Short, heavy rainfall is typical of the Mediterranean, but like many climate extremes in recent years, including the current fires in Los Angeles, nothing is typical about what has happened there recently.
In the fall, deadly floods devastated an arc stretching from Spain to the Balkans and from Morocco to Libya. More than 200 people were killed in Valencia in October, shortly after a deluge poured five times the month’s ordinary precipitation across Europe in a single week.
Scientists say climate change is increasing not only the strength of devastating Mediterranean storms, but also their frequency – and they predict the situation will only get worse.
Coastal areas of the Mediterranean basin have always been prone to extreme precipitation, especially in places with mountains close to the sea.
But it got worse. More rain now falls during extreme precipitation events than a few decades ago.
In some regions, disasters are starting to become the new normal.
Sources: Euro-Mediterranean Center on Climate Change and International Disaster Database
Note: The map shows daily accumulated precipitation for the 99th percentile (top 1 percent) of wet days recorded for the period.
The intensity of these extreme precipitation events is likely to increase in coming decades, said Leone Cavicchia, a scientist at the Euro-Mediterranean Center on Climate Change.
This is partly because the Mediterranean region is already warming 20 percent faster than the world average. And as air temperature increases, so does its ability to hold water.
Source: CMIP6 data via C3S/ECMWF
The projections presented are from an intermediate emissions scenario and reflect changes from 1850-1900.
Climate models suggest that even if heavy precipitation in the Mediterranean region intensifies, average precipitation will decrease. In other words, dry areas will be drier, but when extreme rains arrive, they will be more intense.
A geography tailor-made for flash floods
The mountains, enclosed sea and dry river beds around the Mediterranean Sea make the region particularly prone to flash floods.
Most rivers in the region are quite dry for long periods of the year. When heavy rains arrive, water quickly concentrates in steep river beds and can rise several meters in just a few hours, said Francesco Dottori, associate professor of hydrology at the Pavia University School of Advanced Studies , in Italy.
The Mediterranean Sea is warming more quickly than other bodies of water, in part because it is a virtually enclosed sea. This makes it a powerful source of moisture that winds can carry inland, fueling precipitation systems, often over coastal areas in which much of the Mediterranean population is concentrated.
The strong atmospheric currents of the polar jet stream also play a role in the region’s weather. When the currents oscillate, they form north-south waves whose crests send warm air north and whose troughs send cold air south.
Sometimes when part of the jet stream breaks away, it forms a low pressure system called a cutoff depression. This can persist for days, causing instability when it encounters warmer Mediterranean air.
This is what happened in September, when Storm Boris originated from such a low pressure system and then wreaked havoc in central and southern Europe, where it killed at least 24 people. It was another depression which caused floods in Valencia shortly after, where hundreds of people died. And last year, a low over Greece triggered Storm Daniel, which strengthened as it crossed the Mediterranean to Libya, killing 13,200 people after two dams failed.
Population growth means even more people are at risk
Sources: European Commission, Joint Research Center (JRC)
In recent decades, most coastal areas and floodplains 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 being affected due to urban development and population growth, said Dottori, who helped develop the European Flood Awareness System.
The population of Mediterranean countries has more than doubled since the 1960s. Today, around 250 million people in Mediterranean countries live in river basins, where flooding is more likely.
