Hurrikan-Gefahr an der US-Atlantikküste war während der Mittelalterlichen Wärmephase besonders hoch

Bis zum 14. Januar 2017 kann man sich auf zdf.de noch den Dokumentarfilm”Der Hurrikan von Galveston” anschauen, einem Supersturm der vor mehr als hundert Jahren die Küstenregion von Houston zerstörte. Aus der Film-Beschreibung:

Die Insel-Stadt Galveston wird am 8. September 1900 durch einen Hurrikan ausgelöscht. Nachdem die einzige Brücke zum Festland zerstört war, saßen 37 000 Bewohner in einer tödlichen Falle.

Wieder einmal ist der Kontext wichtig. Die American Geophysical Union (AGU) dokumentierte am 11. Februar 2015 die Hurrikangeschichte der letzten 2000 Jahre an der Nordostküste der USA. Unerwartet: In den Zeiten 150-1150 n.Chr. und 1400-1675 n.Chr. ereigneten sich Hurrikan-reiche Phasen, in denen Stürme auftraten, die jene der letzten 150 Jahre bei weitem übertrafen:

Monster hurricanes reached U.S. Northeast during prehistoric periods of ocean warming

Intense hurricanes possibly more powerful than any storms New England has experienced in recorded history frequently pounded the region during the first millennium, from the peak of the Roman Empire into the height of the Middle Ages, according to a new study. The findings could have implications for the intensity and frequency of hurricanes that the U.S. East and Gulf coasts could experience as ocean temperatures increase as a result of climate change, according to the study’s authors.

A new record of sediment deposits from Cape Cod, Mass., show evidence that 23 severe hurricanes hit New England between the years 250 and 1150, the equivalent of a severe storm about once every 40 years on average. Many of these hurricanes were likely more intense than any that have hit the area in recorded history, according to the study. The prehistoric hurricanes were likely category 3 storms – like Hurricane Katrina — or category 4 storms – like Hurricane Hugo — that would be catastrophic if they hit the region today, according to Jeff Donnelly, a scientist at Woods Hole Oceanographic Institution in Massachusetts and lead author of the new paper accepted for publication in Earth’s Future, a journal of the American Geophysical Union.

The study is the first to find evidence of historically unprecedented hurricane activity along the northern East Coast of the United States, Donnelly said. It also extends the hurricane record for the region by hundreds of years, back to the first century, he said. “These records suggest that the pre-historical interval was unlike what we’ve seen in the last few hundred years,” said Donnelly. The most powerful storm to ever hit Cape Cod in recent history was Hurricane Bob in 1991, a category 2 storm that was one of the costliest in New England history. Storms of that intensity have only reached the region three times since the 1600s, according to Donnelly.

The intense prehistoric hurricanes were fueled in part by warmer sea surface temperatures in the Atlantic Ocean during the ancient period investigated than have been the norm off the U.S. East Coast over the last few hundred years, according to the study. However, as oceans temperatures have slowly inched upward in recent decades, the tropical North Atlantic sea surface has surpassed the warmth of prehistoric levels and is expected to warm further over the next century as the climate heats up, Donnelly said. He said the new study could help scientists better predict the frequency and intensity of hurricanes that could hit the U.S. East and Gulf coasts in the future. “We hope this study broadens our sense of what is possible and what we should expect in a warmer climate,” Donnelly said. “We may need to begin planning for a category 3 hurricane landfall every decade or so rather than every 100 or 200 years.” “The risk may be much greater than we anticipated,” he added.

Donnelly and his colleagues examined sediment deposits from Salt Pond near Falmouth on Cape Cod. The pond is separated from the ocean by a 1.3- to 1.8-meter (4.3- to 5.9-foot) high sand barrier. Over hundreds of years, strong hurricanes have deposited sediment over the barrier and into the pond where it has remained undisturbed. The researchers extracted nine-meter (30-foot) deep sediment cores that they then analyzed in a laboratory. Similar to reading a tree ring to tell the age of a tree and the climate conditions that existed in a given year, scientists can read the sediment cores to tell when intense hurricanes occurred. The study’s authors found evidence of 32 prehistoric hurricanes, along with the remains of three documented storms that occurred in 1991, 1675 and 1635.

