A new paper in Nature Scientific Reports presents the longest reconstruction yet of damaging hydrological events for Italy and the larger Mediterranean region, which will help scientists understand if recent increases in precipitation extremes are unprecedented in a long-term context, and help facilitate a better knowledge of the complex nature of floods and their management.

The study reveals that increases and decreases in the frequency and intensity of damaging hydrological events, such as flooding, extreme snowfall, and drought, have always occurred at regional scale, including the central Mediterranean basin, since 800 CE.

This research also indicates that the trend of more intense Venitian tides appears to be driven by the ground subsidence associated with human activities, rather than from increasing storms (Zavatti, 2019).

According to the study’s author, Nazzareno Diodato, a Fellow Geoscientist with the Global Land Programme (a Future Earth global research project), this study was motivated by the lack of a millennium-long, country-wise analysis of hydrological events, and the access to a rich documentary heritage offered a unique opportunity for such an historical reconstruction of the Italian climate.

To compile this reconstruction, the authors created a storm severity index based on information in historical documents recorded between 800 and 2017, during three main climatic periods: Medieval Warming Period, the Little Ice Age, and Modern Warming.

Documentary data, such as diaries, were used to provide high-resolution information. Diaries are a fantastic source not only for historical weather, but of social, cultural, and economic information from a period in time – but understanding it in context required an interdisciplinary approach, between historians, geographers, and climatologists.

People throughout this historical period noticed and recorded significant flood disasters, regardless of their cause, including “reporters, poets, magistrates of government, as well as religious authorities,” according to the study.

The authors researched Italian archival documents and old books, finding about 1,000 entries which included key words such as rainfall, storm, and flood. They narrowed these entries down to 150 literary works that were most relevant to the region and period of interest.

Surviving historical records increase during the medieval period, and instrumental measurements only occur during and after the late 17^th century, yet the study looked much farther back than these centuries as well. Weather hindcasting was then applied to well-documented anomalies during instrumental periods, to confirm the occurrence and severity of events during pre-instrumental periods.

The text from these sources was made readable by software, and in addition to its historical period, categorized in one of five levels of intensity and by one of seven regions. This way, storms could also be contextualized with other historical events, and how it affected them.

A first key reference is the Opus Chronologicum (Calvisius, 1650), of the German chronologist and astronomer Sethus Calvisius (1556–1615), written in Latin, reporting a detailed and extended list of natural hazards and extreme climatic events since antiquity. Later, father Secondo Lancellotti (1583–1643) published an important book in Italian in Venice (Lancellotti, 1680), listing hydrological events with the year when they occurred.

Other records referring to medieval chronicles capturing climatic information are mainly found in the Annals of Di Meo (1795), which report a wide variety of damaging hydrological events for southern Italy. Among others, literary sources from the Italian Middle Ages include various Medieval Chronicons, such as the Cavense, published in 1753 by Francesco Maria Pratilli (1689–1763), and the Chronica of Salimbene Parmensis (Cantarelli, 1882).

An important and extended documentary source is also the Monumenta Germaniae Historica, published by Georg Heinrich Pertz (1795–1876) and available in different editions (1826, 1844, 1859 and 1866). It includes a full set of carefully prepared sources covering a period ranging from the fall of the Western Roman Empire (476 CE) to the 16th century. Other specific information on abnormal rainfall and floods in Italy can be found in the Annali of Geografia Fisica (Lichtenthal, 1854).