Today we will be looking at hurricanes and their activity for the last two millennia.
You might be wondering how we could possibly know anything about hurricanes two thousand years ago, and that is a fair question. The answer: geology!
More accurately, the ‘paleorecord’ left behind in rocks. In coastal areas, hurricanes can leave deposits of course grained material, which layer in between the usual sedimentary layers. It is not a perfect record–layers can be erased, or not have layers in between to separate different storms. New records from Salt Pond, a coastal pond in Falmouth, Massachusetts, however, has provided a nearly year-by-year record for the authors of today’s article. Written by Jeffrey P. Donnelly et al, “Climate forcing of unprecedented intense-hurricane activity in the last 2000 years” appeared in the American Geophysical Union’s publication Earth’s Future in February 2015.
There are several ways to date paleorecords such as the one taken from Salt Pond. One is using an isotope of cesium (associated with the testing of nuclear weapons). Others have to do with the amounts of certain kinds of pollen, which can be linked to either a non-native plant species being introduced or a plant species being cleared.
Salt Pond is about 5.5 meters (18 feet) deep and 400 meters (437 yards) from the current coastline. The storms recorded here are category 2 or stronger; storms that are weaker did not produce enough waves to carry the coarse grained material far enough inland to reach Salt Pond. Of the event beds found, one was linked to Hurricane Bob (in 1991), and others were linked to storms recorded in 1635 and 1675. The most frequent periods were between the years 1420 and 1675 (ten event beds) and the period of 23 beds between 150 and 1150. These two periods have a much greater frequency of recorded storms–while modern times sport a probability of about one category 2 or above storm per century, these periods are about three per century.
For additional support, records from Mattapoisett Marsh, Massachusetts, the Outer Banks of North Carolina and Thatchpoint Bluehole in the Bahamas also indicate a period of more intense, more frequent hurricanes in 1400-1675. Records from the Gulf of Mexico, the Carribean, and Florida suggest this same period has less frequent hurricanes, with fewer event beds recorded. The heightened activity seems restricted to the east coast of the Atlantic. The 150 to 1150 period also shows heightened activity in the North Atlantic, while the Carribean and Gulf of Mexico remain active till around 1400.
As mentioned before, storms have to be strong enough (and close enough) to Salt Pond to be recorded in its paleorecord. The increase in frequency may then correlate to an increase in intensity of storms. We know hurricanes vary in their path, intensity, and place of origin depending on the climate, although the link is not entirely understood. It does link to sea surface temperature (SST). Using a model previously developed by another group of scientists, actual records and the theoretical model outputs were compared. There is general agreement, though the increase in activity around 1400 for the east coast does not jive with the model’s less favorable hurricane formation conditions at this time. However, at the same time there was a higher sea surface temperature in the North Atlantic and Gulf of Mexico (outside the usual main development region), which may account for the discrepancy.
How exactly hurricanes behave is still being studied. How they will react to any anthropogenic (human-caused) factors have yet to be seen (though it is expected that they would intensify). Typhoons (hurricanes that form in the Northwestern Pacific) appear to be increasing in intensity, albeit the reasons are not yet known. Scientists have many more tools at their disposal than they used to–instruments to take measurements, radar to see the storm coming, and models to predict where they might make landfall. We may not have all the answers yet–and likely never will–but that is why scientists always have something to research.
Carlowicz, Mike. Hurricanes Madeline and Lester. NASA Earth Observatory. 31 Aug. 2016. Web.
A page from NASA Earth Observatory that includes the image used in this post and a brief discussion of the storms.
Donnelly, J. P., A. D. Hawkes, P. Lane, D. MacDonald, B. N. Shuman, M. R. Toomey, P. J. van Hengstum, and J. D. Woodruff (2015), Climate forcing of unprecedented intense-hurricane activity in the last 2000 years, Earth’s Future, 3, 49–65 doi:10.1002/2014EF000274.
Today’s featured article.
National Oceanic and Atmospheric Administration. What is the difference between a hurricane, a cyclone, and a typhoon? NOAA/National Ocean Service, 10 Oct. 2014. Web.
A webpage on the NOAA site discussing hurricanes/typhoons.
Voiland, Adam. Study: Typhoons in the Pacific Northwest are Getting Stronger. NASA Earth Observatory. 16 Sept. 2016. Web.
Linked to at the end of today’s post, this article discusses the strengthening of typhoons, mentioning Super Typhoon Meranti, which fits the discussed trend.