There’s an unwritten rule in climate change reporting that you can point to any bad weather and claim it’s getting worse without bothering to cite any data. A frequent choice in this regard is extreme rainfall, because one of the simplistic truisms of climate science is that warmer air can hold more water ergo we should find extreme rainfall events are getting worse. Let a downpour occur and alarmists say “Gotcha!” Even though in the places where good data go back at least a century, such as Canada and the U.S., there is none of that pesky actual evidence for increasing maximum rainfall events. And now we can add Italy to the list of places where the evidence doesn’t suggest an increasing trend. It used to. But unfortunately the data on which that trend was built, from early in the century, wasn’t very good because the instruments weren’t very precise. When a group of scientists recently put together a more accurate data set, they found the upward trend was washed away completely. We blame climate change, obviously.
Rainfall measurement requires a precise instrument that monitors the rain continuously and gets properly reset each day. And since heavy rain events may only last a few minutes, the instrument needs to be running continuously. Thus older data only show a daily total, and it’s also far from clear how carefully it was reset and measured, whereas modern rainfall instruments record rainfall meticulously and minute-by-minute. Because extreme rainfall can come in short bursts, a modern data set will show higher variability and higher highs than an older data set which only yields an uncertain average over 24 hours.
The authors of the new Italian study took an imaginative approach to interpreting older data. They took recent measurements using both today’s precise instruments and yesterday’s blunter ones. Then they looked at what the instruments of yesteryear found early in the 20th century in the province of Umbria, Italy, and calculated what today’s would have found assuming the same differences would have turned up then as did later. The result was a slight increase in maximum rainfall rates early in the record, and the corrected data showed that no upward trend could be observed.
The authors noted that they were only looking at one small region of the world, and they couldn’t assume that the same change would occur in other places. But they do plan to apply their method to other locations. And maybe they’ll find that claims of increased rates of extreme rainfall were artifacts of changes in rainfall measurement methods. The only prediction we’re prepared to make, however, is that such evidence won’t stop alarmists from claiming otherwise, since their claims were never based on evidence to begin with.