Dr Moses Amweelo
Climate change made the storm that devastated the Libya city of Derna, killing thousands of people, up to 50 times more likely, experts say.
Up to 50% more rain had fallen as a result of human-caused greenhouse gas emissions, climate scientists at the World Weather Attribution (WWA) group found.
Years of conflict in the region compounded the vulnerability of people to flooding, the World Weather Attribution report says.
It turned the extreme weather into a full-scale humanitarian disaster.
The scientists used computer simulations to assess how much more likely such a storm was now, compared with before the 1.1 degree Celsius of warming climate change has already brought. But they cautioned a lack of data, particularly in Libya, meant considerable uncertainties in their findings. Storm Daniel, which brought the deadly rains, had already dumped 910 mm of rain on parts of Greece, Bulgaria and Turkey, killing 28 people. The study found climate change had made this up to 10 times more likely, bringing up to 40% more rain. Storms of this intensity are now reasonably common for this region and can be expected once every 10 years, it warns. But the weather event in northern Libya was much more exceptional, bringing a storm of an intensity, such as would be expected to hit northern Libya once every 300-600 years. As Storm Daniel travelled slowly over the Mediterranean, it drew additional energy from sea temperatures two to three degrees above the September average.
“Storm Daniel was a low pressure (weather system), as we usually have in the Mediterranean,” Kostal Lagouvardos from the National Observatory of Athens says.
“It was not very deep – but it was very early in the season – and it was stagnant and stayed over the south lonian Sea for four/five days.”
That extra warmth fueled stronger winds and meant the air could hold more moisture.
When it hit the northern coast of Libya, it dumped an estimated 400 mm of rain on Derna in just 24 hours.
The average for September in the city is just 1.5 mm, according to Nasa’s Earth Observatory.
The Derna flooding death toll could reach 20 000, according to the city’s mayor. Entire neighbourhoods disappeared into the sea as a huge tsunami-like torrent of water swept the port city in eastern Libya.
Survivors described the situation as ‘’beyond catastrophic”.
Prof Liz Stephens, an expert in climate risks and resilience at Reading University in the UK, says scientists are confident that climate change is supercharging the rainfall associated with such storms.
The Wadi Derna River runs from Libya’s inland mountains through the city of Derna and into the Mediterranean. It is dry for much of the year, but the unusually heavy rain overwhelmed two crucial dams and destroyed several bridges.
Residents of the city, who had been ordered by the local authorities to stay in their homes, reported hearing a loud blast before the city was engulfed in water. The dams would have held back the water initially, with their failure potentially releasing all the water in one go. “The debris caught up in the floodwaters would have added to the destructive power,” says Prof. Stephens.
The upper dam had a storage capacity of 1.5 million cubic metres of water, whilst the lower dam could hold 22.5 million cubic metres.
Each cubic metre of water weighs about one tonne (1 000 kg), so 1.5 million cubic metres of water would weigh 1.5 million tonnes.
Combine that weight with moving downhill, and it can produce enormous power. Witnesses have said the waters were nearly three metres in places. It is estimated that six inches (20cm) of fast-moving flood water is enough to knock someone off their feet, and 2 feet (60cm) is enough to float a car. So, it is no surprise that whole buildings were taken out in the flood. Experts say it is too early to know whether the extreme rainfall was simply too much for the dams to handle, or whether the condition of the structures also played a role.
Based on their observations, the dams are likely to be made from dumped and compacted soil or rocks, which are not as strong as concrete.
“These dams are susceptible to overtopping (when water exceeds a dam’s capacity), while concrete dams can survive overtopping, rockfill dams usually cannot,” says Exeter University’s Prof. Dragan Savic, an expert in hydraulic engineering in the UK. It appears that the upper dam failed first, according to structural engineer Andrew Barr.
He says the water then probably flowed down the rocky river valley towards the lower dam before overwhelming it, resulting in the sudden and catastrophic flooding of the city, which lies trapped between mountains and the sea.
A research paper, published last year, on the hydrology of the Wadi Derna Basin, highlighted that the area “has a high potential for flood risk’’, on the basis of likely historical flood volumes, and that the dams “needed periodic maintenance”.
The report, by civil engineering expert Abdelwanees AR Ashoor from Libya’s University of Omar Al-Mukhtar, said “the current situation in the Derna valley basin requires officials to take immediate measures, carrying out regular maintenance of the existing dams – because in the event of a huge flood, the result will be disastrous for the residents of the valley and the city’’.
Instead, the scientists had to rely on data, based on satellite readings.
However, they are confident climate change played a significant role because there is strong evidence that higher temperatures lead to heavier rainfall, and other studies have shown climate change increases the intensity of weather systems, such as Storm Daniel. This year’s extreme weather events have shown – from heatwaves in Europe to extreme rainfall in Libya – the consequences of climate change increase with every fraction of a degree of warming.
* Dr Moses Amweelo is a former Minister of Works, Transport and Communication. He earned a doctorate in Technical Science, Industrial Engineering and Management from the International Transport Academy (St Petersburg, Russia).