The Historical Record As Evidence For Sea Level Variation

The late 17th century witnessed some of the coldest years since the end of last glacial period. Though this period also saw the dawn of modern instrumental records for changes in climate (Gribbin and Lamb, 1978), probably the most persuasive evidence for the severity of the winters at this time comes from the ample historical documentation. The famous scenes of frozen canals in Holland by contemporary Flemish painters are perhaps the most dramatic examples of winters seldom encountered since in the Netherlands.

The coincidence of extensive historical documentation with the occurrence of the most significant cold period of the late Holocene represents a unique opportunity for climatic research that yields dividends of human interactions to past climates beyond an accounting of temperature. Investigation of sea level variations using historical documents could prove equally rewarding, particularly within the period of the past several centuries which preceded modern mareograph records, but which is still too recent for conventional radiocarbon dating. Woodworth (1999) has used tidal heights recorded in port facilities at Liverpool in the Mersey Estuary since the 18th century to reconstruct sea level variations in southern Britain prior to first tide gauge records of the area. These data indicate a sharp rise in sea levels after the middle of the last century, echoing Horner's (1972) earlier findings of indication of rapidly increasing water levels at London Bridge after the 1780s. Both reconstructions for sea level changes in the Thames are in general agreement with the picture of an upward trend in sea level after about 1820 in the well-known Amsterdam tide staff record (Morner, 1973).

In North America, Kearney and Stevenson (1991) used evidence of land loss in islands of the Chesapeake Bay documented in probate, wills, and other historical records dating from the mid-17th century to infer changes in sea level prior to the period of modern tide gauge records. However, there have yet to be published direct measurements of changes in water level in harbors and estuaries similar to those from Europe. Most likely, the absence of such evidence reflects the large-scale development of North American ports during the 19th century, with demolition or burial of the older colonial structures. For example, remnants of the harbor of New York at the time of its founding as New Amsterdam by the Dutch are occasionally exhumed in the process of new construction, generally covered by landfill and the remains of 19th century structures. A similar story characterizes Baltimore, founded in 1705, whose port really experienced its major growth in the 19th century.

Nevertheless, in North America historical evidence of sea level change in recent centuries has yet to be fully exploited. There continues to be a pressing need for information on sea level changes between the late Middle Ages (Mate 15th century) and the 18th century, a time of dramatic change in global climate, as temperatures worldwide began to plunge into what may have been the coldest period in the last 5000 years, the Little Ice Age. The circumpolar expansion in middle and high latitude glaciers in the Northern Hemisphere (and the Andes in the Southern Hemisphere) (Grove, 1988) argues for a comparable-scale oceanic response. But traditional geological approaches, hampered by the limitations of radiocarbon dating involving materials <500 years old and the problems of mixing and contamination of recent sediments, will not supply the answer. At the same time, the application of historical records to the reconstruction of environmental events is fraught with dangers, especially in the instances of largely subjective accounts. Seldom will there be an exact accounting of water level variations, unless structures affected by such changes are still extant.

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