocean will warm and the ice will melt. One certainty is that water levels will continue to rise faster, we just don’t know how fast. Therefore scientists from the National Oceanic and Atmospheric Administration and the U.S. Army Corps of Engineers have made predictions based on ranges from low to high.

The world’s oceans are the clearest measure of the climate emergency because they absorb more than 90 percent of the heat trapped by greenhouse gases emitted by fossil fuel burning, forest destruction, and other human activities. The other adverse effects of SLR are ocean acidification, species extinction, coral reef die-offs, and huge economic disruption from massive relocation to higher grounds.

The mitigation of SLR requires a holistic, long-term approach with robust and meticulous long-term planning and informed investment in resilient infrastructure. The soft structures that can protect the coast from SLR include mangrove forests, dense coastal vegetation, salt marshes, et cetera. The hard structures include sea walls, bulkheads, revetment, et cetera. If significant mitigation steps are not taken, the current sea level trends can lead to island and lowland submergence.

To ensure long-term resilience, a comprehensive plan is being developed by the Captiva Community Panel and the city of Sanibel. Some scientists have projected that with one meter rise, the J.N. “Ding” Darling National Wildlife Refuge could be 95 percent underwater, and parts of Sanibel may not be habitable. The majority of Captiva may be severely impacted with many of the structures inundated north of central Captiva and in southern sections of Captiva. In fact, except for a strip in the middle, starting at the south end of ‘Tween Waters and ending a half-mile before the Blind Pass bridge, everything else is likely to be underwater. The road access to the central strip may become impassable and therefore not habitable either. This means we will need to abandon Captiva unless mitigation structures are put in place. The time to act is now, before it is too late.

Captiva resident Dr. Suri Sehgal has a long career as a crop scientist, seedsman, entrepreneur and leading expert in the global hybrid seed industry and now operates two nonprofit organizations with his wife, Edda. He is a member of the Captiva Sea Level Rise Committee and chairs the Captiva Island Yacht Club’s Environmental Awareness Committee.