https://phys.org/news/2019-01-seas-faster-thought.html
Study says seas may be rising faster than thought
January 30, 2019 by Barri Bronston, Tulane University
A new Tulane University study questions the reliability of how sea-level
rise in low-lying coastal areas such as southern Louisiana is measured
and suggests that the current method underestimates the severity of the
problem. This research is the focus of a news article published this
week in the journal Science.
Relative sea-level rise, which is a combination of rising water level
and subsiding land, is traditionally measured using tide gauges. But
researchers Molly Keogh and Torbjörn Törnqvist argue that in coastal
Louisiana, tide gauges tell only a part of the story.
Tide gauges in such areas are anchored an average of 20 meters into the
earth rather than at the ground surface. "As a result, tide gauges do
not record subsidence occurring in the shallow subsurface and thus
underestimate rates of relative sea-level rise," said Keogh, a fifth
year Ph.D. student and lead author of the study.
"This study shows that we need to completely rethink how we measure
sea-level rise in rapidly subsiding coastal lowlands" said Törnqvist,
Vokes Geology Professor in the Tulane School of Science and Engineering.
The study, published in the open-access journal Ocean Science, says that
while tide gauges can accurately measure subsidence that occurs below
their foundations, they miss out on the shallow subsidence component.
With at least 60 percent of subsidence occurring in the top 5 meters of
the sediment column, tide gauges are not capturing the primary
contributor to relative sea-level rise.
An alternative approach is to measure shallow subsidence using
surface-elevation tables, inexpensive mechanical instruments that record
surface elevation change in wetlands. Coastal Louisiana already has a
network of more than 300 of these instruments in place. The data can
then be combined with measurements of deep subsidence from GPS data and
satellite measurements of sea-level rise, Keogh said.
Rates of relative sea-level rise obtained from this approach are
substantially higher than rates as inferred from tide-gauge data. "We
therefore conclude that low-elevation coastal zones may be at higher
risk of flooding, and within a shorter time horizon, than previously
assumed," Keogh said.
She said the research has implications for coastal communities across
the globe.
"Around the world, communities in low-lying coastal areas may be more
vulnerable to flooding than we realized. This has implications for
coastal management, city planners and emergency planners. They are
planning based on a certain timeline, and if sea level is rising faster
than what they are planning on, that's going to be a problem."