A new study at Tulane University questions the reliability of how sea levels in lowland coastal areas, such as southern Louisiana, are increasing, and suggests that the current method undermines the severity of the problem. This research is the focus of a news article published this week in the magazine Science.
The relative increase in sea level, which is a combination of rising water levels and falling land, is traditionally measured using tidal gauges. But researchers Molly Kogh and Thorbjorn Torikvist claim that in coastal Louisiana, meteorologists show only part of the story.
Tide gauges in such areas are fastened an average of 20 meters in the ground, and not on the surface of the earth. "As a result, meteorologists do not stop the downhill occurring in the shallow foothills and thus underestimate the rates of relative increase in sea levels," said Kogh, a fifth year doctor. student and chief author of the study.
"This study shows that we need to fully examine how we measure the sea level in the rapid coastal areas of the coastal lowlands," Törnqvist, a professor of geology at Vokes at the Faculty of Science and Engineering, Tulane, said.
The study, published in the daily newspaper with open access Ocean Science, says that while gas gauges can accurately measure the subsidence that appears under their bases, they miss out on the component of the shallow landing. With at least 60 percent of the landings occurring in the first 5 meters of the sedimentary column, meteorologists do not capture the main contributor to the relative increase in sea level.
An alternative approach is to measure shallow subsidence using surface superstructures tables, cheap mechanical instruments that record changes in the height of the surface in the wetlands. Coastal Louisiana already has a network of more than 300 of these instruments. The data can then be combined with deep down-gauge measurements from GPS data and satellite measurements at sea-level level, said Keogh.
Rates of relative sea-level rise obtained from this approach are significantly higher than rates as concluded by tidal-gauge data. "We can therefore conclude that low-lying coastal zones may be at greater risk of flooding and in a shorter time horizon than previously assumed," said Kogh.
She said the survey had implications for coastal communities around the world.
"Around the world, communities in low-lying coastal areas may be more vulnerable to flooding than we have realized. This has implications for coastal management, urban planners and emergency planners. They plan based on a specific time frame and if the sea level increases faster than what they are planning, it will be a problem. "
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Molly E. Keog et al., Measuring the rates of today's relative increase in sea level in coastal zones of low altitude: critical evaluation, Ocean Science (2019). DOI: 10.5194 / os-15-61-2019