Paleotempestology

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 Kerry Emanuel. 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 Kerry Emanuel. Paleotempestology usually tries to identify leftover deposits from past storms, such as overwash deposits in waterbodies close to the coast which is the most commonly applied techniques, oxygen isotope ratio variations caused by tropical cyclone rainfall in trees or speleothems and beach ridges kicked up by storm waves. From these deposits one can then infer the occurrence rate of tropical cyclones - typically the stronger events are the most easily recognizable ones - and sometimes also their intensity, by comparing them to deposits left by historical events. While the findings are prone to confounding factors and only some parts of the world have been investigated, some important findings have been made with the help of paleotempestology. For example, in the Gulf Coast 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 e.g. by shifts in their paths. 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. Tropical cyclones - known depending on location as cyclones, hurricanes or typhoons - with their precipitation, storm surges and winds are highly destructive and deadly phenomena; the 1900 Galveston hurricane claimed over 8,000 fatalities and was the worst natural disaster in United States history, while Hurricane Katrina in 2005 became the costliest hurricane in United States history with over 80 billion dollars damage and over 1600 fatalities. In other parts of the world, a 1970 cyclone killed 300,000 in Bangladesh; Japan in 2004 was hit by 10 typhoons and in 2005 five separate cyclones hit the Cook Islands in a short timeframe; a year later records were broken by Typhoon Saomai in China and Cyclone Larry in Australia. Finally, in 2013 Typhoon Haiyan became one of the most intense tropical cyclones ever recorded and caused 6,000 fatalities in the Philippines. Further, increased coastal development in general and in the United States rapid population growth along hurricane-prone coasts is creating additional attention to the danger posed by tropical cyclones and the interest in the hazard existing for major cities like Miami and New Orleans. Tropical cyclones can also have positive effects on society, for example by bringing rain to drought-prone regions. Finally, there is increasing evidence that tropical cyclone influence the climate themselves by enhancing poleward heat transport. 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 in the Saffir-Simpson scale - have come ashore, making it difficult to estimate the hazard level, and elsewhere the record often goes back less than half a century. Such records may also not be representative for future climates. The realization that one cannot rely solely on historical records to infer past storm activity was a major driving force for the development of paleotempestology. 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. In general, the origin and behaviour of tropical cyclone systems is poorly understood and there is concern that man-made global warming will increase the intensity of tropical cyclones and the frequency of strong events by increasing sea surface temperatures. 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 in atolls, coastal lakes, marshes or reef flats 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.