Paleotempestology is the study of past tropical cyclone activity by means of geological proxies as well as historical documentary records.
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.
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.
The name was coined by Kerry Emanuel of the Massachusetts Institute of Technology;[1] the field has seen increased activity since the 1990s[2] and studies were first carried out in the United States of America[3] on the East Coast.
[4] 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.
Individual layers can be correlated to particular storms in favourable circumstances; in addition they are often separated by a clear boundary from earlier sediments.
[20] Several techniques have been applied to separate out storm overwash deposits from other sediments: Generally, sites suitable for obtaining paleotempestology records are not found along the entire length of the coastline,[21] and depending on the properties of the site such as vegetation cover,[30] they might only track storms approaching from a certain direction.
The storms in the Lake Shelby record have windspeeds of over 190 kilometres per hour (120 mph)[33] as Hurricane Ivan which in 2004 made landfall in the region at that intensity did not leave a deposit.
[53] However, confounding factors like natural variation and soil properties also influence oxygen isotope ratios of tree cellulose.
[60] Historical documents such as county gazettes in China, diaries, logbooks of travellers, official histories and old newspapers can contain information on tropical cyclones.
[69] Speleothems can also store trace elements which can signal tropical cyclone activity[70] and mud layers formed by storm-induced cave flooding.
[58] Droughts on the other hand can cause groundwater levels to drop enough that subsequent storms cannot induce flooding and thus fail to leave a record, as has been noted in Yucatan.
[71] Other techniques: A database of tropical cyclones going back to 6,000 BC has been compiled for the western North Atlantic Ocean.
[92] Tempestite deposits[93] and oxygen isotope ratios in much older rocks have also been used to infer the existence of tropical cyclone activity[60] as far back as the Jurassic.
[96] The recurrence rate, the time gap between storms, is an important metric used to estimate tropical cyclone risk, and it can be determined by paleotempestological research.
[115] This appears to be a stage of increased tropical cyclone activity spanning the region from New York to Puerto Rico,[116] while the last 1,000 years have been inactive both there and in the Gulf Coast.
[122] Paleotempestological data support this theory[123] although additional findings on Long Island and Puerto Rico have demonstrated that storm frequency is more complex[117] as active periods appear to correlate between the three sites.
[130] Furthermore, a tendency to a more northerly storm track may be associated with a strong North Atlantic Oscillation[131] while the Neoglacial cooling is associated with a southward shift.
[117] These patterns (northward shift with warming) has been observed as a consequence of human-induced global warming and the end of the Little Ice Age[159] but also after volcanic eruptions (southward shift with cooling);[161] some volcanic eruptions have been linked to decreased hurricane activity, although this observation is not universal.
[164] During the 1350 to present interval in the Little Ice Age, there were more but weaker storms in the Gulf of Mexico[165] while hurricane activity did not decrease in western Long Island.
[124] Colder waters may have impeded tropical cyclone activity in the Gulf of Mexico during the Little Ice Age.
[167] The Little Ice Age may have been accompanied by more but weaker storms in the South China Sea relative to preceding or following periods,[168][169] leading to increased ship loss rates.
[174] A correlation between hurricane strikes and subsequent wildfire activity[175] and vegetation changes has been noted in the Alabamian[176] and Cuban paleotempestological record.
[179] On the other hand, the Classic Maya collapse may or may not coincide with, and have been caused by, a decrease in tropical cyclone activity.
[182] Paleotempestology has found evidence that the Kamikaze typhoons that impeded the Mongol invasions of Japan did, in fact, exist.
[187] Paleotempestological reconstructions are subject to a number of limitations,[26] including the presence of sites suited for the obtainment of paleotempestological records,[21] changes in the hydrological properties of the site due to e.g. sea level rise[26] which increases the sensitivity to weaker storms[188] and "false positives" caused by for example non-tropical cyclone-related floods, sediment winnowing, wind-driven transport, tides, tsunamis,[26] bioturbation[19] and non-tropical storms such as nor'easters[189] or winter storm, the latter of which however usually result in lower surges.
[194] Not all of the world has been investigated with paleotempestological methods; among the places thus researched are Belize, the Carolinas of North America, northern coasts of the Gulf of Mexico, the northeastern United States,[21] (in a lesser measure) the South Pacific islands and tropical Australia.
[6] In addition, as of 2017[update] there has been little effort in making comprehensive databases of paleotempestological data or in attempting regional reconstructions from local results.