Pluridisciplinary analysis and multi-archive reconstruction of paleofloods: Societal demand, challenges and progress

Abstract

Floods are one of the gravest natural hazards for societies, worsened by population growth, unchecked development, and climate change. From a Global Change perspective, past extreme events merit particular interest because they can be linked to wider climate and environmental changes, introduce perturbations. During the last decade, knowledge of long-term flood frequency and magnitude has been improved by extracting data from different types of archive. But, despite advances in dating methods, proxies and statistical techniques and efforts to identify atmospheric drivers, some fundamental questions remain unresolved. The Special Issue entitled “Pluridisciplinary analysis and multi-archive reconstruction of paleofloods” in the journal Global and Planetary Change addresses these uncertainties and complexities by assembling a selection of studies, which were first presented at the Past Climate Changes (PAGES) Open Scientific Meeting held at Zaragoza in 2017. In this introductory paper, the guest editors outline the 17 research contributions and meta-data from the 17 paleoflood studies were systematically analyzed in terms of i) geographical distribution; ii) methodologies applied; iii) types of archives; iii) numbers of flood series compiled and iv) spatial and temporal resolution of paleoflood data. The data indicate that paleoflood studies focused on fluvial depositional environments show a higher rate of integration with other types of paleoflood archive (mean of 4.5 types of archive) than studies focused on documentary sources (mean of 3.5) and lake sediments (mean of 2.4). We suggest that this strategy of archive integration has been adapted to effectively compensate for the higher uncertainties of fluvial deposition in floodplains. Statistical processing of the meta-data shows quantitative associations between specific types of flood archive and offers a solid platform for designing the optimal approach for multi-archive paleoflood research. A qualitative review and visual comparison of the 17 paleoflood series shows some consistent trends and breaks but also notable differences within and between regions. While a trend of increased flooding since 4-5 ka BP is evident, the lack of synchronicity between breaks and the coeval increases and decreases in fluvial activity is manifest. The majority of studies in the Special Issue do denote the 19th century – including the youngest cool climate pulses during the Little Ice Age – as a particularly flood-rich period. It is more difficult to assess the 20th century because of social changes, population growth and extensive river modification. Despite the mentioned uncertainties, 10 of 14 papers do not record the 20th century as an exceptional flood period. Assessing the effects of human impact on paleoflood calendars and disentangling anthropogenic from natural drivers are major challenges in integrated paleoflood analysis.

It is concluded that the interpretation of flood series is complex as landscapes and flood drivers are heterogeneous and systems show different sensitivities to flood control and drivers. Thus, the study of past floods, from historical and natural archives, is challenging but also offers unparalleled opportunities to document low-frequency, large-magnitude flood events, which occurred under a broad range of climate and/or environmental scenarios, and, probably, the only way to reconstruct robust paleoflood series.