‘Mind-blowing’ marine heat waves put Mediterranean ecosystems at grave risk

‘Mind-blowing’ marine heat waves put Mediterranean ecosystems at grave risk

  • A recent study reveals the widespread effects of climate change-driven marine heat waves on the ecological communities of the Mediterranean Sea.
  • Rises in sea surface temperatures as high as 5° Celsius (9° Fahrenheit) above normal have caused die-offs in 50 different taxonomic groups of animals from around the Mediterranean Basin.
  • These far-reaching impacts of the warming sea could devastate the fisheries on which many of the Mediterranean region’s 400 million people rely.
  • Researchers advocate bolstering and expanding marine protected areas. Although they can’t hold back the warmer waters that have proven deadly to the sea’s rich biodiversity, these sanctuaries can help ensure that these species don’t have to cope simultaneously with other pressures, such as overfishing or pollution.

Heat waves are reshaping life in the Mediterranean, with few of the sea’s coves, bays and shorelines untouched, according to a recent study looking at the impact of marine heat waves between 2015 and 2019.

Surging sea surface temperatures, at times up to 5° Celsius (9° Fahrenheit) above normal and driven by climate change, triggered “unprecedented” die-offs among dozens of species in each of the five years of the study period, the authors reported July 18 in the journal Global Change Biology.

“What is happening is quite dramatic,” Joaquim Garrabou, a marine conservation ecologist at Spain’s Institute of Marine Sciences and the study’s lead author, said in an interview. “It’s like having a forest fire in the marine habitat.”

But unlike the visible destruction that comes with a forest fire, the tranquil surface of the sea belies the upheaval just below, Garrabou said, with little outward sign of “what’s going on under this blue layer.”

What’s going on is the shocking loss of the superlative biodiversity found in the Mediterranean, he said. Though small compared to the world’s great oceans, the Mediterranean Sea is home to 7-10% of all marine species, many of them found nowhere else on Earth. What’s more, seafood harvests from that biological bounty support many of the 400 million people living in the region.

Though small compared to the world’s great oceans, the Mediterranean Sea is home to 7-10% of all marine species, many of them found nowhere else on Earth. Image by Enrico Strocchi via Flickr (CC BY-SA 2.0).
Fishing boats.
Seafood harvests from the Mediterranean Sea support many of the 400 million people living in the region. Image by JanneG via Pixabay (Public domain).

To bring the changes to these coastal ecosystems into sharper focus, Garrabou led an effort to bring together research on the effects of climate change from around the Mediterranean Sea. Ultimately, he said, he hopes this evidence will convince leaders and policymakers of the importance of protecting the basin’s ecological communities.

“If the people understand that what is going on is so serious and that we need to provide solutions, maybe the implementation of these measures will be much easier,” Garrabou said.

The research team began by evaluating sea surface temperatures using satellite data going back to the 1980s, along with localized temperature readings from 75 sites in 11 Mediterranean countries from the online database T-MEDNet. They then analyzed the results of coastal biological surveys from 33 research teams, looking at when, where and what species were involved in large die-offs — what scientists call “mass mortality events.”

The satellite data showed that the Mediterranean’s average surface temperature has risen by 1.2°C (2.2°F) since the mid-1980s, and the five years of the study period were the warmest recorded over that time. In part, that rise is due to the buffer that bodies of water like the Mediterranean have provided against the planet’s warming atmosphere. The world’s marine systems have absorbed 90% of the heat stemming from greenhouse gas emissions, and they’ve also drawn in around one-third of the excess carbon added to the atmosphere by humanity since the beginning of the Industrial Revolution.

Marine heat waves have hit more than three-quarters of the Mediterranean’s surface in the past decade, laying bare the consequences of sponging up that much carbon and heat.

The research by Garrabou and his colleagues shows that those surging temperatures impact a staggering breadth of marine species, with documented mass mortality events between 2015 and 2019 affecting 50 taxonomic groups from categories across the animal kingdom.

Many Mediterranean marine species hail from the higher-latitude, temperate — and in some cases, colder — regions of the Atlantic Ocean as a result of the connection through the Strait of Gibraltar that opened more than 5 million years ago. Through this conduit, the Atlantic has been a critical source of biodiversity for the Mediterranean, allowing the entrance at different times of species with an affinity for cold or warm water depending on climatic fluctuations. Today, Mediterranean marine biodiversity still carries a strong signature of the last glacial age that ended about 17,000 years ago, with many species unable to withstand current warming rates.

This research “starts to uncover the biological mechanism behind the loss of [Mediterranean] biodiversity due to global warming,” said Paolo Albano, a senior scientist at Italy’s Stazione Zoologica Anton Dohrn.

