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Flash floods that ravaged southern and eastern Spain have underscored the lethal threat from rising temperatures that are turning the Mediterranean Sea into a “petrol can”, experts say.
Torrential rain centred on the Valencia region killed at least 158 people this week in one of the worst natural disasters in Spain’s modern history, leaving rescue workers searching for the missing in shell-shocked towns strewn with rubble and upturned cars on Thursday.
The extraordinary downpours were driven in part by higher temperatures in the Mediterranean, a largely enclosed sea whose warmth is a store of energy that can only be released via evaporation, creating the conditions for intense storms.
Francisco Martín León, a meteorologist at global weather network Meteored, said the Mediterranean was acting as a “petrol can” by feeding water vapour — the fuel for rainfall — into the atmosphere. Scientists say that process has been exacerbated by climate change.
“Then the fire that ignites it for us is a cold front or area of low pressure” that crashes into that humidity, as occurred this week, he said. In Spain the phenomenon is known as DANA, an acronym for a high-altitude isolated depression.
Climate change driven by the burning of fossil fuels, and rising greenhouse gas emissions as a result, has put the world on course for the hottest year on record.
Sea surface temperatures globally warmed to 20.83C in September, a near-record for the month. But the increase in the Mediterranean Sea has been especially pronounced: temperatures reached some of the highest daily values on record over the summer and peaked at 28.45C on August 13.
Because the Mediterranean is far smaller and more enclosed than an ocean, with the only exit to the Atlantic through the narrow Strait of Gibraltar, it is “like heating a cup of tea versus a bathtub”, said Michael Byrne, a climate scientist at the University of St Andrews.
It is such an efficient store of heat that temperatures edge down only slowly into autumn.
Álvaro Rodríguez, Spain co-ordinator of the Climate Reality Project founded by former US vice-president Al Gore, said: “That hot sea full of energy is like a Tesla supercharger. It sends up water vapour at full blast.”
The Mediterranean, he noted, was increasingly behaving like the warmer Caribbean Sea. The warmer body of water helps to form hurricanes that can devastate the coasts of the US, Central and South America, as well as the region’s islands.
“The two seas have the same characteristics and we’re starting to see very similar phenomena occur in both of them for one simple reason,” Rodríguez said.
Europe is the world’s fastest warming continent, in part because of its proximity to the Arctic, where melting snow and ice which reflects the sun’s rays is giving way to the darker ground which absorbs heat.
Marilena Oltmanns, a scientist at the UK’s National Oceanography Centre, said the Mediterranean had warmed twice as fast as the global average for oceans in the past 30 years, based on data from the US’ National Oceanic and Atmospheric Administration.
Air holds 7 per cent more water vapour for every degree it warms, according to the laws of thermodynamics. This “leads directly to more rainfall, more flooding and more devastation of the type we’ve seen in Spain this week,” Byrne said.
Aemet, Spain’s state weather agency, said the storm was the most intense to hit the Valencia region since the 1980s.
The head of the Valencia regional government said he had asked the armed forces to assist with rescue operations and distribution of aid, while the mayor of the Valencian town of Alfafar said a lack of help meant some residents were “living with corpses at home”.
Spain began three days of mourning on Thursday.
Researchers are also exploring whether the hotter global climate may be causing storms to stay in one place for longer once they have formed.
Trevor Hoey, director of the centre for flood risk and resilience at the UK’s Brunel University, said some of the humidity sitting above the Mediterranean originated from further afield and that other meteorological factors had contributed to the exceptional downpour.
But he added: “In the last two or three months, every week there have been floods somewhere in different parts of the world. I’m detecting a greater certainty from scientists saying that climate change is a major contributor to these events — and things are only going to get worse.”
Data visualisation by Jana Tauschinski
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