An almost dry Po river runs through the city of Turin in the summer of 2022. Credit: Mauro Ujetto/NurPhoto via Getty Images.
The drought that hit northern Italy in 2022 was unprecedented in more than two centuries, and is part of a long-term trend of more frequent and severe drought in the area.
A team led by Alberto Montanari, from the University of Bologna, published a study in Science Advances1, analysing historical data about the Po River, the longest watercourse in Italy, that for the first seven months of 2022 reached a critically low level, affecting the entire valley and forcing the government to declare a state of emergency.
It is difficult to understand whether hydrological data already show a trend in drought severity that can be linked to climate change. This is due to the limited availability of historical data, and to the fact that many variables influence the flow of rivers, including changes in land use and water withdrawal by the population.
Montanari and his team relied on a time series of river flows spanning from 1807 to 2022, reconstructed by applying statistical analysis to water level measurements taken each year at Pontelagoscuro, near the Po basin outlet. They went through this 216-year long sequence and calculated annual mean river flows for four different time windows in spring and summer, searching for changes in seasonal patterns as a possible driver of the 2022 event. They also studied trends in rainfall, snow precipitation, evaporation, water abstractions, and irrigation to search for possible causes.
They concluded that the 2022 Po drought was by far the worst of the last two centuries, with mean river flow 30% lower than the second worst, and that such an event would occur on average every 600 years. “It means that it was a very rare event, but still a normal one,” says Montanari. “But it is not normal that 6 out of the 10 worst droughts since 1807 have occurred after 2000.”
The 2022 event, in other words, was not an isolated event, but part of a declining trend in river flow. Changes in seasonality, declining amounts of snow, and increasing water evaporation from the soil are the main causes, according to the study. “When we started our analysis we expected to find a flow decline in any season,” says Montanari. “We were surprised to see that the decline in summer flows was compensated by an increase in spring flows.” This is due to earlier snowmelt and reduced snowfall, a clear change induced by warming according to the scientists.
On the other hand, the study did not find a significant correlation with changes in precipitation and water withdrawal, but the actual impact of water withdrawals is unclear due to a lack of long-term data and changes in agricultural practices. "We can conclude with high confidence that global warming is a trigger of the 2022 Po River drought” says Montanari. “We cannot exclude that other factors may have played a role.”
