Neuengland: Starker Golfstrom lässt den Meeresspiegel sinken

Thema heute: Der Meeresspiegel in Amerika. Villanova University gab am 29.8.2018 eine Pressemitteilung heraus, die Hoffnung macht. Ein wärmeres Klima kurbelt das Mangrovenwachstum in den Küstengewässern im Südosten der USA an, sodass neues Sediment entsteht, das einen Teil des Meeresspiegelanstiegs ausgleichen kann:

New Study by Villanova University Biologists Finds Mangrove Expansion may Help Coastal Ecosystems Keep Pace with Sea Level Rise in Warmer Future

Sea level rise and extreme weather events have become harsh realities for those living along the world’s coasts. The record-breaking hurricanes of the past decade in the United States have led to staggering tolls on coastal infrastructure and communities, leading many local governments to consider the benefits of natural coastal barriers. In a landmark study titled “Warming accelerates mangrove expansion and surface elevation gain in a subtropical wetland” a team of Villanova University biologists have documented that coastal wetlands in the southeastern United States are responding positively to rising temperatures both in their growth and in their ability to build soil to keep pace with sea level rise.

Published August 29 in the British Ecological Society’s Journal of Ecology, the study’s results are a ray of sunshine in the climate change forecast. Members of the research team included Glenn A. Coldren, J. Adam Langley, and Samantha Chapman, from Villanova University’s Department of Biology, Villanova, PA and Ilka C. Feller of The Animal-Plant Interaction Lab, Smithsonian Environmental Research Center, Edgewater, MD.

The Villanova research team’s two-year experiment, funded by grants from the National Aeronautics and Space Administration (NASA), was performed at the Kennedy Space Center (KSC) within the Merritt Island National Wildlife Refuge (MINWR) on Merritt Island, Fla., USA.  The KSC was an ideal location to conduct the research being situated at the intersection of two wetland biomes, salt marshes and mangroves. The implications for the KSC are serious since coastal wetlands and sand dunes help protect NASA’s $5.6 billion low-lying infrastructure against rising seas.

The large-scale warming experiment was conducted in place in the MINWR using large passive warming chambers to increase both marsh and mangrove ecosystem air temperatures. The Villanova researchers found that experimental warming both doubled plant height and accelerated the transition from marsh to mangrove. Mangroves are woody trees with more complex roots than their grassy marsh plant counterparts. When subjected to temperatures similar to those that will occur in a warmer future, mangrove plots showed increased surface elevation which is a measure of the wetland’s ability to build soil and keep pace with sea level rise.

“Our study provides some evidence that the ongoing reshuffling of species on earth’s surface could allow for some adaptation to the same global changes that are causing them,” says Chapman. “Conserving and restoring our coastal wetlands can help humans adapt to climate change.” With their unique structure and migration to higher latitudes caused by climate change, mangroves may help coasts keep pace with sea level rise and combat severe weather events like hurricanes. Expansion of these natural barriers in areas like the Kennedy Space Center may enhance the sustainability of coastal communities as they face accelerating sea-level rise in a warmer future.

“The study links the growth of individual plants, and particularly their roots, to the survival of an entire ecosystem. The long-term strength of the mangrove effects we identified may determine what the maps of our southeastern coastlines look like in the future,” says Langley. “This mangrove effect could benefit coastal wetlands around the world.” “Our experiment highlights the impact multiple interacting aspects of climate change, such as warming and sea level rise, can have on the outcome of species invasions resulting from climate change — and on the capacity of those communities to protect shorelines,” concluded Coldren.

Paper: Glenn A. Coldren, J. Adam Langley, Ilka C. Feller, Samantha K. Chapman. Warming accelerates mangrove expansion and surface elevation gain in a subtropical wetland. Journal of Ecology, 2018; DOI: 10.1111/1365-2745.13049

In eine ähnliche Richtung geht diese Arbeit von Zhai et al. 2019 (hatten Sie vorher schonmal von „unhelpful resilience“ gehört?):

Remote sensing of unhelpful resilience to sea level rise caused by mangrove expansion: A case study of islands in Florida Bay, USA