The prehistoric sediments showed that there were two periods of elevated intense hurricane activity on Cape Cod – from 150 to 1150 and 1400 to 1675. The earlier period of powerful hurricane activity matched previous studies that found evidence of high hurricane activity during the same period in more southerly areas of the western North Atlantic Ocean basin – from the Caribbean to the Gulf Coast. The new study suggests that many powerful storms spawned in the tropical Atlantic Ocean between 250 and 1150 also battered the U.S. East Coast. The deposits revealed that these early storms were more frequent, and in some cases were likely more intense, than the most severe hurricanes Cape Cod has seen in historical times, including Hurricane Bob in 1991 and a 1635 hurricane that generated a 20-foot storm surge, according to Donnelly.

High hurricane activity continued in the Caribbean and Gulf of Mexico until 1400, although there was a lull in hurricane activity during this time in New England, according to the new study. A shift in hurricane activity in the North Atlantic occurred around 1400 when activity picked up from the Bahamas to New England until about 1675. The periods of intense hurricanes uncovered by the new research were driven in part by intervals of warm sea surface temperatures that previous research has shown occurred during these time periods, according to the new study.  Previous research has also shown that warmer ocean surface temperatures fuel more powerful storms.

The sediment coring and analysis by Donnelly and his colleagues “is really nice work because it gives us a much longer period perspective on hurricanes,” said Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology in Cambridge. “It gives you something that you otherwise wouldn’t have any knowledge of.” The new research was funded in part by the National Science Foundation’s Division of Ocean Sciences.

Das entsprechende Paper “Climate forcing of unprecedented intense-hurricane activity in the last 2000 years” von Donnelly et al. 2015 kann als pdf kostenlos im Open-Access-Verfahren heruntergeladen werden.

Die Hurrikangeschichte Floridas war Thema einer Studie von Christian Ercolani und Kollegen, die im Oktober 2015 in den Quaternary Science Reviews herauskam. Das Forscherteam fand, dass Hurrikane während der Mittelalterlichen Wärmeperiode häufiger auftraten und in der nachfolgenden Kleinen Eiszeit deutlich weniger wurden. Dies deutet daraufhin, dass das Hurrikangeschehen in Florida an die Oberflächentemperatur des Atlantikwassers gekoppelt ist. Hier der Abstract:

Intense Southwest Florida hurricane landfalls over the past 1000 years
Recent research has proposed that human-induced sea surface temperature (SST) warming has led to an increase in the intensity of hurricanes over the past 30 years. However, this notion has been challenged on the basis that the instrumental record is too short and unreliable to reveal long-term trends in hurricane activity. This study addresses this limitation by investigating hurricane-induced overwash deposits (paleotempestites) behind a barrier island in Naples, FL, USA. Paleotempestologic proxies including grain size, percent calcium carbonate, and fossil shells species were used to distinguish overwash events in two sediment cores spanning the last one thousand years. Two prominent paleotempestites were observed in the top 20 cm of both cores: the first identified as Hurricane Donna in 1960 whereas an older paleotempestite (1900–1930) could represent one of three documented storms in the early 1900s. An active period of hurricane overwash from 1000 to 500 yrs. BP and an inactive period from 500 to 150 yrs. BP correlate with reconstructed SSTs from the Main Development Region (MDR) of the North Atlantic Ocean. We observe an increased number of paleotempestites when MDR SSTs are warmer, coinciding with the Medieval Warm Period, and very few paleotempestites when MDR SSTs are cooler, coinciding with the Little Ice Age. Results from this initial Southwest Florida study indicate that MDR SSTs have been a key long-term climate driver of intense Southwest Florida hurricane strikes.

Highlights:

  • MDR SSTs have been a key long-term climate driver of Southwest Florida hurricanes.

  • We observe an increased number of paleotempestites when MDR SSTs are warmer.

  • An active period of hurricane overwash was observed from 1000 to 500 yrs. BP.

  • An inactive period of hurricane overwash was observed from 500 to 150 yrs. BP.

  • Correlations exist between the Little Ice Age, Medieval Warm Period, and our record.

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