This post is as much prompted by some reactions to the recent UK floodings as similar points made after last summer’s floodings in Germany. I am only considering one kind of argument: There have been higher readings in the past.
Yes, that is likely true.
Let’s only consider readings at the same station. Then I have a problem with comparisons of readings which are in different seasons. It am not sure about the value of comparing a summer flooding with a February flooding. I guess it’s even questionable to compare a November flooding with one in February. Even if we compare only events in February we have to consider their causes. A rainfall related flooding is different from a snow-melt flooding or a flooding with both factors contributing.
So: Yes there have likely been higher water levels in the past. Maybe they have even been similarly caused. But I would only compare cases with similar origins.
OK, let’s add a second point. Claiming that recent precipitation was extreme also tells only part of the story. We expect extremes to occur. More interesting is whether the return periods for certain extremes change. Furthermore, an extreme can be many things. Was the daily, weekly, monthly, annual precipitation extreme? Is the precipitation amount over the last 24 months extreme? Extremes on each of these time-scales may have serious impacts. They may even be dependent. However, it is good to distinguish them similarly to the different definitions of drought. Furthermore, we should consider the occurrence of extremes spatially (compare this twitter-conversation).
End of minor quibble.