Previous studies have found that vegetated coastal areas can increase their elevation indicating resilience to inundation by sea level rise (SLR), but the potential resilience were ignored or showed controversial results (i.e., soil accretion of vegetated areas vs. SLR). To estimate the resilience influences on 15 islands in Florida Bay (Florida, U.S.), our study used indicators (areas of the 15 islands and their mangrove forests) by analyzing 61-yr high-resolution historical aerial photographs and a 27-yr time-series of Landsat images. In these islands, coastal fringes are dominated by mangroves, and inland parts are dominated by brackish or freshwater species. Our results showed that: (1) despite rising sea levels, these low-lying islands significantly increased in area; (2) all of these islands had significant mangrove expansion, and the landward part of expansion led to the replacement of inland non-mangrove habitats; (3) there was a positive relationship between the increase of island area and mangrove expansion in these islands; (4) without the mangrove expansion, simulations showed that all of the islands had decreased areas by 2014 compared with that in 1953. On the basis of our spatial analyses and previous field studies in our study areas, these islands showed resilience to inundation and the mangrove expansion contributed to processes stabilizing these islands under SLR. Meanwhile, the mangrove expansion were partly at the expense of the habitats previously covered by non-mangrove species, thus potentially leading to a loss of plant diversity. Therefore, the mangrove expansion increased unhelpful resilience to maintain islands in a degraded state losing biodiversity, which should be considered in conservation accounting for future SLR. Moreover, the unhelpful resilience can be monitored by remote sensing based indicators, such as island area.

Große Überraschung auch an anderen Segmenten der US-Atlantikküste: Trotz Meeresspiegelanstieg hat sich die Küste zum Teil ins Meer vorgebaut. Armstrong & Lazarus 2019 berichten:

Masked Shoreline Erosion at Large Spatial Scales as a Collective Effect of Beach Nourishment

Sea‐level rise along low‐lying coasts of the world’s passive continental margins should, on average, drive net shoreline retreat over large spatial scales (>102 km). A variety of natural physical factors can influence trends of shoreline erosion and accretion, but trends in recent rates of shoreline change along the U.S. Atlantic Coast reflect an especially puzzling increase in accretion, not erosion. A plausible explanation for the apparent disconnect between environmental forcing and shoreline response along the U.S. Atlantic Coast is the application, since the 1960s, of beach nourishment as the predominant form of mitigation against chronic coastal erosion. Using U.S. Geological Survey shoreline records from 1830–2007 spanning more than 2,500 km of the U.S. Atlantic Coast, we calculate a mean rate of shoreline change, prior to 1960, of −55 cm/year (a negative rate denotes erosion). After 1960, the mean rate reverses to approximately +5 cm/year, indicating widespread apparent accretion despite steady (and, in some places, accelerated) sea‐level rise over the same period. Cumulative sediment input from decades of beach‐nourishment projects may have sufficiently altered shoreline position to mask “true” rates of shoreline change. Our analysis suggests that long‐term rates of shoreline change typically used to assess coastal hazard may be systematically underestimated. We also suggest that the overall effect of beach nourishment along of the U.S. Atlantic Coast is extensive enough to constitute a quantitative signature of coastal geoengineering and may serve as a bellwether for nourishment‐dominated shorelines elsewhere in the world.

Vor einigen Jahren gab es einen Mann in Potsdam, der eine Sintflut für die US-Ostküste kommen sah. Andere Forscher wiedersprachen ihm und die Lokalregierungen entschieden die Katastrophenszenarien zu ignorieren. Das war sicher eine gute Entscheidung. Wissenschaftliche Arbeiten der letzten Jahre konnten zeigen, wie sehr die natürliche Variabilität den Meeresspiegel in der Region beeinflusst.

Die Woods Hole Oceanographic Institution gab am 19.12.2018 eine Pressemitteilung (via ScienceDaily) heraus in der sie erklärt, weshalb der Meeresspiegel in North Carolina und Virginia schneller gestiegen ist als anderswo. Die Lösung: Post-glazialer Rebound! Hier die PM:

Why is sea level rising faster in some places along the US East Coast than others?

Sea levels are rising globally from ocean warming and melting of land ice, but the seas aren’t rising at the same rate everywhere. Sea levels have risen significantly faster in some U.S. East Coast regions compared to others. A new study led by the Woods Hole Oceanographic Institution (WHOI) reveals why.

