WASHINGTON — Oct. 25
In the lecture hall of the
National Academy of Sciences, scientists and policymakers — including the mayor
of Ocean City, Md. — gathered for a discussion today on sea level rise and coastal
disasters. The public forum, sponsored by the Academy’s Natural Disasters Roundtable,
happened to coincide with a report in this week’s Science that concluded observational
estimates of global mean sea level rise for the 20th century may be too high.
While understanding the historical rates of sea level rise is important for making policy decisions, it was clear from the conference that debate over how much damage coastal cities can expect in 30 to 100 years is second to establishing policy that can be implemented to help maintain beaches immediately.
“When it comes to global warming some say there are winners and losers, with some areas doing better climatically than others,” said Bruce Douglas of Florida International University, speaking at the conference. “But when it comes to the consequences of sea level rise, on the whole there are only losers. Unless you count the second row of houses becoming the first row.”
Douglas estimates the United States alone has $3 trillion at stake along beachfront property. “Now is not too soon to consider serious mitigation,” he said. “Sea level will hit a lot of people right away.” Douglas and others at the conference showed that much of the impact will come from erosion and storm surges that hit during high tides.
Reinhard Flick of Scripps Institution of Oceanography pointed out that the El Niño and Southern Oscilation (ENSO) event of 1982 and 1983 struck during California’s highest high tides in 4.5 years. “When the 1997 and 1998 El Niño event hit during lower high tides the weaker coastal structures had already been eliminated,” he said. As a result the state didn’t experience as much damage even though globally the event was considered stronger than the 1982-1983 El Niño.
While coastal cities continue to face the challenges of a depleting shoreline, debate continues over the prevailing factors causing sea level rise. Although tide gauges have for the last century been the primary recorders of sea level change, satellite data in the last decade are providing a more accurate and global record.
The Topex/Poseidon satellite recorded an average of 3.2 ± 0.2 millimeters per year rise in sea level between 1993 and 1998. That average includes the 1997 El Niño and Southern Oscilation event. Cecile Cabanes and colleagues at the Centre Nationale d’Etudes Spatiales in Toulouse, France, measured the global thermal expansion of the oceans, based on temperatures of the upper 500 meters from 1993 to 1998, and found an average sea level rise of 3.1 ± 0.4 millimeters a year. In the Oct. 26 Science, they reported this “striking” agreement between the satellite measurements and a rise in sea level due strictly to thermal expansion of the oceans.
Cabanes and colleagues say other contributions to sea level rise during the last decade are negligible and counterbalance each other. These contributions include an increased amount of water that the atmosphere and continents have introduced into the oceans, the thermal expansion of the oceans at depths greater than 500 meters, and salinity changes that would also affect seawater density.
The report indicates that global warming of Earth’s oceans and their subsequent thermal expansion may be the most important contribution to 21st century sea-level rise. The authors then took this finding a step back in time and used ocean temperature data recorded during the second half of the 20th century to estimate sea level rise and compare with the rates from tide gauges during that period.
“Comparing 21st century satellite observations of sea level rise against a number from the 20th century so we can see what’s happening is not a trivial problem,” said Bruce Douglas of Florida International University.
Unlike the 1993 to 1996 satellite data, which provide a global coverage and are tied to Earth’s center of mass, the longer records of the 20th century tide gauges are sporadically located and often require modification due to land subsidence or tectonic motions. The French oceanographers estimated the average value of the ocean’s thermal expansion from 1955 to 1996 based on global temperature recordings from the National Oceanographic and Atmospheric Administration (NOAA). Then they compared the results to the average thermal expansion they measured based on NOAA temperatures taken near tidal gauges.
The results showed that on a global scale sea level rose 0.5 ± 0.05 millimeters a year due to thermal expansion but, when regionally sub-sampled based on tide-gauge locations, that estimate jumped to 1.4 ± 0.1 millimeters a year. “Subsampling of the global field thus appears to result in an overestimate of thermal expansion by about 0.9 [millimeters a year],” wrote John Church of the Antarctic Cooperative Research Centre and Commonwealth Scientific and Industrial Research Organisation in Tasmania, Australia, in an accompanying Science article.
But, Church continued, because much of the ocean’s thermal expansion has likely occurred in the last few decades, the overestimate “may be too large” to apply to the entire 20th century globally averaged estimates of sea level rise. Douglas is more emphatic. “We may never know from tide gauges. Forty years isn’t a long enough time period to look, because the ocean has variability of multidecadal timescales. You need to look at 70-, 80- and even 90-year periods.”
In the meantime, Mayor James Mathias of Ocean City, Md., is working to replenish about two feet of beach loss a year. “At night I walk along the beach and then stay up till two or three in the morning watching the weather reports to see what’s coming our way,” he says. “Hurricanes and Northeasters move fast. Sea level is creeping upon us always.”