February 9, 2009

1,000 Years of Boston Hurricanes

Filed under: Climate Extremes, Hurricanes

The next highway from Boston to Los Angeles can be paved with articles at odds with the notion that hurricanes are becoming more fierce or frequent or longer-lived thanks to you driving an SUV or flying to Hawaii for a vacation. Our World Climate Report achive is so chalked-full of material on this subject, we wonder if it can stand any more? If the greenhouse crusade would for once say they are wrong on this subject, we would give it up. But with literally millions websites still loudly promoting the link between hurricanes and warming, we are going to stay in business for another essay on the topic.

A research team from the University of Massachusetts, the University of Pittsburgh, and institutions in Germany and Québec focused on hurricane activity near Boston over the past 1,000 years with substantial funding from NSF, NOAA, and UMass. Besonen et al. begin their article noting “The natural variability of hurricane activity on centennial and longer timescales is poorly known because instrumental records extend back just ~130 years, and aircraft reconnaissance and satellite observations only began in the mid-1940’s. Interest is heightened in light of studies suggesting that hurricane activity may increase due to anthropogenic global warming, and, more recently, that such an increase is already perceptible.”

To extend the hurricane record far back in time, the research team headed off to Boston noting that “Lower Mystic Lake is a low elevation (~1 m a.s.l.), fresh water lake situated between Medford and Arlington, MA. The lake has a maximum depth of 24 m, and was formerly confluent with the Upper Mystic Lake until the mid-1860’s when a 3 m high dam was built between the two.” A large hurricane in the area will produce a signal in the sediments that any village idiot could spot, but after 1870, the signal was spoiled by industrialization in New England. Besonen et al. note “The general character of the laminated sedimentation is consistent throughout the record until around ~1870 when the sediments become sapropelic rather abruptly tracking explosive population growth and industrialization in the watershed. Combined with permanent alteration of the lake’s natural hydraulic regime due to dam building in the mid-1860’s as mentioned above, the post-1870 portion of the record shows strong anthropogenic disturbance and dramatically altered sedimentation dynamics.”

OK — spell “sapropelic” and then use it in a sentence! Think sludge from industrial processes and you are not far off. Sapropel is a term used in marine geology to describe dark-colored sediments that are rich in organic matter, and the material is linked uniquely to human industrial activity. Anyway, the material from human activity ruined any chance to use the sediments for hurricane reconstruction past 1870 (let’s all sing a few lines from the Standells’ Dirty Water – “love that dirty water, Oh, Boston, you’re my home”).

Besonen et al. conducted analyses of cores from Lower Mystic Lake (LML) and then calculated statistics such as the threshold detection value (TDV) and robust standard deviation (rstd). Basically, they found “In summary, using guidance provided by the historical portion of the record, we recognize hurricane-related events in the LML varve thickness time series based on two conditions: 1.) the varve must reach or exceed a thickness TDV defined by med +3.5 rstd, and 2.) the varve must also contain a graded bed. Using these criteria, 36 hurricane-related events (7 historic, 29 prehistoric) were recognized the LML record from 1011–1870.”

As seen in the graphic below, considerable variability has existed in the hurricane record in Boston since AD 1011. Besonen et al. conclude “Hurricane frequency, as recorded at LML, has not been constant over the last millennium; the 12th –16th centuries had a significantly higher level of hurricane activity (up to 8 extreme events occurring per century) compared to the 11th and 17 th–19 th centuries when only 2–3 per century was the norm.” Similarly, they conclude “The LML sedimentary record provides a well-controlled and annually-resolved record of category 2–3 hurricane activity in the Boston area over the last millennium. The hurricane signal shows centennial-scale variations in frequency with a period of increased activity between the 12 th–16 th centuries, and decreased activity during the 11th and 17 th–19 th centuries.”


Figure 1. Frequency of hurricane-related deposits in the Lower Mystic Lake record grouped by century. The darker central bars represent the number of extreme events identified using a threshold detection value (TDV) of med +3.5 rstd (robust standard deviation). The flanking light gray bars represent the number of identified extremes using TDVs of med +2.0 rstd. (left) and med +5.0 rstd. (right). Note that given the analysis range (1011–1870), the first and last columns do not span a full century (from Besonen et al., 2008).

This reconstruction shows clearly that the climate system produces a natural variability in climate including substantial variations in hurricane activity. Trends upward (or downward) in the historical period of systematic meteorological observations are often dwarfed by the natural variability seen over the past thousand years – it is a lesson often forgotten in the debate regarding our impact on climate.

Just imagine if a significant category three hurricane crashed into Boston sometime in the near future. The global warming advocates would treat the event like the second coming, we would never hear the end of it, and greenhouse gas emissions would be immediately blamed. New Orleans looked really bad during the Katrina debacle, but in the minds of many, hurricanes are common along the Gulf Coast and Katrina seemed inevitable. But a hurricane in Boston – that would really do the trick to firmly sell the global warming – hurricane link. They would never mind the evidence from Besonen et al. showing that these events were relatively common during the 12th –16th centuries, long before the Industrial Revolution.

Reference:

Besonen, M.R., et al., 2008. A 1,000-year, annually-resolved record of hurricane activity from Boston, Massachusetts, Geophysical Research Letters, 35, L14705, doi:10.1029/2008GL033950.




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