Paleotempestology is the study of past tropical cyclone activity by means of geological proxies as well as historical documentary records. The term was coined by American meteorologist Kerry Emanuel. The usual approach in paleotempestology is the identification of deposits left by storms. Most commonly, these are overwash deposits in waterbodies close to the coast; other means are oxygen isotope ratio variations caused by tropical cyclone rainfall in trees or speleothems (cave deposits), and identifying beach ridges kicked up by storm waves. The occurrence rate of tropical cyclones can then be inferred from these deposits and sometimes also their intensity – typically the stronger events are the most easily recognizable ones –, by comparing them to deposits left by historical events. Paleotempestological research has shown that in the Coast of the Gulf of Mexico and in Australia, the occurrence rate of intense tropical cyclones is about once every few centuries, and there are long-term variations in occurrence which are caused, for example, by shifts in their paths. Common problems in paleotempestology are confounding factors such as tsunami-generated deposits, and the fact that only some parts of the world have been investigated.
History The idea that sediments emplaced by tropical cyclones could be used to reconstruct tropical cyclone history in prehistoric times was developed in the middle 20th century. The earliest publication with this idea was by McKee in 1959, and was inspired by deposits left by Typhoon Ophelia on Jaluit Atoll in 1958, and drew increased attention in the 1990s. The name is a reference to "tempest", "storm".
Definition and rationale Paleotempestology is the estimation of tropical cyclone activity with the help of proxy data. The name was coined by Kerry Emanuel of the Massachusetts Institute of Technology; the field has seen increased activity since the 1990s and studies were first carried out in the United States of America on the East Coast. The realisation that one cannot rely solely on historical records to infer past storm activity was a major driving force for the development of paleotempestology. The historical record in many places is too short (one century at most) to properly determine the hazard produced by tropical cyclones, especially the rare very intense ones which at times are undersampled by historical records; in the United States, for example, only about 150 years of record are available, and only a small number of hurricanes classified as category 4 or 5 – the most destructive ones on the Saffir-Simpson scale – have come ashore, making it difficult to estimate the hazard level. Such records may also not be representative for future weather patterns. Information about past tropical cyclone occurrences can be used to constrain how their occurrences may change in the future, or about how they respond to large-scale climate modes, such as sea surface temperature changes, or to check the accuracy of climate models. In general, the origin and behaviour of tropical cyclone systems is poorly understood, and there is concern that human-caused global warming will increase the intensity of tropical cyclones and the frequency of strong events by increasing sea surface temperatures.
Techniques In general, paleotempestology is a complex field of science that overlaps with other disciplines like climatology and coastal geomorphology. A number of techniques have been used to estimate the past hazards from tropical cyclones. Many of these techniques have also been applied to studying extratropical storms, although research on this field is less advanced than on tropical cyclones.
Overwash deposits Overwash deposits in coastal atolls, coastal lakes, marshes or reef flats or even archeological sites are the most important paleoclimatological evidence of tropical cyclone strikes. When storms hit these areas, currents and waves can overtop barriers, erode these and other beach structures, and lay down deposits in the water bodies behind barriers. Isolated breaches and especially widespread overtopping of coastal barriers during storms can generate fan-like, layered deposits behind the barrier. Individual layers can be correlated to particular storms in favourable circumstances; in addition they are often separated by a clear boundary from earlier sediments. Such deposits have been observed in North Carolina after Hurricane Isabel in 2003, for example. The intensity and impacts of the tropical cyclone can also be inferred from overwash deposits by comparing the deposits to these formed by known storms and analyzing their lithology (their physical characteristics). Additionally, thicker sediment layers usually correspond to stronger storm systems. This procedure is not always clear-cut however. Several techniques have been applied to separate out storm overwash deposits from other sediments:
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