Over the 20th century, sea level has risen about a foot and a half in coastal communities near Cape Hatteras in North Carolina and along the Chesapeake Bay in Virginia. In contrast, New York City and Miami have experienced about a 1-foot rise over the same period, while sea levels farther north in Portland, Maine, rose only about half a foot.

The reason is a phenomenon called „post-glacial rebound,“ explains Chris Piecuch, lead author of a study published on Dec. 20, 2018, in the journal Nature. Essentially, land areas in the Northern Hemisphere that once were covered by mammoth ice sheets during the last Ice Age — such as Canada and parts of the Northeast U.S. — were weighed down like a trampoline with a boulder on it. At the same time, land around the periphery of the ice sheets — along the U.S. mid-Atlantic coast, for example — rose up. As the ice sheets melted from their peak at the Last Glacial Maximum 26,500 years ago, the weighed-down areas gradually rebounded, while the peripheral lands started sinking, creating sort of a see-saw effect. Even though the ice sheets had disappeared by 7,000 years ago, the see-sawing of post-glacial rebound continues to this day.

To explore why sea levels rose faster during the last century in areas such as Norfolk Naval Station in Virginia and the Outer Banks in North Carolina, Piecuch and colleagues gathered tidal gauge measurements of sea levels, GPS satellite data that show how much the land has moved up and down over time, and fossils in sediment from salt marshes, which record past coastal sea levels. They combined all of this observational data with complex geophysical models — something that has not been done before — to give a more complete view of sea level changes since 1900.

The research team found that post-glacial rebound accounted for most of the variation in sea level rise along the East Coast. But, importantly, when that factor was stripped away, the researchers found that „sea level trends increased steadily from Maine all the way down to Florida,“ Piecuch said.

„The cause for that could involve more recent melting of glaciers and ice sheets, groundwater extraction and damming over the last century,“ Piecuch says. „Those effects move ice and water mass around at Earth’s surface, and can impact the planet’s crust, gravity field and sea level.“

„Post-glacial rebound is definitely the most important process causing spatial differences in sea level rise on the U.S. East Coast over the last century. And since that process plays out over millennia, we’re confident projecting its influence centuries into the future,“ Piecuch explains. „But regarding the mass redistribution piece of the puzzle, we’re less certain how that’s going to evolve into the future, which makes it much more difficult to predict sea level rise and its impact on coastal communities.“

Paper: Christopher G. Piecuch, Peter Huybers, Carling C. Hay, Andrew C. Kemp, Christopher M. Little, Jerry X. Mitrovica, Rui M. Ponte, Martin P. Tingley. Origin of spatial variation in US East Coast sea-level trends during 1900–2017. Nature, 2018; 564 (7736): 400 DOI: 10.1038/s41586-018-0787-6

Hier gibt es einen Bericht im Scienmag zur Studie.

Und hier gleich eine weitere Studie zum Thema, diesmal von Domingues et al. 2018. Diese Autoren erklären die Beschleunigung des Anstiegs südlich von Cape Hatteras mit der Erwärmung des Floridastroms, hervorgerufen durch Änderungen im Atlantic Warm Pool:

What Caused the Accelerated Sea Level Changes Along the U.S. East Coast During 2010–2015?

Accelerated sea level rise was observed along the U.S. eastern seaboard south of Cape Hatteras during 2010–2015 with rates 5 times larger than the global average for the same time period. Simultaneously, sea levels decreased rapidly north of Cape Hatteras. In this study, we show that accelerated sea level rise recorded between Key West and Cape Hatteras was predominantly caused by a ~1 °C (0.2 °C/year) warming of the Florida Current during 2010–2015 that was linked to large‐scale changes in the Atlantic Warm Pool. We also show that sea level decline north of Cape Hatteras was caused by an increase in atmospheric pressure combined with shifting wind patterns, with a small contribution from cooling of the water column over the continental shelf. Results presented here emphasize that planning and adaptation efforts may benefit from a more thorough assessment of sea level changes induced by regional processes.

Was stimmt jetzt? Die erste Studie, oder die zweite, oder beide?

