Nearly all non-Alaskan west coast tide stations have also seen high seas – ScienceDaily

These web maps – available online at https://www.vims.edu/research/products/slrc/index.php – project sea level up to 2050 on the basis of 39 Continuous analysis of tide gauges for 32 locations along the US coast from Maine to Alaska. The release of this year's maps was delayed by the closure of the government for 35 days, which prevented the compilation and access to NOAA's latest tide gauge records.

John Boon, Emeritus Professor at VIMS, explains that the report cards provide added value by providing updated sea level forecasts more frequently than those published by NOAA or other agencies.

Boon and his colleagues also use a statistical approach that includes evidence of the recent acceleration of the rate of sea level change in many US tide stations, and insists that they use measures from the US. Relative sea level – changes in the level of water relative to the land. surface on which people live and work. The relative sea level rise in Virginia and in other areas of the east coast and Gulf is due to both rising water levels and sinking of the land.

This year's newsletters, updated with monthly summaries of daily tide gauge readings for the 2018 calendar year, show that trends in sea level change have generally been maintained in the United States. the 32 stations, although the sea level control processes have fluctuated slightly from one region to the other.

Molly Mitchell, the VIMS Marine Scientist who compiled and analyzed this year's tide gauge data, highlighted two features of the 2018 Sea Level Record Sheets.

First, Hurricane Florence in September is a clear indication of what is happening in the Wilmington, North Carolina tide gauge record. "Heavy rainfall in North Carolina has contributed to Wilmington's high water levels throughout the fall," Mitchell said, "although this only happened once." minor impact on the long-term trend since it was a temporary increase. " Florence fell from 20 to over 30 inches of rain over most of the North Carolina coast between September 14 and 18.

Mitchell also notes a noticeable acceleration of sea level rise in five of the eight monitored sites along the coasts of California, Oregon and Washington. "Although the sea level is rising very slowly along the west coast, the models predict that it will start climbing faster," she says. "The report cards of the last two years support this idea." Scientists suggest that this acceleration is due to a change in the wind regimes associated with the Pacific Decadal Oscillation, a pattern of El Niño-like climate variability.

Sea level along the Gulf Coast at Grand Isle, Louisiana and Galveston, Texas continued to skyrocket in 2018 (7.75 millimeters per year at Grand Isle and 6.19 mm / yr). year in Galveston), without showing significant sign of acceleration at any place. If the lack of acceleration persists, the projected sea level by 2050 for these localities will be about equal to that of Norfolk, Virginia (about 0.49 meters). [1.6 feet] above 1992 levels), although their growth rate is currently much higher – the first and third highest of the 32 monitored sites.

There is also a high rate of climb (6.72 mm / yr) at Rockport (Texas) south of Galveston, but the acceleration measured to 2018 is the highest of all locations (0.240 mm / yr2), which gives its 2050 projection a corresponding peak of 0.78 meters (2.6 feet) above 1992 levels. Mitchell and Boon attribute the high rates of climb locally and the strong contrast in acceleration between these stations. the Gulf Coast at their location in a sedimentary basin with a complex history of water and hydrocarbon extraction. Pumping groundwater and hydrocarbons can cause subsidence of the soil, which contributes to the relative rise in sea level.

The value of an annual and localized report card

Since long-term changes in sea level are usually of the order of a few millimeters a year, researchers have generally had little need to issue frequent forecasts of sea-level changes. many sea level projections are global in scope, with a forecast horizon of 2100 – far enough apart to allow linear, easily discernable change. For example, the United Nations has often announced that the absolute sea level will increase from 44 to 74 centimeters (1.4 to 2.4 feet) by the end of the century.

The VIMS team has deliberately adopted a more localized and timely approach, designed to bring maximum value to coastal residents, businesses and governments.

"Our report cards show what sea level has done recently, what is happening right now in your locality.Many studies show that local rates of elevation and sea level acceleration significantly different from the global IPCC and NOAA rates – a key result, as local relative sea-level rise rates give a direct indication of the extent to which dwellings, buildings and roads are threatened by the floods. "

The decision of the team to use a subset of tide gauge data available goes against the traditional approach adopted by NOAA, the agency that operates the official network of tide stations of the country. "NOAA should be commended for its care in ensuring the continuity, consistency and availability of the country's long-term tide datasets," Boon said. "But at the same time, a longer record is not always better, especially when it is obvious that the rate of sea level rise has recently evolved in a non-linear way, as we see along the way. east coast of the United States. "

An earlier analysis by Boon showed that this acceleration had begun in 1987, at the center of a 36-year rolling window starting in 1969, setting the start date of the VIMS report cards. This is decades after the commissioning of many US tide stations, but at a time when many other stations now have complete or near complete recordings. Given recent evidence of ongoing warming, it makes sense, as part of a national comparative study of sea level change, to only analyze observations. carried out during the same period.

Mitchell further explains: "If you cross a threshold such as sea-level rise, what preceded, say, a tide-mark record that began in 1900, is skewed in terms of where you are going. think the ice caps are melting today faster than ever, and if that's true, the data from the previous 90 years will not accurately predict the future. "

The difference between the linear rates used in NOAA's sea level forecasts and the non-linear acceleration rates used in the VIMS report cards can give rise to very different forecasts of our future in terms of Sea level. The extension of NOAA's linear sea-level projections until mid-century for the Norfolk tide gauge, Virginia, indicates that sea level will be here from 0.3 meters (11.8 inches) higher by 2050, while VIMS forecasts – using a nonlinear acceleration rate – measures 0.49 meters or 19.3 inches. This additional 20 centimeters (8 inches) increase in sea level would have major consequences for the lowland region.

In addition, the VIMS team warns that sea levels are likely to experience short-term changes in the future, as they do today. For example, residents and planners in Virginia and other areas likely to experience significant sea-level rise by 2050 must also consider storm surges and other transient forces that raise sea levels. , even higher than the projected average climb value. The 95% confidence bands placed around the quadratic curve projected in the VIMS sea level surveys also point out that mean sea level can vary from month to month by 20 cm (8 inches). ) in relation to the projected annual average.