Rätsel um starken Meeresspiegelanstieg an US-Ostküste aufgeklärt: Schuld hatten die Ozeanzyklen

Thema heute: Der Meeresspiegel in Nordamerika. Die University of California – Berkeley wies im März 2018 darauf hin, dass neben dem Meeresspiegelanstieg auch die Landabsenkung eine große Rolle spielt:

Sinking land will exacerbate flooding from sea level rise in Bay Area
Rising sea levels are predicted to submerge many coastal areas around San Francisco Bay by 2100, but a new study warns that sinking land — primarily the compaction of landfill in places such as Treasure Island and Foster City — will make flooding even worse.

Using precise measurements of subsidence around the Bay Area between 2007 and 2011 from state-of-the-art satellite-based synthetic aperture radar (InSAR), scientists from the University of California, Berkeley, and Arizona State University mapped out the waterfront areas that will be impacted by various estimates of sea level rise by the end of the century. They found that, depending on how fast seas rise, the areas at risk of inundation could be twice what had been estimated from sea level rise only. Previous studies, which did not take subsidence into account, estimated that between 20 and 160 square miles (51 to 413 square kilometers) of San Francisco Bay shoreline face a risk of flooding by the year 2100, depending on how quickly sea levels rise.

Adding the effects of sinking ground along the shoreline, the scientists found that the area threatened by rising seawater rose to between 48 and 166 square miles (125 to 429 square kilometers). “We are only looking at a scenario where we raise the bathtub water a little bit higher and look where the water level would stand,” said senior author Roland Bürgmann, a UC Berkeley professor of earth and planetary science. “But what if we have a 100-year storm, or king tides or other scenarios of peak water-level change? We are providing an average; the actual area that would be flooded by peak rainfall and runoff and storm surges is much larger.”

The data will help state and local agencies plan for the future and provide improved hazard maps for cities and emergency response agencies. “Accurately measuring vertical land motion is an essential component for developing robust projections of flooding exposure for coastal communities worldwide,” said Patrick Barnard, a research geologist with the U.S. Geological Survey in Menlo Park. “This work is an important step forward in providing coastal managers with increasingly more detailed information on the impacts of climate change, and therefore directly supports informed decision-making that can mitigate future impacts.” The low-end estimates of flooding reflect conservative predictions of sea level rise by 2100: about one and a half feet. Those are now being questioned, however, since ice sheets in Greenland and West Antarctica are melting faster than many scientists expected. Today, some extreme estimates are as high as five and a half feet.

That said, the subsidence – which the geologists found to be as high as 10 millimeters per year in some areas – makes less of a difference in extreme cases, Bürgmann noted. Most of the Bay Area is subsiding at less than 2 millimeters per year. “The ground goes down, sea level comes up and flood waters go much farther inland than either change would produce by itself,” said first author Manoochehr Shirzaei, a former UC Berkeley postdoctoral fellow who is now an assistant professor in ASU’s School of Earth and Space Exploration and a member of NASA’s Sea Level Change planning team. Shirzaei and Bürgmann will publish their findings March 7 in the online journal Science Advances.

Combining InSAR and GPS

InSAR, which stands for interferometric synthetic aperture radar, has literally changed our view of Earth’s landscape with its ability to measure elevations to within one millimeter, or four-hundredths of an inch, from Earth orbit. While it has been used to map landscapes worldwide – Bürgmann has used InSAR data to map landslides in Berkeley and land subsidence in Santa Clara County – this may be the first time someone has combined such data with future sea level estimates, he said. The team used continuous GPS monitoring of the Bay Area to link the InSAR data to sea level estimates.

“Flooding from sea level rise is clearly an issue in many coastal urban areas,” Bürgmann said. “This kind of analysis is probably going to be relevant around the world, and could be expanded to a much, much larger scale.” In the Bay Area, one threatened area is Treasure Island, which is located in the Bay midway between San Francisco and Oakland and was created by landfill for the 1939 Golden Gate International Exposition. It is sinking at a rate of one-half to three-quarters of an inch (12 to 20 millimeters) per year. Projections for San Francisco International Airport show that when land subsidence is combined with projected rising sea levels, water will cover approximately half the airport’s runways and taxiways by the year 2100. Parts of Foster City were built in the 1960s on engineered landfill that is now subsiding, presenting a risk of flooding by 2100.

Not all endangered areas are landfill, however. Areas where streams and rivers have deposited mud as they flow into the Bay are also subsiding, partly because of compaction and partly because they are drying out. Other areas are subsiding because of groundwater pumping, which depletes the aquifer and allows the land to sink. In the early 20th century, the Santa Clara Valley at the south end of San Francisco Bay subsided as much as nine feet (three meters) due to groundwater depletion, though that has stabilized with restrictions on pumping. Shirzaei noted that flooding is not the only problem with rising seas and sinking land. When formerly dry land becomes flooded, it causes saltwater contamination of surface and underground water and accelerates coastal erosion and wetland losses. The work was supported by the National Science Foundation, National Aeronautics and Space Administration and Point Reyes Bird Observatory Conservation Science.

