Climate change is cooking our fish before they even leave the sea. Marine heatwaves on the surface of the Mediterranean are breaking records for temperature and duration. Surface heatwaves are affecting as much as half the Mediterranean basin and are wreaking ecological havoc, according to a new study published in the journal Geophysical Research Letters that looked at the period between 1982 and 2017.
However uniform a sea's surface may seem, it isn’t. The problem is not evenly distributed. Among the areas most prone to marine heatwaves are the waters off Israel and Lebanon – which is also where the anomalous heat is the most severe, as much as 2 degrees Celsius.
In three of the past seven years, 2012, 2015 and 2017, Mediterranean heatwaves were record-breaking, says the paper by Sofia Darmaraki of Meteo France and colleagues.
Incidents of anomalously high seawater temperatures have been detected along at least half the world’s coastlines. So far, Mediterranean marine heatwaves have usually been on the surface and have ebbed within a couple of weeks. Temperature anomalies in the depths of the sea last longer and are more extreme.
Both types are getting more common, hotter and are lasting longer, the scientists warn.
Heat stress can devastate marine life and fisheries, which are already suffering from massive overexploitation and pollution. Hot water encourages algae growth– which can drive the development of toxic blooms.
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Darmaraki's study – funded by the MARmaED project, which is a a Marie Sklodowska-Curie European Training Network under Horizon 2020, and disseminated by the American Geophysical Union – is the first to describe the rising frequency of Mediterranean marine heatwaves. It is also one of the first to examine hot spots in the depths of the sea, the team says.
The study, she stresses, does not attempt to link the surface and deep sea heatwaves. Intuitive as the association is, that is for another time and another paper.
Her team checked historic data for anomalies at the surface and at three depths, 23 meters, 41 meters and 55 meters, between 1982 and 2017. That covers the range of depths at which thermal stress-related mass mortality of Mediterranean marine species has been regularly observed in the past, Darmaraki tells Haaretz.
The researchers found that surface heatwaves occur during the summer, but the deeper the water, the later in the year they occur in the depths of the sea.
Marine heatwaves at the surface are more frequent than in the depths, and are bigger, covering almost half the Mediterranean Sea basin. The average increase in temperature is over half a degree Celsius. Subsurface heatwaves are rarer and smaller in scope, but last longer on average. They can raise the water temperature by 1 to 2 degrees.
While the surface heatwaves tend to occur from July to September, the researchers found seasonal shifts in the depths. At 23 meters, heatwaves have historically occurred in September and October. At 41 meters, they have happened in October and November; and at the depth of 55 meters in the Mediterranean, they hit between mid-October and December.
Meanwhile, in Israel and Lebanon
Marine heatwaves can occur anywhere in the world's oceans and seas, and their effect on local marine life can be as cataclysmic as overheating on land.
In 2003, terrestrial Europe experienced deadly heat that killed tens of thousands of people. At the same time, the Mediterranean Sea experienced a large-scale heatwave that decimated sponges, algae, and coral. It warmed the top 15 meters of the water, but was also detectable at 23 meters too.
However uniform the water may look, it isn’t. When it comes to surface heatwaves, as noted, they can be enormous, covering almost half the area of the Mediterranean basin – but the worst-affected areas are the northern part of the basin: the Adriatic Sea, the northern Aegean Sea, the Gulf of Lion (from southern France to Spain), the Tyrrhenian Sea and the Balearic islands, Darmaraki told Haaretz.
The marine heatwaves there aren’t necessarily the longest, but they are the worst in terms of intensity. “This could imply severe impacts on marine ecosystems of the area and their ability to adapt to such intense but not so frequent events,” she adds.
This northern Mediterranean basin area also suffers subsurface heatwaves, but they are not particularly intense.
The award for most intense subsurface hot spots goes to the waters off Israel and Lebanon, as well as Italy, Albania and Greece. The temperature may rise by 1 to 2 degrees, which is a lot.
Why the subsurface heatwaves there are the worst is not clear, Darmaraki acknowledges, but the subject is being investigated.
As for the life forms in the sea, the problem is less creeping temperature increase than extremes, just as on land.
In early 2011, at the height of summer in the southern hemisphere, the west coast of Australia experienced a catastrophic heatwave that raised the ocean temperature from 2 to as much as 5 degrees Celsius (that at a depth of 10 meters!). At the time, it was the highest-magnitude warming event on record. The warming persisted for more than two and a half months, killing fish and bottom-dwelling invertebrates en masse. The seaweed canopy was devastated.
“After the  heatwave, both benthic [bottom-dwelling] and fish communities were markedly distinct from preceding years,” a study of the ecological impact of the event in the journal Nature reported, noting also that warm-water fish such as the western scalyfin and lined dottyback moved in.
Lobsters and the people who eat them did badly from the 2012 heatwave in the Atlantic’s Gulf of Maine. And Pacific marine heatwaves from 2014 to 2016 caused massive toxic algae blooms that closed down an important crab fishery, Darmaraki adds. From 2015 to 2016, a marine heatwave in the Tasman Sea killed off oysters. The list is very long.
A drop in the boiling sea
The role of global warming in the increasing frequency and intensification of marine hot spots is unproven but glaring. “Yes, indeed. It is very likely that global warming has an important role to play in the increase in the frequency, duration and intensity of the marine heatwaves,” Darmaraki concurs. “It has been shown in similar studies of marine heatwaves around the world.”
She is also lead author of a paper published in Climate Dynamics in February 2019 entitled ”Future Evolution of Marine Heatwaves in the Mediterranean Sea” which shows correlation between global warming and marine heatwaves.
Unshockingly, the models indicate that the hotter the atmospheric temperatures, the stronger and more intense the marine heatwaves become. It isn’t only the land around the Mediterranean that seems likely to become unsurvivable by the year 2100. The sea may be as well.
“By 2100 and under RCP8.5 [the worst-case greenhouse-gas emissions scenario], simulations project at least one long-lasting marine heatwave every year, up to three months longer, about 4 times more intense and 42 times more severe than present-day events,” the paper says.
Nor will there be anywhere to hide: “They are expected to occur from June to October and to affect at peak the entire basin.”