A decade ago, Tunisian fisherman Ahmed Chelli’s nets bulged with fish and octopus that he sold at the local market in the Kerkennah islands. Today, he pulls up only “ISIS” — the name locals have given to the blue crabs that have invaded their fishing grounds in the fast-warming waters of the Mediterranean.
“The fisherman, … instead of finding fish to provide an income, he finds something that cuts his nets,” Chelli complained.
For more than 70 days this summer, a marine heatwave cooked the waters of the western Mediterranean.
It was the worst swelter for the western part of the basin in the last four decades, said marine ecologist Joaquim Garrabou at Spain’s Institute of Marine Sciences, who monitors temperature gauges in the sea’s near-coastal waters.
Measurements taken by European Space Agency satellites show that, from June through September, the waters off north Africa and southwest Europe were 2 to 5 degrees Celsius above the 1985-2005 daily averages. Temperatures peaked at nearly 31C in some parts.
By September, populations of sponges, sea-stars, fish, and mollusks were dying en masse in the waters off France and Spain. Corals bleached to bone white.
While tourism drives most of the sea’s economic activity, worth $450 billion in 2017 according to the World Wildlife Fund, there are millions who rely on the sea’s bounty for their livelihoods.
But as climate change makes the Mediterranean among the world’s fastest warming seas – with temperatures rising about 20% faster than the global ocean average – that bounty is under threat.
The rapid warming is due partly to the fact that the Mediterranean is a relatively shallow and contained basin. With an area of about 2.5 million square kilometers (970,000 square miles), it is a “climate change hotspot because it’s a small sea,” Garrabou said.
There are few connections between the sea and the Atlantic Ocean to the west, so there are “not a lot of ways out for warm water,” he said. The overall water temperature is now 0.4C higher on average than it was 30 years ago, data show.
Acute marine heatwaves can form when warm air temperatures coincide with stable ocean conditions – when there is less mixing between the colder, deeper layers of water and the warmer surface layer.
As the world warms, marine heatwaves are expected to become more frequent, according to the U.N. Intergovernmental Panel on Climate Change. Already, climate change has helped to drive the annual number of ocean heatwave days up 54% in the period from 1925 to 2016, a team of international scientists found in 2018.