Am 14. Juni 2019 legte die Woods Hole Oceanographic Institution per Pressemitteilung nach. Diesmal ging es um den Meeresspiegel an der Küste Neuenglands. Es war bereits seit längerem bekannt, dass der Meeresspiegel sinkt, wenn der Golfstrom („AMOC“) stärker ist, und dass der Meeresspiegel dort steigt, wenn sich der Golfstrom abschwächt. Verlockende Idee: Ist der Golfstrom der Auslöser der Meeresspiegelschwankungen? Die Studie fand: Nein. Korrelation bedeutet nicht Kausalität. In Wirklichkeit ist es die NAO die beide separat antreibt. Hier die gut geschriebene PM:

Study Finds No Direct Link Between North Atlantic Ocean Currents, Sea Level Along New England Coast

A new study by the Woods Hole Oceanographic Institution (WHOI) clarifies what influence major currents in the North Atlantic have on sea level along the northeastern United States. The study, published June 13 in the journal Geophysical Research Letters, examined both the strength of the Atlantic Meridional Overturning Circulation (AMOC)—a conveyor belt of currents that move warmer waters north and cooler waters south in the Atlantic—and historical records of sea level in coastal New England.

Scientists had previously noticed that if the AMOC is stronger in a given season or year, sea levels in the northeast U.S. go down. If the AMOC weakens, average sea levels rise considerably,” says Chris Piecuch, a physical oceanographer at WHOI and lead author on the paper. “During the winter of 2009-2010, for example, we saw the AMOC weaken by 30 percent. At the same time, sea level in our region rose by six inches. That doesn’t sound like a lot, but a half-foot of sea level rise, held for months, can have serious coastal impacts.”

“But, it’s been unclear whether those two things—coastal sea level and the AMOC—are linked by cause and effect,” adds Piecuch. Although the study confirmed that AMOC intensity and sea level seem to change at the same time, it found that neither directly causes changes in the behavior of the other. Instead, both seem to be controlled simultaneously by variability in major weather patterns over the North Atlantic, such as the North Atlantic Oscillation (NAO).

Changes in the NAO alter both AMOC and sea level separately,“ says Piecuch. „As the NAO changes, it affects the trade winds, which blow from the east across the tropical Atlantic. When the NAO is high, the trade winds are stronger than normal, which in turn strengthens AMOC. But at the same time, the westerly winds over New England are also stronger than usual. Together with unusually high air pressure on the northeast coast, this lowers the average sea level. It’s wind and pressure that are driving both phenomena.”

According to Piecuch, a study like this was not even possible until recently. For the past few decades, satellite imagery has given scientists a record of movement at the ocean’s surface, but has been unable to detect currents below the surface. Starting in 2004, however, an international team of scientists began maintaining a chain of instruments that stretch across the Atlantic between Florida and Morocco. The instruments, which are collectively called the RAPID array, hold a variety of sensors that measure currents, salinity, and temperature. “RAPID doesn’t resolve the details of every individual current along the way, but it does give us the sum total of the ocean’s behavior, which is what the AMOC represents,” Piecuch notes.

These findings are particularly important for residents along the northeast coast of the U.S., he adds. Existing climate models suggest sea levels will rise globally in the next century due to climate change, but that sea level rise on the New England coast will be greater than the global average. Scientists have traditionally assumed that the heighted future sea level rise in the northeast U.S. is inextricably tied to a weakening of the AMOC, which the climate models also predict. But, given the study’s findings, that assumption might need to be revisited, Piecuch says. “The problem right now is that we only have about 13 years of AMOC data to work with. To get a better sense of how these two things relate to one another in the long term, we’ll need to wait for a longer stretch of observational records to become available,” he says.

Also collaborating on the study were Glen G. Gawarkiewicz and Jiayan Yang of WHOI; Sönke Dangendorf of Universität Siegen in Germany; and Christopher M. Little and Rui M. Ponte of Atmospheric and Environmental Research, Inc.

