In a paper in Nature, Blöschl et al.1 capitalize on a vast assembly of written historical observations to provide a history of flooding for 103 major European river reaches between ad 1500 and 2016. In doing so, they reveal nine flood-rich periods that affected extensive regions in distinct areas of Europe — and find that the most recent of these periods, which might not yet be over, differs in key respects from the others.
Some 0.03% of the European population, on average, are thought to have been affected by flooding annually between 1870 and 2016, at a yearly average cost of 0.8–0.9% of gross domestic product2. Increased flood hazards are widely expected in the future for a substantial area of Europe as a result of climate change2, and so, without effective management and adaptation, these losses will potentially be even greater.
Such measures must be based on the best available knowledge, and require an understanding of long-term flooding patterns. Decision makers must know whether they are living in a flood-rich period (more-frequent flooding, of higher magnitudes or greater extent than usual) or a flood-poor one (fewer floods, with lower magnitudes or less-than-usual extent). Extreme floods in any one river basin in a given year are inherently rare, but the risk is cumulatively higher across large regions such as Europe — so the longer and more spatially extensive these flood histories are, the better.
Fortunately, Europe hosts some of the most abundant and diverse historical documentary sources for any world region, ranging from annals and chronicles to administrative and legal records, correspondence and newspapers (Fig. 1). These sources are replete with observations of extreme weather and hazards such as flooding, given their often severe human impacts, spectacular effects and religious significance as portents or vehicles of divine retribution3. For example, the Gaelic Irish Annals of Connacht for ad 1471 reports4: “Showers of hail fell [on] each side of Beltaine [1st May], with lightning and thunder, destroying much blossom and beans and fruits in all parts of Ireland where they fell. One of these showers, in the east, had stones two or three inches long, which made large wounds on the people they struck … There was another … at the monastery of Boyle; and a boat could have floated over the floor of the great church of the monks, as we have heard from the folk of that place.”
This account highlights the strengths of documentary evidence: it is precisely dated, highly spatially resolved, generally unambiguous about the meteorological conditions involved and explicit regarding human impacts. Such evidence is used as the basis of historical climatology, a field whose origins date back to at least the 1920s5, and which accelerated in the 1960s and 1970s, thanks to pioneers such as Hubert Lamb and Hermann Flohn. Their work furthered the growing recognition that societally meaningful changes in climate had occurred throughout the past few centuries, eroding the idea that the long-term climate was more or less constant during that period6.
Yet despite continued development of the field, historical climatologist Christian Pfister and climate scientist Heinz Wanner remarked7 in 2002 that “many scientists are of the opinion that observations made before the instrumental period are “subjective” and less reliable than natural proxies …”. They went on to argue that “once calibrated and verified … the data are precise and have a spatiotemporal resolution unmatched by any other climate proxy”. Since then, historical climatologists have continued to identify evidence, develop methods to assess its credibility and quantify it to reconstruct past climates8–10. With their compilation of 9,576 floods, Blöschl et al. have built on this foundation to deliver a major contribution to our understanding of European flood history.
Patterns of past precipitation are inherently more spatially variable, and hence harder to reconstruct, than are those of temperature. Reconstructing flooding is even more challenging, because flooding depends not only on precipitation, but also on human landscape usage, from upstream deforestation to damming, bridging and urbanization. It is also related to prevailing temperatures, which have a seasonally and regionally varying role across Europe, influencing evaporation and soil moisture and the timing of spring snowmelt11. Importantly, therefore, Blöschl and colleagues establish an association in timing between all but one flood-rich period and the prevalence of lower-than-usual average temperatures. By contrast, the most recent flood-rich period in Europe (1990 to 2016, when the available data end) — for a region stretching into western and central Europe and northern Italy, and defined by the authors as being one of the most severe — is exceptional for occurring in a warming climate.
Just one highlight of this work is the care taken to control for biases arising from variability in the type and abundance of sources through space and time, a persistent challenge often acknowledged but infrequently addressed3. A further standout feature is the authors’ 3D visualization of the magnitude, duration and geography of each flood-rich period (see Fig. 1 of the paper1 and Supplementary Video 1). This invites a consideration of the internal and external climate processes that potentially drove these periods, and how their regional expression was manifested in, or mediated by, the behaviour of major modes of ocean and atmospheric circulation.
One key suspect noted by Blöschl et al. is the North Atlantic Oscillation (NAO), a fluctuation in the atmospheric pressure gradient between Iceland and the Azores that governs the strength and positioning of moisture-laden winter westerlies (and accompanying storm tracks) over Europe. Depending on the strength of this gradient, the westerlies either bring relatively warm wind and rain to northern and northwestern Europe, leaving southern Europe dry and cool, or flow towards southern Europe, leaving northern and northwestern Europe to experience incursions of cold and dry Arctic air.
But ambiguities remain regarding the mechanisms at play. With the dominant flood season differing by region (winter for northern and northwestern Europe, but summer for central Europe, for example), the wintertime NAO can explain only part of this story. The related role of prevailing temperatures in the observed flood-rich periods also remains an open question. Earth-system modelling, twinned with process-based hydrological models (as the authors note) could be used to capture the complexity of natural and human processes on the ground that enhance or suppress flooding. It could also provide further insight into the extent to which the association of cooler-than-usual conditions with most flood-rich periods is causative or correlative, and how much the most recent flood-rich period is a result of human-induced climate warming.
Blöschl et al. clearly demonstrate the potential of historical climatology, but there are yet more avenues by which to advance this approach to reconstruct past climate. Written evidence of climate can become particularly discontinuous in the deep past, but tree-ring evidence, for example (measurements of tree-ring widths, densities and isotopic composition, which correlate with climate conditions), is more continuous over long periods. However, tree-ring evidence usually reflects only the growing-season climate, whereas written evidence can attest to weather for all seasons. If both sources were more regularly used in a complementary manner12, it would improve our understanding of the climatic signals preserved in each source, and help to resolve perceived conflicts — for instance, the great European drought of ad 1540 is readily apparent in written records, but more ambiguously represented in certain tree-ring evidence13.
Further written sources remain to be discovered, including documents known as weather diaries, which are highly prized in historical climatology for their standardized systematic recording of weather conditions. These can be found not only in the most recent centuries, but sometimes much earlier — as with the incredible Babylonian ‘astronomical diaries’14, which boast systematic daily observations of weather during the first seven centuries bc, and which remain largely unexploited. As Pfister and Wanner stated7 in 2002, “Worldwide, many thousand volumes with daily observations exist, but have not yet been analyzed for their climatic information. Let’s get to work!”