The Med has a fever

By Jack Wittels 

Add unprecedented sea temperatures in the Mediterranean to the grim list of heat-related records being smashed this summer.

The Mediterranean’s surface temperature hit 28.7C (83.7F) last week, the highest median recording for any day since at least 1982, according to Spain’s Institute of Marine Sciences. It’s a record that may soon be surpassed, with the sea typically at its hottest in late August.

“The situation is crazy,” said Melanie Juza, a researcher at the Balearic Islands Coastal Observing and Forecasting System, or SOCIB, a public consortium backed by Spain’s government. “The Mediterranean Sea is responding very rapidly to climate change.”

A pathway leading to Porto Rotondo, during hot weather caused by the Charon heat wave, near Olbia, on the island of Sardinia, Italy, on Tuesday, July 18, 2023.  Photographer: Francesca Volpi/Bloomberg

Spikes in the Mediterranean’s surface temperatures may seem less dramatic than what’s happening on nearby land — a scorching 48C was recently reported for Sardinia and local records have been broken this summer in Rome and Catalonia. But for marine life, these sorts of readings are extreme — and could be deadly.

Take Posidonia oceanica, a type of seagrass that grows in vast underwater meadows. Endemic to the Mediterranean, it’s both an enormous carbon sink and a vital nursery for fish. But if the water gets too warm, its growth can be stunted or, even worse, it can die off. Recent temperatures in the Mediterranean have been high enough to potentially cause damage, according to Mauro Randone, a marine-focused scientist working at the World Wide Fund for Nature’s Mediterranean Marine Initiative.

It’s a similar story for red gorgonians, a type of coral. Typically found at depths of below 25 meters (82 feet), it’s usually safe from swings in sea surface temperature. But marine heatwaves have killed them in the past, and the temperatures seen in recent weeks may also have had an impact, according to Randone.

Source: Balearic Islands Coastal Observing and Forecasting System website

Globally, other marine environments are also suffering from extreme heat. Approximately 44% of the global ocean recently experienced marine heat waves, according to the US National Oceanic and Atmospheric Administration.

Extreme water temperatures are responsible for coral bleaching and harmful algal blooms, among other impacts. One of the most notorious marine heat waves, known as “The Blob,” hit waters off North America’s west coast in the mid-2010s, decimating populations of Pacific cod, seabirds and salmon.

The Mediterranean Sea has its own unique qualities. It’s a hub for biodiversity, hosting up to 18% of known marine species — many of which can only be found there — despite representing just 0.3% of the global ocean by volume.

It’s also unfortunately becoming a “lab for climate change,” said Simon van Gennip, a research scientist at non-profit Mercator Ocean International. “It’s one of the fastest warming parts of the ocean in the world.”

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