October 5, 2007

Big Apple Hurricanes

Imagine if a large hurricane struck New York City during this tropical cyclone season – the devastation would be incredible and during and following the disastrous event, global warming would undoubtedly be blamed for the all that happened to the Big Apple. Believe it or not, this will happen sometime in the not-so-distant future, it’s a virtual lock! New York City has been struck many times in the past by tropical cyclones, and it is just a matter of time before another hurricane passes directly over the city. Officials there are fully aware of the threat, as this brochure attests, providing plenty of information about hurricane evacuation zones located throughout the metropolitan area.

As we have detailed many times in the past, there is a considerable debate in the climate community regarding the future of hurricane activity. There are several prominent scientists arguing that recent global warming has increased tropical sea surface temperatures (SSTs) that can support more hurricanes, stronger events with much higher destructive powers, and hurricanes that simply last longer. These scientists and their many followers have suggested that Katrina and other recent hurricanes have been made more destructive given the ongoing buildup of greenhouse gases. Many others have raised substantial doubts about the proposed link between greenhouse gases and hurricane activity (we have covered many of their research papers, for the latest, see here). Nonetheless, should a large hurricane pass over downtown Manhattan, scientists promoting the greenhouse link would breathlessly appear on our televisions 24 hours a day.

A recent article in Geochemistry, Geophysics, Geosystems by geological scientists at Brown University and Woods Hole Oceanographic Institution focuses on hurricanes in New York City (well, western Long Island). The fact that the scientists come from geology programs suggests that we may be taking a look at New York City hurricanes over a long period of time. Scileppi and Donnelly begin their article noting that “Historical records show that New York City is at risk of being struck by a hurricane. Four documented strong hurricanes (Category 2 or higher on the Saffir-Simpson Scale) with high storm surges (~3 m) have made landfall in the New York City area since 1693 with the last occurring in 1893. Population growth during the 20th century has significantly increased the risk to lives and property should a strong hurricane recur today. The frequency of hurricane landfalls is difficult to estimate from the instrumental and documentary records due to the relative rarity of these events and the short historical observation period.”

Here is the trick to documenting hurricanes in the past in the Big Apple – they note “When a hurricane makes landfall, waves and storm surge can overtop coastal barriers, depositing sandy overwash fans on backbarrier salt marshes and tidal flats or within coastal ponds. Long-term records are formed as organic-rich sediments accumulate over storm-induced deposits, preserving coarse overwash layers.” These deposits can be dated using a variety of methods, and just like magic, hurricanes of the past are pretty clearly identified.

The pair of geologists was able to reconstruct storm surge levels in the New York City area, and as seen in Figure 1, largest surges are found in 1788, 1821, and 1893. Maybe it is just us at World Climate Report, but we fail to see any trend upward coincident with the buildup of greenhouse gases. Furthermore, we notice that the three large surges occurred during a time when, last we checked, the Northern Hemisphere was considerably cooler than what we have today.


Figure 1. Storm surge heights relative to modern mean sea level which accompanied the 1788, 1821, and 1893 hurricanes inferred from historic archives and the most extreme flooding events of the 20th century recorded by the Battery Park (New York City) tide gauge from 1920 to present (from Scileppi and Donnelly, 2007)

In terms of actual hurricane activity, they state “Four historically documented hurricanes that caused approximately 3 m of storm surge made landfall in the New York City area in 1893, 1821, 1788, and likely 1693” (see Figure 2). They correctly note in terms of any linkage with sea surface temperatures (SSTs), “Interestingly, several major hurricanes occur in the western Long Island record during the latter part of the Little Ice Age (~1550–1850 A.D.) when SSTs were generally colder than present. According to paleoclimate estimates, SSTs were likely 2 °C cooler than present in the Caribbean, 1°C cooler than present in the Florida Keys during the latter part of the Little Ice Age, and 1°C cooler than present during the 17th and 18th centuries at the Bermuda Rise.”


Figure 2. Tracks of hurricanes impacting the western Long Island region and study sites. Dashed track of 1693 indicates uncertainty with respect to the path of this hurricane. The gray track of 1985 (Gloria) indicates that this hurricane was a near-miss storm for the western Long Island study sites given that storm surge is typically highest on the right side of a hurricane (from Scileppi and Donnelly, 2007)

Here’s a quandary for the global warming crusade. The scientists state that “Despite significantly cooler than modern SSTs in the Atlantic during the latter half of the Little Ice Age, the frequency of intense hurricane landfalls increased during this time.” Had these two found an increase in hurricane activity given warmer conditions, they would be paraded right down Broadway. But finding increased activity in colder periods means no parade, but a feature in World Climate Report.

In terms of hurricane activity in New York City, staying cool is not necessarily staying safe.

Reference:

Scileppi, E. and J.P. Donnelly. 2007. Sedimentary evidence of hurricane strikes in western Long Island, New York. Geochemistry, Geophysics, Geosystems, 8(6), 1-25.




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