Paper: Christopher G. Piecuch, Sönke Dangendorf, Glen G. Gawarkiewicz, Christopher M. Little, Rui M. Ponte, Jiayan Yang. How is New England Coastal Sea Level Related to the Atlantic Meridional Overturning Circulation at 26° N? Geophysical Research Letters, 2019; DOI: 10.1029/2019GL083073

Und gleich noch ein Paper zum Golfstrom („AMOC“) und dem Meeresspiegel, nämlich von Little et al. 2019. Hierbei handelt es sich um ein Review des Kenntnisstandes:

The Relationship Between U.S. East Coast Sea Level and the Atlantic Meridional Overturning Circulation: A Review

Scientific and societal interest in the relationship between the Atlantic Meridional Overturning Circulation (AMOC) and U.S. East Coast sea level has intensified over the past decade, largely due to (1) projected, and potentially ongoing, enhancement of sea level rise associated with AMOC weakening and (2) the potential for observations of U.S. East Coast sea level to inform reconstructions of North Atlantic circulation and climate. These implications have inspired a wealth of model‐ and observation‐based analyses. Here, we review this research, finding consistent support in numerical models for an antiphase relationship between AMOC strength and dynamic sea level. However, simulations exhibit substantial along‐coast and intermodel differences in the amplitude of AMOC‐associated dynamic sea level variability. Observational analyses focusing on shorter (generally less than decadal) timescales show robust relationships between some components of the North Atlantic large‐scale circulation and coastal sea level variability, but the causal relationships between different observational metrics, AMOC, and sea level are often unclear. We highlight the importance of existing and future research seeking to understand relationships between AMOC and its component currents, the role of ageostrophic processes near the coast, and the interplay of local and remote forcing. Such research will help reconcile the results of different numerical simulations with each other and with observations, inform the physical origins of covariability, and reveal the sensitivity of scaling relationships to forcing, timescale, and model representation. This information will, in turn, provide a more complete characterization of uncertainty in relevant relationships, leading to more robust reconstructions and projections.

Und nun die Anwendung des gerade Gelernten. Goddard et al. 2015 berichteten von einem extremen Meeresspiegelanstieg von satten 12,8 cm nördlich von New York City in den zwei Jahren 2009 und 2010. Wahnsinn! Was steckte hinter diesem Ereignis? Es war eine vorübergehende Abschwächung des Golfstroms, unterstützt von einer negativen NAO. Abstract:

An extreme event of sea-level rise along the Northeast coast of North America in 2009–2010

The coastal sea levels along the Northeast Coast of North America show significant year-to-year fluctuations in a general upward trend. The analysis of long-term tide gauge records identified an extreme sea-level rise (SLR) event during 2009–10. Within this 2-year period, the coastal sea level north of New York City jumped by 128 mm. This magnitude of interannual SLR is unprecedented (a 1-in-850 year event) during the entire history of the tide gauge records. Here we show that this extreme SLR event is a combined effect of two factors: an observed 30% downturn of the Atlantic meridional overturning circulation during 2009–10, and a significant negative North Atlantic Oscillation index. The extreme nature of the 2009–10 SLR event suggests that such a significant downturn of the Atlantic overturning circulation is very unusual. During the twenty-first century, climate models project an increase in magnitude and frequency of extreme interannual SLR events along this densely populated coast.

Zum Abschluss für heute noch ein Vergleich der Meeresspiegelentwicklung von US-Ostküste und -Westküste von Boretti 2019:

The Nonlinear Pattern of Sea Levels: A Case Study of North America

Here I analyze the relative sea level signals from the tide gauges of North America. Linear and parabolic fittings are used to compute relative rates of rise and accelerations. There are 20 long-term-trend (LTT) tide gauges along the (Pacific) West Coast of North America. The average relative rate of rise is −0.38 mm/year, and the average acceleration is +0.0012 mm/year2. There are 33 LTT tide gauges of the (Atlantic) East Coast of North America. The average relative sea level rise is 2.22 mm/year, and the average acceleration is +0.0027 mm/year2.

Haben Sie das mitbekommen: Ein negativer Meeresspiegelanstieg an der US-Westküste, also ein Meeresspiegelabfall! Die Strandhäuser der Hollywoodstars scheinen also sicher zu sein.

Genug Meeresspiegel für heute. Bleiben Sie gesund in diesen schwierigen Zeiten!