Mediterranean coast.
The satellite data from the study showed that the Mediterranean’s average surface temperature has risen by 1.2°C (2.2°F) since the mid-1980s. Image by Katerina Katopis / Ocean Image Bank.

Species migrations northward or deeper down have occurred, but the confines of the Mediterranean Sea mean there’s only so far they can go.

The researchers report the occurrence of mass mortality events along “thousands of kilometers of coastline” down to depths of 45 meters (148 feet).

That’s “quite deep,” Albano said, “but still, this depth is not enough to buffer benthic communities from the effects of warming.”

Thomas Wernberg, a professor of marine botany at the University of Western Australia, said Garrabou’s comprehensive study reveals that the patterns of biodiversity loss due to heat waves are “extensive.”

“It is really scary and mind-blowing,” said Wernberg, who also was not involved in the research. “We are not talking about individual isolated effects in some rare corner of the planet on some particularly sensitive critter. We’re talking about whole ecosystem impact.”

Around the world, climate-warming greenhouse gas emissions from fossil fuel use, deforestation and agriculture continue apace, forcing average global atmospheric temperatures upward. And the Mediterranean happens to be a climate change hotspot, with atmospheric warming increasing nearly twice as quickly as the rest of the world; the region has already warmed by more than 1.5°C (2.7°F) over pre-industrial levels.

Scientists recently projected in the journal Reviews of Geophysics that if global emissions continue at the current rate — the so-called business-as-usual scenario — a total spike of 5°C (9°F) in atmospheric temperatures by 2100 in the Eastern Mediterranean and Middle East is possible.

Marine heatwaves in the Mediterranean.
The ongoing climate change means marine heat waves — and their effects on life in the Mediterranean — will continue, and there’s no time to spare to avoid the worst effects. Image courtesy of Garrabou et al. (2022).

Garrabou said ongoing climate change means marine heat waves — and their effects on life in the Mediterranean — will continue, and there’s no time to spare to avoid the worst effects.

Stopping the heat waves will require a swift drawdown of the amount of carbon emitted by humans. Even then, reversing the warming in the Mediterranean Sea would take a century or more, Wernberg said. But society has yet to fully make strides toward such a bold step.

“The tragedy here really is that we have known about these potential changes for about 30 years,” he said. Had humanity acted as the first signs of global warming were coming into focus, “We would not have been facing a future where the train had already left the station.

“Now, it just takes a long time to stop that train,“ Wernberg added.

Still, investing now in preserving and expanding marine protected areas (MPAs) in the hardest-hit parts of the sea could help ecological communities weather heat waves, Garrabou said. Currently, MPAs cover only about 8% of the Mediterranean.

But while boundaries of a protected area can’t hold back destructive surges of warm water, Wernberg said, well-protected MPAs could help ensure that ecosystems aren’t confronted by numerous assaults. If MPAs reduce overfishing or chemical pollution, for example, it could give species a better chance to survive the impacts of warming, he said.

“It’s about simply not adding multiple stresses onto these ecosystems,” Wernberg said.

Albano said he favors stringent protections for deeper areas, down to perhaps 150 m (492 ft). These deep-down assemblages of native plants and animals survive intact below the thermocline, where the tendrils of warmer waters have yet to reach. The hope is that these species, many of which can also live in shallower waters, will repopulate the waters above — if they somehow do cool down in the future.

Human-led restoration of degraded marine habitats is also worth pursuing, Garrabou added. But he acknowledged that such efforts are costly and “limited” in scale compared to the impacts of the basin-wide battering by heat waves that he and his colleagues have documented.

Still, we must use every tool at our disposal to maintain what remains of the Mediterranean’s remarkable biodiversity, he said.

“There is no magic solution for this,” Garrabou added. But, “If we keep the ecosystems or the habitats in the best condition possible, they can face these climate change effects in a better way.”

Banner image: A moray eel among corals in the Mediterranean Sea. Image by Enrico Strocchi via Flickr (CC BY-SA 2.0).

John Cannon is a staff features writer with Mongabay. Find him on Twitter: @johnccannon


Garrabou, J., Gómez‐Gras, D., Medrano, A., Cerrano, C., Ponti, M., Schlegel, R., … Harmelin, J.-G. (2022). Marine heatwaves drive recurrent mass mortalities in the Mediterranean Sea. Global Change Biology28(19), 5708-5725. doi:10.1111/gcb.16301

Zittis, G., Almazroui, M., Alpert, P., Ciais, P., Cramer, W., Dahdal, Y., … Lelieveld, J. (2022). Climate change and weather extremes in the Eastern Mediterranean and Middle East. Reviews of Geophysics60(3), e2021RG000762. doi:10.1029/2021RG000762

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