Again another once-in-a-lifetime flood? - Exploring ways to communicate climate extremes

(Hydro-)meteorological extreme events account for a large portion of immediate exposure to and long-term impact by climate change for humans, ecosystems and infrastructure worldwide. These extremes, especially high-impact events, are often communicated by means of return periods, e.g., a 1-in-100 years flood, a 1-in-50 years drought. Frequency increases are already observed for a range of meteorological extremes, in parts also at temporal scales of merely a few decades (i.e., less than a human lifetime). Distributional shifts of atmospheric variables, e.g., temperature or precipitation, lead to further frequency increases of (current) extremes and the emergence of yet unseen, sometimes compound and cascading events. Therefore, one could think that the (decreasing) rareness of extreme events forms a good basis for illustrating the impacts of climate change among the broad public.

However, the common reaction is similar to the headline introduction: Again another once-in-a-lifetime flood? But we just had one last year … In other cases, the new regularity of extreme events often seems to cause habituation or fatalism rather than concern.

Apparently, there is a discrepancy between the scientific understanding of extremes on the one side and the public perception and personal experience on the other side. In this contribution, we try to illustrate this discrepancy and explore ways to approximate the different perceptions.

We start by showing how prominently return periods are used explicitly and implicitly in grasping extremes (both in science and everyday-talking). A clear scientific definition will follow and hint to potential sources of misunderstanding. We next present examples where return periods are used for communicating (hydro-)meteorological hazards to the general public: 1. teaching the meaning and understanding of meteorological data with a focus on extremes with the “Living with changing climate”-course of the Finnish Climate University initiative (https://climateuniversity.fi/portfolio-items/living_with_changing_climate/), which is targeting people from outside academia as well, and 2. results from the Bavarian-Québec ClimEx project (https://climex-project.org) formatted as a hydrometeorological atlas. Further, we discuss ideas of tailoring return periods to peoples’ life experience e.g. by referring to literature and using 80-years as the averaged life expectation.

We conclude by summarizing where we think that these examples succeed or struggle in clarifying the meaning and implications of extremes and provide an outlook on further ideas and narratives to bridge the gap between science and society.