Paper: Global climate change and local land subsidence exacerbate inundation risk to the San Francisco Bay Area (Science Advances)

Talke et al. 2018 präsentierten Meeresspiegelpegelmessungen aus dem Hafen von Boston für die letzten 200 Jahre. In der ganzen Zeit stieg der Meeresspiegel um 28 cm an. Dies entspricht einem Anstieg von 1,45 mm pro Jahr. Hier gehts zum Paper, und hier zur Pressemitteilung.

Auf Climate Audit diskutierte Steve McIntyre im November 2017 die verschiedenen Versuche, Meeresspiegel-Hockeysticks für die US Ostküste zu produzieren. Was hatten Anhänger des Aarmismus übersehen? Trompetenstoß: Die Ozeanzyklen, speziell ENSO und NAO, wie die University of Florida am 9. August 2017 bekanntgab:

East Coast’s rapidly rising seas explained

University of Florida scientists discover cause of Atlantic coastline’s sea level rise hot spots

When the Indian River Lagoon on Florida’s Atlantic coast became much saltier after 2011, Arnoldo Valle-Levinson began to investigate. The UF professor of civil and coastal engineering sciences in the College of Engineering checked local tidal gauges, revealing that seas in the region were rising nearly 10 times faster than the long-term rate recorded in that region. When he reviewed tidal data for the entire eastern seaboard, he found similar numbers for all the tide gauge stations south of Cape Hatteras, revealing the regional extent of the “hot spot.”  Sea level rise hot spots — bursts of accelerated sea rise that last three to five years — happen along the U.S. East Coast thanks to a one-two punch from naturally occurring climate variations, according to a new study lead by Valle-Levinson.

After UF scientists identified the hot spot reaching from Cape Hatteras to Miami, they probed the causes by analyzing tidal and climate data for the U.S. eastern seaboard. The study, published online today in Geophysical Research Letters, shows that seas rose in the southeastern U.S. between 2011 and 2015 by more than six times the global average sea level rise that is already happening due to human-induced global warming. The study’s findings suggest that future sea level rise resulting from global warming will also have these hot spot periods superimposed on top of steadily rising seas, said study co-author Andrea Dutton, assistant professor in UF’s department of geological sciences in the College of Liberal Arts and Sciences. “The important point here is that smooth projections of sea level rise do not capture this variability, so adverse effects of sea level rise may occur before they are predicted to happen,” Dutton said. “The entire U.S. Atlantic coastline is vulnerable to these hot spots that may amplify the severity of coastal flooding.”

The combined effects of El Niño (ENSO) and the North Atlantic Oscillation (NAO), both of which are naturally occurring climate processes, drove the recent hot spot, according to the study. Study authors also discovered similar hot spots at various positions along the U.S. eastern seaboard over the past century. They found that these past hot spots are also explained by the combined influence of ENSO and NAO. The finding challenges previous arguments that a hot spot north of Cape Hatteras over the past few decades was due to a slowdown of circulation in the North Atlantic, which is itself due to global warming. Instead, study authors discovered the combination of these two naturally occurring ocean-atmosphere processes explained both the timing and the location of hot spots observed along the entire U.S. Atlantic coast, Dutton said.

While a slowdown of circulation in the North Atlantic can further exacerbate sea level rise in the northeast, it does not explain the accelerations observed in the southeast, and was not required to explain the hot spots observed in the northeast, according to the study. The authors found that hot spots observed over the past century were created by the influence of ENSO that affects the amount of water that accumulates in the western portion of the North Atlantic and causes seas to rise along the entire U.S. Atlantic coast. This sea level rise is then concentrated to the north or south by the NAO, which is a measure of the atmospheric pressure difference between Iceland and the Azores.

Valle-Levinson said hot spots are difficult to predict and it’s not clear if the hot spots will worsen with time. By decreasing emissions, he said we may be able to stabilize rising seas long-term, but the trend will likely be difficult to reverse. “It’s amazing to see construction along the East Coast. That’s the worst place to build anything,” said Valle-Levinson, who described the future for some southeastern U.S. cities as “Venice-like.” “We need to understand that the ocean is coming.” The study was also co-authored by Jonathan Martin, a UF professor of geological sciences in the College of Liberal Arts and Sciences.

Das Virginia Institute of Marine Science gibt offenbar eine Art von “Karteikarte” für US-Küstenpegel und ihren Wasserstandsverlauf heraus. Hier nachlesen. Wer noch viel mehr über die US-Meeresspiegelmessungen wissen möchte, wird bei Judith Cury fündig.