Short, heavy rainfall is typical of the Mediterranean, but like many climate extremes in recent years, including the current fires in Los Angeles, there’s nothing typical about what’s happening there recently.
In autumn, deadly floods wreaked havoc in ports from Spain to the Balkans and from Morocco to Libya. More than 200 people were killed in Valencia in October, shortly after the flood five times the usual monthly amount of rainfall in the whole of Europe in one week.
Scientists say climate change is increasing not only the strength of devastating storms in the Mediterranean, but also their frequency – and they predict it will get worse.
The coastal areas of the Mediterranean basin have always been prone to extreme rainfall, especially in places where mountains are close to the sea.
But it got worse. More rain now falls during extreme precipitation events than just a few decades ago.
In some areas, disaster is beginning to feel like the new normal.
Sources: Euro-Mediterranean Climate Change Center and International Disaster Database
Note: Map shows daily accumulated precipitation for the 99th percentile (top 1 percent) of recorded rainy days for the time period.
The intensity of these extreme rainfall events is likely to increase in the coming decades, said Leone Cavicchia, a scientist at the Euro-Mediterranean Center for Climate Change.
This is partly because the Mediterranean area is already warming 20 percent faster than the global average. And as the temperature of the air rises, so does its ability to hold water.
Source: CMIP6 data via C3S/ECMWF
The projections shown are from the medium emissions scenario and reflect changes compared to 1850-1900.
Climate models suggest that even as heavy rains intensify in the Mediterranean region, average rainfall will decrease. In other words, dry areas will be drier, but when extreme rains come, they will be more intense.
Geography tailored for flash floods
The mountains, closed sea and dry riverbeds around the Mediterranean Sea make this area particularly vulnerable to flash floods.
Most waterways in the region are quite dry for long periods of the year. When heavy rains come, water quickly concentrates in steep riverbeds and can rise several meters in just a few hours, said Francesco Dottori, an associate professor of hydrology at the University’s School of Advanced Studies in Pavia, Italy.
The Mediterranean Sea warms faster than other bodies of water, partly because it is practically a closed sea. This makes it a powerful source of moisture that winds can carry inland, feeding rain systems, often over coastal areas where most of the Mediterranean population is concentrated.
The strong atmospheric currents of the polar jet stream also play a role in the region’s weather. As the currents oscillate, they create waves in a north-south direction whose crests send warm air northward and whose troughs send cold air southward.
Sometimes, when part of the jet stream separates, it forms a low pressure system known as a low pressure boundary. This can last for days, causing instability when it encounters warmer Mediterranean air.
This happened in September, when Storm Boris formed as one such low-pressure system and continued to wreak havoc across central and southern Europe, killing at least 24 people. It was another record low that soon after caused flooding in Valencia, where hundreds died. And last year, a border low over Greece triggered Storm Daniel, which strengthened as it crossed the Mediterranean into Libya, killing 13,200 after two dams burst.
A growing population means even more people are at risk
Sources: European Commission, Joint Research Center (JRC)
Over recent decades, most coastal and floodplain areas in the Mediterranean region have also become densely urbanized, leaving little space for waterways. These changes not only increase the risk of flooding – they also put more people at risk.
Floods have generally become less deadly thanks to improvements in flood protection structures and early warning systems. But more homes and properties are affected because of urban development and population growth, said Mr Dottori, who helped develop the European Flood Awareness System.
The population of Mediterranean countries has more than doubled since the 1960s. Today, about 250 million people in Mediterranean countries live in river basins, where floods are more likely.
