In the short time span since 2009 to the present day, there have been more than 10 extreme events in Italy, with over 70 dead and with damage to infrastructures amounting to hundreds of millions of euro. In the future, owing to the effects of climatic changes, we should expect an increase in their frequency and the intensity of these events which can no longer be referred to as "extreme" if the aforementioned is taken to signify "abnormal".
What does science understand by "probability"? We are used to using this term in everyday speech when assessing the possibility of something happening as part of daily life, however we only demand certainty from science when resolving our issues, hence dismissing ourselves from making choices.

Uncertainty and probability

If we are speaking abut meteorological events, natural risks and climatic estimations, we must quickly learn that uncertainty and probability go hand-in-hand, even if they may appear as unpleasant and foreign concepts to the majority of people. Is false certainty or probable information which may become an effective support to decide on the actions to adopt, on a personal level and as a community, in order to provide advance notice of risks more beneficial?
A variable measure of uncertainty characterizes all actions of the warning system, the network structure (national and local) used to oversee, monitor and manage adverse events. The meteorological measure is used to activate the warning chain operation overseeing extreme weather events which is able to identify, using extremely variable temporal and spacial elements, dependent on the event, the approach of an intense or violent atmospheric phenomenon which may be disastrous across the country and within natural environments with strong social presence.

The state of the art of meteorology (a reminder that it is a science) indicates, that it is not yet possible, and perhaps never will be, to accurately foresee the details of convective systems (for example, the V-shape systems responsible for many cloud-bursts) which increasingly more often, are able to threaten our cities. Although it is possible to foresee the conditions which are likely to motivate these convective systems a few days in advance, the time and intensity of the phenomenon remain uncertain and the "prognosis" only appears once the event is underway.

Dangerous percentages

Uncertainty is intrinsic to atmospheric dynamics, which can be described more or less accurately using meteorological methods, and is not possible to reset even with a perfect model, owing to the fact that the initial conditions underpinning the forecast are inevitably filled with errors, at the very least, of scale, which, with time grow more significant, affecting the forecast results. The forecasts will be more or less reliable, sometimes close to 100% reliable (unfortunately, this is only the case in few circumstances) and sometimes close to the other extreme, which are close to 10% reliable. However if they are not clearly defined beforehand, firstly amongst these same previsors and then consumers, what does the percentage relative to the forecast actually mean; the addition of these numbers does not help transform a "useless" deterministic forecast (event will occur/event will not occur) into a "useful" probable forecast. If a percentage is not usable, or it does not help evaluate uncertainty, even where it is simply not understood by the user, it may become more misleading then false certainty, just like all the ambiguous messages which leave lots of room for interpretation and subjective deformation.

A language understood by the public

There are two aspects to usability which require attention:

• what type of probable information is really required by users, with the distinction made between the general public and political decision-makers;
• how clear and unequivocal are the probable messages expressed.

The first point definitely calls into question social research, direct communication with those who are to receive and manage the information. In America, where there is no snobbery between the different sciences and between science and society, projects have already been developed for “the strategic implementation governing the production and communication of provisional, uncertain information”, which involves subjects from different business spheres, meteorologists, hydrologists, economists, sociologists, communication experts. This cross-sectional approach seems to be well suited in terms of resolving critical standstills between science and society, as it favors dialog and reciprocal understanding.
Defining clarity of a probable message is not completely detached from the first point, as evidently it also involves the de-codification process on behalf of the recipient. It does however require reflection on, primarily, the code and language, not just verbal, used to communicate. One solution proposed is a scale which associates the probability of it occurring to each term (e.g. IPPC Likelihood scale), enabling simple conversion. The same idea is valid for the graphical representation of a forecast, which should provide accurate and not ambiguous instructions in the form of symbols, numbers and colors.

A culture where the risk is shared

Let's pretend that the “probability system” has worked well up to this point, we must then ask ourselves what those receiving and able to correctly interpret this uncertain information, can do. If we are talking about a probable extreme event, we must try and work out what each person must do after having realistically evaluated, according to valid scientific methods, the risk which is to manifest itself. The subsequent stage unravels in the real world of people, assets and infrastructures, theoretically exemplary scientific concepts. What does the Mayor of Genoa (to quote an example) do when aware of a 30% probability of 300 mm of rain falling in just a few hours time within an unclearly defined area of the city? What does Mr. Bianchi do with the same information? It is clear that the same forecast will means different things to the two different people when we consider the different cost-advantages ratio associated with the possible contrasting actions.
They both, with different responsibilities, need to make decisions which will minimize the risk. This signifies that they need to know which actions to carry out, that they must be in a position to select one amongst the various hypothetical scenarios and be prepared to make these choices. This all falls under the concept of preventive preparation and the “risk culture” during peace time, when the normal situation enables the system to take on board and digest the educational stage.
It is during this dormant risk time, under normal circumstances, that we must work to transfer all the bearing of uncertainty related to extreme events, which begins with forecasts of uncertainty, which come into being, as strange as it may seem, and which always forms a part of daily life. We must work on making quantification of the uncertainty relevant and useful, as outrage is always displayed owing to the inaccuracy of science or dishonesty, simply due to lack of understanding. We must therefore look to change the language, by replacing probability with a frequency of occurrence, which is perhaps a more accessible concept (the forecast cloud-burst will occur 6 times out of 10); replacing numbers where possible, with more concrete images, comparing the forecast with something which has already been observed and tested.

When all is said done, the will of scientific to make itself understood, media will to pass on honest information, political and academic will to develop educational and training programs and the will of people to actually want to understand and not insist on and look for certainties which do not exist.

by Carlo Cacciamani and Alessandra DeSavino
ARPA SIMC (Hydro-Meteo-Climate. Service) - Emilia Romagna